Distinct properties of uridine transport systems in growing, quiescent and serum-stimulated hamster embryo cells.

The kinetics of uridine uptake in growing, quiescent and serum-activated hamster embryo cells are investigated. The maximum velocity of uridine uptake in growing hamster embryo cells, is lower than in the methylcholanthrene transformed hamster cell line (MCT). This kinetic constant is further reduced in quiescent cells. The Km values in growing and in quiescent hamster embryo cells, as well as in MCT cells are of the same magnitude. Distinct alterations in the pattern of inhibition by nitrobenzyl 6-mercaptoinosin (NBMI) are detected as growing hamster embryo cells become quiescent. In quiescent cells the maximum level of inhibition is lower and the apparent Ki value for the inhibition is much higher. These changes are due to the lower apparent K'm values of NBMI-bound carriers and to the slower rate of formation of the carrier-inhibitor complex. The changes in the kinetic properties of the carriers are partly reversed by serum-activation. The number of inhibitor binding sites (i.e. nucleoside carriers) does not increase by serum-stimulation of quiescent cells (0.36 and 0.34-10(5) sites/cell in quiescent and serum-stimulated cells, respectively). It is implied that the reduction in uridine transport in quiescent cells is probably due to changes in the turnover of the carriers. These changes may be connected with the observed alterations in the properties of carriers or their immediate environment in quiescent cells.