Effect of cell density on energy-dependent calcium uptake by Balb/c 3T3 membranes is independent of protein synthesis and attachment to substratum.

Membranes isolated from subconfluent cultures of Balb/c 3T3 cells have low energy-dependent calcium uptake activity. Replating confluent cells at low density results in a prompt fall of energy-dependent calcium uptake by membrane fractions. The level to which uptake activity falls is a function of the density at which the cells are plated (Moore and Pastan, '77b). To determine if regulation of energy-dependent uptake of calcium by membrane fractions is dependent upon attachment to a substrate and to further characterize conditions that regulate the process, we examined calcium uptake activity of membranes isolated from cells in suspension. With cells in suspension energy-dependent calcium uptake activity of isolated membranes falls promptly if cells are diluted to a low density (less than 10(5) cells/ml) and is a function of cell density. When cells in suspension at low cell densities are concentrated to high cell densities (greater than 2 x 10(6) cells/ml), calcium uptake activity of the isolated membrane fraction is increased as a function of cell density. These changes of membrane calcium uptake activity occur promptly and do not require protein synthesis.