Effect of cell density on growth rate and amino acid transport in simian virus 40-transformed 3T3 cells.

Rate of proliferation and amino acid transport were assessed in simian virus 40 (SV40)-transformed 3T3 cells by measurements of growth rate quotient and L-serine uptake via System ASC, respectively. Growth rate and cell density of the cultures were varied by modifying: (a) the number of cells initially plated; and (b) the period spent by the cells in culture. The growth rate quotient of SV40 3T3 cells was not correlated with cell density. Sparse cultures exhibited marked fluctuations in their growth rate as a function of time, whereas, under comparable conditions, crowded cultures retained some form of growth control by density. Transport activity by System ASC decreased as a function of increased cell density following a complex trend described by a double-exponential equation. The density-dependent decrease in amino acid transport was not accompanied by a parallel change in the rate of cell proliferation. These results indicate that alterations in amino acid transport are not linked with cell growth and suggest that an increase in transport activity is not a prerequisite for an optimal rate of proliferation in SV40-transformed 3T3 cells.