| Abstract: | Possible coupling of bovine adrenal medullary opioid receptors to islet-activating protein (IAP, pertussis toxin)-sensitive GTP-binding proteins was investigated by studying effects of guanyl-5'-yl imidodiphosphate (Gpp(NH)p) and IAP treatment of membranes on opioid binding. Gpp(NH)p inhibited 3H]D-Ala2-D-Leu5-enkephalin (3H]DADLE) binding by increasing the dissociation constant of 3H]DADLE and membranes, and enhanced slightly 3H]diprenorphine binding. IAP treatment of membranes reduced 3H]DADLE binding and abolished almost completely the Gpp(NH)p inhibition of 3H]DADLE binding. Treatment of membranes with IAP and 32P]NAD resulted in radio-labeling of membrane proteins of approximately 39,000 dalton. DADLE inhibited adenylate cyclase activity in rat brain caudate nucleus. However, DADLE, beta-endorphin, levorphanol and dynorphin A(1-13) did not show any significant inhibitory action on bovine adrenal medullary adenylate cyclase activity. These results suggest that bovine adrenal medullary opioid (DADLE) receptors are linked to IAP-sensitive GTP-binding proteins which are not directly coupled to adenylate cyclase. |
