Thyroid hormone is considered a permissive hormone. comment.

• Thyroid hormones are required for the actions of other hormones on target tissues.
• This permissive action is also a characteristic of gonadal and adrenal steroids on some of their target tissues.
• Thyroid hormone induce GH production in cultured rat pituitary tumor (GH1) cells.
• Glucocorticoids also stimulate GH secretion, but only in the presence of thyroid hormones.
• There is a dramatic synergistic activation of GH production when both hormones are present together in the medium, suggesting that the ability of glucocorticoids to stimulate GH synthesis is controlled by thyroid hormones.
• The enzyme Ornithine decarboxylase (ODC) is essential for polyamine biosynthesis and is intimately related to the regulation of nucleic acid and protein biosynthesis.
• Growth hormone stimulate ODC activity in brain tissue, but only in the presence of thyroid  hormones.
• Thyroid hormones, however, do not stimulate brain ODC activity.
• This demonstrates dramatically the permissive action of a hormone. In the liver in contrast, GH and thyroid hormone stimulate ODC activity independently of each other.
• These results demonstrate the tissue specificity to the actions of these hormones within an individual animal.

Some physiological roles and actions of thyroid hormones:

a) Permissive action:

1. Enhances lipolytic response of adipose tissue to hormones.
2. Required for growth-promoting activity of GH.
3. Increases activity of the sympathoadrenal system.

b) Regulates basal metabolic rate:

1. Increases mitochondrial oxidative phosphorylation.
2. Required for bone growth and maturation.
3. Required for hepatic conversion of carotene to vitamin A.

c) Induces enzyme synthesis:

1. Na+/K+ ATPase
2. Carbamoyl phosphate synthetase.

Give the causes and consequences of :

(A) DIABETES INSIPIDUS:

Causes: 

The control of Arginine Vasopressin (AVP) release and regulation of fluid balance occurs primarily in response to alterations in blood toxicity and blood volume so that a relative constancy of these two parameters is maintained.

Impairment of neurohypophysial system to synthesize or release AVP results in pituitary (neurogenic) diabetes insipidus. Failure to synthesize AVP can also occur as a result of disease processes that destroy hypothalamus or posterior pituitary. It can also result from traumatic or surgical injury to hypothalamus.

In nephrogenic (AVP resistant ) diabetes insipidus (NDI), there is defect in urinary concentration mechanism at the level of kidney. It involves failure to generate intracellular level of cAMP within distal tubular cells of kidney in response to AVP. Kidney specific V2 vasopressin receptor is defective in this disorder (either Xq28 gene or AQP2 gene is mutated).

Consequences:

The affected person feels excessive thirst and passes large amount of urine frequently. Even if a person drinks less amount of water, urine formation remains unaffected. The patient passes pale and diluted urine. Some of the complications of diabetes insipidus are :

• Dehydration: It can cause dry mouth, fever, headache, weight loss etc.
• Electrolyte Imbalance: It can cause nausea, cerebral edema, seizures, shock and even coma in serious conditions.
• Hypotension/ low blood pressure : Dizziness may occur due to low blood pressure which can damage heart and brain.
• Thrombosis: Due to decreased blood volume and circulatory fluids, heart is not able to pump properly and can cause shock to many body organs.

(B) CUSHING SYNDROME:

Causes:

Cushing syndrome occurs due to excessive and prolonged secretion and action of glucocorticoids.

Primary hypercortisolism may be due to an adrenal cortical tumor (adenoma or carcinoma). Cushing syndrome of secondary origin may derive from a pituitary tumor secreting excess of ACTH, or to ectopic ACTH production, or to pituitary corticotroph hyperplasia.

In patients with adrenal tumor, plasma cortisol levels are high but concentration of ACTH is low. But in bilateral adrenal hyperplasia, both plasma cortisol and ACTH levels are elevated.

Consequences:

Excess cortisol secretion can lead to central (truncal) obesity, hypertension, glucose intolerance, hirsutism, osteoporosis, polyuria, polydipsia and hyperpigmentation. Hyperglycemia may lead to steroid diabetes. The change in fat distribution is due to lipolytic action of ACTH but now insulin is exerting a lipogenic effect in some areas of the body. Loss of protein matrix of bones causes severe osteoporosis affecting spinal column. Excess of androgens produced are the cause of hirsutism in female. Hyperpigmentation may be present in Cushing's disease of secondary origin due to presence of increased circulating level of ACTH.

(C) ADDISON'S DISEASE:

Causes:

Addison's disease occurs due to insufficient amounts of cortisol and often aldosterone as well. These hormones give instructions to virtually every organ and tissue in our body.

Primary adrenocortical failure occurs when the adrenal gland is totally damaged which may be due to body attacking itself (autoimmune disease). Most often etiology is due to bilateral tubercular destruction of glands. Atrophy due to tuberculosis involves medulla as well as cortex whereas in idiopathic atrophy, only cortex is affected. In Addison's disease of secondary and tertiary origin, the defect may reside at the level of pituitary and hypothalamus respectively. A failure of hypothalamus to secrete CRH or absence of functional pituitary corticotrophs could explain the etiology of some forms of adrenal insufficiency.

Consequences:

Addison's disease cause extreme fatigue, weight loss, decreased appetite, hypotension, salt craving, hypoglycemia, muscle or joints pain, depression, nausea, diarrhea, increased melanin pigmentation of skin (because of absence of cortisol feedback of hypothalamus, which releases excessive ACTH), body hair loss or sexual dysfunction in women, high potassium and low sodium levels.

(D) GIGANTISM:

Causes:

Gigantism is almost always caused by a benign adenoma in the pituitary gland. An adenoma is a non-cancerous tumor and in case of gigantism causes secretion of too much growth hormone. Pituitary tumors can be small in size (micro-adenoma) or large (macro-adenoma). However, in gigantism they are frequently large and invade nearby brain tissue. The size of adenoma directly affects the signs and symptoms experienced by the individual.

Consequences:

Gigantism involves accelerated growth leading to above-normal expected height for a child of their age. The effect of growth hormone on bone, ligaments and soft tissue swelling can lead to coarse facial features, excessively large hands and feet with thick fingers and toes, prominent forehead and jaws.

Larger adenomas can damage the function of normal pituitary gland causing failure of secretion of other hormones. It may also cause headaches, visual field disturbances and nausea. In some cases, puberty may be delayed if pressure from adenoma on pituitary gland results in failure of sex hormone secretion and thus allows further growth of skeleton.

Gigantism may be complicated by diabetes, high blood pressure, enlargement of internal organs especially the heart.

 

Insulin receptor is an excellent example of divergence of different signaling pathway?

The insulin receptor is a multifunctional protein encoded by a modular gene. Certain discrete domain within the insulin-receptor structure subserve specific functional properties. This organizational model of the insulin receptor predict the existence of divergent signaling pathway facilitating specific bioeffects.
Although insulin play the dominant role in the control of carbohydrates metabolism in human and other vertebrates, it also profoundly affect growth process in animal. Children with diabetes, for e.g., fail to grow even though GH level are normal, whereas infant of diabetic mother with islet hyperplasia and hyper-insulinism are of increased stature. This might not be too surprising considering the structural similarities among insulin, the IGFs, and their receptors. The growth promoting action of insulin are amply documented, and protein catabolism is accelerated in the absence of insulin. Insulin is required for the full anabolic effect of GH an action that may be because insulin, through its action on glucose uptake by muscle, provide the energy substrate necessary for protein synthesis. Insulin also increase the incorporation of amino acid into muscle by an action that is independent on its effect on glucose metabolism. This may result from the direct action of insulin on the transport of amino acid into cell, as well as activation of ribosomal translational capacity, a protein synthesis per second is not dependent on glucose availability or RNA synthesis. Insulin stimulates the growth of immature hypophysectomized rats, but this action is manifest only when the protein-sparing action of insulin is enhanced by the concomitant feeding of a highly carbohydrate diet.
Insulin at a high concentration, as in familial insulin resistance, stimulates general body growth through low-affinity binding to IGF receptor.

Distinguish the effect of estrogen and progesterone on:

(A) ENDOMETRIUM:

The role of estrogen is to regeneration and growth of the endometrium prior to ovulation. It leads to thicken of uterine lining during the pre-ovulatory phase of the cycle. One of the progesterone's most important function is to cause the endometrium to secrete special protein during second half of menstrual cycle, preparing it to receive and nourish an implanted fertilized egg. If implantation does not occur, estrogen and progesterone level drops, the endometrium breaks down and menstruation occur.

(B) CERVICAL MUCUS:

The cervix is the lowest portion of uterus. It undergoes hormonally induced changes during ovarian cycle. Under the influence of estrogen during the follicular phase, the mucus secreted by the cervix become clear and thin. This change, which is most pronounced when estrogen is at its peak and ovulation is approaching, facilitate the passage of sperm through cervical canal. After ovulation, under the influence of progesterone, the mucus becomes thick and sticky, essentially plugging up the cervical opening. Sperm cannot penetrate this thick mucus barrier.

(C) MYOMETRIAL:

Progesterone withdrawal or enable to create myometrial contraction. During late pregnancy the progesterone level drop. As progesterone decreases sensitivity is increased to oxytocin.

(D) MAMMARY GLAND:

Estrogen is primary hormone that stimulate breast growth during puberty, progesterone act with estrogen to maintain female reproductive health and female sex characteristics. Progesterone has several effect on breast tissue during puberty, menstruation and pregnancy. It also influences the growth of cancerous tissue in the heart. Progesterone levels begin to rise and peak during menses. Progesterone that occur during menstrual cycle contributes to cause of the characteristic breast tenderness during menstruation. During pregnancy, progesterone promote further development of glandular tissue in breast so that it will secrete milk.

(E) GONADOTROPHIN SECRETION:

The primary mechanism of action of hormonal contraceptives is that they suppress the secretion of gonadotrophins (FSH,LH). A woman receives a combination of progesterone and estrogen or just estrogen. Progesterone and estrogen naturally inhibit gonadotrophins secretion during luteal phase of cycle. The goal is to suppress ovulation.  

What changes bring about induction of labor leading parturition?

Many factors are known to be involved in the process of labor, the sequences of events that induces and temporarily organizes labor:

• One model for the initiation of labor in the human suggests that as a result of the rising estrogen levels during gestation, the contraction of uterine OT receptors increases.
• Rapid fetal growth near term increase uterine distention which may also contribute to increase in uterine OT receptors near term.
• The increased number of OT receptors probably lower the threshold of the uterus to a level where myometrial contractions are initiated.
• Oxytocin may bind to decidual receptors and stimulate progesterone biosynthesis. The coupling of OT receptors activation and PG biosynthesis may be the crucial event in the initiation of labor.
• During spontaneous labor there is an increase in plasma OT levels in 6the fetal umbilical artery compared to umbilical vein, indicating a flow of OT from the fetus towards maternal compartment.
• Oxytocin derived from the fetus may provide the stimulus for the increased production of PGs at the onset of labor.
• Hypothalamoneurohypophysical maturation within the fetus, leading to fetal adrenal activation and a decline in progesterone biosynthesis as well as increased neurohypophysial hormone secretion, may provide final essential directive induction of parturition.

Name the stimulus and hormone which initiate lactation.

Prolactin is the most important hormone for the actual initiation of lactation postpartum. Milk secretion is regulated by PRL through an action on plasma-lemmal receptors of mammary gland Alveolar secretory cells.
It is ,however, the fall in estrogen and progesterone levels which gives the stimulus to lactation. Baby suckling also provides stimulus to lactation.

Justify the following:

1. TSH levels are low in grave's disease while in hashimoto's TSH levels are high.

TSH levels are low in grave's disease because the pituitary gland will try to compensate for the excess T3 and T4 hormones in the blood. It will stop producing TSH in an attempt to stop production of the thyroid hormones whereas, in hashimoto's disease, TSH levels are high because the brain thinks the pituitary gland is not making enough thyroid stimulating hormone which in return, is not producing enough thyroid hormones and needs stimulation.

2. All steroid hormone receptors have DNA binding domain.

Steroid hormones mediate their actions through genomic mechanisms. The hormone enters the cell by passive diffusion. It binds to the hormone-binding domain of the receptor. This binding results in a conformational change exposing the receptor's DNA binding domain and enabling it to bind to the nuclear chromatin. This binding to DNA initiate transcription of mRNA and ultimately translation into androgen-specific proteins.

The DNA-binding domain, a 66-68 amino acid sequence is encoded by exons B and C which code the first and second (C-terminal) zinc finger motifs respectively. The first zinc finger specifies the receptor's DNA recognition and the second is mainly responsible for dimerization of two receptor molecules during the association with DNA. The androgen binding C-terminal domain consists of about 252 amino acids and is encoded by exons D-H.

 3. DOPA is an important agent in the treatment of Parkinson disease.

Levodopa is the most commonly prescribed medicine for Parkinson's disease. It works when brain cells change it into dopamine. Dopamine is a neurotransmitter present in the brain which sends signals that helps in the movement of the body. People suffering from Parkinson's disease don't have enough dopamine in their brains to control their movements. There are many dopamine agonists available which are recommended first than levodopa therapy.

4. Ras protein requires Sos for its activation.

In the mechanism of activation of the INSR protein kinase, an adaptor protein functions to bring together two proteins: IRS-1 and the protein Sos that must interact to enable signal transduction. In addition to Grb2's SH2 domain, it also has SH3 that binds to proline-rich region of Sos recruiting Sos to the growing receptor complex. When bound to Grb2, Sos acts as a guanosine nucleotide exchange factor (GEF) catalyzing the replacement of bound GDP with GTP and hence Ras is activated. Ras can exist in two conformations either active GTP bound or GDP bound inactive.

5. Goiter is a symptom of both hyper and hypo-thyroidism.

Failure of the thyroid to produce T4 and T3 may lead to the development of a goiter since, in the absence of any negative feedback to the hypothalamus and pituitary gland, there is excessive secretion of TSH. In the absence of thyroid hormone production TSH stimulation leads to hypertrophy of the thyroid follicular cells which then causes the gland to enlarge.

Goiter may also develop under conditions of hyperthyroidism due to auto-antibodies against thyroid TSH- receptors causing it to produce excess thyroid. This overstimulation causes the thyroid to swell.

6. Hyperglycemia is not a confirmatory symptom for diabetes mellitus.

Hyperglycemia is an abnormally high blood glucose level. It is not always related to lack of insulin or insulin sensitivity and diabetes is characterized by high amounts of sugar in the bloodstream that occurs as a result of a lack of insulin or insulin sensitivity.

Kidney is an endocrine organ. Comment.

Kidney is an endocrine organ. It produces three important hormones: erythropoietin, calcitriol (1,25-dihydroxycholecalciferol) and renin. They also synthesize prostaglandins which affect many processes in kidney.

ERYTHROPOIETIN:

• It is a peptide hormone which regulates erythropoiesis.
• It is a glycoprotein containing 165 amino acids. Its receptors are present on the membrane of RBC precursors. Binding of the hormone reduces apoptosis of these cells-multiple cell survive and therefore complete their development into mature erythrocytes.
• In adults, approx. 90% of the erythropoietin is synthesized in the kidneys (interstitial cells) and remaining amount in liver.
• Erythropoietin is also abused as doping substance especially in endurance athletics.

CALCITRIOL:

(1,25-dihydroxycholecalciferol) Final activation of vitamin D to the active hormone calcitriol takes place in kidneys 1-hydroxylation of 25 hydroxycholecalciferol to 1,25-dihydroxycholecalciferol. Calcitriol stimulate the small intestine for protein synthesis allowing absorption of Ca2+ and phosphates. This ensures availability of Ca2+ and phosphate for bone growth.

RENIN:

Renin is a part of renin-angiotensin aldosterone (RAAS). In case of insufficient blood flow to the system, kidneys (decrease in blood volume). Cells of renal juxtaglomerular apparatus begin synthesis of protein renin. Renin is an enzyme which catalyzes the conversion of angiotensinogen to angiotensin I. Angiotensin I is then converted to angiotensin II by angiotensin converting enzyme which stimulates aldosterone synthesis and causes vasoconstriction.

Justify the following:

1.  ANF affects natriuresis and diuresis.

ANF is a humoral substance which regulates volume overload by inhibiting, reabsorption by the renal tubules, thereby promoting natriuresis and diuresis. ANF affects diuresis and natriuresis by several physiological functions:

1. It inhibits aldosterone production by the adrenal glomerulus cells.
2. It inhibits the release of renin (which indirectly stimulates aldosterone secretion).
3. It inhibits vasopressin secretion from the pituitary as well as the action of vasopressin at the level of kidney.
4. It causes relaxation of blood vessels (possibly by antagonizing the vasoconstrictor actions of AII).

All these ANF actions effectively reduce the retention of Na+ and water and therefore, decreased volume expansion.

2.  If the neural connection between the hypothalamus and pituitary were severed then secretion of some pituitary hormones area affected.

Since the secretion of the pituitary principles of the pars distalia and pars intermedia is under the control of hypothalamic releasing and inhibitory hormones. So if the neural connection between the hypothalamus and pituitary are severed then some hormones will not be secreted or synthesized. For e.g., the cell bodies for the hormone vasopressin and oxytocin originate within the hypothalamus and secreted through the posterior lobe of pituitary and when the connection is severed several hormones might not be there which are required by the human body.

3.  Catecholamines can produce prolonged hyperglycemia.

Epinephrine increase both the force and rate of the heartbeat through stimulation of cardiac muscle beta-adrenergic. There are no other examples known where beta-adrenergic induce muscle contraction. The particular distribution of vascular smooth muscle ARs provide a mechanism for shunting of blood to various body compartments during stress. For e.g., blood is shunted from the skin, mucosa, connective tissue and kidneys; vascular smooth muscles of these organs possess alpha-ARs. In contrast, smooth muscle of coronary arteries as well as skeletal muscle possess beta-ARs. If dominant alpha-ARs were present, blood flow to these vital organs would be diminished when these organs were maximally active. The decreased blood flow to the kidneys reduces glucose clearance from the circulation, and this reduced clearance may be primarily responsible for the prolonged hyperglycemia induced by catecholamines.

4.  PDGF can play an important role in the development of atherosclerosis.

Platelets also release one or more chemical messengers that stimulate contraction of the injured vessels to prevent further loss of blood. At the site of injury platelets release PDGF in response to thrombin. Its role is to induce proliferation of smooth muscle cells within the arterial wall. Although PDGF serves an important role in the healing process of the vascular system, it may play a dominant part in the development of atherosclerosis as PDGF is a potent vasoconstrictor, and it is possible that this peptide may be responsible for the increased vasoreactivity that occurs primarily at the site of arthrosclerosis lesions. One of the main causes for atherosclerosis is smooth muscle proliferation which is done by PDGF.

5.  A male athlete taking large amount of an androgenic steroid hormones becomes sterile.

When using steroids, users try to maximize the anabolic (tissues growth stimulatory) effect and minimize the androgenic (male type) effect. These steroids have a significant effect on gonadal function. The effects are individual specific in some users sperm production ceases totally and in other the changes are minor.

The functioning of the testicles is controlled by hypophysis. Androgenic steroids prevent the normal functioning of the hypophysis. The testicles own testosterone production and concentration decreases and as consequences, sperm production decreases and may cease totally. These steroids also have an effect on structure of sperm cells. Since they decrease testosterone concentration in blood circulation which is insufficient normal libido and the hormone imbalances cease sexual apathy.

6.  Tissue responsiveness towards a hormone does not remain constant.

Yes, it is true that tissue responsiveness towards a hormone does not remain constant. Some hormones can simultaneously stimulate a number of different tissues which implies that each of these diverse tissues possess receptors for the hormones. Insulin stimulates glucose uptake by hepatocytes, fat cells and certain muscle cells and interact with many other cell types. Thus, insulin is able to rapidly lower extracellular glucose levels, which is one of its functions. If more than one tissue responds to a particular hormone at one time, it is often the case that all these different physiological responses would complement the physiological process being regulated. For e.g., parathyroid hormone (PTH) elevates serum Ca2+ levels by releasing Ca2+ from bone, stimulating calcium uptake from the gut and preventing Ca2+ loss from the kidney. Each of these individual responses to PTH is important in elevating Ca2+ levels.

Tissue responses to a hormone are determined by the presence of specific receptors. If each type of all possessed all of the specific hormone receptors then all cells might be expected to respond to all chemical messengers. If this were the case, all the cells would response and this would result in uncoordinated muscle contraction. and relaxation and an uncontrolled secretion of numerous cellular products. Therefore, cells do not possess receptors for all hormones and the magnitude for an individual tissue's response to a particular hormone is determined in large part by the numbers of receptors for the hormone that are present in those target cells.

Describe the effect of calcitonin, PTH and vitamin D on osteoclasts.

Osteoclasts are type of bone cell that removes bone tissue by removing its mineralize matrix and breaking up the organic bone. This process is known as bone resorption.

• Since calcitonin is released from parafollicular cells in direct relation to circulating levels of Ca2+. It is suggested that calcitonin function to prevent elevated Ca2+ i.e., it prevents "postprandial hypercalcemia" i.e., absorption of Ca2+ from food during meal. Thus, calcitonin lower blood Ca2+ levels by inhibiting the osteoclast activity in bones.
• Ca2+ exchange is regulated by hormones, the most important of which is the parathyroid hormone (PTH), secreted by parathyroid gland. This hormone increases blood Ca2+ level. A major target tissue of PTH is bone, specifically stromal osteoblast cells of the bone marrow. In response of PTH, these osteoblast cells secrete one/more cytokines that then stimulate osteoclastic activity leading to the resorption of bone. In this way, demineralization of bone results in elevated levels of Ca2+.
• Hypocalcemia is stimulatory to PTH secretion from the parathyroid. PTH stimulates renal cortical 1-alpha-OHase activity and 1,25 (OH)2 D3 biosynthesis. 1,25 (OH)2 D3 stimulates Ca2+ reabsorption from the gut, increase release of Ca2+ reabsorption from the kidney. Vitamin D causes demineralization to provide Ca2+ and possibly PO43-, to maintain the critical plasma levels of these ions. However, Ca2+ and PO43- are provided for accretion of new bone.

How does thyroid hormone regulate BMR?

Thyroxine is the main hormone produced by the thyroid gland and control the body's basal metabolic rate (BMR). A person's basal metabolic rate refer to the minimum number of calories needs to survive in the absence of any activity. Every cell in the body depends on thyroid hormone for proper metabolism. The iodine containing hormone control neural development as well as releasing fat from most of the fat cell controlling metabolism and affecting growth. A lack of thyroxine contribute to growth disorder. Too much thyroxine causes a BMR that is too high. Too much thyroid hormone in blood stream can create extreme in body weight due to difference in basal metabolic rate. Thyroid hormone stimulate diverse metabolic activities leading to an increase in basal metabolic rate. Increased thyroid hormone level stimulate fat metabolism and mobilization, leading to increase of fatty acid in plasma. Also enhances oxidation of fatty acids in many tissues. Also thyroid hormones stimulate almost all aspect of carbohydrate metabolism, including enhancement of insulin-dependent entry of glucose into cells. Thyroid hormones regulate the rate of cellular metabolism. Hyperthyroidism-in which there is increase in production of thyroid hormone- leads to high BMR, while hypothyroidism- in which thyroid hormone are depleted-cause a low BMR. 

TRANSCRIPTION IN PROKARYOTES (steps of transcription part 2)

There are 3 steps of Transcription: a) Initiation b) Elongation c)Termination.

INITIATION

Initiation is further divided into three phases : 

1. Isomerization   2. Abortive Transcription  3. Promoter Escaping

ISOMERIZATION:

Transition from closed to open complex involves structural changes in enzyme. This reveals the template and non-template strands. This melting occurs between -11 to +2 position. And does not require ATP hydrolysis instead result of conformational change.

Two bases in non-template strand of -10 element (A11 and T7) flip out from stacking and insert into pockets within the sigma protein. By stabilizing the single stranded form of -10 element, these interactions drive melting of promoter region. 

THERE ARE FIVE CHANNELS INTO THE ENZYME:

1. NTP-uptake channel: It allows ribonucleotides to enter the active center to incorporate into RNA chain.
2. RNA exit channel: It allow growing RNA chain to leave the enzyme.
3. Downstream DNA channel: Downstream ds DNA enter the active center cleft through this.
4. Non-template strand (NT) channel: Non-template strand exit the active and travel across surface of enzyme.
5. Template strand channel:  Template strand follow a path through the active cleft and exits through T channel.

The double helix reforms at -11 in upstream DNA behind the enzyme.

STRUCTURAL CHANGES:

• Pincers at front of enzyme clamp down tightly on downstream DNA.
• Major shift in position of alpha-NTD (amino terminal region of sigma).

When not bound to DNA, sigma region 1.1 lies within the active site blocking the path that in open complex, followed by template strand. In open complex, 1.1 shifts some 50 A and found on outside of enzyme.

Region 1.1 of sigma is highly negatively charged. Thus, it act as molecular mimic of DNA. The space in active site is highly positively charged.

Why most transcripts start with the same nucleotide?

For the requirement of specific interactions between enzyme and initiating nucleotide. The structure of open complex shows that sigma region 3/4 linker interacts with the template strand, organizing it in correct conformation and location to allow initiation.

During initial transcription, RNA Pol. remains stationary and pulls downstream DNA into itself. This process is called 'scrunching process'.

The active center of RNA Pol. is translocated forward relative to the DNA template and synthesize short transcripts before aborting, then repeats this cycle until it escapes the promoter. Three models for figuring this were as follows:

• Transient excursions : Polymerase moves along the DNA.
• Inch worming: Front part of enzyme moves along the DNA, but back part remain at promoter.
• Scrunching : Enzyme remains stationary and pulls the DNA into itself.

ABORTIVE TRANSCRIPTION:

Initially primary transcript of 7-8 nucleotides is formed which is not able to push the linker sigma 3/4 region present at RNA exit channel and release from the active site. This process is repeated until more than 8 nucleotides are formed.

Once 9-10 length is formed, the transcript cannot accommodate within the region where it hybridizes to DNA and must start threading into RNA exit channel.

Promoter escape is associated with breaking of all interaction between Pol. and promoter and any regulatory proteins.

Scrunching is a way to store and mobilize energy during transcription initiation.

PROMOTER ESCAPING:

During promoter escaping, release of sigma 3/4 region weak the elongating complex (RNA Pol. + transcript). Since sigma is responsible for association of RNA Pol. to promoter region. Once the sigma 3/4 region is released, it weakens the association between RNA Pol. and promoter.

ELONGATION:

• 8-9 nucleotides remain bound to the DNA template.
• DNA passes through enzyme and enter the catalytic site where the strand separate and ribonucleotides enter the active site to their channels and adding to the growing channel.
• The enzyme add one nucleotide at a time to the growing RNA transcript.
• The enzyme steps forward as a molecular motor.
• The size of the bubble i.e., length of DNA is not double helical, remains constant as 1 base pair separated ahead and 1 base pair is formed.

PROOFREADING:

RNA Pol. performs two types of proof reading:

1. PYROPHOSPHOROLYTIC EDITING:

• Enzyme uses its active site, in a simple back reaction, to catalyze the removal of an incorrectly inserted ribonucleotide by reincorporation of PPi.
• The enzyme then incorporates another ribonucleotide in its place in growing RNA chain.

2. HYDROLYTIC EDITING:

• The Pol. backtracks by one or more nucleotides and cleaves the error-RNA.
• It is stimulated by Gre factors, which enhances hydrolytic editing function and serves as elongation stimulating factors means (Pol. elongates efficiently and overcome 'arrest'.)

RNA Pol. can become arrested and need removing:

In case of damaged DNA strand, the consequences of arrest cause roadblock to other Pol attempting to transcribe same gene. Transcription-coupled repair (TRCF) proteins has an ATPase activity. It binds dsDNA upstream of Pol. and use ATPase motor to translocate along DNA until it encounters RNA Pol. The collision pushes Pol. forward, allowing it to restart elongation or dissociation of ternary complex of RNA Pol. , template DNA and RNA transcript. This terminates transcription.

TERMINATION:

It is a normal and important function at the end of genes. Sequences called terminators trigger the elongating polymerase to dissociate from DNA and release RNA chain it has made. It is of two types:

Rho-dependent Termination:

• It requires rho proteins and contains rut sites.
• Rho is ring shaped protein with 6 identical subunits, binds to ssRNA as it exits the polymerase.
• This protein has ATPase activity. Once attached to the transcripts, Rho uses ATP hydrolysis energy to induce termination.
• Rho binds to RNA Pol. throughout the transcription cycle.
• The role of translocation by Rho is to tighten the resulting RNA loop and when sufficiently tight, Pol. elongation ceases.
• The rut sites for rho utilization consist of stretches of 40 nucleotides that do not fold into secondary structure. They remain single stranded and are rich in C residues.
• Rho only terminates those transcripts still being transcribed beyond the end of gene or operon.
• Rho fails to bind any transcript that is being translated.

Rho-independent Termination:

• There is no involvement of other factors.
• Rho-independent terminators are also called intrinsic terminators.
• It consist of two sequence elements: a short inverted repeats of 20 nucleotides , followed by stretch of eight AT base pair.
• These elements affect the Pol. after transcription..
• After transcription, the resulting RNA can form a stem loop structure (hairpin) by base-pairing itself. This hairpin causes termination by disrupting the elongation complex.
• The hairpin works efficient terminator when followed by stretch of AU base pair.
• Because AU base pair are weakest of all base pair they are more easily disrupted by the effect of stem-loop on transcribing Pol. 
• And thus RNA will more readily dissociate.

TRANSCRIPTION IN PROKARYOTES: (Intro- part 1)

Important points:

• RNA polymerase initiate transcription de novo without primers.
• Multiple RNA Polymerase can transcribe the same gene at same time. Thus, a cell can synthesize large number of transcripts from single gene in short time.
• DNA replication is more accurate because DNA is the molecule in which genetic material is stored and by replication it is passed on. Any mistake that arises during replication can therefore easily be catastrophic. It becomes permanent in the genome of that individual and also get passed on subsequent generations.
• Transcription, in contrast, produces only transient copies and normally several from each transcribed region. Thus, a mistake during transcription will rarely do harm than render one out of many transient transcripts defective.

RNA Polymerase ;

In prokaryotes (bacteria):

• Only a single type of RNA Polymerase.
• Alone can synthesize all types of RNA.
• The core enzyme comprises of :- 2 copies of alpha-subunits = alpha' and alpha "(homologous to RPB3 and RPB11), 1 each copy of beta, beta', omega subunits (homologous to RPB1, RPB2 and RPB6).

In eubacteria or eukaryotic cell:

• Three types of RNA pol. are there which synthesize different mRNA:
• RNA Pol I = pre ribosomal RNA, 18S RNA, 28 RNA, 5.8s RNA
• RNA Pol II = pre mRNA, snRNA, LINE sequences, miRNA
• RNA Pol III = tRNA, 5srRNA.

Plants have Pol IV and Pol V.

Shape of RNA Pol. resembles crab claw in which two largest subunits beta' and beta (or RPB1 and RPB2) forms the pincers. And the active center cleft contains two Mg2+ ions.

TRANSCRIPTIONAL UNIT:

Sequence of nucleotides in DNA that codes for a single RNA molecule, along with the sequences necessary for its transcription; normally contains promoters, RNA-coding sequences and terminator.

+1 is the site from where transcription starts. Bases under +1 are called downstream and outside the Transcription unit or before start point are called upstream elements.

 PROMOTER is the DNA sequence that initially binds the RNA Pol. together with other factors and initiates the process. 

• They also determine the rate of initiation. 
• They are cis acting elements (present near the gene).
• They can be in upstream or downstream elements.
• They can be strong or weak sequences.
• Promoters with sequences closer to the consensus sequences are generally stronger.
• By strength means, how many transcripts it initiates in a given time and it is influenced by how well - a) promoter binds the polymerase initially b)how efficiently it supports isomerization (closed to open complex)  c) how ideally polymerase can escape.

CONSENSUS SEQUENCES:

These are the conserved sequences derived by aligning number of (300) sequences known to function as promoter.

In case of prokaryotes, there are two consensus sequences:

1. Present at -10 position (contain six base pair AT rich region) (5'- TATAAT-3'). This sequence is present at non-template or sense strand.

2. Present at -35 position ( TTGACA). 

These two sequences are centered and are present at upstream region. And are separated by a non-specific stretch of 17-19 nucleotides.

Promoter region contain four different parts:

• Consensus sequences (-10, -35)
• Upstream (UP) elements
• -10 extended elements
• Discriminator region (before -10)

SIGMA FACTOR:

RNA Pol. as a core enzyme combine with the sigma factor or subunit to form holoenzyme.

sigma 70 can be divided into four regions: 

• Region 4  = Two helices form common DNA-binding motif called helix-turn-helix. First helix inserts in major groove and interact with bases of -35 region. -35 provides binding energy to secure Pol. to promoter. Second helix lies top of groove, making contact with DNA backbone. E.g., transcriptional activator and repressor in bacterial cells.
• Region 2 = Interact with -10 region by alpha-helix. Alpha helix contains several essential aromatic amino acids that interact with bases on non-template strand and stabilize melted DNA. -10 is within that element that DNA melting is initiated. This region is bound to single stranded -10 element and also of the entire intact open complex. Two bases of non-template flipped out and inserted into pocket within sigma protein where they stabilize the unwound state of promoter.
• Region 3 = Extended -10 element is recognized by alpha helix in region 3. Interact with two specific base pair of this element.
• Region 1.2  recognizes the discriminator region.
• UP elements are recognized by carboxyl terminal domain of sigma called alpha-CTD. While the alpha-NTD is embedded in the body of enzyme.

Sigma 2 and 4 region are separated by 75 A when sigma is bound in holoenzyme. This is same between centers of -10 and -35. This distance is provided by region 3.2. Therefore, it is known as Linker DNA or sigma 3/4.

Differences between replication and transcription.

REPLICATION:

• The process of making two daughter strands where each daughter strand contains half of the original DNA double helix.
• The purpose of replication is to conserve the entire genome for next generation.
• Enzyme required- DNA Polymerase.
• It occurs- in the S phase of cell cycle. , Along the both strands of DNA.
• It is the preparation for cell division.
• Replicated DNA strand remains hydrogen bonded to its template DNA strand.
• This process requires RNA primer to start the process.
• Product remains within the nucleus.
• It involves unwinding and splitting of entire genome.
• More accurate than transcription.
• Speed is generally more as compared to transcription. 

TRANSCRIPTION:

• The process of synthesis of RNA using DNA as a template is called transcription.
• Purpose- To make RNA copies of individual genes.
• Enzyme required- RNA Polymerase.
• Occurrence- in G1 and G2 phases of cell cycle, Along one strand of DNA. 
• It is the preparation for protein translation.
• Transcribed RNA strand separates from its DNA template strand.
• No primer is required.
• Most part of product i.e., mRNA passes from nucleus to cytoplasm.
• It involves unwinding of only those genes which are to be transcribed.
• Comparatively less accurate than replication.
• Speed is less as compared to replication.

Outline the mechanism of insulin.

a) Insulin acts on specific receptor located on the cell membrane of practically every cell, but the density depends upon the cell type, liver and fat cells are very rich.

b) The insulin receptor is a receptor tyrosine kinase (RTK) which is a heterotetrametric glycoprotein consisting two extracellular alpha- and two transmembrane beta- subunits linked together by disulfide bonds, orienting across the cell membrane as a heterodimer.

c) The alpha -subunits carry insulin binding sites while beta-subunits have tyrosine kinase activity.

 STEPS:

1. IRS 1 phosphorylated by the insulin receptor activates PI3K by binding to its SH2 domain. PI3K converts PIP2 to PIP3 (PI3K is phospho-inositide 3 kinase).
2. PKB bound to PIP3 is phosphorylated by PDK-1. The activated PKB phosphorylates GSK3 on the serine residue, inactivating it.
3. The glycogen synthase kinase 3 (GSK3) is inactivated by phosphorylation, thus it cannot convert glycogen synthase (GS) to its inactive form by phosphorylation so GS remains active.
4. Synthesis of glycogen from glucose is accelerated rapidly in liver and muscles.
5. PKB stimulates movement of glucose transporter GLUT-4 from internal membrane vesicles to plasma membrane, thus increasing the uptake of glucose.

Direct and indirect effects of growth hormone.

GROWTH HORMONE:

The hormone responsible for general somatic growth. It accounts for 4% to 10% of the wet weight of the anterior pituitary in human adult (5 to 10 mg per gland). it is a polypeptide synthesized by somatotrophs of the pars distalis. It is derived from prohormone in pituitary cells but is rapidly converted to GH by proteolysis. Growth hormone exerts its effects both directly and indirectly.

• GH regulates growth indirectly through hepatic IGF-I then affects growth through its action on skeletal tissues and many other connective tissue and organs. Several hormones (e.g., prolactin, placental lactogen, insulin) may also exert part of their growth promoting activity through effects on hepatic IGF production. The structures of human placental lactogen (hpL) and prolactin (PRL) are similar to that of GH, IGF levels remains normal in hypophysectomies pregnant rats even in the absence of PRL and GH, these levels decline promptly postpartum. Thus other GH related factors may be important in IGF production during particular physiological states such as pregnancy.
• Growth hormone has a direct action on number of target cells in addition to its action on hepatic IGF production. These GH actions have been described in diabetogenic in nature. For example GH stimulates lipolysis which provides substrate for glucose formation and thus has a sparing effect on direct glucose utilization.

What happens when a person which takes drugs inhibit angiotensin converting enzyme. What will happen to aldosterone secretion, when the person goes on low sodium diet?

Angiotensin is converted into a smaller peptide, angiotensin II (a hormone) by an enzyme. Angiotensin II is a substance that narrows the blood vessels and releases hormones that can raise your blood pressure. By inhibiting the converting enzyme, the blood vessels relaxes and the constriction of blood vessels is also prevented. This also leads to the low blood pressure and lessens the energy the heart has to expend from beat to beat.
Aldosterone acts on certain collecting tubules of the kidney to cause resorption of Na+ from the urine. When a person goes on low sodium diet, there is a decrease in serum sodium concentration, which will trigger the release of aldosterone from adrenals. And thus, the collecting tubules will reabsorb the Na+ from urine.

Give schematic representation of hypothalamic hypophyseal portal system.

The hypothalamus is the basal part of the diencephalon lying below the thalamus. It forms the walls and lower part of the third ventricle of the brain.

• It induces the optic chiasm, the tuber cinereum, the infundibulum and the mammillary bodies.
• The lower part of the tuber cinereum, which is richly supplied with blood vessels that drain into the pituitary stalk and then in turn empty into a secondary plexus in the anterior pituitary, is referred to as the median eminence.
• The vascular link between the median eminence and the pituitary gland is known as the hypophysial portal system.
• The hypophyseal portal system is a system of blood vessels in the brain. 
• Its main function is the transport and exchange of hormones to allow a fast communication between both glands.
• Within the hypothalamus are clusters of neurons, hypothalamic nuclei, which are symmetrically located around the third ventricle.
• The supraoptic nuclei (SON) and paraventricular nuclei (PVN) are composed of cell bodies whose axons extend into the median eminence and then into the neurohyposis.
• This particular nerve tract, consisting of neuronal axons from the SON and PVN, is the supraopticoparaventriculohypophysial tract. 
• Other major nuclear groups within the hypothalamus include the ventromedial nuclei, arcuate nucleus, lateral tuberal nuclei and dorsomedial nuclei.
• The neural hypophyseal stalk and ventromedial region of the hypothalamic arcuate nucleus receive arterial blood from ascending and descending infundibular branches and capillaries, coming from the arteries of the superior hypophyseal arterial system.
• Small ascending vessels arising from the anastomoses that connect the upper and lower hypophyseal arterial system also supply blood to hypophyseal vessels.
• The majority of these branches penetrate into the neural tissue to break up into capillaries for rapid hormone exchange.

Explain the action of : Caffeine, Viagra and Pertussis toxin.

CAFFEINE:

It is the most widely consumed central nervous system stimulant. Mobilization of intracellular calcium and inhibition of specific phosphodiesterase only occur at high non-physiological concentrations of caffeine. Caffeine increases energy metabolism throughout the brain but decreases at the same time cerebral blood flow, including a brain hypo perfusion.  Caffeine activates adrenaline neurons  and seems to affect the local release of dopamine. It exerts obvious effects on anxiety and sleep which vary according to individual sensitivity. The CNS does not seem to develop a great tolerance to the effects of caffeine although dependence and withdrawal symptoms are reported. Caffeine increases the release of acid in the stomach. Sometimes leading to an upset stomach or heart burn. It has been associated with increased urine volume and frequently causing the blood to lose water and electrolytes like Na+/K+.

VIAGRA:

It belongs to the class of PDE5 inhibitor drugs which inhibit or block the chemicals known as Phosphodiesterase inhibitor drugs. The sexual stimulation releases nitric oxide in the corpus cavernous, the two parallel channels situated at the sides along the length of the penis. The release of NO activates the enzyme guanylyl cyclase which results in the increased levels of cGMP responsible for penile erection.

Viagra works by inhibiting the chemical PD type 5 which is responsible for the erections turning flaccid. This prolongs the effect of the muscle relaxation in the corpus cavernous area of the penis allowing the blood to flow into the spongy tissues of the penis causing an erection. It does not have any direct relevant effect on the corpus cavernous area in the human penis but enhances the effect of NO by inhibiting PDE5 which is responsible for degradation of cGMP in the corpus cavernous. When sexual stimulation causes local release of NO the inhibition of PDE5 by Viagra causes increase level of cGMP in corpus cavernous, resulting in smooth muscle relaxation and inflow of blood to the corpus cavernous resulting in the required erections of the penis.

 PERTUSSIS TOXIN:

It is a secreted protein exotoxin and an important virulence factor produced exclusively by beta-pertussis. It is a complex multisubunit toxin with an AB5 configuration (one active subunit plus five binding subunits). It is a pentameric ring composed of subunits S2, S3 two S4, S5 which bind to various glycoconjugate molecules in the surface of target cell.

• S2 and S3 each contain two different carbohydrates binding domains, which most likely accounts for the capacity of PT to bind to and intoxicate virtually any mammalian cell.
• The enzymatic activity of PT residues in the A-subunit also known as S1.
• S1 hydrolyzes cellular NAD and transfers the released ADP-ribose to a specific cysteine residue near the C-terminus of the alpha-subunit of heterotrimeric G-proteins of the Gi family in mammalian cells.
• This modification results in inhibition of Gi protein-coupled signaling pathways, causing a variety of downstream effects.
• Pertussis toxins also binds to glycosylated molecules on the surface of mammalian cells and enters cells by endocytosis.

Differences between Menarche and Menopause.

Menarche:

1. It is the first menstrual cycle or the first menstrual bleeding in human female.
2. It is the culmination of a series of physiological and anatomic process of puberty.
3. There is secretion of estrogen by the ovaries in response to pituitary hormones.
4. It usually occurs between 10-14 years.

Menopause:

1. Also known as climacteric, it is the time when women lives, the menstrual periods stops permanently.
2. It is usually a natural change. It can occur earlier in those who smoke, tobacco etc.
3. Decrease in the hormone production.
4. It usually occurs between 45 to 50 years.

Difference between Diabetes melitus type-1 and Diabetes melitus type-II.

DIABETES MELITUS TYPE-1:

1. It is an autoimmune disease which means it results from immune system mistakenly attacking part of body. Immune system incorrectly targets insulin producing beta-cells in pancreas.
2. It is often diagnosed in childhood.
3. Often associated with higher than normal ketones levels at diagnosis.
4. Need to inject themselves with insulin to compensate for death of beta cells. It is insulin dependent.
5. Often hereditary and unpreventable.

DIABETES MELITUS TYPE-II:

1. It is characterized by the body losing its ability to respond to insulin. It is known as insulin resistance.
2. Usually diagnosed in over 30 years old.
3. Often associated with high blood pressure or cholesterol level at diagnosis.
4. The body compensates for the ineffectiveness of the insulin by producing more, but cannot always produce enough.
5. It can be hereditary but excess weight, lack of exercise and unhealthy diet increases it chances. 

Differentiate between PKA and PKG.

PKA :

1. Protein kinase-A (PKA) is a family of enzymes whose activity is dependent on cellular level of cyclic AMP (cAMP).
2. PKA is also known as cAMP dependent protein kinase.
3. Inactive PKA contains two identical catalytic subunits (C) and two identical regulatory subunits (R).
4. Binding of two cAMP molecules to each PKA regulatory subunit. Regulatory subunits move out of the active site of catalytic subunit and R2C2 complex dissociates.
5. PKA is active when phosphorylated and can begin process of mobilizing glycogen stored in muscle and liver in anticipation of need of energy as signaled by epinephrine.

PKG:

1. Protein kinase G is a serine/threonine specific protein kinase that is activated by cGMP.
2. PKG is also known as cGMP dependent protein kinase.
3. The catalytic and regulatory domain of this enzyme are in a single polypeptide.
4. Binding of cGMP forces the pseudo-substrate out of the binding site to target protein containing PKG consensus sequences.
5. PKG activity modulates several targets involved in muscular contraction.

Differentiate between Osteoporosis and Osteomalacia.

OSTEOPOROSIS:

1. It is a progressive bone disease that is characterized by decrease in bone mass and density which leads to increase risk of fracture.
2. It can be classified into a) Postmenopausal (type-1) osteoporosis b)Senile (type-2) osteoporosis.
3. Weakened bones that are no longer able to support body weight can break even on slight pressure. Most common fracture occur in spine, wrist and hip bones.
4. Bone density measurement by a method called DEXA (Dual Energy X-ray Absorptiometry) is the most effective way to assess osteoporosis risk.
5. Hormone replacement therapy (HRT) consisting estrogen alone or progesterone and estrogen has been found useful in reducing menopausal vasomotor symptoms.
6. Postmenopausal osteoporosis occur after menopause is related to decrease in gonadal steroids.
7. Ageing patients are to be maintained at optimum state of health (important to evaluate exposure to sunlight and vitamin-D intake and recommend vitamin-D prophylaxis to patients at risk. 

OSTEOMALACIA:

1. It is the softening of bones caused by impaired bone metabolism primarily due to inadequate levels of calcium and vitamin-D.
2. It is classified as Vitamin-D responsive or vitamin-D resistant.
3. Bone pain especially in hips is very common symptom. Dull aching pain can spread from hips to lower back, pelvis, legs and even ribs.
4. Blood test to check low levels of vitamin D, calcium, phosphorus in blood can help diagnose osteomalacia. A bone biopsy may be required to definitively diagnose osteomalacia.
5. Treatment can be simply as taking oral supplements for vitamin D, calcium and phosphorus.
6. No such role of gonadal steroids is involved.
7. Several unusual forms of vitamin D resistant rickets have been described which involves end organ hyposensitivity (pseudo vitamin-D deficiency) to biologically active vitamin-D. 

How is nitric oxide formed and how does it induces smooth muscle relaxation?

Nitric oxide (NO) is produced from the amino acid L-arginine by the enzymatic action of nitric oxide synthase (NOS), present in many mammalian tissues. Co-factors for NOS include: oxygen, NADPH, tetrahydrobiopterin and flavin adenine nucleotides. Under normal, basal conditions in blood vessels, NO is continually being produced by cNOS. The activity of cNOS is calcium-and calmodulin-dependent.
When NO forms, it has a half-life of only a few seconds, in large part because superoxide anion has a higher affinity for NO.
NO avidly binds to the heme moiety of hemoglobin (in RBCs) and in the heme moiety of the enzyme guanylyl cyclase, which is found in vascular smooth muscle cells. Therefore, when NO is formed by vascular endothelium, it rapidly diffuses into the blood and subsequently broken down. It also diffuses into the vascular smooth muscle cells adjacent to the endothelium where it binds to and activates guanylyl cyclase. This enzyme catalyzes the dephosphorylation of GTP to cGMP, which serves as a second messenger for many important cellular functions, particularly for signaling smooth muscle relaxation.
cGMP induces smooth muscle relaxation by multiple mechanisms including:

1. increased intracellular cGMP, which inhibits calcium entry into the cell and decreases intracellular calcium concentrations.
2. Activates K+ channels, which leads to hyperpolarization and relaxation.
3. Stimulates a cGMP-dependent protein kinase that activates myosin light chain phosphatase, the enzyme that dephosphorylates myosin light chains, which leads to smooth muscle relaxation.

How is cortisol effective in treatment of arthritis?

Corticosteroids are used in arthritis for two reasons:

1. First they are anti-inflammatory i.e., they inhibit the inflammatory and allergic reactions.
2. And second, they are immunosuppressive i.e., they reduce the activity of immune system.

Glucocorticoids stabilize the lysosomal membranes and prevents the secretion of enzymes that normally occurs during inflammation. They also inhibit the infiltration of leukocytes into the affected tissue.

Interleukin-1 (IL-1), polypeptides produced by activated monocytes has an important role in eliciting a variety of physiological reactions observed during infection and inflammation. Stimulation of ACTH release is in one of those important nonimmune responses, which is induced by IL-1-beta. Increased hypothalamic-pituitary-adrenal activity results in an increased secretion of glucocorticoids that suppress immune responses, and hence, unfavorable overstimulation of the immune reaction, during infection or other conditions.

Lesion in hypothalamus leads to hyperprolactinemia.

Lesions of specific areas of the hypothalamus leads to specific defects in pituitary hormone secretion. For example, Prolactin (PRL) is an anterior pituitary hormone which is secreted in a pulsatile manner. Unlike other tropic hormones secreted by the anterior pituitary gland, PRL secretion is controlled primarily by inhibition from the hypothalamus and it is not subject to negative feedback directly or indirectly by peripheral hormones. This explains that a mass lesion in the hypothalamic or pituitary stalk area, results in a decrease of anterior pituitary hormones whereas it causes the elevation in PRL concentrations . Thyrotropin-releasing-hormone (TRH) is the main PRL-releasing factor and thus hyperprolactinemia is found in primary hypothyroidism. Other factors, like GHRH can also stimulate PRL release. Thus, the possible mechanism of elevated PRL levels is pituitary stalk compression by a tumor, leading to a decrease of PRL inhibition by dopamine from the hypothalamus and resulting into hyperprolactinemia.

Discuss the role of:

Role of ANF (Atrial Natriuretic Factor) :

Guanylyl cyclase in the kidney is activated by a peptide hormone atrial natriuretic factor (ANF) which is released by the cells in cardiac atrium when the heart is stretched by increased blood volume. Carried in the blood to the kidney, ANF activates guanylyl cyclase in the cells of collecting ducts. The resulting rise in [cGMP] triggers increased renal excretion of Na+ and consequently of water driven by change in osmotic pressure. Water loss reduce the blood volume countering the stimulus that initially lead to ANF secretion. Vascular smooth muscle also have an ANF receptor guanylyl cyclase ; on binding to this receptor, ANF causes relaxation (vasodilation) of blood vessels which increases blood flow and hence decrease blood pressure.

Role of Vasopressin:

The hyperosmolarity (increase in fluid Na+ concentration) and hypovolemia (decrease in fluid blood volume) are detected by  osmoreceptors or baroreceptors, sensory cell that monitor changes in Na+ concentration or volume (pressure) respectively of blood. Integration of this information within the hypothalamus results in the release of hormone, vasopressin from the neurons that are components of the posterior pituitary gland. This hormone is also referred to as anti-diuretic hormone (ADH) (or more specifically arginine vasopressin). AVP is released into the blood and acts on certain cells of kidney collecting tubules to cause water reabsorption. Vasopressin also causes the contraction of certain smooth muscles of the vasculature, which results in partial restoration of blood pressure.

Role of NO (Nitric oxide):

A distinctly different type of guanylyl cyclase is a cytosolic protein with a tightly associated heme group, an enzyme activated by nitric oxide. Nitric oxide is produced from arginine by Ca2+ dependent NO synthase present in mammalian tissue, and diffuse from its cell of origin into nearby cells. NO is sufficiently non-polar to cross plasma membrane without carrier.

NO induced relaxation of cardiac muscle is the same response brought about by nitroglycerin and other nitro vasodilators taken to relieve angina pectoris, the pain caused by contraction of heart deprived of oxygen because of blocked coronary arteries. Nitric oxide is unstable and its action is brief, within seconds of its formation it undergoes oxidation to nitrite to nitrate. Nitro vasodilators produce long lasting relaxation of cardiac muscle because they break down over several hours, yielding a steady stream of NO. NO can dilate the smooth muscle of the blood vessels. With this dilation, the vessels can relax and allow blood to flow easily through them and quite possibly lower blood pressure.

How do Renin Angiotensin system help in Na+ homeostasis?

The Renin-Angiotensin System play an important role in regulating blood volume and systemic vascular resistance which together influence cardiac output and arterial pressure. There are two important components to this system 1) Renin 2)Angiotensin. Renin which is released primarily by the kidney, stimulates the formation of angiotensin in blood and tissues. Renin is a proteolytic enzyme which is released into the circulation by the kidneys when :

1. Sympathetic nerve activation
2. Renal artery hypotension
3. Decreased sodium delivery to the distal tubules of kidneys.

Although the sodium ion (Na+) is the major electrolyte in blood, Na+ is continuously being lost from the body in urine and sweat. Specialized cells in the walls of certain blood vessels within the kidney can act as osmoreceptors and continuously monitor the Na+ concentration of the blood. If a drop in Na+ concentration and hence blood osmolarity is noted, these cells release a substance, renin, which acting as an enzyme, is able to split a plasma protein into a smaller peptide. This peptide is then acted on by yet another enzyme to release a still smaller peptide, angiotensin II, a hormone. Angiotensin II then stimulates certain cells of the adrenal cortex to release aldosterone, another hormone. This hormone then acts on certain collecting tubules of the kidney to cause resorption of Na+ from the urine. Thus a complex set of hormonal actions are brought into play to maintain the proper levels of blood Na+. 

How TSH stimulates thyroid hormone synthesis?

Follicular thyroid hormone synthesis is regulated through the action of thyrotropin (thyroid stimulating hormone; TSH). Continued stimulation of thyroid by TSH results in immediate activation of follicular cell thyroid hormone synthesizing activity (follicular cells become columnar and luminal content of colloid is decreased). TSH interacts with follicular cell membrane receptors with resulting activation of adenylate cyclase and cAMP production.

All the subsequent follicular activities may be mediated through cAMP induced gene expression and activation of NIS, TG, peroxidase and proteins involved in iodine uptake and thyroid hormone synthesis.

In response to TSH, oxygen consumption is increased and glucose is taken up from medium and metabolized via pentose monophosphate shunt to generate NADPH. NADPH is required for reduction of molecular oxygen to hydrogen peroxide. the H2O2 is then used in oxidation of iodine to active form in the presence of thyroidal peroxidase.

As TG is secreted into follicular lumen, organification of iodine into tyrosine residues of TG may occur to yield 3 monoiodotyrosin (MIT) or 3,5-diiodotyrosine (DIT). Two DIT moieties couples to yield T4. Coupling of a MIT to a DIT yields T3.

On stimulation of thyroid by TSH, there is enhanced pinocytotic activity at apical follicular membrane. Thus, colloid is actively engulfed and carried to cell by endocytosis process. Active removal of colloid from  lumen can be visualized as reabsorption lacunae. 

Write the action of:

HIGH Ca2+ ON PTH:

High extracellular Ca2+ level would allow Ca2+ to interact with acidic residues of extracellular domain of seven spanning membrane G protein coupled receptors. Binding of Ca2+ to receptor results in conformational change in receptor such that coupling to G-protein results in  lowered cAMP levels and decreased PTH secretion. High Ca2+ level may cause a person to feel run down, cause to sleep poorly, make them more irritable than usual, and even cause a disease in memory.

AROMATASE ON TESTOSTERONE:

Aromatase belongs to cytochrome P450 family. During aromatization reactions, aromatase forms an electron transfer complex with NADPH cytochrome P450 reductase. This enzyme converts androstenedione to estrogen and testosterone to estradiol by aromatizing androgens. Factors that increase the activity of aromatase include age, obesity, insulin gonadotrophins, alcohol and smoking.

CHOLERA TOXIN ON G-PROTEIN:

Cholera toxin activates cAMP production. The A subunit of the toxin activates the heterotrimeric G-protein, GS-alpha, via ADP ribosylation. The modified GS-alpha loses its GTPase activity but remains constitutively active in its GTP bound state, causing a continuous stimulation of adenylate cyclase. It leads to excessive cAMP production. The result is secretion of chloride ions from the cell.

DEIODINASE ON MIT AND DIT:

TG tyrosyl residues are iodinated to form MIT and DIT which on subsequent oxidative coupling forms iodothyronines i.e., T3 and T4. MIT, DIT along with T3 and T4 are transported into the cell in the form of colloid droplets. These droplets fuse with lysosome to form secondary lysosome. The iodinated tyrosine i.e., MIT and DIT are released into cytosol through lysosomal proteolysis which is then deiodinated by deiodinase enzyme and the iodide formed is recycled for use within the cell.

HYPOXIA ON ERYTHROPOIETIN:

Hypoxia is a state of low oxygen tension in tissues. Erythropoietin counteract tissue hypoxia by increasing systemic oxygen carrying capacity. It induces augmentation of red blood cells mass by stimulating the formation and differentiation of erythroid precursor cells in bone marrow. An oxygen sensor that controls hypoxia induced erythropoietin synthesis is located in kidney. Changes in oxygen flux may be detected by tubular epithelial cells and transmitted to adjacent peritubular endothelial cells, resulting in induction of gene for EP. Several signaling molecules such as transcription factor Hypoxia-inducible factor 1 (HIF-1) are involved in EP transcriptional response to cellular hypoxia.

L-AROMATIC AMINO ACID DECARBOXYLASE ON DOPA.

L-Aromatic Amino acid decarboxylase (AAAD) is an essential enzyme for the formation of catecholamines, indolamines and trace amines. It is a non-specific decarboxylase found in many tissues and helps in converting L-DOPA to dopamine by the decarboxylation of L-DOPA. This enzyme is important when treating patients with Parkinson's disease.

3-beta HYDROXY STEROID DEHYDROGENASE AND delta 5,4 ISOMERASE ON DEHYDROEPIANDROSTERONE:

3-beta hydroxy steroid dehydrogenase and delta-5,4 isomerase acts on dehydroandrosterone i.e., a 17- hydroxysteroid and is converted by side chain cleavage to androstenedione i.e., 17-ketosteroid. This androstenedione is further converted into testosterone which regulates male reproductive behavior. 

Erythropoietin

Erythropoietin (EPO) is also known as hematopoietic or hemopoietin is a glycoprotein cytokine secrete by the kidney in response to cellular hypoxia; it stimulates red blood cells production (erythropoiesis) in the bone marrow. Low levels of EPO (around 10 mU/mL) are constantly secreted sufficient to compensate for normal red blood cell turn over. Common causes of cellular hypoxia resulting in elevated levels of EPO (upto 10,000 mU/mL) include by anemia and hypoxemia due to chronic lung disease. Erythropoietin is produced by intenstitial fibroblasts in the kidney in close association with peritubular capillary and proximal convoluted tubule. It is also produced in perisinusoidal cells in the liver. EPO is highly glycosylated (40% of total molecular weight ) with half life in blood around five hours.
Erythropoietin is an essential hormone for red blood cells production. Without it definitive erythropoiesis does not take place.
Erythropoietin has been shown its effects by binding to the erythropoietin receptor (EPOR). EPO binds to the erythropoietin receptor on the red cell progenitor surface and activates a JAK 2 signaling cascade. This initiate STAT5, PIK3, and Ras MAPK pathways. This results in differentiation, survival and proliferation of erythroid cell.