Stimulation of uridine uptake in 3T3 cells is associated with increased ATP affinity of uridine-phosphorylating system [16]

CELLS maintained in culture may be brought into a quiescent state by transfer to suboptimal medium and subsequently activated to proliferate by addition of complete medium¹. An early event during activation with serum is the stimulation of uridine uptake². Recent work has shown that it is not uridine transport but the subsequent trapping of uridine within the cell, that is enhanced^3,4. We report here a study of the trapping of uridine by 3T3 fibroblasts in situ in which we have varied separately both the uridine and the ATP levels within the cells. Stimulation of uridine uptake as a result of serum or insulin activation was found to be associated with a dramatic increase in the half-saturation concentration of the uridine trapping system for the substrate ATP, with relatively little change in the maximum velocity of uptake or in the affinity for uridine.