The aim of this study was to investigate the role of mitochondrial ionic homeostasis in promoting reoxygenation-induced hypercontracture in cardiac muscle. Mitochondrial membrane potential and intramitochondrial Ca2+ concentration ([Ca2+]) were measured using confocal imaging in guinea pig ventricular myocytes exposed to anoxia and reoxygenation. Anoxia produced a variable, but often profound, mitochondrial depolarization. Some cells mounted a recovery of their mitochondrial membrane potential during reoxygenation; the depolarization was sustained in other cells. Recovery of the mitochondrial membrane potential seemed essential to avoid reoxygenation-induced hypercontracture. Reoxygenation also caused a sizable elevation in intramitochondrial [Ca2+], the amplitude of which was correlated with the likelihood of a cell undergoing hypercontracture. A sustained Ca2+load analogous to that seen during reoxygenation was imposed on cardiac mitochondria through permeabilization of the plasma membrane. Elevation of intracellular [Ca2+] to 800 nM caused a substantial mitochondrial depolarization. We propose that the conditions seen in guinea pig ventricular myocytes during reoxygenation are well suited to produce Ca2+-dependent mitochondrial depolarization, which may play a significant role in promoting irreversible cell injury.

The aim of this study was to evaluate whether the magnitude and time course of the intracellular acidification observed in anoxic cardiac myocytes was sufficient to protect against reoxygenation-induced hypercontracture. Cytosolic [Ca2+], [Na+], and pH were measured using fluorescent indicators in myocytes that were first subjected to both anoxia and glucose deprivation and then oxygen and glucose restoration 15-30 min after the onset of rigor. The cytosol underwent a profound acidification early in anoxia (pH 7.21 to 6.84) that reached a plateau at the time of rigor contracture. In contrast, [Na+] rose throughout anoxia. Cytosolic [Ca2+] underwent little rise during anoxia, but reoxygenation induced a large spike in [Ca2+]. Reoxygenation also induced a significant secondary acidification of the cytosol that was apparently induced by the spike in [Ca2+]. The characteristics of this secondary acidification were deemed sufficient to provide partial protection against the hypercontracture associated with the reoxygenation-induced [Ca2+] transient.