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Space-filling model of arginine vasopressin
|RNA expression pattern|
Space-filling model of arginine vasopressin
|RNA expression pattern|
Arginine vasopressin (AVP), also known as vasopressin, argipressin or antidiuretic hormone (ADH), is a neurohypophysial hormone found in most mammals. Its two primary functions are to retain water in the body and to constrict blood vessels. Vasopressin regulates the body's retention of water by acting to increase water absorption in the collecting ducts of the kidney nephron. Vasopressin increases water permeability of the kidney's collecting duct and distal convoluted tubule by inducing translocation of aquaporin-CD water channels in the kidney nephron collecting duct plasma membrane. 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, by 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 may also be released directly into the brain, and accumulating evidence suggests it plays an important role in social behavior, sexual motivation and bonding, and maternal responses to stress.
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. At 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 vole ventral forebrain regions have been shown to facilitate and coordinate reward circuits during partner preference formation, critical for pair bond formation.
A very similar substance, lysine vasopressin (LVP) or lypressin, has the same function in pigs and is often used in human therapy.
Vasopressin has two effects by which it contributes to increased urine osmolarity (increased concentration) and decreased water excretion. These are:
Vasopressin increases peripheral vascular resistance (vasoconstriction) 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.
Vasopressin released within the brain has many actions:
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. 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 toward 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 affects 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. 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.
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 to cholecystokinin (CCK) secreted by the small intestine:
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:
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). Vasopressin is produced by magnocellular neurosecretory neurons in the Paraventricular nucleus of hypothalamus (PVN) and Supraoptic nucleus (SON). It then travels down the axon 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. However there are two other sources of AVP with important local effects:
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||Second messenger system||Locations||Actions|
|AVPR1A||Phosphatidylinositol/calcium||Liver, kidney, peripheral vasculature, brain||Vasoconstriction, gluconeogenesis, platelet aggregation, and release of factor VIII and von Willebrand factor; social recognition, circadian tau|
|AVPR1B or AVPR3||Phosphatidylinositol/calcium||Pituitary gland, brain||Adrenocorticotropic hormone secretion in response to stress; social interpretation of olfactory cues|
|AVPR2||Adenylate cyclase/cAMP||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 factor and surface expression of P-selectin through exocytosis of Weibel-Palade bodies from endothelial cells|
|VACM-1||Phosphatidylinositol/calcium||Vascular endothelium and renal collecting tubules||Increases cytosolic calcium and acts as an inverse agonist of cAMP accumulation|
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 is Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Arg-Gly, with the cysteine residues forming a disulfide bond. 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.
Below 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|
|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||Some New World monkeys, northern tree shrews|
|Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Ile-Gly-NH2||Mesotocin||Most marsupials, all birds, reptiles, amphibians, lungfishes, coelacanths|
|Invertebrate VP/OT Superfamily|
|Cys-Phe-Ile-Arg-Asn-Cys-Pro-Lys-Gly-NH2||Lys-Connopressin||Geography & imperial cone snail, pond snail, sea hare, leech|
|Cys-Ile-Ile-Arg-Asn-Cys-Pro-Arg-Gly-NH2||Arg-Connopressin||Striped cone snail|
|†Vasotocin is the evolutionary progenitor of all the vertebrate neurohypophysial hormones.|
Decreased AVP 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 may lead to hyponatremia. In many cases, the AVP secretion is appropriate (due to severe hypovolemia), and the state is labelled "hypovolemic hyponatremia". In certain disease states (heart failure, nephrotic syndrome) the body fluid volume is increased but AVP production is not suppressed for various reasons; this state is labelled "hypervolemic hyponatremia". A proportion of cases of hyponatremia feature neither hyper- nor hypovolemia. In this group (labelled "euvolemic hyponatremia"), AVP secretion is either driven by a lack of cortisol or thyroxine (hypoadrenalism and hypothyroidism, respectively) or a very low level of urinary solute excretion (potomania, low-protein diet), or it is entirely inappropriate. This last category is classified as the syndrome of inappropriate antidiuretic hormone (SIADH).
SIADH in turn can be caused by a number of problems. Some forms of cancer can cause SIADH, particularly small cell lung carcinoma but also a number of other tumors. A variety of diseases affecting the brain or the lung (infections, bleeding) can be the driver behind SIADH. A number of drugs has been associated with SIADH, such as certain antidepressants (serotonin reuptake inhibitors and tricyclic antidepressants), the anticonvulsant carbamazepine, oxytocin (used to induce and stimulate labor), and the chemotherapy drug vincristine. Finally, it can occur without a clear explanation.
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.
Injection of vasopressors for the treatment of cardiac arrest was first suggested in the literature in 1896 when Austrian scientist Dr. R. Gottlieb described the vasopressor epinephrine as an "infusion of a solution of suprarenal extract [that] would restore circulation when the blood pressure had been lowered to unrecordable levels by chloral hydrate." Modern interest in vasopressors as a treatment for cardiac arrest stem mostly from canine studies performed in the 1960s by anesthesiologists Dr. John W. Pearson and Dr. Joseph Stafford Redding in which they demonstrated improved outcomes with the use of adjunct intracardiac epinephrine injection during resuscitation attempts after induced cardiac arrest. Also contributing to the idea that vasopressors may be useful treatments in cardiac arrest are studies performed in the early to mid 1990's that found significantly higher levels of endogenous serum vasopressin in adults after successful resuscitation from out-of-hospital cardiac arrest compared to those who did not live. Results of animal models have supported the use of either vasopressin or epinephrine in cardiac arrest resuscitation attempts, showing improved coronary perfusion pressure and overall improvement in short-term survival as well as neurological outcomes.
|RR (95% CI)|
|Failure of ROSC||0.81 (0.58-1.12)|
|Death before hospital admission||0.72 (0.38-1.39)|
|Death within 24 hours||0.74 (0.38-1.43)|
|Death before hospital discharge||0.96 (0.87-1.05)|
|Number of deaths and neurologically impaired survivors||1.00 (0.94-1.07)|
Although both vasopressors, vasopressin and epinephrine differ in that vasopressin does not have direct effects on cardiac contractility as epinephrine does. Thus, vasopressin is theorized to be of increased benefit over epinephrine in cardiac arrest due to its properties of not increasing myocardial and cerebral oxygen demands. This idea has led to the advent of several studies searching for the presence of a clinical difference in benefit of these two treatment choices. Initial small studies demonstrated improved outcomes with vasopressin in comparison to epinephrine. However, subsequent studies have not all been in agreement. Several randomized controlled trials have been unable to reproduce positive results with vasopressin treatment in both return of spontaneous circulation (ROSC) and survival to hospital discharge, including a systematic review and meta-analysis completed in 2005 that found no evidence of a significant difference with vasopressin in five studied outcomes (see Table 1).
|RR (95% CI)||p value|
|Survival to hospital admission||1.42 (1.02-2.04)||0.05|
|In subgroup: PEA||1.30 (0.90-2.06)||0.02|
|In subgroup: Collapse to ED arrival time of 15–30 minutes||1.22 (1.01-1.49)||0.05|
|In subgroup: Collapse to ED arrival time of 30–45 minutes||1.11 (1.00-1.24)||0.05|
|Survival to hospital discharge||3.69 (1.52-8.95)|
There is no current evidence of significant survival benefit with improved neurological outcomes in patients given combinations of both epinephrine and vasopressin during cardiac arrest. A systematic review from 2008 did, however, find one study that showed a statistically significant improvement in ROSC and survival to hospital discharge with this combination treatment; unfortunately, those patients that survived to hospital discharge had overall poor outcomes and many suffered permanent, severe neurological damage. A more recently published clinical trial out of Singapore has shown similar results, finding combination treatment to only improve the rate of survival to hospital admission, especially in the subgroup analysis of patients with longer "collapse to emergency department" arrival times of 15 to 45 minutes. Table 2 lists all statistically significant findings of a correlation between combined treatment and positive outcomes found in these two studies.
The 2010 American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care recommend the consideration of vasopressor treatment in the form of epinephrine in adults with cardiac arrest (Class IIb, LOE A recommendation). Due to the absence of evidence that vasopressin administered instead of or in addition to epinephrine has significant positive outcomes, the guidelines do not currently contain vasopressin as a part of the cardiac arrest algorithms. It does, however, allow for one dose of vasopressin to replace either the first or second dose of epinephrine in the treatment of cardiac arrest (Class IIb, LOE A recommendation).