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

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⁺≥Li⁺>K⁺>choline⁺; ITP will replace GTP but ATP, UTP, or CTP will not. The apparent K(m) for Na⁺ is 20 mM and for GTP it is 1 μM. Under saturating Na⁺ and GTP conditions, the apparent K(i) for AMEA-directed inhibition is 20 nM for basal and 100 nM for prostaglandin E₁-activated adenylate cyclase activity. For both cyclase activities, maximal inhibition is only partial (i.e., ~55% of control in each case). In intact viable NG108-15 cells, the decrease in basal and prostaglandin E₁-stimulated intracellular cyclic AMP concentrations by AMEA is also dependent upon extracellular Na⁺. The enkephalin-directed reductions in cyclic AMP 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.