Overview

JG (granular) cells are epithelioid smooth muscle cells of the tunica media of the afferent arteriole just before it enters the glomerulus. They store renin in dense secretory granules and release it by exocytosis. Three inputs control secretion: (1) intrarenal baroreceptor — reduced stretch of the afferent arteriole wall (low perfusion pressure) stimulates release; (2) macula densa — low distal NaCl signals via PGE₂ and NO; (3) sympathetic nerves — noradrenaline acting on β1-adrenoceptors (cAMP). Angiotensin II (via AT1, raising intracellular Ca²⁺) and ANP inhibit release.

Renin is an aspartyl protease that cleaves angiotensinogen to angiotensin I — the rate-limiting step of the RAAS.

Location: Media of the afferent arteriole at the vascular pole of the glomerulus.

Anatomy

Histology: Plump epithelioid cells with PAS-positive renin granules.

Function

Synthesise, store and secrete renin
Act as intrarenal baroreceptors

Secretions: Renin (and its precursor prorenin).
Regulation: ↑ cAMP (β1, PGE₂) and low stretch; ↓ intracellular Ca²⁺ (angiotensin II, high pressure), ANP.

Clinical significance

Primary vs secondary hyperaldosteronism

High aldosterone with low renin (aldosterone:renin ratio high) suggests primary hyperaldosteronism (Conn syndrome); high aldosterone with high renin suggests secondary causes such as renal artery stenosis, diuretics, heart failure or a reninoma.

Mnemonics

Angiotensin II actions

"ATII is VAST" — Vasoconstriction, Aldosterone release, Sodium (PCT) and water retention with thirst and ADH, Tightens the efferent arteriole (maintains GFR).

Compiled from standard anatomy and physiology references — see the Urinary System sources. For education only; not medical advice.