Vasopressin, also known as antidiuretic hormone (ADH), is a neurohypophysial hormone found in most mammals. In most species it contains arginine and is thus also called arginine vasopressin (AVP) or argipressin. 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 reabsorption in the kidney's collecting ducts, the tubules which receive the very dilute urine produced by the functional unit of the kidney, the nephrons.
Vasopressin is a peptide hormone that increases water permeability of the kidney's collecting duct and distal convoluted tubule by inducing translocation of aquaporin-CD water channels in the plasma membrane of collecting duct cells. 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 vasopressin 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 pair bonding, and maternal responses to stress. It has a very short half-life between 16-24 minutes.
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Physiology
Function
Vasopressin regulates the body's water retention. It is released from the brain 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.
Kidney
Vasopressin has three main effects:
- Increasing the water permeability of distal convoluted tubule and collecting duct cells in the kidney, thus allowing water reabsorption and excretion of more concentrated urine, i.e., antidiuresis. This occurs through increased transcription and insertion of water channels (Aquaporin-2) into the apical membrane of distal convoluted 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. This effect is mediated by V2 receptors. 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.
- Increasing permeability of the inner medullary portion of the collecting duct to urea by regulating the cell surface expression of urea transporters, 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.
- Acute increase of sodium absorption across the ascending loop of henle. This adds to the countercurrent multiplication which aids in proper water reabsorption later in the distal tubule and collecting duct.
Serum osmolarity/osmolality is also affected by vasopressin due to its role in keeping proper electrolytic balance in the blood stream. Improper balance can lead to dehydration, alkalosis, acidosis or other life-threatening changes. The hormone ADH is partly responsible for this process by controlling the amount of water the body retains from the kidney when filtering the blood stream.
Central nervous system
Vasopressin released within the brain has many actions:
- It is likely that vasopressin acts in conjunction with corticotropin-releasing hormone to modulate the release of corticosteroids from the adrenal gland in response to stress, particularly during pregnancy and lactation in mammals.
- Selective AVPr1a blockade in the ventral pallidum has been shown to prevent partner preference in prairie voles, suggesting that these receptors in this ventral forebrain region are crucial for pair bonding.
- Recent evidence suggests that vasopressin may have analgesic effects. The analgesia effects of vasopressin were found to be dependent on both stress and sex.
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.
Regulation
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 in rats.
- Angiotensin II stimulates AVP secretion, in keeping with its general pressor and pro-volumic effects on the body.
- Atrial natriuretic peptide inhibits AVP secretion, in part by inhibiting Angiotensin II-induced stimulation of AVP secretion.
- Cortisol inhibits secretion of antidiuretic hormone.
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). 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:
- AVP is also synthesized by magnocellular neurosecretory neurons at the PVN, transported and released at the median eminence, which then travels through the hypophyseal portal system to the anterior pituitary where it stimulates corticotropic cells synergistically with CRH to produce ACTH (by itself it is a weak secretagogue).
- Vasopressin is also released into the brain by several different populations of smaller patterns.
Receptors
The following describes the actions of AVP:
Structure and relation to 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 (argipressin) is Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Arg-Gly-NH2, with the cysteine residues forming a disulfide bond and the C-terminus of the sequence converted to a primary amide. Lysine vasopressin (lypressin) has a lysine in place of the arginine as the eighth amino acid, and is found in pigs and some related animals, whereas arginine vasopressin is found in humans.
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 secrete vasopressin are adjacent to magnocellular neurons that secrete 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:
Role in disease
There may be a connection between arginine vasopressin and autism.
Lack of vasopressin
Decreased AVP release (neurogenic -- i.e. due to alcohol intoxication or tumour) or decreased renal sensitivity to AVP (nephrogenic, i.e. by mutation of V2 receptor or AQP) leads to diabetes insipidus, a condition featuring hypernatremia (increased blood sodium concentration), polyuria (excess urine production), and polydipsia (thirst).
Excess vasopressin
Syndrome of Inappropriate Antidiuretic Hormone secretion (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. It has also been associated with fluoroquinolones (including ciprofloxacin and moxifloxacin). Finally, it can occur without a clear explanation. Hyponatremia can be treated pharmaceutically through the use of vasopressin receptor antagonists.
Pharmacokinetics/therapeutics
Vasopressin is administered through an intravenous device, intramuscular injection or a subcutaneous injection. The duration of action depends on the mode of administration and ranges from thirty minutes to two hours. It has a half life of ten to twenty minutes. It is widely distributed throughout the body and remains in the extracellular fluid. It is degraded by the liver and excreted through the kidneys.
Contraindications
The use of vasopressin is contraindicated in the presence of hypersentivity to beef or pork proteins, increased BUN and chronic renal failure. It recommended that it be cautiously used in instances of perioperative polyuria, sensitivity to the drug, asthma, seizures, heart failure, a comatose state, migraine headaches, and cardiovascular disease.
Interactions
- alcohol - may lower the antiduretic effect
- carbamazepine, chloropropamide, clofibrate, tricyclic antidepressants fludrocortisone may raise the diruretic effect
- lithium, demeclocycline, heparin or norepinephrine may lower the antidiuretic effect
- vasopressor effect may be higher with the concurrent use of ganglionic blocking medications
Indications
Vasopressin is used to manage anti-diuretic hormone deficiency. It has off-label uses and is used in the treatment of vasodilatory shock, gastrointestinal bleeding, ventricular tachycardia and ventricular defibrillation. Vasopressin is used to treat diabetes insipidus related to low levels of antiduretic hormone.It is available as Pressyn.
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.
Vasopressin infusions are also used as second line therapy in septic shock patients not responding to fluid resuscitation or infusions of catecholamines (e.g., dopamine or norepinephrine).
The role of vasopressin analogues in cardiac arrest
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.
Vasopressin vs. epinephrine
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.
Vasopressin and epinephrine vs. epinephrine alone
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.
2010 American Heart Association Guidelines
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). 2015 AHA GUIDELINES VASOPRESSIN has been removed from the 2015 AHA GUIDELINES Update for CPR and ECC.
Surgery for congenital heart disease
Vasopressin is used in managing hemodynamic instability in neonates and older children recovering from cardiac surgery. There is evidence that some children recovering from cardiac surgery have relative vasopressin deficiency, such that their endogenous plasma concentrations of arginine vasopressin are lower than what would be expected in this clinical setting. Though low endogenous vasopressin concentrations in and of themselves do not cause hemodynamic instability, neonates and children recovering from cardiac surgery who develop hemodynamic instability and have low endogenous vasopressin concentrations are optimal candidates for this surgery. Unfortunately, measurement of endogenous vasopressin concentration is time consuming and cumbersome, and not practical for bedside application. Copeptin, a more stable and easily measured product of pro-AVP processing, may be a means of identifying patients with low endogenous vasopressin concentrations. Further research is needed. Also, systemic corticosteroids have been shown to suppress endogenous vasopressin production and release. Neonates and children recovering from cardiac surgery who are receiving systemic corticosteroid therapy may also be optimal candidates for vasopressin therapy should hemodynamic instability be present.
Vasopressin receptor inhibition
A vasopressin receptor antagonist is an agent that interferes with action at the vasopressin receptors. They can be used in the treatment of hyponatremia.
Side effects and adverse reactions
The most common side effects during treatment with vasopressin are dizziness, angina, chest pain, abdominal cramps, heartburn, nausea, vomiting, trembling, fever, water intoxication, pounding sensation in the head, diarrhea, nausea, vomiting, sweating, and paleness.flatulence. The most severe adverse reactions are mycardial infarction and hypersensitivy.
History
Vasopressin was elucidated and synthesized for the first time by Vincent du Vigneaud. 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."
Animal studies
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.
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