Effect of external sodium and calcium on calcium efflux in frog striated muscle.

The effect of media with different ionic composition on calcium efflux from the dorsal head of semitendinosus muscles of Rana pipiens was studied. The reduction in the fractional loss of 45Ca, when going from normal Ringer's solution to an ONa--OCa medium, was 60%. Withdrawal of only Na or Ca from the external medium also caused a significant drop in the fractional loss (33% and 34%, respectively). The effect of different concentrations of Ca (studied on the absence of the external Na) was also studied. It was found that a linear function could describe the relationship between the calcium-dependent calcium efflux and the external calcium concentration. These results indicate that calcium efflux from frog muscle fibers consists of three major components: one that is dependent on the presence of calcium in the external medium, one that is dependent on the presence of sodium in the external medium, and one that persists in the absence of these two cations.