Is the sarcolemmal Na+ -K+ ATPase involved in active calcium transport?

Sarcolemmal preparations from rat and guinea pig cardiac muscle consisting mostly of inside-out vesicles were found to accumulate Ca2+ in the presence of ATP. The normalized rate of calcium uptake (the rate of calcium accumulation divided by its initial content in vesicles) correlated with the Na+ -K+ ATPase activity of the preparation. ATP-dependent calcium uptake by sarcolemmal vesicles was inhibited by 40-50% by cardenolids, digitoxigenin and ouabain, when the latter was included inside the vesicles. The Ca2+ gradient formed in the presence of ATP was dissipated by the addition of external sodium; Li+ was found to be ineffective in this process. External sodium also caused calcium release from vesicles pre-equilibrated with calcium in the medium. In the energy-dependent calcium uptake, the membrane-bound creatine kinase ATP-regenerating system was found to be the most effective energy source for active calcium transport. The results obtained may be interpreted to show that either the cardiac sarcolemma contains an energy-dependent calcium transport system or the sarcolemmal Na+ -K+ ATPase is able to transport calcium instead of sodium.