Muscarinic receptors in isolated gastric fundic mucosal cells. Binding-activity relationships

Muscarinic receptors are involved in the control of gastric acid secretion. The characteristics of (3H)-N-methyl-scopolamine [(3H)-NMS] binding to isolated cells from rabbit fundic gastric mucosa and inhibition of this binding by muscarinic agonists, antagonists and other pharmacological agents known to regulate acid secretion are reported. Specific binding for (3H)-NMS was described: antagonists interact with high affinity sites (KD = 0.5 nM) whereas binding curves for agonists clearly deviated from the simple mass action isotherm with a flattening of the curve suggesting the presence of more than one class of sites. The low affinity sites for agonists are in the micromolar range. Pirenzine, a gastroselective antimuscarinic compound, known to differentiate between M1 and M2 sites, inhibited (3H)-NMS binding with an IC50 of 0.05 microM. On the same gastric cell population, muscarinic agonist carbachol stimulated (14C)-aminopyrine accumulation in a dose-dependent manner with an ED-50 of 10 microM, value close to that needed to 50% inhibit (3H)-NMS binding. This stimulation was competitively inhibited by muscarinic antagonists and pA2-values for atropine, QNB and pirenzepine, calculated from linear Schild plots, were in the following order: 9.2 for atropine, 8.6 for QNB and 7.0 for pirenzepine. In conclusion, fundic gastric mucosal cells from rabbit, isolated with collagenase and EDTA, contained specific muscarinic receptors coupled to the acid secretory mechanism and pirenzepine interact with these receptors with an intermediate affinity suggesting the presence of functional M2-sites.