Subcontracture depolarizations increase sarcoplasmic ionized calcium in frog skeletal muscle

The hypothesis that the sarcoplasmic concentration of ionized calcium (Cai) of frog skeletal muscle may control its rate of metabolism was examined by monitoring the changes in Cai due to perturbations that are known, from the work of previous investigators, to alter the rate of metabolism. Cai was measured with aequorin, which was microinjected into isolated fibers in sufficient quantity to detect basal Cai. When these fibers were exposed to 5-18 mM KCl, 75 mM RbCl2, or 100 mM CsCl2, there was an increase in the aequorin signal. The potassium-evoked increase in the aequorin signal was diminished by an increase in the extracellular concentration of Ca or by Ca-free media containing 3.6 mM Mg, Mn, Sr, or Zn but was enhanced by subcontracture concentrations of caffeine or media containing nitrate instead of chloride. These results are consistent with the hypothesis that Cai may control the rate of metabolism in frog skeletal muscle fibers.