Q.24 Reabsorption of water via the distal convoluted tubule is by which of the following?

(1) Rennin and angiotensin

(2) Aldosterone

(3) Vasopressin

(4) Glucocorticoid

Water Reabsorption in DCT: Vasopressin

Vasopressin (ADH) facilitates water reabsorption in the distal convoluted tubule (DCT) by increasing aquaporin channel insertion in the collecting duct, which follows the DCT. The correct answer is option (3) Vasopressin.

Correct Answer

Vasopressin (Option 3). Released from the posterior pituitary in response to high plasma osmolarity, vasopressin binds V2 receptors on principal cells of the late DCT and collecting ducts, triggering cAMP-mediated insertion of aquaporin-2 channels into the apical membrane for osmotic water reabsorption.

Option Explanations

Rennin and angiotensin (Option 1)

Rennin (renin) from juxtaglomerular cells initiates the renin-angiotensin-aldosterone system (RAAS), leading to angiotensin II, which primarily vasoconstricts and stimulates aldosterone. They promote sodium reabsorption in proximal tubule and early DCT but not direct water reabsorption there.

Aldosterone (Option 2)

Aldosterone, a mineralocorticoid from adrenal cortex, acts on principal cells in late DCT and collecting ducts to increase Na+ reabsorption via ENaC channels and K+ secretion. This indirectly enhances water retention by osmosis but primarily targets sodium, not water directly.

Vasopressin (Option 3)

Vasopressin (antidiuretic hormone, ADH) specifically regulates water permeability in the distal nephron without sodium transport, making urine concentrated during dehydration. Its action defines the final urine volume adjustment.

Glucocorticoid (Option 4)

Glucocorticoids like cortisol influence proximal tubule gluconeogenesis and sodium retention systemically but lack specific receptors for water channel regulation in the DCT or collecting ducts.

Nephron Reabsorption Summary

Water handling progresses from obligatory (proximal tubule, 65%) to facultative (DCT/collecting duct, ADH-dependent).