Calcium Movement in Cardiac Mitochondria

Abstract

According to a prevalent view large mitochondrial Ca2+ fluxes are normal and physiological. Thus, mitochondria are significant dynamic buffers of cytosolic Ca2+([Ca2+]i) signals in many tissues including the heart. Here a quantitative investigation of Ca2+ movement through the inner mitochondrial membrane (IMM) has been conducted in isolated rat cardiomyocytes. In these cells, thousands of intermyofibrillar mitochondria are exposed to local [Ca2+]i that rises from 100 nM at rest to as high as 5 uM with each heartbeat. Moreover, the large electrical potential across the IMM (ΔΨmito) of −180 mV provide a strong thermodynamic driving force for movement of Ca2+ from the cytosol into the mitochondrial matrix. Here, halting mitochondrial Ca2+ uptake by depolarizing ΔΨmito is used to investigate the effects of mitochondrial fluxes on cell-wide and subcellular [Ca2+]i signals ([Ca2+]i transients and Ca2+ sparks). We find that rapid loss of ΔΨmito lead to no effect on [Ca2+]i signals. Using a mitochondrially targeted Ca2+-sensitive fluorescent protein, we make direct measurements of mitochondrial matrix Ca2+ ([Ca2+]m). We find that under quiescent conditions [Ca2+]m is nearly the same as [Ca2+]i (i.e., 100 nM), which suggest that although the mitochondria are buffer-capable under quiescent conditions they do not act as dynamic buffer of physiological [Ca2+]i signals. Quantitative analysis using a computational model of mitochondrial Ca2+ cycling suggest that if mitochondrial Ca2+ fluxes were 100 fold larger than its current estimates it would have had a significant influence on [Ca2+]i signals. These findings are consistent with recent quantitative analysis of uptake measurements that have been reported in diverse tissues including liver, cardiac and skeletal muscle. Furthermore, using these novel quantitative tools, we investigate the regulation of mitochondrial function by [Ca2+]m. In doing so, new insights into the physiological and pathophysiological roles of [Ca2+]m are gained.