Calcium Movement in Cardiac Mitochondria

Existing theory suggests that mitochondria act as significant, dynamic buffers of cytosolic calcium ([Ca2+](i)) in heart. These buffers can remove up to one-third of the Ca2+ that enters the cytosol during the [Ca2+](i) transients that underlie contractions. However, few quantitative experiments have been presented to test this hypothesis. Here, we investigate the influence of Ca2+ movement across the inner mitochondrial membrane during both subcellular and global cellular cytosolic Ca2+ signals (i.e., Ca2+ sparks and [Ca2+](i) transients, respectively) in isolated rat cardiomyocytes. By rapidly turning off the mitochondria using depolarization of the inner mitochondrial membrane potential (Delta Psi(m)), the role of the mitochondria in buffering cytosolic Ca2+ signals was investigated. We show here that rapid loss of Delta Psi(m) leads to no significant changes in cytosolic Ca2+ signals. Second, we make direct measurements of mitochondrial [Ca2+] ([Ca2+](m)) using a mitochondrially targeted Ca2+ probe (MityCam) and these data suggest that [Ca2+](m) is near the [Ca2+](i) level (similar to 100 nM) under quiescent conditions. These two findings indicate that although the mitochondrial matrix is fully buffer-capable under quiescent conditions, it does not function as a significant dynamic buffer during physiological Ca2+ signaling. Finally, quantitative analysis using a computational model of mitochondrial Ca2+ cycling suggests that mitochondrial Ca2+ uptake would need to be at least similar to 100-fold greater than the current estimates of Ca2+ influx for mitochondria to influence measurably cytosolic [Ca2+] signals under physiological conditions. Combined, these experiments and computational investigations show that mitochondrial Ca2+ uptake does not significantly alter cytosolic Ca2+ signals under normal conditions and indicates that mitochondria do not act as important dynamic buffers of [Ca2+](i) under physiological conditions in heart.