M3 muscarinic receptors mediate pepsinogen secretion via polyphosphoinositide hydrolysis in guinea pig gastric chief cells.

The muscarinic receptor system involved in pepsinogen secretion from isolated guinea pig gastric chief cells was investigated by evaluating the effect of muscarinic receptor antagonists on carbamylcholine (CCh)-stimulated chief cell responses. CCh stimulated the hydrolysis of polyphosphoinositide in chief cells at the same concentrations as it stimulated pepsinogen secretion. Each of five different muscarinic receptor antagonists, atropine, pirenzepine, 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP), AF-DX116 and scopolamine, inhibited both pepsinogen secretion and inositol phosphate accumulation stimulated by graded concentrations of CCh. The pA2 values of the antagonists calculated from data on inositol phosphate accumulation and pepsinogen secretion (atropine = scopolamine = 4-DAMP greater than pirenzepine greater than AF-DX116) suggest that the muscarinic acetylcholine receptor in gastric chief cells is the M3 subtype. On the other hand, CCh did not affect the adenylate cyclase/cAMP signaling pathway in gastric chief cells. All pA2 values of the antagonists were also in agreement with the Ki values determined by [3H]NMS binding to chief cells. Furthermore, GTP gamma S reduced [3H]acetylcholine binding to chief cell membranes in a concentration-dependent manner. The present study, therefore, suggests that muscarinic M3 receptors, which may be coupled to a G protein, mediate pepsinogen secretion, probably by activation of the polyphosphoinositide second messenger system in guinea pig gastric chief cells.