β₁-Adrenoreceptor activation contributes to ischemia-reperfusion damage as well as playing a role in ischemic preconditioning

Protein kinase A (PKA) activation has been implicated in early-phase ischemic preconditioning. We recently found that during ischemia PKA activation causes inactivation of cytochrome-c oxidase (CcO) and contributes to myocardial damage due to ischemia-reperfusion. It may be that β-adrenergic stimulation during ischemia via endogenous catecholamine release activates PKA. Thus β-adrenergic stimulation may mediate both myocardial protection and damage during ischemia. The present studies were designed to determine the role of the β₁-adrenergic receptor (β₁-AR) in myocardial ischemic damage and ischemic preconditioning. Langendorff-perfused rabbit hearts underwent 30-min ischemia by anterior coronary artery ligation followed by 2-h reperfusion. Occlusion-reperfusion damage was evaluated by delineating the nonperfused volume of myocardium at risk and volume of myocardial necrosis after 2-h reperfusion. In some hearts ischemic preconditioning was accomplished by two 5-min episodes of global low-flow ischemia separated by 10 min before coronary occlusion-reperfusion. Orthogonal electrocardiograms were recorded, and coronary flow was monitored by a drip count. Three hearts from each experimental group were used to determine mitochondrial CcO and aconitase activities. Two-hour reperfusion after occlusion caused an additional decrease in CcO activity vs. that after 30-min occlusion alone. Blocking the β₁-AR during occlusion-reperfusion reversed CcO activity depression and preserved myocardium at risk for necrosis. Similarly, mitochondrial aconitase activity exhibited a parallel response after occlusion-reperfusion as well as for the other interventions. Furthermore, classic ischemic preconditioning had no effect on CcO depression. However, blocking the β₁-AR during preconditioning eliminated the cardioprotection. If the β₁-AR was blocked after preconditioning, the myocardium was preserved. Interestingly, in both of the latter cases the depression in CcO activity was reversed. Thus the β₁-AR plays a dual role in myocardial ischemic damage. Our findings may lead to therapeutic strategies for preserving myocardium at risk for infarction, especially in coronary reperfusion intervention.