Secretion of ADH increases in response to fluid loss, such as dehydration. Hemorrhaging causes an increase in ADH secretion in order to maintain the body's fluid balance. Very strenuous exercise, emotional or physical stress, and drugs such as nicotine or barbiturates all increase the secretion of ADH in order to decrease the amount of urine excreted.
Arginine vasopressin (AVP), also known as vasopressin, argipressin or antidiuretic hormone (ADH), is a neurohypophysial hormone found in most mammals, including humans.[1] Vasopressin is a peptide hormone that controls the reabsorption of molecules in the tubules of the kidneys by affecting the tissue's permeability. It also increases peripheral vascular resistance, which in turn increases arterial blood pressure. It plays a key role in homeostasis, and the regulation of water, glucose, and salts in the blood. It is derived from a preprohormone precursor that is synthesized in the hypothalamus and stored in vesicles at the posterior pituitary. Most of it is stored in the posterior pituitary to be released into the bloodstream; however, some AVP is also released directly into the brain, where it plays an important role in social behavior and bonding.
One of the most important roles of AVP is to regulate the body's retention of water; it is released when the body is dehydrated and causes the kidneys to conserve water, thus concentrating the urine, and reducing urine volume. In high concentrations, it also raises blood pressure by inducing moderate vasoconstriction. In addition, it has a variety of neurological effects on the brain, having been found, for example, to influence pair-bonding in voles. The high-density distributions of vasopressin receptor AVPr1a in prairie voleventral forebrain regions have been shown to facilitate and coordinate reward circuits during partner preference formation, critical for pair bond formation.[2]
A very similar substance, lysine vasopressin (LVP) or lypressin, has the same function in pigs and is often used in human therapy.
[edit]Kidney
Vasopressin has two effects by which it contributes to increased urine osmolality (increased concentration) and decreased water excretion. These are:
1.) Increasing the water permeability of distal tubule and collecting duct cells in the kidney, thus allowing water reabsorption and excretion of more concentrated urine, i.e., antidiuresis. This occurs through insertion of water channels (Aquaporin-2) into the apical membrane of distal tubule and collecting duct epithelial cells. Aquaporins allow water to move down their osmotic gradient and out of the nephron, increasing the amount of water re-absorbed from the filtrate (forming urine) back into the bloodstream.
V2 receptors, which are G protein-coupled receptors on the basolateral plasma membrane of the epithelial cells, couple to the heterotrimeric G-protein Gs, which activates adenylyl cyclases III and VI to convert ATP into cAMP, plus 2 inorganic phosphates. The rise in cAMP then triggers the insertion of aquaporin-2 water channels by exocytosis of intracellular vesicles, recycling endosomes. Vasopressin also increases the concentration of calcium in the collecting duct cells, by episodic release from intracellular stores. Vasopressin, acting through cAMP, also increases transcription of the aquaporin-2 gene, thus increasing the total number of aquaporin-2 molecules in collecting duct cells.
Cyclic-AMP activates protein kinase A (PKA) by binding to its regulatory subunits and allowing them to detach from the catalytic subunits. Detachment exposes the catalytic site in the enzyme, allowing it to add phosphate groups to proteins (including the aquaporin-2 protein), which alters their functions.
2.) Increasing permeability of the inner medullary portion of the collecting duct to urea, which facilitates its reabsorption into the medullary interstitium as it travels down the concentration gradient created by removing water from the connecting tubule, cortical collecting duct, and outer medullary collecting duct.
Cardiovascular system
Vasopressin increases peripheral vascular resistance and thus increases arterial blood pressure. This effect appears small in healthy individuals; however it becomes an important compensatory mechanism for restoring blood pressure in hypovolemic shock such as that which occurs during hemorrhage.
[edit]Central nervous system
Vasopressin released within the brain has many actions:
§ It has been implicated in memory formation, including delayed reflexes, image, short- and long-term memory, though the mechanism remains unknown; these findings are controversial. However, the synthetic vasopressin analogue desmopressin has come to interest as a likely nootropic.
§ Vasopressin is released into the brain in a circadian rhythm by neurons of the supraoptic nucleus.
§ Vasopressin released from centrally-projecting hypothalamic neurons is involved in aggression, blood pressure regulation and temperature regulation.
§ Selective AVPr1a blockade in the ventral pallidum has been shown to prevent partner preference, suggesting that these receptors in this ventral forebrain region are crucial for pair bonding.[2]
In recent years, there has been particular interest in the role of vasopressin in social behavior. It is thought that vasopressin, released into the brain during sexual activity, initiates and sustains patterns of activity that support the pair-bond between the sexual partners; in particular, vasopressin seems to induce the male to become aggressive towards other males.[3]
Evidence for this comes from experimental studies in several species, which indicate that the precise distribution of vasopressin and vasopressin receptors in the brain is associated with species-typical patterns of social behavior. In particular, there are consistent differences between monogamous species and promiscuous species in the distribution of AVP receptors, and sometimes in the distribution of vasopressin-containing axons, even when closely-related species are compared.[3] Moreover, studies involving either injecting AVP agonists into the brain or blocking the actions of AVP support the hypothesis that vasopressin is involved in aggression towards other males. There is also evidence that differences in the AVP receptor gene between individual members of a species might be predictive of differences in social behavior. One study has suggested that genetic variation in male humans effects pair-bonding behavior. The brain of males uses vasopressin as a reward for forming lasting bonds with a mate, and men with one or two of the genetic alleles are more likely to experience marital discord. The partners of the men with two of the alleles affecting vasopressin reception state disappointing levels of satisfaction, affection, and cohesion.[4] Vasopressin receptors distributed along the reward circuit pathway, to be specific in the ventral pallidum, are activated when AVP is released during social interactions such as mating, in monogamous prairie voles. The activation of the reward circuitry reinforces this behavior, leading to conditioned partner preference, and thereby initiates the formation of a pair bond.[5]
[edit]Control
Vasopressin is secreted from the posterior pituitary gland in response to reductions in plasma volume, in response to increases in the plasma osmolality, and in response tocholecystokinin secreted by the small intestine:
§ Secretion in response to reduced plasma volume is activated by pressure receptors in the veins, atria, and carotids.
§ Secretion in response to increases in plasma osmotic pressure is mediated by osmoreceptors in the hypothalamus.
§ Secretion in response to increases in plasma cholecystokinin is mediated by an unknown pathway.
The neurons that make AVP, in the hypothalamic supraoptic nuclei (SON) and paraventricular nuclei (PVN), are themselves osmoreceptors, but they also receive synaptic input from other osmoreceptors located in regions adjacent to the anterior wall of the third ventricle. These regions include the organum vasculosum of the lamina terminalis and the subfornical organ.
Many factors influence the secretion of vasopressin:
§ Ethanol (alcohol) reduces the calcium-dependent secretion of AVP by blocking voltage-gated calcium channels in neurohypophyseal nerve terminals.[6]
§ Angiotensin II stimulates AVP secretion, in keeping with its general pressor and pro-volemic effects on the body.[7]
§ Atrial natriuretic peptide inhibits AVP secretion, in part by inhibiting Angiotensin II-induced stimulation of AVP secretion.[7]
[edit]Secretion
The main stimulus for secretion of vasopressin is increased osmolality of plasma. Reduced volume of extracellular fluid also has this effect, but is a less sensitive mechanism.
The AVP that is measured in peripheral blood is almost all derived from secretion from the posterior pituitary gland (except in cases of AVP-secreting tumours). However there are two other sources of AVP with important local effects:
§ Vasopressin is produced in the PVN and SON and travels down the axons through the infundibulum within neurosecretory granules that are found within Herring bodies, localized swellings of the axons and nerve terminals. These carry the peptide directly to the posterior pituitary gland, where it is stored until released into the blood.
§ Vasopressin is also released into the brain by several different populations of smaller neurons (see below).
[edit]Receptors
Below is a table summarizing some of the actions of AVP at its four receptors, differently expressed in different tissues and exerting different actions:
| Type | Locations | Actions | |
| Vasoconstriction, gluconeogenesis, platelet aggregation, and release of factor VIII and von Willebrand factor; social recognition,[8] circadian tau[9] | |||
| Adrenocorticotropic hormone secretion in response to stress;[10] social interpretation of olfactory cues[11] | |||
| Basolateral membrane of the cells lining the collecting ducts of the kidneys (especially the cortical and outer medullary collecting ducts) | Insertion of aquaporin-2 (AQP2) channels (water channels). This allows water to be reabsorbed down an osmotic gradient, and so the urine is more concentrated. Release of von Willebrand factorand surface expression of P-selectin through exocytosis of Weibel-Palade bodies from endothelial cells[12][13] | ||
| Vascular endothelium and renal collecting tubules | Increases cytosolic calcium and acts as an inverse agonist of cAMP accumulation[14] |
[edit]Structure and relation to oxytocin
Chemical structure of the argipressin (indicating that this compound is of the vasopressin family with an arginine at the 8th amino acid position.
Chemical structure of oxytocin
The vasopressins are peptides consisting of nine amino acids (nonapeptides). (NB: the value in the table above of 164 amino acids is that obtained before the hormone is activated by cleavage). The amino acid sequence of arginine vasopressin isCys-Tyr-Phe-Gln-Asn-Cys-Pro-Arg-Gly, with the cysteine residues forming a sulfur bridge. Lysine vasopressin has a lysine in place of the arginine.
The structure of oxytocin is very similar to that of the vasopressins: It is also a nonapeptide with a disulfide bridge and its amino acid sequence differs at only two positions (see table below). The two genes are located on the same chromosome separated by a relatively small distance of less than 15,000 bases in most species. The magnocellular neurons that make vasopressin are adjacent to magnocellular neurons that make oxytocin, and are similar in many respects. The similarity of the two peptides can cause some cross-reactions: oxytocin has a slight antidiuretic function, and high levels of AVP can cause uterine contractions.[15][16]
Here is a table showing the superfamily of vasopressin and oxytocin neuropeptides:
| Vertebrate Vasopressin Family | ||
| Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Arg-Gly-NH2 | Argipressin(AVP, ADH) | Most mammals |
| Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Lys-Gly-NH2 | Lypressin(LVP) | Pigs, hippos, warthogs, some marsupials |
| Cys-Phe-Phe-Gln-Asn-Cys-Pro-Arg-Gly-NH2 | Some marsupials | |
| Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Arg-Gly-NH2 | Non-mammals | |
| Vertebrate Oxytocin Family | ||
| Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH2 | Oxytocin(OXT) | Most mammals, ratfish |
| Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Pro-Gly-NH2 | Prol-Oxytocin | |
| Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Ile-Gly-NH2 | Most marsupials, all birds, reptiles,amphibians, lungfishes, coelacanths | |
| Cys-Tyr-Ile-Gln-Ser-Cys-Pro-Ile-Gly-NH2 | ||
| Cys-Tyr-Ile-Ser-Asn-Cys-Pro-Ile-Gly-NH2 | ||
| Cys-Tyr-Ile-Ser-Asn-Cys-Pro-Gln-Gly-NH2 | ||
| Cys-Tyr-Ile-Asn/Gln-Asn-Cys-Pro-Leu/Val-Gly-NH2 | Various tocins | |
| Invertebrate VP/OT Superfamily | ||
| Cys-Leu-Ile-Thr-Asn-Cys-Pro-Arg-Gly-NH2 | ||
| Cys-Phe-Val-Arg-Asn-Cys-Pro-Thr-Gly-NH2 | ||
| Cys-Phe-Ile-Arg-Asn-Cys-Pro-Lys-Gly-NH2 | Geography & imperial cone snail, pond snail, sea hare, leech | |
| Cys-Ile-Ile-Arg-Asn-Cys-Pro-Arg-Gly-NH2 | Striped cone snail | |
| Cys-Tyr-Phe-Arg-Asn-Cys-Pro-Ile-Gly-NH2 | ||
| Cys-Phe-Trp-Thr-Ser-Cys-Pro-Ile-Gly-NH2 | Octopus | |
| †Vasotocin is the evolutionary progenitor of all the vertebrate neurohypophysial hormones.[17] | ||
[edit]Role in disease
Decreased vasopressin release or decreased renal sensitivity to AVP leads to diabetes insipidus, a condition featuring hypernatremia (increased blood sodium concentration), polyuria(excess urine production), and polydipsia (thirst).
High levels of AVP secretion (syndrome of inappropriate antidiuretic hormone, SIADH) and resultant hyponatremia (low blood sodium levels) occurs in brain diseases and conditions of thelungs (Small cell lung carcinoma). In the perioperative period, the effects of surgical stress and some commonly used medications (e.g., opiates, syntocinon, anti-emetics) lead to a similar state of excess vasopressin secretion. This may cause mild hyponatremia for several days.
Hyponatremia can be treated pharmaceutically through the use of vasopressin receptor antagonists. These include the approved drug Vaprisol and the phase III drug lixivaptan.
[edit]Pharmacology
[edit]Vasopressin analogues
Vasopressin agonists are used therapeutically in various conditions, and its long-acting synthetic analogue desmopressin is used in conditions featuring low vasopressin secretion, as well as for control of bleeding (in some forms of von Willebrand disease and in mild haemophilia A) and in extreme cases of bedwetting by children. Terlipressin and related analogues are used as vasoconstrictors in certain conditions. Use of vasopressin analogues for esophageal varices commenced in 1970.[18]
Vasopressin infusion has been used as a second line of management in septic shock patients not responding to high dose of inotropes (e.g., dopamine or norepinephrine). It had been shown to be more effective than epinephrine in asystolic cardiac arrest.[19] While not all studies are in agreement, a 2006 study of out-of hospital cardiac arrests has added to the evidence for the superiority of AVP in this situation, but these studies relied on sub-group analysis and better designed prospective studies show no benefit in ACLS.[20][21]
[edit]Vasopressin receptor inhibition
Main article: vasopressin receptor antagonist
A vasopressin receptor antagonist is an agent that interferes with action at the vasopressin receptors. They can be used in the treatment of hyponatremia.[22]
Arginine vasopressin has attracted attention as a potentially important neurohormonal mediator of the heart failure (HF) syndrome and hyponatremic states in humans1 because vasopressin influences renal handling of free water, vasoconstriction, and myocyte biology.2,3 Several vasopressin antagonists are under development,4 and one of these agents, conivaptan, recently received US Food and Drug Administration approval for short-term intravenous treatment in patients with euvolemic or hypervolemic hyponatremia.
The Role of Vasopressin in HF and Hyponatremia
A neurohypophysial hormone, vasopressin (also called antidiuretic hormone [ADH]), affects free water reabsorption by the kidney, body fluid osmolality, blood volume, vasoconstriction, and myocardial contractile function.2,3 Vasopressin is synthesized by neurosecretory cells located predominantly in the supraoptic and paraventricular hypothalamic nuclei. These neurons have axons terminating in the neural lobe of the posterior pituitary (neurohypophysis) that release vasopressin and oxytocin.5
Physiology of Regulation of Vasopressin Release
Normally, the dominant stimulant for vasopressin release is a change in plasma tonicity, plasma volume depletion, or blood pressure, the last 2 mediated by arterial baroreceptors. Osmoreceptors in the anterior hypothalamus sense the increase in serum osmolality and stimulate secretion of vasopressin from the posterior pituitary. In an attempt to normalize plasma osmolality, vasopressin acts on the V2 renal receptors, increasing free water reabsorption by insertion of protein water channels, aquaporins, in the luminal membranes of the principal cells of the renal collecting ducts.6
Receptor/Effector Mechanisms
The 3 vasopressin receptor subtypes belong to a family of rhodopsin-like G-protein–coupled receptors.7 V1a (vascular) receptors are located on several cell types, including vascular smooth muscle cells and cardiomyocytes (Table 1), with effects on the maintenance and regulation of vascular tone and possibly myocardial function.3
Figure 1. Vasopressin V1 receptor activation. The binding of arginine vasopressin (AVP) to its V1 receptor (V1R) stimulates membrane-bound phospholipase (PLCB) via stimulation of a G-coupled protein (Gq), which in turn results in inositol triphosphate (IP3) formation and mobilization of intracellular Ca2+ (icCa2+). A separate phosphorylation cascade occurs via diacylglycerol (DAG) and protein kinase C (PKC), which has downstream effects, including vascular smooth muscle (VSM) vasoconstriction, cell growth, adrenocorticotrophic hormone (ACTH) release, and platelet aggregation.
V2 (renal) receptors, expressed on the basolateral membrane of the renal collecting ducts, mediate the antidiuretic effects of vasopressin. The intracellular effects of this receptor subtype are mediated by the adenylate cyclase signaling pathway (Figure 2). Intracellular events triggered by binding of vasopressin to the V2 receptor include increased de novo synthesis and “shuttling” of aquaporin 2 water channels (AQP-2) from cytoplasmic vesicles to the luminal surface of the renal collecting duct cells, where they are inserted into the cell membrane and facilitate water transport across the collecting duct cells (Figure 2).
Figure 2. Vasopressin V2 receptor activation. The binding of arginine vasopressin (AVP) to the V2 vasopressin receptor (V2R) stimulates a Gs-coupled protein that activates adenylyl cyclase, in turn causing production of cAMP to activate protein kinase A (PKA). This pathway increases the exocytosis of aquaporin water channel–containing vesicles (AQMCV) and inhibits endocytosis of the vesicles, both resulting in increases in aquaporin 2 (AQ2) channel formation and apical membrane insertion. This allows an increase in the permeability of water from the collecting duct (CD).
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