The juxtaglomerular apparatus regulates blood pressure via release of which enzyme?

• A. Angiotensin converting enzyme
• B. Renin
• C. Aldosterone synthase
• D. Erythropoietin

Answer: (B)

The juxtaglomerular apparatus regulates blood pressure by releasing renin, the enzyme that kick-starts the renin‑angiotensin‑aldosterone system. Juxtaglomerular (JG) cells in the afferent arteriole secrete renin in response to signals such as reduced renal perfusion, decreased NaCl delivery to the macula densa, or sympathetic stimulation. Renin cleaves circulating angiotensinogen (from the liver) to angiotensin I, which is then converted by angiotensin‑converting enzyme to angiotensin II. Angiotensin II causes vasoconstriction and stimulates aldosterone release, promoting sodium and water reabsorption to raise blood pressure. So renin is the initiating enzyme released by the JGA.

The other enzymes listed are not produced by the JGA: ACE acts downstream to convert angiotensin I to II and is abundant on pulmonary and other endothelium; aldosterone synthase is in the adrenal cortex and synthesizes aldosterone; erythropoietin is produced by kidney interstitial cells in response to hypoxia.