Activation of a Na +-dependent amino acid transport system in preimplantation mouse embryos

The uptake of l-methionine-methyl- 3H and l-leucine- 3H from completely defined medium into acid-soluble fractions of preimplantation mouse embryos has been studied. Late four-cell embryos and early blastocysts raised in vitro can concentrate both amino acids by processes which exhibit saturable, Michaelis-Menten type kinetics, characteristic of carrier-mediated active transport systems. This uptake is temperature-sensitive and inhibited by certain amino acids which compete for the same uptake sites. Methionine uptake seems to be mediated by a single transport system ( K m = 6.25 × 10 −5 M) at the four-cell stage. Complex kinetics suggest that two distinct transport systems exist at the early blastocyst stage ( K m = 6.25 × 10 −5 M; 8.9 × 10 −4 M). V max values (mg/embryo/15 min) for methionine and leucine transport increase significantly from the late four-cell stage to the blastocyst stage, suggesting that additional carriers are produced or activated during development. Most importantly, leucine and methionine transport is Na +-independent at the four-cell stage, methionine transport is partially dependent at the morula stage, and both amino acids are completely Na +-dependent at the blastocyst stage. The cumulative results suggest that preimplantation embryos accumulate leucine and methionine by specific, chemically mediated, active transport systems. The qualitative and quantitative developmental changes in cell membrane function may represent preparatory steps for subsequent growth of embryonic and/or trophoblastic cells.