Effects of stimulation on intracellular calcium in frog sartorius muscle

The cellular state and distribution of calcium were investigated in frog sartorius muscle at rest and following periods of stimulation. The purpose of the experiments was to define the relative importance of the various cell organelles in the regulation of intracellular calcium. Stimulation increased 45CA uptake by whole muscle and decreased the rate of exchange of cellular calcium in washout experiments. The change in washout characteristics appeared to be the results of a shift of 45Ca from rapidly exchanging to a slowly exchanging pool of cellular calcium. The effect of stimulation on the subcellular distribution 45Ca and calcium was studied by differential centrifugation. The data revealed a heterogeneous distribution of the intracellular calcium, with the highest concentrations in the mitochondrial, heavy reticular, and microsomal fractions. In resting muscle, mitochondria accounted for about 25% of the total tissue 45Ca; however, in stimulated preparations over 40% of the 45Ca was recovered in this fraction. The increase in radioactivity is the consequence of accelerated turnover because stimulation produced little change in the subcellular distribution of total calcium. The relatively profound effect of stimulation on mitochondrial calcium exchange suggests that this organelle plays a role in the movements of intracellular calcium associated with the cycle of contraction and relaxation in skeletal muscle.