Chronology of critical events in neonatal rat ventricular myocytes occurring during reperfusion after simulated ischemia

While an ischemic insult poses a lethal danger to myocardial cells, a significant proportion of cardiac myocytes remain viable throughout the ischemic episode and die, paradoxically, only after the blood flow is reinstated. Despite decades of research, the actual chronology of critical events leading to cardiomyocyte death during the reperfusion phase remains poorly understood. Arguably, identification of the pivotal event in this setting is necessary to design effective strategies aimed at salvaging the myocardium after an ischemic attack. Here we used neonatal rat ventricular myocytes (NRVMs) subjected to 20-30 min of simulated ischemia followed by 1 hour of reperfusion. Using different combinations of spectrally-compatible fluorescent indicators, we analyzed the relative timing of the following events: (1) abnormal increase in cytoplasmic [Ca2+] (T-CaCy); (2) abnormal increase in mitochondrial [Ca2+] (T-CaMi); (3) loss of mitochondrial inner membrane potential (Delta Psi(m)) indicating mitochondrial permeability transitions (T-MPT); (4) sacrolemmal permeabilization (SP) to the normally impermeable small fluorophore TO-PRO3 (T-SP). In additional experiments we also analyzed the timing of abnormal uptake of Zn2+ into the cytoplasm (T-ZnCy) relative to T-CaCy and T-SP. We focused on those NRVMs which survived anoxia, as evidenced by at least 50% recovery of Delta Psi(m) and the absence of detectable SP. In these cells, we found a consistent sequence of critical events in the order, from first to last, of T-CaCy, T-CaMi, T-MPT, T-SP. After detecting T-CaCy and T-CaMi, abrupt switches between 1.1 mM and nominally zero [Ca2+] in the perfusate quickly propagated to the cytoplasmic and mitochondrial [Ca2+]. Depletion of the sarcoplasmic reticulum with ryanodine (5 mu M)/thapsigargin (1 mu M) accelerated all events without changing their order. In the presence of ZnCl2 (10-30 mu M) in the perfusate we found a consistent timing sequence T-CaCy < T-Zn <= T-SP. In some cells ZnCl2 interfered with Ca2+ uptake, causing steps or gaps in the [Ca2+](Cy) curve, a phenomenon never observed in the absence of ZnCl2. Together, these findings suggest an evolving permeabilization of NRVM's sarcolemma during reoxygenation, in which the expansion of the pore size determines the timing of critical events, including T-MPT.