Na +-dependent amino acid transport in preimplantation mouse embryos : II. Metabolic inhibitors and nature of the cation requirement

Experiments were conducted in order to determine the energy source and nature of the cation dependency of [ 3H]methionine transport in preimplantation mouse embryos. The energy source of methionine transport was studied at the late four-cell and early blastocyst stages. The embryos, raised in vitro, were incubated for 1 hr in inhibitor(s) of energy metabolism and then transferred for 1 hr to medium that contained inhibitor(s) and 3H-methionine. These inhibitor studies suggest that respiration and glycolysis are needed to maintain uptake of methionine in early blastocysts. Late four-cell embryos seem to utilize respiration alone for transport. The cation dependency of methionine transport was studied at the late morula and early blastocyst stages. The kinetics of methionine uptake by early blastocysts in Na +-depleted media indicate a competitive type of inhibition. The uptake of methionine by early blastocysts is relatively resistant to ouabain and unaffected by K +-free medium. In contrast, methionine uptake by late morula-stage embryos is markedly inhibited by ouabain and K +-free medium in 1 hr. These results suggest that 1) Na + serves to increase the affinity of methionine for the carrier in early blastocysts, 2) the cation gradients do not supply a major fraction of the energy required for methionine transport, and/or the gradients are difficult to perturb once the blastocyst has formed, and 3) putative Na + pumps may be localized on the blastocoelic surface of the blastocysts.