Energy-dependent calcium uptake activity in cultured mouse fibroblast microsomes. Regulation of the uptake system by cell density.

Calcium has been implicated as a factor regulating the motility and proliferation of cultured fibroblasts. Since little is known about how the intracellular concentration of calcium is regulated in cultured fibroblasts, we have characterized the microsomal calcium transport activity of Balb/c 3T3 cells, an established line of mouse fibroblasts. Calcium uptake by the microsomal fraction is enhanced by oxalate, but is unaffected by azide, oligomycin, or antimycin A, substances that inhibit mitochondrial calcium uptake. In these respects, the microsomal activity is similar to the skeletal muscle sarcoplasmic reticulum calcium pump and the microsomal systems described in other nonmuscle tissues. In growing cultures, the specific activity of the microsomal calcium transport system increases up to &fold as a function of cell density. When confluent cells with high calcium transport activity are replated at a low density, activity falls rapidly to a low level, but if replated at a high density, it does not. Serum concentration also affects calcium transport. Cells propagated in 2.5% serum have increased calcium uptake. Some transformed cell lines have diminished calcium uptake, whereas others do not. The addition of calcium or CAMP derivatives to the culture medium produces a small increase in microsomal calcium uptake. The addition of methyl xanthines to the culture medium inhibits microsomal calcium uptake. Therefore, these compounds cannot be used to enhance the effect of CAMP derivatives. We suggest that microsomal calcium uptake may serve as a regulatory mechanism to control cytoplasmic calcium levels and thereby modulate fibroblast proliferation and movement.