Coupling of opiate receptors to adenylate cyclase: requirement for Na+ and GTP.

Abstract

Inhibition of the adenylate cyclase activity in homogenates of mouse neuroblastoma-glioma hybrid cells (NG108-15) by the opioid peptide [D-Ala2,Met5]enkephalin amide (AMEA) requires the presence of Na+ and GTP. In this process, the selectivity for monovalent cations is Na+ .gtoreq. Li+ > K+ > choline+; ITP will replace GTP but ATP, UTP, or CTP will not. The apparent Km for Na+ is 20 mM and for GTP it is 1 .mu.M. Under saturating Na+ and GTP conditions, the apparent Ki [inhibition constant] for AMEA-directed inhibition is 20 nM for basal and 100 nM for prostaglandin[PG]E1-activated adenylate cyclase activity. For both cyclase activities, maximal inhibition is only partial (i.e., .apprxeq. 55% of control in each case). In intact viable NG108-15 cells, the decrease in basal and PGE1-stimulated intracellular cyclic[c]AMP concentrations by AMEA is also dependent upon extracellular Na+. The enkephalin-directed reductions in cAMP concentrations are at least 75%. The specificity of the monovalent cation requirement for enkephalin action on intact cells is the same as for enkephalin regulation of homogenate adenylate cyclase activity. Based on these data, a model is presented in which the transfer of information from opiate receptors to adenylate cyclase requires active separate membrane components, which correspond to the sites of action of Na+ and GTP in this process.