Intracellular Mg2+ homeostasis of Ehrlich ascites tumor cells

Treatment of Ehrlich ascites tumor cells with the cation ionophore A23187 results in cellular accumulation of calcium from the extracellular medium only when cellular metabolism is inhibited by uncouplers and iodoacetate. In the metabolically active cells A23187 induces a substantial loss of intracellular magnesium at extracellular Mg2+ concentrations below 1 mM. By contrast, in the inhibited cells A23187 induces a conspicuous loss of magnesium at concentrations of magnesium in the medium ranging from 0.2 to 5 mM. This cellular magnesium loss is accompanied by an increase in intracellular calcium. At 10 mM external Mg2+, A23187 provokes an intracellular magnesium increase in both metabolically active and inhibited cells with a corresponding H+ ejection. Intracellular free Mg2+ in both control and metabolically inhibited cells was estimated through a null-point titration by monitoring changes in extracellular Mg2+ concentration due to the addition of A23187 to cell suspensions with the metallochromic indicator Antipyrylazo III. Free intracellular Mg2+ was 0.4 and 1.2 mM in metabolically active cells and in cells treated with metabolic inhibitors, respectively. Under these conditions the cellular ATP was 30.9 and 2.3 nmol/mg dry wt, respectively. The results lead to the conclusion that in intact Ehrlich ascites tumor cells the level of ATP mainly regulates the intracellular free Mg2+ concentration and, in turn, intracellular free Mg2+ determines the entry of calcium into the cell after A23187 addition.