Isolated parietal cells: [3H]QNB binding to putative cholinergic receptors

Abstract

The tritiated muscarinic cholinergic antagonist quinuclidinyl benzilate, [3H]QNB, was used as a direct probe for the detection and characterization of muscarinic cholinergic receptors associated with the particulate fraction of isolated and purified rat gastric mucosal parietal cells. Specific binding is saturable (Bmax [maximum number of binding sites] = 55 fmol/mg protein, Kd = 0.78 nM), shows a single population of binding sites, and has appropriate pharmacological specificity. Nanomolar concentrations of muscarinic cholinergic antagonists, i.e., atropine and scopolamine, inhibit [3H]QNB binding by 50%, but micromolar concentrations are needed for agonists, i.e., acetylcholine, oxotremorine and carbamylcholine. Binding is stereoselective as shown by the more than 1000-fold difference in inhibitory potencies of the stereoisomers of benzetimide. Noncholinergic agents, including pentagastrin, histamine and the H2-receptor antagonists cimetidine and metiamide have little or no effect on [3H]QNB binding at concentrations of 100 .mu.M. These data support the existence of specific parietal cell muscarinic cholinergic receptors with which the secretagog acetylcholine may directly interact to initiate gastric acid secretion.