Allosteric interactions between muscarinic agonist binding sites and effector sites demonstrated by the use of bisquaternary pyridinium oximes

Abstract

Agonist binding to muscarinic receptors from rat brain stem and cerebral cortex was studied using bisquaternary pyridinium oximes for detecting possible interactions between agonist binding sites and sites of the effector guanosine 5′ (β, γ-imino) triphosphate (Gpp(NH)p) and Co 2+. Pretreatment of either brain stem or cortical homogenates with 200 μM 1-(2-hydroxyiminoethylpyridinium) 1-(3-phenylcarboxypyridinium) dimethylether (HGG-12) reduced the affinity of muscarinic agonists. No change was observed in the relative proportions of high (R H) and low (R L) affinity agonist binding sites. However, the oxime affected the processes of interconversion between these sites. Thus, unlike in control membranes, HGG-12 treated brain stem membranes, Gpp(NH)p could not induce conversion of R H to R L, and in cortical membranes Co 2+ could not induce conversion of R L to R H. These results suggest that HGG-12 inactivates a component which is involved in both processes of induced-interconversion. Induced-interconversion between R H and R L was not affected in membranes treated with HGG-12 in the presence of carbamylcholine in concentrations at which mainly R H is occupied by the agonist. The occupation of R H by carbamylcholine protected both R H and R L from the effects of the oxime. The possible role of the molecular events involved is discussed.