Regucalcin increases Ca2+-ATPase activity and ATP-dependent calcium uptake in the microsomes of rat kidney cortex

The effect of regucalcin, a Ca2+-binding protein, on Ca2+ transport system in rat renal cortex microsomes was investigated. The presence of regucalcin (10(-8) to 10(-6) M) in the reaction mixture caused a significant increase in Ca2+-ATPase activity and ATP-dependent Ca-45(2+) uptake in the microsomes. Regucalcin (10(-7) M) increased Ca2+-ATPase activity independently of increasing concentrations of CaCl2. The microsomal Ca2+-ATPase activity and Ca-45(2+) uptake were markedly decreased by the presence of vanadate (0.1 mM) or N-ethylmaleimide (NEM; 5 mM) in the absence or presence of regucalcin. Dithiothreitol (DTT; 5 mM) markedly elevated Ca2+-ATPase activity and Ca-45(2+) uptake in the microsomes. The DTT effects were not further enhanced by regucalcin (10(-7) M). Meanwhile, the microsomal Ca2+-ATPase activity and Ca-45(2+) uptake were significantly decreased by the presence of dibutyryl cyclic AMP (DcAMP; 10(-5) and 10(-3) M) or inositol 1,4,5-trisphosphate (IP3; 10(-7) and 10(-5) M). The effect of regucalcin (10(-7) M) on Ca(2+)ATPase activity and Ca-45(2+) uptake was weakened in the presence of DcAMP or IP3. The present results demonstrate that regucalcin has a stimulatory effect on ATP-dependent Ca2+ uptake in the microsomes of rat renal cortex due to acting on the thiol groups of Ca2+-ATPase.