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

Abstract

Calmodulin prepared from human red cell hemolysates significantly increased Ca2+ uptake into dog cardiac microsomal preparations enriched in sarcoplasic reticulum in a dose-dependent manner. The stimulation of Ca uptake by calmodulin was additive to that stimulation produced by maximal stimulatory concentration of cAMP dependent protein kinase and cAMP, indicating separate mechanisms of action and potentially different modulatory roles for these 2 systems in the control of Ca transport. K+ significantly decreased calmodulin stimulation of Ca 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 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 investigated. Calmodulin reduced 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-2-Amino-2-(hydroxymethyl)-1,3-propanediol-ATP (0.5 mM) was significantly stimulated by calmodulin. Calmodulin stimulates Ca2+ transport in cardiac sarcoplasmic reticulum by increasing the turnover rate of the transport process.