Transport of cationic and zwitterionic amino acids in preimplantation rat conceptuses

The ability of preimplantation rat conceptuses to take up several amino acids was examined under a variety of conditions, and the characteristics of uptake were compared to those determined previously for mouse conceptuses. Mediated leucine transport in two-cell rat conceptuses is Na⁺-independent and inhibited almost completely by 2-amino-endobicyclo[2.2.1]heptane-2-carboxylic acid (BCH), so it resembles system L which predominates in two-cell mouse conceptuses. System L becomes less conspicuous than homoarginine-sensitive, Na⁺-independent leucine transport (provisionally designated system b^O,+) by the time rat conceptuses develop into blastocysts, as is also the case for mouse conceptuses. In contrast to leucine transport, system b^O,+ appears to be the most conspicuous transporter of cationic amino acids throughout preimplantation development of both species. A Na⁺-independent cation-preferring amino acid transport process also appears to be present in rat as well as in mouse conceptuses. Moreover, rat conceptuses resemble mouse conceptuses because Na⁺-dependent transport system Gly activity virtually disappears from them by the time they form blastocysts. Unlike mouse conceptuses, however, Na⁺-dependent system B^O,+ activity appears to be present throughout preimplantation development of rat conceptuses, whereas it has not been detected until at least the two-cell stage in the mouse. Although system B^O,+ becomes more conspicuous in mouse than in rat conceptuses by the time they form blastocysts, system B^O,+ activity appears to increase when blastocysts of both species are removed from the uterus just prior to implantation. The latter observation is consistent with the possibility that system B^O,+ activity is controlled, in part, by the uterus near the time of implantation, although further studies are needed to verify this possibility. Similarities as well as differences in the amino acid transport processes present in conceptuses of rats and mice may eventually be understood best in relation to the environments in which they develop in vitro and in situ.