Dissociation of cell density and cell cycle effects on the rate of transport of alpha-aminoisobutyric acid in 3T3 cells.

The rate of transport of alpha-aminoisobutyric acid (AIB) has been measured in Swiss 3T3 cells at different cell densities in the range 10(3) to 5 x 10(4) cells/cm2. There is a pronounced increase in the rate of Na+-dependent AIB uptake below 4 x 10(3) cells/cm2, which can be observed both in growing cells (10% serum) or in cells arrested early in the G1 (G0) portion of the cell cycle in medium containing plasma-derived serum. The rate of AIB transport of cells arrested in G0 can be inhibited by the addition of a plasma membrane-enriched fraction prepared from 3T3 cells. The results are interpreted in terms of two elements that control the rate of AIB transport, one dependent on the position in the cell cycle and the other on cell contact. Preliminary observations suggest that BALB 3T3 cells behave in a similar way.