Comparison of ATPase activity of cardiac sarcoplasmic reticulum fraction from rat and dog.

The purpose of the present study was to compare the ATPase activities of cardiac SR in two species in which the different intrinsic myocardial contractility can only partially be explained by the different properties of cardiac myosins. In cardiac SR isolated from rat heart, the total ATPase activity was 1512.5 +/- 23.3 nmol Pi/mg protein/min, nearly four times as high as in dog cardiac SR (408.8 +/- 28.9 nmol Pi/mg protein/min). The Ca2+-activated ATPase in rat cardiac SR represented only 23.8% of the total ATPase activity, while in dog cardiac SR it was approximately 50% of the total. Thus, the specific Ca2+-activated ATPase was nearly two times higher in the cardiac SR of the rat than in that of the dog. This higher rate of ATP hydrolysis in rat cardiac SR may be, at least in part, responsible for the increased intensity and shorter duration of the active state in the rat myocardium. Polyacrylamide gel electrophoresis of SR showed that the relative amount of Ca2+-pump protein was two times higher in dog heart, similar to the percentage of Ca2+-activated ATPase activity. At the same time, the specific Ca2+-activated ATPase activity and the relative amount of Ca2+ pump protein in both the rat and dog cardiac SR were inversely related.