Intracellular Calcium Binding and Release in Frog Heart

The capacities and affinities of intracellular calcium-binding sites have been studied in frog ventricles, in which the concentration of Ca(++) in the sarcoplasm can be controlled as a result of treatment with EDTA. The total calcium content of calcium-depleted and nondepleted muscles at rest and muscles generating considerable tension was 0.8, 1.4, and 5.4 µmol/g of muscle, respectively. Net movement of calcium into or out of the cells occurred without change in tension when the sarcoplasmic concentration of Ca(++) was either of two values, less than 10(-7) M or approximately 5 x 10(-7) M. These data can be explained by the presence of two groups of intracellular calcium sinks which compete with the contractile proteins, one with a capacity of about 0.6 µmol/g and an affinity constant greater than 10(7) M(-1) and a second with a capacity of 4.0 µmol/g and an affinity constant of about 2 x 10(6) M(-1). The higher affinity calcium is released by anoxia, oligomycin, or abrupt changes in sarcoplasmic Ca(++). Muscles soaked in Sr-Ringer's contain electron densities in the sarcoplasmic reticulum and to a lesser extent in the mitochondria.