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

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 Co²⁺. 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 Co²⁺ 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.