Stimulatory effect of hormonal signaling factors on (Ca(2+)-Mg2+)-ATPase activity in rat liver plasma membranes: cross talk with regucalcin.

The effect of hormonal signaling factors on (Ca(2+)-Mg2+)-ATPase activity in rat liver plasma membranes was investigated. The presence of inositol-glycan (10(-7)-10(-5) M), dibutyryl cAMP (10(-4) and 10(-3) M) or inositol 1,4,5-trisphosphate (IP3; 10(-6) and 10(-5) M) in the enzyme reaction mixture produced a significant increase in (Ca(2+)-Mg2+)-ATPase activity. These effects were completely inhibited by the presence of vanadate (10(-4) M), an inhibitor of the enzyme phosphorylation, and N-ethylmaleimide (5 x 10(-3) M), a SH group modifying reagent. Meanwhile, regucalcin, a Ca(2+)-binding protein isolated from rat liver cytosol, increased the enzyme activity by binding to the SH groups of (Ca(2+)-Mg2+)-ATPase in liver plasma membranes. The presence of regucalcin (0.25 microM) with an effective concentration completely inhibited the effect of inositol-glycan (10(-5) M) to increase (Ca(2+)-Mg2+)-ATPase activity, while the effect of dibutyryl cAMP (10(-3) M) or IP3 (10(-5) M) was not altered. The inositol-glycan effect was not modulated by the presence of dibutyryl cAMP or IP3. Now, the preincubation of the plasma membranes with regucalcin did not modify the effect of inositol-glycan on the enzyme activity, suggesting that regucalcin competes with inositol-glycan for the binding to the plasma membranes. The present results suggest that there may be a cross talk with regucalcin and hormonal signaling factors in the regulation of (Ca(2+)-Mg2+)-ATPase activity in liver plasma membranes.