Tissue uridine pools: evidence in vivo of a concentrative mechanism for uridine uptake.

Pools of free uridine, ranging from 7.3 to 38.0 nmol/g wet weight, have been detected in a variety of freeze-clamped murine tissues. These concentrations average 10-fold greater than that detected in plasma. The kinetics of these pools after an i.v. tracer dose of [3H]uridine suggest that the initial rapid disappearance of [3H]uridine from plasma (t1/2 = 2 min) reflects distribution into tissues as well as catabolism by the liver. Subsequently, the tissue uridine pools turn over with half-lives of 13 to 18 h. Analyses of the activity of the proximal enzymes in uridine metabolism (uridine phosphorylase and uridine kinase) suggest that the phosphorylase correlates with the size of tissue uridine pools. Further evidence for this is seen in the sustained 5- to 15-fold increase in both tissue and plasma uridine concentrations after treatment with benzylacyclouridine, a potent uridine phosphorylase inhibitor. In contrast, a nonphysiological dose of exogenous uridine (250 mg/kg) briefly increases the plasma concentration of uridine to over 1 mM but it returns to below 10 microM within 1 h. Under these conditions as well, tissue concentrations of uridine increase 5- to 10-fold in most tissues, 20-fold in spleen, and 70-fold in kidney. High cellular concentrations of free uridine relative to medium are also observed in dispersed murine splenocytes. Furthermore, splenocytes incubated in 5 microM [3H]uridine achieved a 2-fold higher intracellular concentration of [3H]uridine in less than 1 min independent of phosphorylation. Thymidine was not concentrated in this system nor did nitrobenzylthioinosine inhibit [3H]uridine uptake. These findings suggest that in normal tissues and explanted cells, pools of uridine are sustained by a concentrative transport mechanism and constitute a previously unrecognized reservoir of pyrimidine nucleosides in tissues.