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 on vascular smooth muscle cells (angyocytes). Their stimulation by the hormone leads to marked vasoconstriction (narrowing of vessel lumens).
- $V_2$ receptors: localized in the renal collecting ducts. Activation of these receptors increases 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 conivaptan. It blocks both $V_{1A}$ and $V_2$ receptors simultaneously, thereby preventing both vasoconstriction and excessive water reabsorption.
- Selective blockers: The primary representative is tolvaptan. This drug selectively blocks only renal $V_2$ receptors.
Notably, both groups of agents have undergone clinical evaluation for fluid-retention states.
Pathophysiological Role of Endothelin
Endothelin is a biologically active 21-amino acid peptide. It exhibits potent vasoconstrictive and marked pro-inflammatory actions.
In the body, endothelin plays an active role in the pathogenesis of several severe conditions, including acute heart failure, tissue ischemia, and vascular spasms. It poses a particular danger during myocardial infarction: at this time, blood levels of endothelin rise sharply, leading to critical constriction of the coronary arteries, which further worsens the pumping function of the damaged heart.
Tezosentan: An Endothelin Receptor Blocker
To neutralize the pathological effects of endothelin, the specific antagonist tezosentan is used.
- Mechanism of action: The drug blocks endothelin receptors of both types (ET-A and ET-B). These receptors are located directly within the vascular wall—in the endothelium and vascular smooth muscle cells.
- Pharmacokinetics and administration: Tezosentan is designed for intravenous administration. Its distinguishing feature is a short half-life, which provides high controllability of its effect by allowing clinicians to easily titrate the dose in real time.
Other Direct-Acting Vasodilators
In the management of acute heart failure, alongside endothelin antagonists, vasopressin antagonists, and natriuretic peptide analogs, direct-acting myotropic agents are also utilized.
Their mechanism involves the direct relaxation of vascular smooth muscle, leading to vasodilation and a subsequent decrease in blood pressure. Their characteristic action is a moderate and relatively short-lived effect.
- Bendazol: Typically prescribed as part of combination therapy.
- Magnesium sulfate: Widely used in hypertensive emergencies. The preferred route of administration in such settings is intramuscular.
Precautions: Intravenous administration of magnesium sulfate carries a high risk of a life-threatening complication: respiratory center depression.