Alpha adrenergic contributions to dysrhythmia during myocardial ischemia and reperfusion in cats

Alpha compared to beta adrenergic contributions to dysrhythmias induced by left anterior descending coronary occlusion and by reperfusion were assessed in chloralose-anesthetized cats (n = 96). Alpha receptor blockade with either phentolamine or prazosin significantly reduced the number of premature ventricular complexes during coronary reperfusion (321±62-14±10 premature ventricular complexes, P < 0.001), abolished early ventricular fibrillation (from 25% in controls to 0%), and prevented the increase in idioventricular rate seen with coronary reperfusion. However, β-receptor blockade was without effect. Ventricular dysrhythmias induced by coronary occlusion alone (without reperfusion) were attenuated markedly by α-receptor blockade under conditions in which perfusion (measured with radiolabeled microspheres) within ischemic zones was not affected. Alternative sympatholytic interventions including pretreatment with 6-hydroxydopamine to deplete myocardial norepinephrine from 8.8±1.4 to 0.83±0.2 ng/mg protein and render the heart unresponsive to tyramine (120 μg/kg) attenuated dysrhythmias induced by both coronary occlusion and reperfusion in a fashion identical to that seen with α-receptor blockade. Although efferent sympathetic activation induced by left stellate nerve stimulation increased idioventricular rate from 66±6 to 144±7 beats/min (P < 0.01) before coronary occlusion, this response was blocked by propranolol but not by phentolamine. In contrast, during reperfusion the increase in idioventricular rate induced by left stellate nerve stimulation (to 203±14) was not inhibited by propranolol but was abolished by phentolamine (79±10). Intracoronary methoxamine (0.1 μM) in animals depleted of myocardial catecholamines by 6-hydroxydopamine pretreatment did not affect idioventricular rate before coronary occlusion. However, early after coronary reperfusion, methoxamine increased idioventricular rate from 33±7 to 123±21 beats/min (P < 0.01). Thus, enhanced α-adrenergic responsiveness occurs during myocardial ischemia and appears to be a primary mediator of the electrophysiological derangements and resulting malignant dysrhythmias induced by catecholamines during myocardial ischemia and reperfusion.