title: Muscarinic receptor subtypes on rat pancreatic acini: secretion and binding studies.
abstract: Characterization of muscarinic receptor subtypes on rat pancreatic acinar cells was examined by using specific muscarinic receptor antagonists to study amylase secretion and binding of [N-methyl-3H]scopolamine ([3H]NMS). Rat pancreatic acini were dispersed in HEPES-Ringer buffer and incubated with acetylcholine +/- 4-diphenylacetoxy-N-methylpiperadine-methiodide (4-DAMP, a specific M2 muscarinic receptor antagonist) or +/- pirenzepine (a specific M1 muscarinic receptor antagonist). 4-DAMP (10(-9) to 10(-6) M) caused a progressive parallel rightward shift in the acetylcholine dose-response curve without a change in maximal amylase release. Only high concentrations of pirenzepine (10(-6) to 10(-4) M) caused a rightward shift in the dose-response curve to acetylcholine. Schild analysis of the data indicated an inhibitory constant (Ki) of 200 pM for 4-DAMP and 183 nM for pirenzepine. The slope of the Schild regression lines was not different from unity, suggesting competitive inhibition. Binding of 50 pM [3H]NMS was specific, rapid, and saturable. [3H]NMS binding was displaced by increasing concentrations of 4-DAMP or pirenzepine with apparent Ki's of 102 pM and 330 nM, respectively, and similar maximal binding levels of 60 fmol/mg prot. We have demonstrated that 4-DAMP has an approximately 1,000-fold greater potency than pirenzepine to inhibit amylase release and binding, indicating that cholinergic-stimulated amylase release from pancreatic acini is mediated by M2 muscarinic receptors.
mesh_terms: D000109:Acetylcholine / Q000494:pharmacology; D000681:Amylases / Q000378:metabolism*; D000818:Animals; D001570:Benzodiazepinones / Q000494:pharmacology; D001665:Binding Sites; D008297:Male; D019832:N-Methylscopolamine; D010179:Pancreas / Q000378:metabolism*; D010880:Piperidines / Q000494:pharmacology; D010890:Pirenzepine; D051381:Rats; D011919:Rats, Inbred Strains; D011976:Receptors, Muscarinic / Q000187:drug effects / Q000302:isolation & purification / Q000378:metabolism*; D012602:Scopolamine Derivatives / Q000378:metabolism; D014316:Tritium
chemical_list: D001570:Benzodiazepinones; D010880:Piperidines; D011976:Receptors, Muscarinic; D012602:Scopolamine Derivatives; D014316:Tritium; D010890:Pirenzepine; C042375:4-diphenylacetoxy-1,1-dimethylpiperidinium; D000681:Amylases; D000109:Acetylcholine; D019832:N-Methylscopolamine