Question

Describe the cellular effects (where and how) antidiuretic hormone (ADH) and aldosterone act on and signal the collecting duct and, finally, give a physiological example of what triggers the production of ADH and aldosterone (6 marks

Answer 1

# Vasopressin or ADH is a posterior pituitary hormone. It acts on kidney to reabsorb water and produce concentrated urine. ADH when released from posterior pituitary acts on the proximal tubule, collecting tubule and collecting duct. ADH binds to V2 receptors in the kidney which activatescAMP second messenger pathway resulting in increased cAMP formation. cAMP promotes insertion of AQP2 channels into the renal tubular cells resulting in increased water reabsorption.
# Aldosterone is secreted by the adrenal cortex. It helps to regulate blood pressure and is a part of RAS system. It increases water and sodium retention.
Aldosterone binds to the mineralocorticoid receptors present in the principal cells of collecting duct. This binding activates Na+/K+ pump in the basolateral membrane and cause 3 Na+ out of the cell into interstitial space and 1K+ to enter the cell. As sodium concentration increases in interstitial fluid it is reabsorbed into the blood along with water. Potassium is excreted in the urine.
# When the blood volume and blood pressure drops, it results in low perfusion of the kidneys which stimulates release of renin from kidneys. Renin acts on liver to secrete angiotensinogen which is converted to angiotensin ll. Angiotensin II stimulates the release of aldosterone and ADH. Aldosterone and ADH acts on kidney to increase salt and water retention. This restores the blood pressure and volume back to normal.