Temperature dependence of uridine transport in quiescent and serum-stimulated 3T3 cells.

(1) The kinetics of uptake of uridine into 3T3 cells have been measured as a function of concentration in the temperature range 5-37 degrees C, for both quiescent and serum-stimulated cells. (2) The maximun velocity of uridine uptake is increased some ten-fold by adding serum, but the hald-saturation concentration is not systematically affected in this temperature range. (3) A detailed study of the temperature dependence of the maximum velocity of transport in the range 4-43 degrees C shows that the activation energy of uridine transport is not increased following serum activation. (4) The data suggest that any change in membrane fluidity that might occur as a result of serum activation does not in itself lead to a more rapid rate of turn over of the individual uridine carriers. It would appear, rather, that there is an increase in the number of functional uridine carriers.