Physiological significance of Ca uptake by mitochondria in the heart in comparison with that by cardiac sarcoplasmic reticulum.

It was investigated whether mitochondria play a significant role in the physiological regulation of the contractile process by Ca2+ in cardiac muscle in comparison with the sarcoplasmic reticulum (SR). Ca uptake activities of chicken cardiac SR and rabbit cardiac mitochondria were measured by means of centrifugation, dual-wave-length spectrophotometric and Millipore filtration methods. The maximum Ca uptake capacity of cardiac SR was usually 50-60 nmoles/mg protein and the apparent binding constant was 2.0 X 10(6) M-1. The apparent Ca-binding constant of cardiac mitochondria under limited loading conditions was 2.4 X 10(5) M-1 at pH 7.4 and 5.9 X 10(4) M-1 at pH 6.8. In the presence of 100 muM Ca2+ at 28-29 degrees, the estimated initial rate of Ca uptake of cardiac SR ranged from 20 to 30 nmoles Ca/mg-sec, while that of mitochondria was 4.6 nmoles Ca/mg-sec under limited loading conditions at pH 7.4 and 0.64 nmoles Ca/mg-sec under massive loading conditions at pH 6.8, which was much closer to physiological conditions. In the presence of low Ca2+ concentrations, the initial rate of Ca uptake of cardiac SR was 0.5 nmoles Ca/mg-sec at 3.5 X 10(-7) M Ca2+ and that of mitochondria under massive loading conditions at 1 X 10(-6) M Ca2+ was 0.02 nmoles Ca/mg-sec at pH 7.4 and 0.004 nmoles Ca/mg-sec at pH 6.8. The Ca uptake activities were also examined using glycerol-extracted cardiac muscle fibers. Cardiac SR, 1.7 mg/ml, reduced the tension of maximally contracted cardiac muscle fibers to a level corresponding to about 30% of maximum tension, but in the presence of 14.3 mg/ml of mitochondria the maximum tensions of both skeletal muscle and cardiac muscle fibers were maintained for at least 3 min. From these results the time course of relaxation of cardiac muscle induced by cardiac SR or mitochondria was calculated. It was concluded that, in the physiological contraction of cardiac muscle, the SR plays a major role in controlling intracellular Ca2+ movement; the Ca uptake of mitochondria is relatively insignificant. When the cardiac muscle contracts maximally, SR alone cannot relax the cardiac muscle without the aid of other Ca removing system.