A Kinetic Study of the Energy‐Linked Influx of Ca2+ into Heart Mitochondria

The energy‐linked uptake of Ca2+ by heart mitochondria is inhibited by Mg2+ and this has been exploited to devise an inhibitor‐stop method which has permitted direct measurements of the kinetics of Ca2+ uptake. The initial rate of Ca2+ influx has also been measured directly using a Ca2+‐selective electrode. Half‐maximal rate of uptake is attained at 12–15 μM Ca2+ and the maximum velocity is about 1.4 nmol of Ca2+/mg of protein per s at 25°C. The rate versus Ca2+ ‐concentration curve is hyperbolic but becomes sigmoidal in the presence of Mg2+. Inorganic phosphate stimulates the initial rate of Ca2+ influx 2 to 4 times.