Beta-alanine but not taurine can function as an organic osmolyte in preimplantation mouse embryos cultured from fertilized eggs

Early preimplantation mouse embryos are susceptible to the detrimental effects of increased osmolarity and, paradoxically, their in vitro development is significantly compromised by osmolarities near that of oviductal fluid. In vitro development can be restored, however, by several compounds that are accumulated by 1-cell embryos to act as organic osmolytes, providing intracellular osmotic support and cell volume regulation. Taurine, a substrate of the beta-amino acid transporter that functions as an organic osmolyte transporter in other cells, had been proposed to function as an organic osmolyte in mouse embryos. Here, however, we found that taurine is neither able to provide protection for in vitro embryo development against increased osmolarity nor is it accumulated to higher intracellular levels as osmolarity is increased, indicating that it cannot function as an organic osmolyte in early preimplantation embryos. In contrast, beta-alanine, the other major substrate of the beta-amino acid transporter, both protects against increased osmolarity and is accumulated to somewhat higher levels as osmolarity is increased, indicating that it is able to function as an organic osmolyte in embryos. However, we also found that beta-alanine is displaced from embryos by glycine-the most effective organic osmolyte in embryos previously identified-and beta-alanine does not increase protection above that afforded by glycine at concentrations near those in vivo. Thus, the beta-amino acid transporter is likely present in early preimplantation embryos to supply beta-amino acids such as taurine for purposes other than to serve as organic osmolytes.