What is the mechanism of action for vaptans?

Vaptans, specifically tolvaptan and conivaptan, are classified as vasopressin receptor antagonists. Their primary mechanism of action is to inhibit the resorption of water in the kidneys, particularly in the collecting ducts. By blocking the action of vasopressin (antidiuretic hormone, ADH), which normally promotes water reabsorption, vaptans lead to increased excretion of free water. This results in a condition called aquaresis, where the body eliminates excess water, making vaptans particularly useful in managing conditions like hyponatremia associated with heart failure or cirrhosis.

The other options do not accurately reflect the action of vaptans. They do not promote sodium retention or directly increase potassium excretion; rather, they primarily focus on water balance. Furthermore, while vasopressin plays a role in vascular tone, vaptans are not primarily considered vasodilators; their main effect is on fluid management rather than vasodilation. Understanding this mechanism is crucial for effective therapeutic use in clinical scenarios.