Regulatory role of ca^<2+> -binging protein requcalcin in signal transduction system and its mechanism

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The requlatory role of Ca^<2+> -binding protein requcalcin in signal trasnduction system of liver cells was investigated, and the following results were obtained.1) Requcalcin activates directly (Ca^<2+>-Mg^<2+>)-ATPase in liver plasma membranes, an the activation is not involved in the Ca^<2+>-dependent phosphorylation of the enzyme.2) The activating mechanism of requcalcin on (Ca^<2+>-Mg^<2+>)-ATPase is not involved on GTP-binding protein which modulates the receptor-mediated hormonal effect in rat liver plasma menbranes.3) Regucalcin stimulates Ca^<2+> release from liver nuclei, and the effect is not influenced by calmodulin antagonist, suggesting that regucalcin can regulate the Ca^2 transport system in rat liver nuclei.4) Arachidonic acid inhibited Ca^<2+> uptake and stimulated Ca^<2+> release in rat liver nuclei.5) NAD^+ has an inhibitory effect on nuclear Ca^<2+> uptake.6) Liver nuclei were Ca^<2+> -ATPase which can taken up Ca^<2+>.Rgucalcin had no effect on the enzyme activity, although arachidonic acid and NAD^+ inhibited the enzyme activity.