How does the hormone ADH control the reabsorption of water in the collecting duct of the nephron?

SouravNovember 9, 2024

Answer

Antidiuretic hormone (ADH), also known as vasopressin, plays a crucial role in regulating water reabsorption in the collecting ducts of the nephron. Here’s how ADH functions in this context:

Mechanism of Action of ADH

1. Release and Response:
   • ADH is synthesized in the hypothalamus and released from the posterior pituitary gland in response to increased blood osmolarity (high concentration of solutes) or decreased blood volume. This release signals the kidneys to conserve water.
2. Binding to Receptors:
   • ADH primarily targets the principal cells in the collecting ducts. It binds to V2 receptors, which are G-protein coupled receptors located on the basolateral membrane of these cells.
3. Activation of Intracellular Signaling:
   • The binding of ADH activates adenylate cyclase, leading to an increase in cyclic AMP (cAMP) levels within the cell. This cAMP acts as a second messenger, activating protein kinase A (PKA).
4. Aquaporin-2 Insertion:
   • The activation of PKA triggers a series of phosphorylation events that result in the movement of intracellular vesicles containing aquaporin-2 (AQP2) water channels to the apical membrane of the principal cells.
   • Once inserted into the membrane, AQP2 channels significantly increase the permeability of the collecting duct to water.
5. Water Reabsorption:
   • With AQP2 channels present, water can move from the tubular fluid (urine) back into the interstitial fluid and then into the bloodstream by osmosis, driven by the osmotic gradient established by solutes (mainly sodium and urea) in the medullary interstitium.
   • This process allows for substantial reabsorption of water, leading to more concentrated urine and helping to dilute blood plasma osmolarity.

Importance for Osmoregulation

• Conservation of Water: By increasing water reabsorption, ADH helps maintain body fluid balance, particularly during states of dehydration or when blood osmolarity is elevated.
• Urine Concentration: The presence of ADH allows for the production of concentrated urine, which is essential for preventing excessive water loss and maintaining homeostasis.
• Response to Hydration Status: When hydration status improves (e.g., after drinking water), ADH secretion decreases, leading to reduced insertion of AQP2 channels and consequently less water reabsorption, resulting in more dilute urine.