Intracellular calcium pools and their metabolic dependence in normal versus simian virus 40‐transformed human fibroblasts

Previous studies indicate that although normal and Simian virus (SV40)‐transformed WI38 human fibroblasts have similar levels of intracellular Ca⁺⁺ on a per mg protein basis, their ability to maintain this intracellular Ca⁺⁺ against a low concentration of extracellular Ca⁺⁺ differs markedly. The transformed but not the normal cells rapidly lose Ca⁺⁺ when exposed to low extracellular Ca⁺⁺, suggesting Ca⁺⁺ transport and/or sequestration differ in the two cell types. In this study we have extended our investigations of Ca⁺⁺ metabolism in the two cell types. We observe that normal WI38 cells, when exposed to metabolic inhibitors to deplete intracellular ATP, undergo a twofold increase in intracellular Ca⁺⁺ levels. Under similar conditions and over the same time course, no comparable change in Ca⁺⁺ level is observed in the SV40‐transformed cell, despite the extensive depletion of ATP. ⁴⁵Ca⁺⁺ desaturation curves indicate that the bulk of the net increase in cell Ca⁺⁺ following ATP depletion of the normal WI38 cell comes to reside in a slowly exchanging Ca⁺⁺ pool. The data also indicate that glycolysis, and not oxidative phosphorylation, drives the active extrusion of Ca⁺⁺ from these cells, an observation consistent with previous studies on the Na⁺‐K⁺ pump in other cell types. Finally, the data indicate that in these cells mitochondria do not appear to be the major subcellular organelle responsible for regulation of at least the two cellular Ca⁺⁺ pools measurable using isotope desaturation analysis. This is based on the inability of the respiratory inhibitor rotenone to alter significantly the size of either of these Ca⁺⁺ pools. These pools compose 80–90% of total cell Ca⁺⁺ in both cell types.