Acetylcholine stimulates cortisol secretion through the M3 muscarinic receptor linked to a polyphosphoinositide-specific phospholipase C in bovine adrenal fasciculata/reticularis cells

Zona fasciculata/reticularis (ZFR) cells, isolated from the bovine adrenal cortex, secreted cortisol in response to acetylcholine (AcCh). The response was present in freshly isolated cells and in cells maintained in primary culture, reaching a maximum after 48–72 h and thereafter declining. Cells maintained in primary culture for 72 h secreted cortisol with an ED 50 at 1.2 × 10 −6 M. The potent inhibition of AcCh-stimulated secretion by atropine, and the relative ineffectiveness of nicotine or nicotinic antagonists, were consistent with a predominantly muscarinic response to AcCh in these cells. A selective M1-receptor agonist, McN-A-343, had no effect on cortisol secretion whereas the M3 antagonist, hexahydro-sila-difenidol, produced a dose-dependent inhibition of AcCh-stimulated cortisol secretion. These findings are consistent with AcCh mediating its effects on cortisol secretion through an M3 receptor. While AcCh had no effect on cAMP formation, a dose-dependent increase in [ 3H]phosphoinositols (identified using high-performance liquid chromatography (HPLC)) occurred in a manner that was not dependent on an influx of extracellular Ca 2+. Detailed HPLC analysis of the formation of 3H-labelled phosphoinositols and glycerophosphoinositols from pre-labelled cells over the period 0–15 min showed that the earliest significant rise was in Ins(1,4,5)P 3 at 5 s, followed by later rises in InsP 1, InsP 2 and Ins(1,3,4)P 3. Additional studies using cells loaded with fura-2 indicator revealed a 1.6-fold increase in [Ca 2+] i from a mean resting value of 75 nM in response to 10 −4 M AcCh. Furthermore, the rise in Ca 2+ was not abolished by lowering extracellular Ca 2+ to resting cytosolic levels, suggesting the mobilisation of an intracellular pool. These observations indicate that AcCh promotes rapid activation of a Ca 2+-independent and polyphosphoinositide-specific phospholipase C, and that the Ins(1,4,5)P 3 formed releases Ca 2+ from an intracellular pool. The stimulation by AcCh of this signal transduction mechanism is consistent with our conclusion, based on the effects of the selective muscarinic agonist and antagonist on cortisol secretion, that the AcCh receptor is of the M3 subtype. We conclude that AcCh, acting through an M3 receptor coupled to phospholipase C, regulates cortisol secretion at the cellular level in bovine adrenal ZFR cells.