Uridine transport by mouse blastocysts

Transport of uridine by mouse early blastocysts is a saturable process. Kinetic studies of uptake by the blastocysts reveal an apparent K_m of 1.6 μM and V_max of 0.0063 pmole/min/embryo at 37°C. Uridine uptake is reduced when thymidine, adenosine, deoxyuridine, cytidine, or deoxyadenosine is added to the medium. These findings suggest that transport of these compounds may occur at the same or overlapping sites in the cell membrane. Inhibition of transport by dinitrophenol and KCN suggests a coupling of transport to phosphorylation and energy metabolism, probably through the phosphorylation of uridine to form UTP, the principal intracellular metabolite of uridine. However, since phosphorylation of uridine is not measurable separately from the transport process in the intact embryo, it has not been determined whether uridine uptake by the embryos occurs by facilitated diffusion or by active transport.