Involvement of a high-affinity GTPase in the inhibitory coupling of striatal muscarinic receptors to adenylate cyclase

The stimulation of GTP hydrolysis has been proposed as a mechanism by which hormones inhibit receptor-coupled adenylate cyclase activity. The present study attempts to verify whether this mechanism is also operative in transmitter-mediated receptor-coupled attenuation of adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] located in synaptic plasma membrane preparations. As a model, we used the inhibition of adenylate cyclase activity by muscarinic receptor activation in rat striatum. This striatal preparation contains high-affinity GTPase (EC 3.6.1-) activity which is stimulated when the recognition site for muscarinic agonists is occupied. Acetylcholine (ACh), but not nicotine, increases the Vmax of the high-affinity GTPase, and the stimulatory effect is antagonized by atropine but not by d-tubocurarine. The rank order of potency of various cholinergic agonists to stimulate GTPase correlates with their ability to inhibit adenylate cyclase activity of striatal membranes. Pre-exposure of striatal membranes to guanosine-5'-O-(3-thiotriphosphate) causes a parallel decrease in the basal and ACh-stimulated GTPase activities and in the ACh-induced inhibition of adenylate cyclase. Treatment of the membranes with cholera toxin does not affect the ACh-stimulated GTPase activity but amplifies the extent of adenylate cyclase inhibition elicited by the cholinergic agonist. These results indicate that the stimulation of a high-affinity GTPase parallels the inhibitory coupling of central muscarinic receptors to adenylate cyclase.