title: [3H]pirenzepine and (-)-[3H]quinuclidinyl benzilate binding to rat cerebral cortical and cardiac muscarinic cholinergic sites. I. Characterization and regulation of agonist binding to putative muscarinic subtypes.
abstract: The binding and regulation of selected muscarinic agonists to putative subtypes in rat cerebral cortex and heart were studied. Parallel inhibition studies of [3H]pirenzepine ([3H]PZ) and (-)-[3H]quinuclidinylbenzilate [(-)-[3H]QNB]-labeled membranes were done with and without 30 microM guanyl-5'-yl imidodiphosphate [Gpp(NH)p] at 25 degrees C in 10 mM Na-K-phosphate buffer which enhances PZ binding affinity and in modified Krebs-phosphate buffer, which mimics physiological conditions. Classical agonists such as carbachol, oxotremorine and acetylcholine inhibited (-)-[3H]QNB binding to membranes with shallow Hill values (nH less than 1), were better fit to a 2-state model, were Gpp(NH)p-regulated and showed lower affinity in modified Krebs-phosphate buffer than in 10 mM Na-K-phosphate buffer. Some agonists were not significantly better fit to a 2-state model in [3H]PZ-labeled cortical membranes, especially in 10 mM Na-K-phosphate buffer. Whereas putative M1 and M2 binding sites distinguished by PZ possessed multiple agonist affinity states, as judged by carbachol, and agonist binding to [3H]PZ-labeled sites were Gpp(NH)p modulated, the partial agonist pilocarpine and nonclassical agonist McN-A-343 [3-(m-chlorophenylcarbamoyloxy)-2-butynyl trimethylammonium chloride] showed little Gpp(NH)p-induced shift in [3H]PZ-labeled cortical membranes in physiological conditions. Agonist binding to (-)-[3H]QNB-labeled putative M2 cardiac sites was more sensitive to Gpp(NH)p than (-)-[3H]QNB-labeled cortical sites. Carbachol and acetylcholine showed significant selectivity for putative M2 sites.(ABSTRACT TRUNCATED AT 250 WORDS)
mesh_terms: D000109:Acetylcholine / Q000378:metabolism; D000818:Animals; D001570:Benzodiazepinones / Q000378:metabolism*; D002021:Buffers; D002217:Carbachol / Q000378:metabolism; D002540:Cerebral Cortex / Q000378:metabolism*; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D066298:In Vitro Techniques; D008297:Male; D009206:Myocardium / Q000378:metabolism*; D010277:Parasympathomimetics / Q000378:metabolism*; D010890:Pirenzepine; D011812:Quinuclidines / Q000378:metabolism*; D011813:Quinuclidinyl Benzilate / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D011976:Receptors, Muscarinic / Q000032:analysis / Q000378:metabolism*; D014316:Tritium
chemical_list: D001570:Benzodiazepinones; D002021:Buffers; D010277:Parasympathomimetics; D011812:Quinuclidines; D011976:Receptors, Muscarinic; D014316:Tritium; D006165:Guanylyl Imidodiphosphate; D010890:Pirenzepine; D011813:Quinuclidinyl Benzilate; D002217:Carbachol; D000109:Acetylcholine