During moderate haemorrhage, plasma arginine vasopressin rises markedly even though osmolality is unchanged. The receptor subtype mediating the vascular contribution to blood pressure maintenance is:

• A V2 receptors on collecting duct cells acting through cAMP
• B Beta 2 adrenergic receptors activated by vasopressin
• C V1b receptors on anterior pituitary corticotrophs
• D V1a receptors on vascular smooth muscle acting through phospholipase C and IP3 ✓

Explanation

Haemorrhage releases vasopressin through baroreceptor unloading rather than osmotic stimuli. V1a receptors on vascular smooth muscle couple to Gq, activate phospholipase C, generate IP3, release intracellular calcium and cause potent vasoconstriction. V2 receptors mediate water reabsorption in the collecting duct via cAMP insertion of aquaporin 2, which conserves volume but does not constrict vessels. V1b receptors regulate ACTH secretion from the pituitary and have no vascular role.

Reference: Ganong's Review of Medical Physiology, 26th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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