Characterization of calmodulin effects on calcium transport in cardiac microsomes enriched in sarcoplasmic reticulum

Calmodulin prepared from red cell hemolysates was found to significantly increase Ca2+ uptake into cardiac microsomal preparations enriched in sarcoplasic reticulum in a dose-dependent manner. The stimulation of calcium uptake by calmodulin was additive to that stimulation produced by maximal stimulatory concentrations of adenosine cyclic 3',5'-phosphate (cAMP) dependent protein kinase and cAMP, indicating separate mechanisms of action and potentially different modulatory roles for these two systems in the control of calcium transport. K+ significantly decreased calmodulin stimulation of calcium uptake, while in the absence of calmodulin, K+ increased Ca2+ uptake. In the absence of K+, calmodulin increased Ca2+ uptake to levels observed at maximal K+ concentrations without calmodulin present. Na+ produced effects similar to those of K+ in this preparation both in the presence and absence of calmodulin. The effect of calmodulin on the intermediate steps of the (Mg2+,Ca2+)ATPase in cardiac sarcoplasmic reticulum was also investigated. Calmodulin was found to reduce the steady-state level of the Ca2+-dependent phosphoprotein (ECaP) and increase the (Mg2+,Ca2+)ATPase activity of this preparation. Dephosphorylation of ECaP in the presence of Tris-ATP (0.5 mM) was significantly stimulated by calmodulin. These studies indicate that calmodulin stimulates Ca2+ transport in cardiac sarcoplasmic reticulum by increasing the turnover rate of the transport process.