A possible explanation for the peculiar contractile behavior displayed by rat heart muscle.

Because it is generally agreed that the sarcoplasmic reticulum plays an important role in regulating the intracellular availability of Ca2+, the ability of rat microsomal fractions to accumulate Ca2+ was compared with that of microsomal fractions similarly prepared from guinea pig heart muscle. Despite a relatively high level of basic ATPase enzyme activity (19.2 +/- 2.6 muM Pi/mg of microsomal protein/10 min) rat microsomal fractions consistently accumulated significantly (p less than 0.001) less Ca2+ than did the guinea pig preparations, irrespective of whether the incubation medium contained oxalate. The microsomal yield obtained from the rat hearts was not significantly different (p less than 0.8) from that obtained for guinea pig heart muscle. Rat mitochondria similarly accumulated significantly less Ca2+ than did the guinea pig mitochondria. These observations substantiate a general hypothesis that rat heart cells may possess a relatively high intracellular concentration of free Ca.