Changes in the activities of amino acid transport systems b 0,+ and L during development of preimplantation mouse conceptuses

Abstract

Uptake of leucine, lysine and arginine was predominantly Na +-indepdent in mouse conceptuses through the 8-cell stage of development, and two compoents of saturable transport were detected for each of these amino acids. Uptake of cationic substrates from solutions near 1 μM was inhibited most strongly by bulky cationic and zwitterionic amino acids whose carbon skeletons do not branch at the α or β positions. By this criterion, system b 0,+ accounted for most of the Na +-independent arginine and lysine transport in eggs and conceptuses throughout preimplantation development. A small, leucine-resistant, cation-preferring component of amino acid transport was also detected in these cells. Leucine uptake was inhibited most strongly by bicyclic, branched-chain or benzenoid, zwitterionic amino acids in eggs and conceptuses prio to formation of blastocysts. Therefore, it apperaed to be taken up mainly by system L, while system b 0,+ accounted for a smaller portion of leucine uptake during this development period. In blastocysts, in contrast, system L was less conspicuous, and system b 0,+ was primarily responsible for Na +-independent leucine uptake. The V max values for transport of amino acids by system 0,+ increased by up to 30-fold in conceptuses between the 1-cell and blastocyst stages. In contrast, the V max value for leucine transport via system L decreased while the K m value increased between these two development stages. Although several explanations for these changes are possible, we favor the hypothesis that the density of system L tranport sites in plasma membranes decreases while the number of system b 0,+ sites increases during development of blastocysts from 1-cell conceptuses.