Antidiuretic: Definition, Metabolism, Effects and Structural Analogs

It is a nano peptide (nine amino acids) synthesized in the hypothalamus, transported and stored in the posterior lobe of the pituitary gland that releases it into the bloodstream.

The antidiuretic hormone, ADH, also called vasopressin and arginine-vasopressin, has antidiuretic and vasopressor actions.

Vasopressin metabolism

Vasopressin is a nano peptide with a disulfide bond between two cysteine ​​residues. Its structure resembles that of oxytocin.

Secretion

Formed in the supraoptic and paraventricular nuclei of the hypothalamus by cleavage of a 168 amino acid preprohormone and then a prohormone, vasopressin is transported to the posterior lobe of the pituitary gland that stores it.

Its release depends mainly on two factors: hyperosmolality and blood volume and, in addition, on the effects of certain medications.

Hyperosmolality

The increase in the osmolality of the blood supplying the hypothalamic-pituitary complex causes vasopressin secretion. In patients with central diabetes insipidus, there is no response to increased osmolality.

Blood volume

The decrease in the volume of extracellular fluids and blood pressure decreases the stimulation of the baroreceptors located in the atria, pulmonary veins, carotid sinus, increasing the secretion of vasopressin, an increase has opposite effects. In addition, angiotensin facilitates the secretion of ADH;

Effects of medications

Tricyclic antidepressants, fluoxetine and the other serotonin reuptake inhibitors, nicotine, neuroleptics, and carbamazepine can increase ADH secretion. When this increase is significant, it results in fluid retention, usually detected by dilution hyponatremia.

This syndrome is referred to in the literature as SIADH or syndrome of inappropriate antidiuretic hormone secretion.

Other medications such as phenytoin, mineralocorticoids, and glucocorticoids can decrease its secretion.

Ethanol also reduces it. Modifications of the effect of the hormone at the renal level can also be observed.

In patients with central diabetes insipidus, there is insufficient or no secretion of ADH. In patients with nephrogenic diabetes insipidus, there is ADH secretion but no response at the renal level.

catabolism

The plasma half-life of ADH is approximately 15 to 30 minutes. It is inactivated by plasma and tissue endopeptidases, particularly in the kidney and liver.

Effects of vasopressin

The effects of vasopressin are the result of the stimulation of receptors V1 and V2, V1 mainly responsible for vasoconstriction, V2 for the antidiuretic effect

The V1 receptors are coupled by the G protein to phospholipase C. Their activation causes the hydrolysis of PIP2 in IP3 and DAG, which induces an increase in the intracellular calcium concentration, responsible for vasoconstriction.

V2 receptors are G protein coupled to adenyl cyclase. Its activation causes an increase in cAMP which, through protein kinases, induces the activation of aqueous channels called type 2 aquaporins or AQP2 located mainly in the renal collecting duct.

Under the influence of vasopressin AQP2 migrate from the cytoplasm to the apical membrane. In nephrogenic diabetes insipidus there are AQP2 alterations.

Antidiuretic action (V2)

ADH increases the water permeability of the collecting ducts in the cortical and medullary part of the kidney.

It induces the incorporation of aquaporins in the apical membrane of the collecting ducts and induces their opening, which allows the reabsorption of water.

A certain number of medicines modify the activity of ADH in the kidney:

Enhancers

Chlorpropamide, acetaminophen, carbamazepine, indomethacin, and other non-steroidal anti-inflammatory drugs can increase ADH activity and cause fluid retention.

Chlorpropamide, a hypoglycemic sulfonamide, potentiates the action of very low concentrations of vasopressin.

When used for the treatment of diabetes mellitus, chlorpropamide can cause, as an adverse effect, an apparent syndrome of inappropriate ADH secretion.

In the treatment of central diabetes insipidus, when ADH analogs cannot be used, chlorpropamide is used alone or sometimes in combination with a thiazide diuretic.

Antagonists

Lithium, demeclocycline, and methoxyflurane decrease the renal activity of ADH. The polyuria observed in some patients treated with lithium is explained by this mechanism.

Demeclocycline is the only tetracycline with an antagonistic effect on ADH in the kidney. Reduces water retention induced by excessive secretion of antidiuretic hormones, often of neoplastic origin. When used as an antibiotic, demeclocycline can cause nephrogenic diabetes insipidus with polyuria, polydipsia.

Phototoxic reactions can occur in patients treated with demeclocycline and exposure to sunlight should be avoided.

Furthermore, thiazide diuretics have, in patients with nephrogenic diabetes insipidus, a paradoxical antidiuretic effect, probably due to the depletion of sodium in the tubular fluid. They are the main treatment, often combined with amiloride, for nephrogenic diabetes insipidus.

Vasoconstrictor effect (V1)

At doses higher than those necessary to induce water retention, ADH induces vasoconstriction. The plasma concentration of vasopressin may be sufficient to increase peripheral resistance and blood pressure.

The decrease in cutaneous blood flow observed in smokers could be the consequence of an increase in vasopressin secretion under the influence of nicotine.

The development of specific V1 antagonists would allow better specification of the role of vasopressin in physiology and pathology, and probably to find therapeutic applications.

Role in hemostasis:

ADH has a platelet aggregation effect under stress conditions where its secretion is greatly increased.

It has a favorable action in patients with hemophilia or von Willebrand disease by increasing the secretion of Factor VIII and Factor von Willebrand. The product generally used in these indications is desmopressin, which decreases bleeding time.

Other effects

ADH stimulates the secretion of ACTH.

Experimental studies showed that ADH facilitates memorization, while oxytocin could have an inverse effect.

Furthermore, vasopressin by activating receptors present in the brain is involved in sexual behavior, at least in certain animal species.

Structural analogs of vassopressin

The vasopressor effect is seldom desirable, vasopressin analogs such as desmopressin with strong antidiuretic activity and low vasopressor activity have been obtained. The following table compares the effects of vasopressin and desmopressin.

Desmopressin differs from vasopressin, by the loss of the amine group in cysteine, which increases its antidiuretic activity and prolongs its action, and by the substitution of L-arginine by D-arginine, which considerably reduces its affinity for receptors. V1 and its vasopressor effect.

The main therapeutic use of structural analogs of vasopressin with little vasoconstrictor effect is the treatment of central diabetes insipidus, that is, due to insufficient hormonal secretion.

The analog used for its antidiuretic effect is desmopressin. Being a polypeptide, desmopressin is poorly absorbed from the digestive tract and partially inactivated by enzymes in the digestive tract.

It is administered by injection, nasal spray and even orally, but in the latter case a very high dose is administered to compensate for its low bioavailability.

In addition to its use in diabetes insipidus, desmopressin is used in the treatment of enuresis (children> 5 years), factor VIII defective bleeding (hemophilia A) or von Willebrand factor. It is also used in functional examination to assess the concentrating power of the kidney.

During its use as an anti-hemorrhagic drug, desmopressin can induce fluid retention with dilutional hyponatraemia.

Terlipressin, an inactive prodrug that is progressively converted to lypressin in the body, is not used in the treatment of diabetes insipidus, but rather in the treatment of gastrointestinal bleeding due to ruptured esophageal varices due to its vasoconstrictive effect on the portal circulation. Terlipressin can adversely induce a hypertensive peak.

According to recent studies, vasopressin is as effective as adrenaline in treating cardiac arrest and certain shocks with excessive vasodilation. In these indications, it could be used as an alternative to adrenaline.

Vasopressin receptor antagonists

Vasopressin receptor antagonists can reduce vasoconstriction by inhibiting V1 receptors and increase acresis by inhibiting V2 receptors.

Conivaptan is a non-peptide antagonist of the V1A and V2 vasopressin receptors. The main pharmacological effect of conivaptan is to induce acresis (improved excretion of free water) with an increase in plasma sodium concentration.

It is marketed as Vaprisol * and is used for the treatment of hyponatremia of the syndrome of inappropriate antidiuretic hormone secretion.

Conivaptan

Generic names of drugs that have V2 vasopressin antagonist effects end with “vaptan.”

Other vaptans under investigation are tolvaptan, lixivaptan, satavaptan … Its indications are hyponatremia of the syndrome of inappropriate secretion of antidiuretic hormone and, perhaps, in some circumstances, congestive heart failure and cirrhosis.