MITOCHONDRIAL CA2-CONTRACTION CYCLE - EFFECTS OF PACING AND HORMONAL-STIMULATION( TRANSIENTS IN CARDIAC MYOCYTES DURING THE EXCITATION)

Using laser scanning confocal microscopy, our objective was to measure mitochondrial, nuclear, and cytosolic free ionized Ca2+ in adult rabb it cardiac myocytes loaded with Ca2+-indicating fluorophores, When myo cytes were loaded with Flue 3 at 37 degrees C, the fluorophore was loa ded extensively into the cytosol and nucleus, but poorly into mitochon dria, and Flue 3 fluorescence transients after field stimulation were confined to the cytosol and nucleus. In contrast, after loading at 4 d egrees C, Flue 3 also entered mitochondria, and large transients of mi tochondrial Flue 3 fluorescence then occurred after stimulation. Isopr oterenol (1 mu M) increased the magnitude of Ca2+ transients and their subsequent rate of decay, an effect more marked in the cytosol and nu cleus than in mitochondria. As pacing frequency was increased from 0.5 to 2 Hz, diastolic mitochondrial Ca2+ rose markedly in the absence bu t not in the presence of isoproterenol. Resting Ca2+ estimated by Indo 1 ratio imaging using UV/visible laser scanning confocal microscopy w as about 200 nM in all compartments. During field stimulation, Ca2+ tr ansiently increased to 671, 522, and 487 nM in cytosol, interfibrillar mitochondria, and perinuclear mitochondria, respectively. Isoproteren ol increased these respective peak values to 1280, 750, and 573 nM, Th ese results were consistent with those obtained in Flue 3 experiments. We conclude that rapid mitochondrial Ca2+ transients occur during exc itation-contraction coupling in adult rabbit cardiac myocytes, which m ay be important in matching mitochondrial metabolism to myocardial ATP demand during changes in cardiac output.