Physiological Significance of Ca Uptake by Mitochondria in the Heart in Comparison with that by Cardiac Sarcoplasmic Reticulum1

Abstract

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 2.0×10⁵ M₋₁. The apparent Ca-binding constant of cardiac mitochondria under limited loading conditions was 2.4×10₅ M⁻¹ at pH 7.4 and 5.9×104 M₋₁ at pH 6.8. In the presence of 100 * M Ca₂₊ at 28–29°, 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 3.5×10–7 M Ca₂₊ and that of mitochondria under massive loading conditions at 1×10⁻⁶M 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.