| Abstract: | Effects of Na+ and K+ on Ca2+ transport by sarcoplasmic reticulum vesicles were studied in a medium containing high Mg2+ and ATP (2mM) and low Ca2+ (0.44μM) concentrations. Under these conditions, Na+ and K+ inhibit Ca2+ uptake. ATPase activity and membrane phosphorylation by ATP. Since the concentrations of ATP and Ca2+ used are consistent with relaxation in vivo, the results suggest that under physiological resting conditions the Ca2+ pump of the sarcoplasmic reticulum operates below its maximal capacity. |
