title: High-affinity agonist binding to rat brainstem muscarinic receptors is eliminated by low pH.
abstract: The effects of hydrogen ion concentration on high-affinity agonist binding to muscarinic receptors were determined in rat brainstem membranes using [3H]oxotremorine-M as a probe. [3H]Oxotremorine-M bound with high affinity to a small subpopulation of brainstem muscarinic receptors (10% of the receptors labelled by [3H]methylscopolamine). [3H]Oxotremorine-M binding was constant between pH 7.0 and 9.0; the number of high-affinity sites decreased below pH 7.0 and at pH 5.0 no binding was detected. This decrease was irreversible; when brainstem membranes were incubated for 1 h at low pH and then returned to pH 8.0, agonist binding was not restored. In contrast, the total number of receptors, i.e. the number of [3H]antagonist binding sites, was not affected by prolonged incubation at low pH. Agonist affinity for the surviving [3H]oxotremorine binding sites and the sensitivity of agonist binding to guanine nucleotides were not altered in media of low pH (pH 5.5). These findings suggest that [3H]oxotremorine-M binds only to receptors which are functionally coupled to guanine nucleotide-dependent regulatory proteins and that this coupling is irreversibly inactivated in media of low pH.
mesh_terms: D000818:Animals; D001933:Brain Stem / Q000378:metabolism*; D002217:Carbachol / Q000494:pharmacology; D006165:Guanylyl Imidodiphosphate / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D008297:Male; D010095:Oxotremorine / Q000031:analogs & derivatives / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D011976:Receptors, Muscarinic / Q000378:metabolism*; D012601:Scopolamine / Q000378:metabolism
chemical_list: D011976:Receptors, Muscarinic; D006165:Guanylyl Imidodiphosphate; D010095:Oxotremorine; C042743:oxotremorine M; D002217:Carbachol; D012601:Scopolamine