Regulatory effect of arachidonic acid on the calcium transport system in rat liver nuclei

Mechanism of arachidonic acid-induced Ca2+ mobilization in liver nuclei

Arachidonic acid treatment in isolated liver nuclei resulted in a rapid and transient increase of Ca2+ concentration in the nucleoplasm which was monitored with the Ca(2+)-sensitive dye fura-2 dextran. This effect was associated with a decrease of Ca2+ concentration in the nuclear envelope as measured with fura-2 AM. Our results indicate that arachidonic acid causes a Ca2+ release from the nuclear envelope to the nucleoplasm similar to that evoked by inositol trisphosphate (IP3). The arachidonic acid-induced Ca2+ mobilization in the nucleus was not due to the metabolites of arachidonic acid. Experiments performed in the presence of ATP and Ca2+ indicate that arachidonic acid-induced Ca2+ mobilization in the nucleus takes place in a non ATP-dependent way. Taken together, these results suggest that arachidonic acid may contribute to the regulation of nuclear Ca2+ mobilization.

Effect of nuclear Ca2+ uptake inhibitors on Ca(2+)-activated DNA fragmentation in rat liver nuclei

The effect of nuclear Ca2+ uptake inhibitors on the Ca(2+)-activated DNA fragmentation in rat liver nuclei was investigated. The addition of Ca2+ (40 microM) into the reaction mixture containing liver nuclei in the presence of 2.0 mM ATP caused a remarkable increase in nuclear DNA fragmentation. This Ca(2+)-activated DNA fragmentation was not seen in the absence of ATP, because nuclear Ca2+ uptake is not initiated without ATP addition. Moreover, the presence of various reagents (10 microM arachidonic acid, 2.0 mM NAD+, 10 microM zinc sulfate and 0.2 mM N-ethylmaleimide), which could inhibit Ca(2+)-ATPase activity and Ca2+ uptake in the nuclei, produced a significant inhibition of the Ca(2+)-activated DNA fragmentation in the nuclei. The results show that the Ca(2+)-activated DNA fragmentation is involved in the uptake of Ca2+ by the nuclei, suggesting a role of Ca2+ transport system in the regulation of liver nuclear functions.

Enhancement of nuclear Ca(2+)-ATPase activity in regenerating rat liver: involvement of nuclear DNA increase

The alteration of calcium content, Ca(2+)-ATPase activity, DNA content and DNA fragmentation in the nuclei of regenerating rat liver was investigated. Liver was surgically removed about 70% of that of sham-operated rats. The reduced liver weight by partial hepatectomy was completely restored at 3 days after the surgery. Regenerating liver significantly increased Ca(2+)-ATPase activity and DNA content in the nuclei between 1 and 5 days after hepatectomy. The nuclear calcium content was clearly increased from 2 days after hepatectomy. The increase of Ca(2+)-ATPase activity in regenerating liver was clearly inhibited by the presence of trifluoperazine (10 microM), staurosporine (2.5 microM) and dibucaine (10 microM), which are inhibitors of calmodulin and protein kinase, in the enzyme reaction mixture. However, the nuclear enzyme activity in normal rat liver was not significantly altered by these inhibitors. Meanwhile, the increase of nuclear DNA content in regenerating liver was completely blocked by the administration of trifluoperazine (2.5 mg/100 g body weight), suggesting an involvement of calmodulin. Now, the nuclear DNA fragmentation was significantly decreased in regenerating liver, suggesting that this decrease is partly contributed to the increase in nuclear DNA content. The present study clearly demonstrates that regenerating liver enhances nuclear Ca(2+)-ATPase activity and induces a corresponding elevation of nuclear calcium content. This Ca(2+)-signaling system may be involved in the regulation of nuclear DNA functions in regenerating rat liver.

Effect of phorbol 12-myristate 13-acetate on Ca2+-ATPase activity in rat liver nuclei

The effect of phorbol 12-myristate 13-acetate (PMA) on Ca(2+)-ATPase activity in rat liver nuclei was investigated. Ca(2+)-ATPase activity was calculated by subtracting Mg(2+)-ATPase activity from (Ca(2+)-Mg(2+)-ATPase activity. The nuclear Ca(2+)-ATPase activity was significantly increased by the presence of PMA (2-20 microM) in the enzyme reaction mixture; the maximum effect was seen at 10 microM. The PMA (10 microM)-increased Ca(2+)-ATPase activity was not blocked by the presence of staurosporine (2 microM) or dibucaine (2 and 10 microM), an inhibitor of protein kinase. Meanwhile, vanadate (20 and 100 microM) caused a significant reduction in the nuclear Ca(2+)-ATPase activity increased by PMA (10 microM). The present finding suggests that PMA has an activating effect on liver nuclear Ca(2+)-ATPase independent of protein kinase.

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.

Characterization of Ca(2+)-stimulated adenosine 5'-triphosphatase and Ca2+ sequestering in rat liver nuclei

The role of Ca(2+)-stimulated adenosine 5'-triphosphatase (Ca(2+)-ATPase) in Ca2+ sequestering of rat liver nuclei was investigated. Ca(2+)-ATPase activity was calculated by subtracting Mg(2+)-ATPase activity from (Ca(2+)-Mg2+)-ATPase activity. Ca2+ uptake and release were determined with a Ca2+ electrode. Nuclear Ca(2+)-ATPase activity increased linearly in the range of 10-40 microM Ca2+ addition. With those concentrations, Ca2+ was completely taken up by the nuclei dependently on ATP (2 mM). Nuclear Ca(2+)-ATPase activity was decreased significantly by the presence of arachidonic acid (25 and 50 microM), nicotinamide-adenine dinucleotide (NAD+; 2 mM) and zinc sulfate (2.5 and 5.0 microM). These reagents caused a significant decrease in the nuclear Ca2+ uptake and a corresponding elevation in Ca2+ release from the nuclei. Moreover, calmodulin (10 micrograms/ml) increased significantly nuclear Ca(2+)-ATPase activity, and this increase was not seen in the presence of trifluoperazine (10 microM), an antagonist of calmodulin. The present findings suggest that Ca(2+)-ATPase plays a role in Ca2+ sequestering by rat liver nuclei, and that calmodulin is an activator. Moreover, the inhibition of Ca(2+)-ATPase may evoke Ca2+ release from the Ca(2+)-loaded nuclei.