A Computational Model of Cytosolic and Mitochondrial [Ca<SUP>2＋</SUP>] in Paced Rat Ventricular Myocytes

A Computational Model of Cytosolic and Mitochondrial [Ca<SUP>2＋</SUP>] in Paced Rat Ventricular Myocytes

We carried out a series of experiment demonstrating the role of mitochondria in the cytosolic and mitochondrial Ca^2＋ transients and compared the results with those from computer simulation. In rat ventricular myocytes, increasing the rate of stimulation (1∼3 Hz) made both the diastolic and systolic [Ca^2＋] bigger in mitochondria as well as in cytosol. As L-type Ca^2＋ channel has key influence on the amplitude of Ca^2＋-induced Ca^2＋ release, the relation between stimulus frequency and the amplitude of Ca^2＋ transients was examined under the low density (1/10 of control) of L-type Ca^2＋ channel in model simulation, where the relation was reversed. In experiment, block of Ca^2＋ uniporter on mitochondrial inner membrane significantly reduced the amplitude of mitochondrial Ca^2＋ transients, while it failed to affect the cytosolic Ca^2＋ transients. In computer simulation, the amplitude of cytosolic Ca^2＋ transients was not affected by removal of Ca^2＋ uniporter. The application of carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone (FCCP) known as a protonophore on mitochondrial membrane to rat ventricular myocytes gradually increased the diastolic [Ca^2＋] in cytosol and eventually abolished the Ca^2＋ transients, which was similarly reproduced in computer simulation. The model study suggests that the relative contribution of L-type Ca^2＋ channel to total transsarcolemmal Ca^2＋ flux could determine whether the cytosolic Ca^2＋ transients become bigger or smaller with higher stimulus frequency. The present study also suggests that cytosolic Ca^2＋ affects mitochondrial Ca^2＋ in a beat-to-beat manner, however, removal of Ca^2＋ influx mechanism into mitochondria does not affect the amplitude of cytosolic Ca^2＋ transients.