Development of system B 0,+ and a broad-scope Na +-dependent transporter of zwitterionic amino acids in preimplantation mouse conceptuses

Abstract

The nature and ontogeny of Na +-dependent l-alanine transport was examined in mouse eggs and preimplantation conceptuses. Mediated l-alanine uptake was not detected in fertilized or unfertilized eggs, but a small amount of Na +-dependent l-alanine transport was detected in 2-cell conceptuses. Na +-dependent alanine transport was more rapid at the 8-cell stage of development, and more than 10-fold faster in blastocysts than in 8-cell conceptuses. Analog inhibition analyses were consistent with the interpretation that l-lysine-sensitive and l-lysine-resistant components of transport were present at the 2-cell, 8-cell and blastocyst stages of development. The range of amino acids and their analogs that inhibited the most conspicuous component of alanine transport in blastocysts was consistent with the conclusion that system B 0,+ is largely responsible for l-alanine uptake in these conceptuses. Moreover, system B 0,+, but not other known systems in blastocysts, became susceptible to activation as these conceptuses approached the time of implantation, so this activation could be involved in implantation. Although the data are consistent with the possibility that system B 0,+ is also present in 2-cell and 8-cell conceptuses, the relatively slow l-alanine transport in conceptuses at these earlier stages of development precluded more detailed study of their ability to take up alanine. Similarly, the less conspicuous l-lysine-resistant component of l-alanine transport in blastocysts also may be present in conceptuses as early as the 2-cell stage. The l-lysine-resistant component of l-alanine transport could not be attributed to residual system B 0,+ activity, however, because it was inhibited more strongly by trans-OH- l-proline than l-arginine, whereas the reverse was the case for system B 0,+. Similarly, l-tryptophan and l-leucine each inhibited system B 0,+ more strongly than l-serine or l-cysteine, whereas all four of these amino acids inhibited the l-lysine-resistant component equally well. Moreover, a Hofstee plot for l-alanine influx was consistent with the interpretation that at least two mediated components of Na +-dependent l-alanine transport are present in blastocysts. The less conspicuous component of l-alanine transport in blastocysts was relatively susceptible to inhibition by l-leucine and l-tryptophan, but it resisted inhibition by the ‘model’ system A substrate, MeAIB, and the system ASC inhibitors, l-penicillamine and cationic amino acids. Therefore, the zwitterion-preferring Na +-dependent transport process in blastocysts appears to be sufficiently distinct from systems A and ASC as well as from B 0,+ to warrant its own provisional designation as system B.