An Unusual Active Hexose Transport System in Human and Mouse Preimplantation Embryos

In a metabolic study of human and mouse preimplantation embryos (preembryos), we measured glucose uptake and phosphorylation with nonradioactive 2-deoxyglucose (DG) as tracer. Initial experiments indicated an active hexose transport capacity, a property thought to be restricted in mammals to intestinal villi and kidney tubules [Baly, D. L. \& Horuk, R. (1988) Biochim. Biophys. Acta 947, 571-590]. Significant findings are as follows: (i) During a 60-min incubation with a low level of DG, mouse blastocyst DG rose to levels up to 30 times that of the medium. {The intestinal active system does not transport DG [Crane, R. K. (1960) Physiol. Rev. 40, 789-825].} (ii) Active preembryo transport was not blocked (as it would have been in the intestine) by phlorizin [Alvarado, F. \& Crane, R. K. (1962) Biochem. Biophys. Acta 56, 170-172 and Sacktor, B. (1989) Kidney Int. 36, 342-350] or by replacement of Na+with choline+or K+[Crane (1960) and Sacktor (1989)]. (iii) Transport of DG was blocked by cytochalasin B (which is not true for the intestinal transporter). We conclude that a distinct active hexose transporter and at least one facilitated transporter are present in preembryos, perhaps appearing in tandem on different membranes during formation of the increasingly complex preembryo structure.