Facilitated diffusion and sodium-dependent transport of purine and pyrimidine nucleosides in rat liver

Abstract

In mammalian cells, nucleoside transport usually is mediated by facilitated diffusion. In addition, a Na +-dependent, concentrative nucleoside transport system has been detected in several tissues but not the liver. To further clarify hepatic nucleoside transport mechanisms, we measured the uptake of [2- 14C]uridine (2 to 100 μmol/L) and of [8- 14C]adenosine (10 to 75 μmol/L) by the isolated perfused rat liver in the presence or absence of extracellular sodium or specific inhibitors of facilitated nucleoside diffusion. Uridine transport and metabolism were monitored by the release of labeled catabolites including 14CO 2, which indicated complete degradation of the pyrimidine. Adenosine, uridine and uridine catabolites were measured in the effluent perfusate by reversed-phase high-performance liquid chromatography and a radioactivity flow monitor. The existence of a Na +-dependent nucleoside transport system could be inferred from the following observations: (a) Sodium depletion caused a strong inhibition of nucleoside transport reflected by an up to threefold and 15-fold increase in extracellular uridine and adenosine, respectively. The sodium-dependent transport of uridine was saturated when the influent uridine concentration was raised beyond 20 μmol/L. No such saturation was observed for much higher concentrations of adenosine used (10 to 75 μmol/L). (b) Na +-free perfusion resulted in a strong suppression of the release of uridine catabolites by the liver. Complete uridine breakdown was depressed to 7% of the amount of 14CO 2 released in the presence of sodium and at influent uridine concentrations below 20 μmol/L. (c) Inhibition of uridine (10 μmol/L) transport and degradation was observed after coperfusion with adenosine, deoxyadenosine, guanosine and deoxyguanosine. No effect was obtained with cytidine, deoxycytidine, thymidine or hypoxanthine (100 μmol/L each), indicating a specificity identical to that reported for one of the sodium-dependent transport systems from other tissues. (d) Na +-free perfusion further enhanced the effects on nucleoside uptake of specific inhibitors of the nucleoside facilitated-diffusion transporter (dipyridamol, S-(4-nitrobenzyl)-6-thioinosine; 50μmol/L). The results of our experiments demonstrate the presence of a sodium-dependent and a sodium-independent nucleoside transport system in the isolated perfused rat liver.