Involvement of Ca(2+)-stimulated adenosine 5'-triphosphatase in the Ca2+ releasing mechanism of rat liver nuclei.

The regulatory role of Ca(2+)-stimulated adenosine 5'-triphosphatase (Ca(2+)-ATPase) in Ca2+ transport system of rat liver nuclei was investigated. Ca2+ uptake and release were determined with a Ca2+ electrode. Ca(2+)-ATPase activity was calculated by subtracting Mg(2+)-ATPase activity from (Ca(2+)-Mg2+)-ATPase activity. The release of Ca2+ from the Ca(2+)-loaded nuclei was evoked progressively after Ca2+ uptake with 1.0 mM ATP addition, while it was only slightly in the case of 2.0 mM ATP addition, indicating that the consumption of ATP causes a leak of Ca2+ from the Ca(2+)-loaded nuclei. The presence of N-ethylmaleimide (NEM; 0.1 mM) caused an inhibition of nuclear Ca2+ uptake and induced a promotion of Ca2+ release from the Ca(2+)-loaded nuclei. NEM (0.1 and 0.2 mM) markedly inhibited nuclear Ca(2+)-ATPase activity. This inhibition was completely blocked by the presence of dithiothreitol (DTT; 0.1 and 0.5 mM). Also, DTT inhibited the effect of NEM (0.1 mM) on nuclear Ca2+ uptake and release. Meanwhile, verapamil and diltiazem (10 microM), a blocker of Ca2+ channels, did not prevent the NAD+ (1.0 and 2.0 mM), zinc sulfate (1.0 and 2.5 microM) and arachidonic acid (10 microM)-induced increase in nuclear Ca2+ release, suggesting that Ca2+ channels do not involve on Ca2+ release from the nuclei. These results indicates that an inhibition of nuclear Ca(2+)-ATPase activity causes the decrease in nuclear Ca2+ uptake and the release of Ca2+ from the Ca(2+)-loaded nuclei. The present finding suggests that Ca(2+)-ATPase plays a critical role in the regulatory mechanism of Ca2+ uptake and release in rat liver nuclei.