Uptake of Thymidine into Isolated Rat Hepatocytes. Evidence for Two Transport Systems

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Abstract
Thymidine transport was studied in isolated rat hepatocytes. In these cells no phosphorylation of the substrate by thymidine kinase occurred subsequent to transport. Results from studies of the concentration-dependent uptake of thymidine indicated 2 transport systems with about 80-fold differences in their kinetic constants. These systems were denoted as high affinity [Km = 5.3 .mu.M, V = 0.47 pmol/(106 cells .times. s)] and low affinity systems [Km = 480 .mu.M, V = 37.6 pmol/(106 cells .times. s)]. From intracellular to extracellular distribution ratios of [3H]thymidine it could be concluded that the uptake by the high affinity system was a concentrative process while the transport by the low affinity system was non-concentrative. The uptake of [3H]-thymidine by the high affinity system could only be inhibited by unlabeled thymidine. All other nucleosides tested (uridine, 2''-deoxycytidine and 2''-deoxyguanosine) were equally effective in inhibiting the low affinity system competitively. The results would suggest that in hepatocytes lacking phosphorylation by thymidine kinase, thymidine is taken up by a high and a low affinity system working in tandem. The high affinity system seems to be an active transport process with narrow substrate specificity. Thymidine uptake by the low affinity system is a facilitated diffusion process. This system is considered to be a common transport route for nucleosides of different structures.