ATP stimulates Ca²⁺ uptake and increases the free Ca²⁺ concentration in isolated rat liver nuclei

Addition of ATP to a highly purified fraction of rat liver nuclei incubated with submicromolar concentrations of Ca²⁺ and trace amounts of ⁴⁵Ca²⁺ resulted in the rapid accumulation of ⁴⁵Ca²⁺ in the nuclei. This was associated with an increase in intranuclear free Ca²⁺ concentration as measured with the fluorescent dye 1-[2-(5-carboxyoxazol-2-yl)-6-aminobenzofuran-5-oxy]-2-(2'-amino-5'-me0 thylphenoxy)ethane-N,N,N',N'-tetraacetic acid (fura-2). Inhibitors of microsomal and mitochondrial Ca²⁺ translocases had no effect on nuclear Ca²⁺ sequestration, indicating that it was distinct from previously known intracellular Ca²⁺-transporting systems. Ca²⁺ uptake and the associated increase in intranuclear free Ca²⁺ concentration were prevented by calmidazolium, a potent calmodulin antagonist. Partial characterization of the ATP-stimulated nuclear Ca²⁺ uptake showed that maximal rates of Ca²⁺ uptake and increase in intranuclear free Ca²⁺ level occurred at concentrations of Ca²⁺ normally present in the cytosol of mammalian cells. Together, these results show that a distinct, ATP- and calmodulin-dependent Ca²⁺ uptake system exists in liver nuclei. This system may play an important role in the regulation of intranuclear Ca²⁺-dependent processes.