Kinetics of Early Changes in Phosphate and Uridine Transport and Cyclic AMP Levels Stimulated by Serum in Density-Inhibited 3T3 Cells

A kinetic study of the early changes in uridine- and phosphate-transport rates and 3':5'-cyclic AMP levels induced by the addition of serum to quiescent 3T3 cells revealed that the increase in phosphate uptake and the decrease in the intracellular concentration of 3':5'-cyclic AMP immediately follow the addition of serum. In contrast, the increase in uridine transport occurred after a lag of several minutes and was clearly preceded by the other changes. The activation of phosphate transport involved an increase in V(max), while a more complex pattern was observed for uridine transport. A possible role of phosphate in connection with the other early events was also studied. The addition of phosphate to quiescent cells produced a time-dependent decrease in 3':5'-cyclic AMP levels and this ion was required for full activation of uridine transport by serum. The results show that the increase in phosphate transport is a primary event in the reinitiation of growth and suggest that the increase in phosphate transport may be connected with subsequent metabolic steps.