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' (beta, gamma-imino) triphosphate (Gpp(NH)p) and Co2+. Pretreatment of either brain stem or cortical homogenates with 200 microM 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 (RH) and low (RL) 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 RH to RL, and in cortical membranes Co2+ could not induce conversion of RL to RH. These results suggest that HGG-12 inactivates a component which is involved in both processes of induced-interconversion. Induced-interconversion between RH and RL was not affected in membranes treated with HGG-12 in the presence of carbamylcholine in concentrations at which mainly RH is occupied by the agonist. The occupation of RH by carbamylcholine protected both RH and RL from the effects of the oxime. The possible role of the molecular events involved is discussed.