title: Agonist mediated conformational changes of solubilized calf forebrain muscarinic acetylcholine receptors.
abstract: Muscarinic receptors in calf forebrain membranes can be identified by the specific binding of the radiolabelled antagonist [3H]dexetimide. These receptors (2.8 pM/mg protein) comprise two non-interconvertible subpopulations with respectively high and low agonist affinity but with the same antagonist affinity. For all the agonists tested the low affinity sites represent 85 +/- 5% of the total receptor population. 0.5% Digitonin solubilized extracts contain 0.8 pM muscarinic receptor/mg protein. In contrast with the membranes, these extracts contain only sites with low agonist affinity. The alkylating reagent N-ethylmaleimide causes an increase of the acetylcholine affinity for the low affinity sites in membranes as well as for the solubilized sites. This effect is time dependent until a maximal 3-fold increase in affinity is attained. The rate of N-ethylmaleimide action is enhanced by the concomitant presence of agonists. In contrast, N-ethylmaleimide does not affect antagonist binding. This suggests that agonists mediate a conformational change of both the membrane bound low affinity muscarinic sites and of the solubilized sites, resulting in their increased susceptibility towards NEM alkylation.
mesh_terms: D000478:Alkylation; D000818:Animals; D001921:Brain / Q000378:metabolism; D002417:Cattle; D003909:Dexetimide / Q000378:metabolism; D005033:Ethylmaleimide / Q000494:pharmacology; D010277:Parasympathomimetics / Q000494:pharmacology*; D011487:Protein Conformation / Q000187:drug effects; D011976:Receptors, Muscarinic / Q000187:drug effects* / Q000378:metabolism; D012995:Solubility
chemical_list: D010277:Parasympathomimetics; D011976:Receptors, Muscarinic; D003909:Dexetimide; D005033:Ethylmaleimide