Vasopressin and its receptors
Vasopressin is a hormone secreted by the posterior pituitary gland. In the human body, it exerts its physiological effects by stimulating specific receptors that have different localizations and functions:
- $V_{1A}$ receptors: located in the smooth muscle cells of the vascular wall (angiomyocytes). Their stimulation by the hormone leads to pronounced vasoconstriction (narrowing of the vascular lumen).
- $V_2$ receptors: located in the renal collecting ducts. Activation of these receptors causes an increase in the reabsorption of water from the primary urine back into the bloodstream.
Classification of vasopressin antagonists
Drugs that block the action of vasopressin are used to eliminate excessive vascular tone and fluid retention. Depending on their receptor affinity, they are divided into two main groups:
- Non-selective blockers: this group includes the drug conivaptan. It is capable of blocking both $V_{1A}$ and $V_2$ receptors simultaneously, thereby preventing both vasoconstriction and excessive water reabsorption.
- Selective blockers: the main representative is tolvaptan. This drug selectively blocks only renal $V_2$ receptors.
It is important to note that at the time these materials were compiled, both drug groups were undergoing Phase III clinical trials.
Pathophysiological role of endothelin
Endothelin is a biologically active peptide whose molecule consists of 21 amino acids. It has potent vasoconstrictive and pronounced pro-inflammatory effects.
In the body, endothelin takes an active part in the pathogenesis of a number of severe conditions, including the development of acute heart failure, tissue ischemia, and blood vessel spasms. It poses a particular danger during myocardial infarction: at this moment, the level of endothelin in the blood rises sharply. This leads to critical narrowing of the coronary arteries, which further impairs the pumping function of the damaged heart.
Tezosentan: an endothelin receptor blocker
To neutralize the pathological effects of endothelin, a specific antagonist is used — tezosentan.
- Mechanism of action: the drug blocks two types of endothelin receptors (ET-A and ET-B). These receptors are located directly in the vascular wall — in the endothelium and angiomyocytes.
- Pharmacokinetics and administration: tezosentan is intended for intravenous administration. Its distinctive feature is a short half-life. This ensures high controllability of the effect, as it allows the physician to easily titrate the drug dose in real time.
Drug status: undergoing Phase III clinical trials.
Other direct-acting vasodilators
In the treatment of acute heart failure, in addition to endothelin antagonists, vasopressin antagonists, and natriuretic peptide analogues, direct myotropic agents are used.
Their mechanism consists in the direct relaxation of vascular smooth muscle. This leads to the expansion of the vascular bed and a subsequent decrease in blood pressure. A characteristic feature of their action is a moderate and short-lived effect.
- Bendazole (Dibazole): as a rule, it is prescribed not in isolation, but as part of combination therapy.
- Magnesium sulfate: widely used for hypertensive crises. The optimal route of administration in such cases is intramuscular.
Precautions: intravenous administration of magnesium sulfate carries a high risk of a life-threatening complication — respiratory center depression.
To easily remember the selectivity of vasopressin blockers, pay attention to the first letters: Tolvaptan — Tonly $V_2$ (selective), Conivaptan — Combo of $V_{1A}$ and $V_2$ (non-selective).
Class and exam questions
Where are vasopressin $V_{1A}$ and $V_2$ receptors located?
$V_{1A}$ receptors are located in angiomyocytes (vascular smooth muscle), and $V_2$ receptors are located in the renal collecting ducts.
Why is endothelin release dangerous during myocardial infarction?
Endothelin causes severe coronary artery spasm. This exacerbates ischemia and significantly impairs the heart's pumping function.
What is the advantage of tezosentan's short half-life?
A short half-life makes it easy to manage the drug's effect and safely titrate its dose during intravenous administration.
What complication is possible with intravenous administration of magnesium sulfate?
There is a high risk of respiratory center depression when administering this drug intravenously.
What else to study on this topic
- Natriuretic peptide analogues in acute heart failure
- Pathogenesis of acute heart failure and the role of vasoconstrictors
- Mechanisms of direct myotropic action of bendazole
- Management of hypertensive crises: algorithms and drug selection
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