Characterization of Ca²⁺-stimulated adenosine 5′-triphosphatase and Ca²⁺ sequestering in rat liver nuclei

Abstract

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