renine

Renine, commonly spelled renin, is a proteolytic enzyme that initiates the renin-angiotensin system by cleaving angiotensinogen to angiotensin I. Angiotensin I is then converted to the potent vasoconstrictor angiotensin II by angiotensin-converting enzyme, linking renin activity to blood pressure regulation and electrolyte balance.

Renine is produced primarily by the juxtaglomerular cells of the kidney, within the walls of the afferent arterioles. It is synthesized as prorenin, an inactive precursor, and is secreted into the circulation where proteolytic processing activates it. Small amounts of renin and prorenin can be produced in other tissues, including the brain, heart, placenta, and adipose tissue, but the kidney remains the dominant source.

Function and regulation: Renine catalyzes the rate-limiting step of the renin-angiotensin system by generating angiotensin I from angiotensinogen. Angiotensin II, produced downstream, promotes vasoconstriction and stimulates aldosterone release, increasing sodium and water reabsorption and elevating blood pressure. Renine release is tightly regulated by multiple signals: decreased renal perfusion pressure or effective circulating volume, decreased sodium chloride delivery to the macula densa, and sympathetic nervous system stimulation via beta-1 receptors. Angiotensin II and aldosterone provide negative feedback to reduce renin secretion.

Clinical relevance: Renine is a therapeutic target in hypertension and certain cardiac and kidney diseases. Direct renin inhibitors, such as aliskiren, reduce renin activity. Renin levels or activity (plasma renin activity or direct renin concentration) are used in diagnostic assessments of hypertension and electrolyte disorders. Rare renin-secreting tumors (reninomas) have been described but are uncommon.