Characterization of cholinergic receptors mediating pepsinogen secretion from chief cells

By use of a highly enriched preparation of chief cells (greater than 90% pure) prepared from guinea pig stomach, the cholinergic receptors regulating pepsinogen secretion were studied. Each of five different muscarinic cholinergic agonists, but not the nicotinic cholinergic agonist nicotine, stimulated pepsinogen release. Carbamylcholine-stimulated pepsinogen release was inhibited by each of seven different cholinergic receptor antagonists with relative potencies of N-methylscopolamine greater than scopolamine = 4-diphenylacetoxy-N-methylpiperidine methiodide = atropine much greater than pirenzepine greater than AF-DX-116 much greater than tetraethylammonium. Binding of [N-methyl-3H]scopolamine ([3H]NMS) was time and temperature dependent, reversible, saturable, and specific. Analysis of [3H]NMS binding demonstrated a Kd of 1.3 nM for NMS with a binding capacity of 61 fmol/mg protein or 5,920 sites/chief cell. With the agonists carbamylcholine, acetylcholine, or muscarine, the receptor population could be divided into two classes of receptor sites: a class with high affinity (Kd, 12-53 microM) representing 73% of the binding sites and a class with low affinity (Kd, 2-5 mM) representing 27% of the binding sites. Each of the antagonists had equal affinity for both receptor populations. There was a close correlation between the ability of antagonists to inhibit [3H]NMS binding and carbamylcholine-stimulated pepsinogen release. Each agonist was 29- to 63-fold more potent at stimulating pepsinogen release than interacting with high-affinity receptors and 2,000- to 11,000-fold more potent than interacting with low-affinity receptors, suggesting spare receptors. Studies with the alkylating agent, propylbenzilylcholine mustard, demonstrated that there was a receptor reserve of 50-80%.(ABSTRACT TRUNCATED AT 250 WORDS)