Carbachol-induced potentiation and inhibition of acid secretion by guinea pig gastric gland.

The effects of muscarinic ligands on acid secretion were examined by estimating the accumulation of [14C]aminopyrine in gastric glands isolated from guinea pigs. The accumulation of [14C]aminopyrine in the presence of 0.1 mM histamine was potentiated by 1 microM carbachol but suppressed by 1 mM. These two effects of carbachol were abolished by atropine, pirenzepine and AF-DX 116. Assuming that the binding of carbachol to one site (Site 1) increases [14C]aminopyrine accumulation but its binding to the other site (Site 2) reduces [14C]aminopyrine accumulation, we analysed the dose-response curves for the carbachol effects in the absence and presence of different concentrations of atropine, pirenzepine and AF-DX 116. The dissociation constants determined for these ligands at Sites 1 and 2 were as follows: carbachol, 0.28 and 7.1 microM; atropine, 0.28 and 0.54 nM; pirenzepine, 45 and 560 nM; and AF-DX 116, 380 and 4400 nM, respectively. The binding of [3H]N-methylscopolamine to the gastric glands indicated the presence of two populations of binding sites with different affinities for the above ligands, other than atropine. The apparent dissociation constants, which were estimated by analysing the displacement curves for [3H]N-methylscopolamine binding, were as follows: carbachol, 0.18 microM (10%) and 31 microM (90%); atropine, 1.24 nM; pirenzepine, 15 nM (16%) and 220 nM (84%); and AF-DX 116, 370 nM (10%) and 2970 nM (90%). These results suggest that there are two kinds of muscarinic acetylcholine receptors in the guinea pig gastric gland, one potentiating and the other inhibiting the acid secretion induced by histamine.