Vasopressin

Discovery

Vasopressin is a mammalian VP family peptide with vasopressor and antidiuretic actions. Disorders of VP and V2aR cause central and nephrogenic diabetes insipidus, respectively. Vasopressor and the antidiuretic effects of VP were first reported in 1895 and 1913, respectively. Arginine vasopressin (AVP) was isolated in 1951, and chemically synthesized in 1955.

Properties

Mr 1084 (AVP), 1056 (LVP), 1068 (phenypressin); pI, 10.9 (AVP). The peptides are freely soluble in water. AVP solution in water at >10^-4M is stable for more than a year at -20°C, with increased stability under acidic conditions using acetic acid.

Structure

The N-terminal six aa residues of the nonapeptides are flanked by two cysteine residues (positions 1 and 6) forming an intramolecular ring while the C-terminal extension with arginine or lysine residue confers a basic peptide property. The C-terminus is amidated. VPs are found only in mammals. Three peptides have been found . Most mammals, including humans, have AVP while LysineVP (LVP) and phenypressin were discovered from pigs and marsupials, respectively.

Gene, mRNA, and precursor

The human preproAVP gene located on the short arm of chromosome 20 (20p13) consists of three exons and two introns.2 The oxytocin (OT) gene is located in the vicinity of the AVP gene in a tail-to-tail configuration, and they are transcribed in opposite directions. Human AVP mRNA has 595 bp encoding a signal peptide, a mature AVP with processing and amidation sites (GlyLys-Arg), a neurophysin, and a glycoprotein (copeptin) . Processing of the prohormone occurs in the acidic pH of large dense core vesicles. The neurophysin noncovalently binds with the mature peptide in the vesicles and functions as an intravesicular chaperone and carrier protein while the function of copeptin is still unknown. The AVP gene has glucocorticoid and cAMP responsive elements, and AP-1/2 regulatory elements in the promoter region. A downstream region of the AVP gene is also important as an enhancer.

Agonists

A number of peptide and orally-active nonpeptide agonists and antagonists have been investigated. Recent research to find coupling-selective ligands and to design bivalent ligands found that potent selective agonists are [Phe2 , Orn8 ]VP and F-180 (V1a agonists), d[Cha4 ]AVP (V1b agonist), and dDAVP and dVDAVP (V2a agonists). The selectivities vary between human and rat receptors.

Antagonists

Potent selective antagonists are d(CH2)5[Tyr(Me)2 ] AVP and SR49059 (V1a antagonists); SSR149415 (V1b antagonist); and OPC31260, OPC41061, and SR121463 (V2 antagonists).

Receptors

Three functional receptors have been identified (V1aR, V1bR, and V2aR). In humans, the length (aa residues), chromosomal location, and number of exons are as follows: V1aR, 418, 12q14-15, two exons; V1bR, 424, 1q32, two exons; and V2aR, 371, Xq28, three exons. AVP binds to three receptors with high affinity (Kd<2 nM). GPCRs including VP and OT receptors may also function as homo/hetero-dimers/oligomers.

Biological functions

The most prominent function of AVP is the antidiuretic action through V2aR/aquaporin-2 (AQP2) in the inner medullary collecting duct of the kidney. AVP administration also induces a large increase in blood pressure. This effect is caused by vascular contraction, basoreflex, renin production, and aldosterone and glucocorticoid releases from the adrenal gland, all of which are mediated via V1aR. In addition, adrenocorticotropic hormone (ACTH) and catecholamine release via V1bR and body fluid retention via V2aR contribute to blood pressure regulation. In the pituitary, AVP synergistically controls the secretion of ACTH with corticotropin-releasing hormone (CRH). AVP also regulates metabolic function via V1aR and V1bR. In the central nervous system, AVP is involved in social behaviors, learning, memory, aggression, anxiety, depression, water and food intake, circadian rhythm, and thermoregulation.

Regulation of synthesis and release

The synthesis and release of AVP are stimulated by the elevation of plasma osmolality and decreases in blood volume and blood pressure. Magnocellular AVP neurons receive signals of various neurotransmitters and neuromodulators from the circumventricular organs and medulla, including GABA, noradrenaline, dopamine, glutamic acid, atrial natriuretic peptide, and angiotensin II. In the parvocellular PVN, adrenalectomyincreases AVP synthesis while elevation is restored by dexamethasone administration. Direct regulation by androgen and estrogen is shown in the BNST and amygdala. Various peptide hormones (such as galanin, enkephalin, neuropeptide Y, vasoactive intestinal peptide, dynorphin) are colocalized in AVP neurons, and are involved in the regulation of AVP and OT release.

Clinical implications

Approximately 90% of CNDI patients are males with mutations in the V2aR gene on the X-chromosome. Besides CNDI, gain-of-function mutations in V2aR cause nephrogenic syndrome of inappropriate antidiuresis (NSIAD), characterized by excessive sodium excretion, low plasma sodium concentration, and low plasma osmolality. The syndrome of inappropriate antidiuretic hormone secretion (SIADH) is typically associated with measurably elevated AVP levels and consequent V2aR hyperactivity.Due to the equimolar secretion with VP, plasma copeptin levels can be a prognostic marker in acute diseases, and a promising marker in the diagnosis of VP-related disorders.

Description

Vasopressin, also called antidiuretic hormone (A D H) or arginine vasopressin (AVP), is another hormone key to water homeostasis and blood pressure regulation. A rginine vasopressin is produced in the neurones of the hypothalamus and stored in vesicles within the posterior pituitary. Vasopressin is released in response to increased blood osmolality detected by hypothalamic osmoreceptors; systemic hypotension or hypovolaemia detected by cardiopulmonary baroreceptors of the great veins and atria; or angiotensin I I acting on the hypothalamus.

Uses

Vasopressin is a peptide hormone secreted by the hypothalamus via the posterior pituitary. Its primary role is the regulation of body fluid balance. It is secreted in response to hypotension and promotes retention of water by action on specific cAMP-coupled V2 receptors. It causes vasoconstriction by stimulating V1 receptors in VSM and is particularly potent in hypotensive patients. It is increasingly used in the treatment of refractory vasodilatory shock which is resistant to catecholamines. The vasopressin analogue desmopressin is used to treat diabetes insipidus and in the management of von Willebrand's disease. Terlipressin (another analogue) is used to limit bleeding from oesophageal varices in patients with portal hypertension. When used for the management of low SVR states, vasopressin is administered as an i.v. infusion at a rate of 0.01–0.1 units min–1.

Uses

Hormone (antidiuretic).

brand name

Pitressin (Parke-Davis).

Biological Functions

Human vasopressin, or Arg-vasopressin, is chemically very similar to oxytocin and therefore sometimes is referred to as [Phe3, Arg8]oxytocin. The physiological role of vasopressin is the regulation of water reabsorption in the renal tubules (an antidiuretic action, thus often referred to as the antidiuretic hormone). In high doses, vasopressin promotes the contraction of arterioles and capillaries, resulting in an increase in blood pressure, thus the name vasopressin. An inadequate output of pituitary antidiuretic hormone can cause diabetes insipidus, which is characterized by the chronic excretion of large amounts of pale urine and results in dehydration and extreme thirst.

Clinical Use

lly as a long-lasting and selective V2aR agonist for the treatment of central diabetes insipidus. For X-CNDI, the use of nonpeptide antagonists (pharmacological chaperones) and nonpeptide agonists (vaptans) has been tested. These cell-permeable reagents stabilize ERretained misfolded V2aR mutants, allowing the receptors to reach the plasma membrane or to initiate a cAMP response, leading to AQP2 translocation. Strategies bypassing nonfunctional or mislocalized V2aR to induce AQP2 translocation and phosphorylation have also been tested. OPC41061 (V2aR antagonist, tolvaptan) has been approved in the United States and Europe for the treatment of hyponatraemia in SIADH. Recently, the VP system (V1aR) was proposed as a novel therapeutic target for castration-resistant prostate cancer. The results of clinical trials to improve autism spectrum disorder by targeting the AVP/V1aR pathway have been reported.

Clinical Use

The nonapeptide vasopressin is well known for its role on fluid metabolism, but it also is a key regulator of the HPA axis. Stress stimulates the release of vasopressin in the pituitary gland, where it strongly potentiates the effects of CRF on adrenocorticotropic hormone release. These findings suggest that HPA axis dysregulation in depression might be associated with the development of centrally acting vasopressin receptor antagonists for the treatment of depression.