Mitochondrial free calcium transients during excitation‐contraction coupling in rabbit cardiac myocytes

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Abstract
Mitochondrial free Ca2+ may regulate mitochondrial ATP production during cardiac exercise. Here, using laser scanning confocal microscopy of adult rabbit cardiac myocytes co‐loaded with Fluo‐3 to measure free Ca2+ and tetramethylrhodamine methylester to identify mitochondria, we measured cytosolic and mitochondrial Ca2+ transients during the contractile cycle. In resting cells, cytosolic and mitochondrial Fluo‐3 signals were similar. During electrical pacing, transients of Fluo‐3 fluorescence occurred in both the cytosolic and mitochondrial compartments. Both the mitochondrial and the cytosolic transients were potentiated by isoproterenol. These experiments show directly that mitochondrial free Ca2+ rises and falls during excitation‐contraction coupling in cardiac myocytes and that changes of mitochondrial Ca2+ are kinetically component to regulate mitochondrial metabolism on a beat‐to‐beat basis.