Alanine and leucine transport in unfertilized pig oocytes and early blastocysts.

Amino acid transport is facilitated by specific transporters within the plasma membrane of the cell. In mouse oocytes and cleavage-stage conceptus Na(+)-dependent L-alanine and L-leucine transport are nearly undetectable. Sodium-dependent transport via system B0,+ in the mouse conceptus increases greatly between the 8-cell and blastocyst stages. By contrast, data presented here for the pig show that L-alanine and L-leucine transport is mainly Na(+)-dependent in the oocyte; this Na(+)-dependent component of transport becomes undetectable by the blastocyst stage. The Na(+)-dependent component of transport in oocytes is inhibited by BCH (2-aminoendo-bicyclo[2.2.1] hexane-2-carboxylic acid) and L-lysine and thus could be a form of system B0,+. In both oocytes and blastocysts Na(+)-independent L-leucine transport is inhibited by BCH, which is consistent with the presence of system L. The dramatic decrease in Na(+)-dependent amino acid transport activity could occur in pig conceptuses in association with the onset of RNA synthesis during the 4-cell stage. Regardless of the precise time during development at which it occurs, however, this dramatic, developmentally regulated decrease in Na(+)-dependent alanine and leucine transport activity contrasts sharply with the large increase in Na(+)-dependent system B0,+ activity that occurs during preimplantation development of murine conceptuses. Elucidation of the molecular mechanisms by which these changes occur should contribute to an understanding of regulation of gene expression during early development.