The expression of regucalcin, a regulatory protein of Ca(2+) signaling, and its effect on Ca(2+) pump activity in the microsomes (sarcoplasmic reticulum) of rat heart muscle was investigated. The expression of regucalcin mRNA was demonstrated by reverse transcription-polymerase chain reaction (RT-PCR) analysis in heart muscle using rat regucalcin-specific primers. Results with Western blot analysis showed that regucalcin protein was present in the cytoplasm, although it was not detected in the microsomes. Microsomal Ca(2+)-ATPase activity was significantly increased in the presence of regucalcin (10(-10)-10(-8) M) in the enzyme reaction mixture. This increase was not seen in the presence of thapsigargin (TP) (10(-5) M), a specific inhibitor of the microsomal Ca(2+) pump enzyme. Regucalcin (10(-10)-10(-8) M) significantly stimulated ATP-dependent (45)Ca(2+) uptake by the microsomes. The effect of regucalcin (10(-8) M) in increasing microsomal Ca(2+)-ATPase activity was completely prevented in the presence of digitonin (10(-3) or 10(-2)%), which has a solubilizing effect on membranous lipid, or N-ethylmaleimide (NEM), a modifying reagent of sulfhydryl (SH) groups. Dithiothreitol (DTT; 5 mM), a protecting reagent of SH groups, increased markedly Ca(2+)-ATPase activity. In the presence of DTT (5 mM), regucalcin could not significantly enhance the enzyme activity. Also, the effect of regucalcin in increasing Ca(2+)-ATPase activity was completely inhibited by the addition of vanadate (1 mM), an inhibitor of phosphorylation of enzyme. In addition, the effect of regucalcin on Ca(2+)-ATPase activity was not significantly modulated in the presence of dibutyryl cyclic AMP (10(-4) M), inositol 1,4,5-trisphosphate (10(-3) M), or calmodulin (5 microg/ml) which is an intracellular signaling factor. The present study demonstrates that regucalcin can activate Ca(2+) pump activity in rat heart microsomes, and that the protein may act the SH groups of Ca(2+)-ATPase by binding to microsomal membranes.