Vasopressin (Antidiuretic Hormone) Reduces Water Loss Through the Kidneys
The correct answer is A. Vasopressin. Vasopressin, also known as antidiuretic hormone (ADH) or arginine vasopressin (AVP), is the primary hormone that reduces water loss through the kidneys by increasing water reabsorption in the renal collecting ducts 1, 2.
Mechanism of Action
Vasopressin works through a specific molecular pathway to conserve water:
Vasopressin binds to V2 receptors on the basolateral membrane of collecting duct principal cells, initiating a G-protein coupled signaling cascade that activates adenylyl cyclase, increases cyclic AMP production, and stimulates protein kinase A 2, 3.
This signaling triggers the insertion of aquaporin-2 (AQP2) water channels into the apical membrane of principal cells through exocytosis, allowing water to enter the cell from the tubular lumen 2, 4.
Water then exits through constitutively expressed AQP3 and AQP4 channels in the basolateral membrane, moving from the hypotonic tubular fluid into the hypertonic medullary interstitium, thereby concentrating the urine and reducing water excretion 2, 4.
At lower concentrations typical for antidiuretic function, vasopressin inhibits water diuresis via renal V2 receptors, while at higher pressor doses it causes vasoconstriction through V1 receptors 3.
Clinical Significance
The critical role of vasopressin in water conservation is demonstrated by disease states:
In congenital nephrogenic diabetes insipidus, mutations in either the V2 receptor or AQP2 genes prevent vasopressin from exerting its antidiuretic effect, resulting in inability to concentrate urine despite high ADH levels, with patients presenting with polyuria, inappropriately dilute urine (osmolality <200 mOsm/kg), and hypernatremia 1, 2.
Without vasopressin influence, a person would need to ingest 5-15 liters of water daily to match urinary losses, highlighting the hormone's essential role in water conservation 5.
Conversely, excessive vasopressin secretion (SIADH) reduces urine output to as little as 500 mL per day in adults, causing dangerous dilution of blood volume and intracellular expansion 6, 5.
Why the Other Options Are Incorrect
Angiotensin (Option B) primarily causes vasoconstriction and stimulates aldosterone release, which promotes sodium retention with secondary water retention, but does not directly reduce water loss through the kidneys 2.
Cortisone (Option C) is a glucocorticoid that does not have a primary role in renal water conservation 1.
Renin (Option D) is an enzyme that initiates the renin-angiotensin-aldosterone system but does not directly affect renal water handling 2.