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

1. (1) The kinetics of uptake of uridine into 3T3 cells have been measured as a function of concentration in the temperature range 5-37 °C, for both quiscent and serum-stimulated cells. 2. (2) The maximum velocity of uridine uptake is increased some ten-fold by adding serum, but the half-saturation concentration is not systematically affected in this temperature range. 3. (3) A detailed study of the temperature dependence of the maximum velocity of transport in the range 4-43 °C shows that the activation energy of uridine transport is not increased following serum activation. 4. (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 turnover of the individual uridine carriers. It would appear, rather, that there is an increase in the number of functional uridine carriers.