β-Receptors and adenylate cyclase: Comparison of nonischemic, ischemic, and postmortem tissue

We compared the effects of myocardial ischemia and postmortem changes on β-adrenergic receptors and their coupling to adenylate cyclase activity. The effects of 1 h of left circumflex coronary artery occlusion were examined in eight conscious calves, which were then anesthetized with pentobarbital sodium, and the left ventricle was divided into nonischemic and ischemic regions. A crude membrane fraction was prepared from each region and from the nonischemic tissue 1 h postmortem. β-Adrenergic receptor density increased (152 ± 55%) and decreases in basal (-21 ± 6.1%), isoproterenol-stimulated (-25 ± 8.0%), 5'-guanylylimidodiphosphate [Gpp(NH)p]-stimulated (-17 ± 5.8%), fluoride-stimulated (-26 ± 5.8%), and forskolin-stimulated (-31 ± 8.4%) adenylate cyclase activities were observed in the ischemic myocardium compared with nonischemic myocardium. Similarly, in postmortem samples, β-adrenergic receptor density rose 58 ± 16%, whereas decreases in basal (-48 ± 8.7%), isoproterenol-stimulated (-61 ± 7.8%), Gpp(NH)p-stimulated (-58 ± 7.0%), fluoride-stimulated (-64 ± 6.1%), and forskolin-stimulated (-52 ± 6.2%) adenylate cyclase activities were observed. Agonist-binding competition curves with isoproterenol were shifted, indicating that β-adrenergic receptors were binding agonists with low affinity in both the ischemic and postmortem myocardium. The marked, but directionally opposite, changes in receptor density and adenylate cyclase that occur postmortem indicate the importance of prompt processing of tissues. The striking similarity in response of β-adrenergic receptor agonist and antagonist binding and adenylate cyclase activity in ischemic and postmortem tissue raises the speculation that similar mechanisms may operate under both conditions.