Pharmacological analysis of the inhibition by pirenzepine and atropine of vagal-stimulated acid secretion in the isolated stomach of the mouse.

The muscarinic receptors involved in the vagal stimulation of gastric acid secretion in the mouse isolated stomach assay have been examined by analysing the effects of pirenzepine and atropine on fully-defined frequency-effect curves. Both atropine and pirenzepine produced concentration-dependent inhibition of vagal-stimulated acid secretion in a manner consistent with a model describing competitive antagonism of endogenous acetylcholine, which was assumed to be released by vagal stimulation. The results obtained are quite compatible with the hypothesis that vagal stimulation involves muscarinic receptors which are homogeneous with those previously found on histamine and oxyntic cells in the mouse stomach assay. These results find no evidence for muscarinic receptor heterogeneity and reinforce the hypothesis that the selectivity of pirenzepine in vivo relative to atropine is due to the loss of atropine into the gastric secretion.