Involvement of Ca2+-stimulated adenosine 5'-triphosphatase in the Ca2+ releasing mechanism of rat liver nuclei
1994
Yamaguchi, M. | Oishi, K.
The regulatory role of Ca2+-Stimulated adenosine 5'-triphosphatase (Ca2+-ATPase) in Ca2+ transport system of rat liver nuclei was investigated. Ca2+ uptake and release were determined with a Ca2+ electrode. Ca2+-ATPase activity was calculated by subtracting Mg2+-ATPase activity from (Ca 2+-Mg2+)-ATPase activity. The release of Ca2+ from the Ca2+-loaded nuclei was evoked progressively after Ca2+ uptake with 1.0 millimole ATP addition, while it was only slightly in the case of 2.0 millimole ATP addition, indicating that the consumption of ATP causes a leak of Ca2+ from the Ca2+-loaded nuclei. The presence of N-ethylmaleimide (NEM; 0.1 millimole) caused an inhibition of nuclear Ca2+ uptake and induced a promotion of Ca2+ release from the Ca2+-loaded nuclei. NEM (0.1 and 0.2 millimole) markedly inhibited nuclear Ca2+-ATPase activity. This inhibition was completely blocked by the presence of dithiothreitol (DTT, 0.1 and 0.5 millimole). Also, DTT inhibited the effect of NEM (0.1 millimole) on nuclear Ca2+ uptake and release. Meanwhile, verapamil and diltiazem (10 micrometer), a blocker of Ca2+ channels, did not prevent the NAD+ (1.0 and 2.0 millimole), zinc sulfate (1.0 and 2.5 micrometer) and arachidonic acid (10 micrometer)-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 Ca2+-ATPase activity causes the decrease in nuclear Ca2+ uptake and the release of Ca2+ from the Ca2+-loaded nuclei. The present finding suggests that Ca2+-ATPase plays a critical role in the regulatory mechanism of Ca2+ uptake and release in rat liver nuclei.
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