Inhibitory effect of regucalcin on Ca2+/calmodulin-dependent protein kinase activity in rat renal cortex cytosol

The effect of regucalcin on Ca2+/calmodulin-dependent protein kinase activity in the cytosol of rat renal cortex was investigated. Regucalcin is a calcium-binding protein which exists in rat liver and renal cortex. Protein kinase activity in renal cortex cytosol was markedly increased by the addition of CaCl2 (0.5 mM) plus calmodulin (10 mu g/ml) in the enzyme reaction mixture. This increase was completely prevented by the addition of trifluoperazine (25 mu M), an antagonist of calmodulin. The cytosolic Ca2+/calmodulin-dependent protein kinase activity was clearly inhibited by the addition of regucalcin; an appreciable effect of regucalcin was seen at 0.01 mu M. The cytosolic Ca2+/calmodulin-dependent protein kinase activity was fairly increased by increasing concentrations of added Ca2+ (100-1000 mu M). This increase was markedly blocked by the presence of regucalcin (0.1 mu M) The inhibitory effect of regucalcin on the protein kinase activity was also seen with varying concentrations of calmodulin (2-20 mu g/ml). These results demonstrate that regucalcin can regulate Ca2+/calmodulin-dependent protein kinase activity in renal cortex cells.