OPIOID RECEPTOR AGONISTS ACTIVATE PERTUSSIS-TOXIN-SENSITIVE G-PROTEINS AND INHIBIT ADENYLYL-CYCLASE IN CANINE CARDIAC SARCOLEMMA

Although both opioid receptors and endogenous opioids are abundant in cardiac tissues, the signal transduction pathways of opioids in cardia c sarcolemmal membranes have yet to be identified. In highly purified canine cardiac sarcolemmal membranes, binding of the opioid receptor a ntagonist [H-3]diprenorphine and effects of mu, delta and kappa agonis ts on low K-m GTPase and adenylyl cyclase were measured. Equilibrium b inding of [H-3]diprenorphine revealed a maximal binding capacity of 7. 2 pmol/mg protein and a K-d of 1.3 nmol/l. In the presence of GTP, (D- Pen(2,5),p-Cl-Phe(4))enkephalin and (D-Arg(6))dynorphin A 1-13 fragmen t both inhibited adenylyl cyclase by 20-25% (from 206 +/- 30 to 164 +/ - 28 pmol . min(-1). mg protein(-1), 164 a 28 pmolmin-' EC(50) 6 mu mo l/L and from 254 +/- 109 to 204 +/- 90 pmol . min(-1). mg protein(-1), EC(50) 8 mu mol/L, respectively; P<0.001). Both substances stimulated low K-m GTPase by 20% and 13%, respectively (from 12.7 +/- 3.0 to 15. 2 +/- 3.7 pmol . min(-1). mg protein(-1), EC(50) 12 mu mol/L, P<0.01, and from 9.1 +/- 2.8 to 10.4 +/- 3.2 pmol . min(-1). mg protein(-1), E C(50) 6 mu mol/L, P<0.05, respectively). These effects were blocked by the opioid receptor antagonist naltrexone and by pretreatment of sarc olemmal membranes with pertussis toxin. The mu opioid receptor agonist s (D-Ala(2), Me Phe(4), Gly-[ol](5))enkephalin and morphiceptin had no effect on either cardiac adenylyl cyclase or low K-m GTPase activitie s. These data suggest that in cardiac sarcolemma, opioid receptors are coupled to pertussis toxin sensitive G proteins and mediate inhibitio n of adenylyl cyclase activity.