Regulation of cADP-ribose-induced Ca2+ release by Mg2+ and inorganic phosphate

cADP-ribose (cADPr) has recently been shown to release Ca2+ from an intracellular store of permeabilized T lymphocyte cell lines (Guse, A. H., da Silva, C. P., Emmrich, F., Ashamu, G. A., Potter, B. V. L., and Mayr, G. W. (1995) J. Immunol. 155, 3353-3359). Using permeabilized Jurkat and HPB. ALL T lymphocytes, the effects of varying concentrations of inorganic phosphate and Mg2+ on cADPr-induced Ca2+ release were investigated. cADPr-induced Ca2+ release was dependent on the concentration of inorganic phosphate, showing very low Ca2+ release activity between 0.5 and 2 mM inorganic phosphate. At 4 to 5 mM inorganic phosphate, the cADPr-induced Ca2+ release was much more pronounced, reaching maximal values at 10 mM inorganic phosphate. The underlying mechanism for this stimulatory effect was an increased loading of the cADPr-sensitive Ca2+ store, which was demonstrated by enhanced resequestration of Ca2+ selectively into the cADPr-sensitive Ca2+ store. The free Mg2+ concentration also influenced cADPr-induced Ca2+ release in permeabilized cells: at 0 and 8.58 mM the release was nearly completely abolished, whereas at 1.06 mM maximal Ca2+ release by cADPr was observed. High performance liquid chromatographic analysis of exogenously added cADPr revealed that the catabolism of cADPr at varying Mg2+ and Pi concentrations had only minor relevance for the modulatory effects observed. To correlate the effects of inorganic phosphate and Mg2+ on cADPr-induced Ca2+ release observed in the permeabilized cell preparations, measurements of these ions in intact Jurkat T lymphocytes were carried out. Intact Jurkat T cells stimulated via the T cell receptor middle dotCD3 complex did not respond with significant elevation of the free intracellular Mg2+ concentration. In contrast, stimulation via the T cell receptor middle dotCD3 complex resulted in an increase in the intracellular inorganic phosphate concentration. These data indicate a role for the intracellular inorganic phosphate concentration in the regulation of cADPr-mediated Ca2+ release in T lymphocytes.