Which of the following vasopressin receptors MOST likely mediates vasopressin antidiuretic action?
## **Core Concept**
The question tests understanding of vasopressin (also known as antidiuretic hormone, ADH) and its receptor subtypes. Vasopressin plays a crucial role in regulating water balance in the body by promoting water reabsorption in the kidneys. This action is primarily mediated through a specific subtype of vasopressin receptors.
## **Why the Correct Answer is Right**
The correct answer, **V2 receptors**, are predominantly found in the kidneys, specifically in the collecting ducts. Vasopressin binding to **V2 receptors** activates **Gs proteins**, which in turn stimulate **adenylate cyclase**. This leads to an increase in intracellular **cAMP** levels, triggering a series of events that result in the insertion of **aquaporin-2 (AQP2) water channels** into the apical membrane of collecting duct cells. This increases water permeability, allowing water reabsorption into the bloodstream and concentrating the urine. This mechanism underlies the antidiuretic action of vasopressin.
## **Why Each Wrong Option is Incorrect**
- **Option A (V1A receptors):** These receptors are primarily found in vascular smooth muscle and mediate vasoconstriction, not antidiuretic effects. Their activation leads to increased blood pressure, which is not directly related to the antidiuretic action of vasopressin.
- **Option B (V1B receptors):** Also known as **V3 receptors**, they are involved in regulating adrenocorticotropic hormone (ACTH) secretion from the anterior pituitary gland. They do not play a primary role in the antidiuretic action of vasopressin.
- **Option D:** This option is not labeled, but based on the context, any other receptor not specifically implicated in the renal effects of vasopressin would be incorrect.
## **Clinical Pearl / High-Yield Fact**
A key clinical correlation is the use of **desmopressin**, a synthetic analogue of vasopressin that selectively activates **V2 receptors**. It is used to treat conditions like **central diabetes insipidus**, where there's a deficiency in vasopressin production, leading to an inability to concentrate urine.
## **Correct Answer:** .