We studied possible coupling of opioid receptors to GTP-binding proteins to clarify the mechanism (s) of opioid action in bovine adrenal medullary membranes. Guanylyl imidodiphosphate (Gpp (NH) p) reduced the binding of [3H] D-Ala2-D-Leu5-enkephalin ([3 H] DADLE) to bovine adrenal medullary membranes dose-dependently, and enhanced the binding of [3H] diprenorphine to them. Gpp (NH) p (0.1 mM) enhanced the Kd value of the [3 H] DADLE binding from 2.9nM to 3.9nM, but did not change its Bmax. Pretreatment of bovine adrenal medullary membranes with pertussis toxin (PT) reduced the [3 H] DADLE binding. The Gpp (NH) p inhibition for [3 H] DADLE binding was diminished by the PT-pretreatment. On the other hand, the [3H] diprenorphine binding to PT-pretreated membranes was higher than that to control membranes. Levorphanol inhibited the adenylate cyclase activity of the rat caudate nucleus crude synaptosomal fraction, but did not change that of bovine adrenal medullary membranes.
These results suggest that opioid receptors in bovine adrenal medullary membranes are coupled to PT-sensitive GTP-binding protein which may not influence on adenylate cyclase.
