Functional changes in cation-preferring amino acid transport during development of preimplantation mouse conceptuses

In a previous study, a Na(+)-independent, cation-preferring amino acid transport system was detected in preimplantation mouse blastocysts. The system resisted Na(+)-dependent inhibition by homoserine and so resembled the lysosomal system c more than it resembled the plasmalemmal system y+. We now report the presence of a cation-preferring system in unfertilized and fertilized eggs and cleavage-state conceptuses which also resists Na(+)-dependent inhibition by homoserine. The systems in 1-cell conceptuses and blastocysts are, however, insensitive to changes in pH in the interval of 6.0 to 8.0 and, thus, different from the pH-sensitive system c. Moreover, the relative strengths of the interactions of a variety of basic amino acids with the systems in conceptuses do not correspond well with the relative strengths of their interactions with either system c or system y+. Similarly, the system in 1-cell conceptuses can be distinguished from the system in blastocysts because L-arginine interacts about equally well with each of these systems, whereas the system in 1-cell conceptuses is inhibited more strongly than the system in blastocysts by most other basic amino acids. In addition, inhibition of the system in 1-cell conceptuses by some basic amino acids is Na(+)-stimulated, whereas Na+ does not affect inhibition of the system in blastocysts. Finally, L-tryptophan inhibits the system in blastocysts better than L-histidine or D-arginine do, but the reverse is true for the system in 1-cell conceptuses. Therefore, the relative activities of at least two forms of a novel, cation preferring amino acid transport process change during development of blastocysts from fertilized eggs. For convenience, the forms of the cation-preferring transport processes that seem to predominate at the 1-cell and blastocysts stages are provisionally designated systems b+1 and b+2, respectively, although these two systems need not represent entirely different gene products.