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 cardiac sarcolemmal membranes have yet to be identified. In highly purified canine cardiac sarcolemmal membranes, binding of the opioid receptor antagonist [3H]diprenorphine and effects of mu, delta and kappa agonists on low Km GTPase and adenylyl cyclase were measured. Equilibrium binding of [3H]diprenorphine revealed a maximal binding capacity of 7.2 pmol/mg protein and a Kd of 1.3 nmol/l. In the presence of GTP, (D-Pen2,5, p-Cl-Phe4) enkephalin and (D-Arg6) dynorphin A 1-13 fragment both inhibited adenylyl cyclase by 20-25% (from 206 +/- 30 to 164 +/- 28 pmol.min-1.mg protein-1, EC50 6 mumol/L, and from 254 +/- 109 to 204 +/- 90 pmol.min-1.mg protein-1, EC50 8 mumol/L, respectively; P < 0.001). Both substances stimulated low Km GTPase by 20% and 13%, respectively (from 12.7 +/- 3.0 to 15.2 +/- 3.7 pmol.min-1.mg protein-1, EC50 12 mumol/L, P < 0.01, and from 9.1 +/- 2.8 to 10.4 +/- 3.2 pmol.min-1.mg protein-1, EC50 6 mumol/L, P < 0.05, respectively). These effects were blocked by the opioid receptor antagonist naltrexone and by pretreatment of sarcolemmal membranes with pertussis toxin. The mu opioid receptor agonists (D-Ala2, Me Phe4, Gly-[ol]5)enkephalin and morphiceptin had no effect on either adenylyl cyclase or low Km GTPase activities. These data suggest that in cardiac sarcolemma, opioid receptors are coupled to pertussis toxin sensitive G proteins and mediate inhibition of adenylyl cyclase activity.