Effects of Mitochondrial Membrane Depolarization on Cellular Function in Cardiac Myocytes

Abstract

Mitochondria are the primary sites of ATP generation in heart cells. The voltage gradient across the mitochondrial inner membrane (ΔΨmito) is a critical feature and this in turn is controlled in part by the mitochondria permeability transition pore (mPTP), whose molecular identity remains elusive. Here we used used illumination-dependent subcellular mitochondrial depolarization to investigate cardiac mitochondrial function in cells exposed to low (nM) concentrations of the fluorescent mitochondrial reporter tetramethyl rhodamine methyl ester (TMRM). The relationship between mPTP gating (as measured by ΔΨmito depolarization) and subcellular myocyte function was examined in single cardiac myocytes. ROS production was measured using dichlorofluorescin (DCF). [Ca2+]i was measured with intracellular fluo-4. There was a time-dependent increase in the depolarization of those mitochondria exposed to visible light but nearby mitochondria in the same cell but kept in the dark remained normally polarized. Only rarely did the illuminated and depolarized mitochondria repolarize following the cessation of illumination. We have also investigated the hypothesis that the illuminated mitochondrial depolarization is due to a ROS-dependent mechanism. How Ca2+ signaling, ROS, ΔΨmito are inter-related will be discussed. Additionally the mitochondrial depolarization dependent actions on other myocyte functions (contraction, [Ca2+]i transients, Ca2+ instability, membrane currents) will be discussed.