Organic osmolytes and embryos: substrates of the Gly and beta transport systems protect mouse zygotes against the effects of raised osmolarity.

Mouse embryo development is identically inhibited by raised osmolarity, whether produced by added NaCl or raffinose, demonstrating that high osmolarity is itself detrimental to embryos. In the face of increased osmolarity, cells in the brain and kidney, and likely many other cells, accumulate nonperturbing organic osmolytes in their cytoplasm. In the presence of any of a number of organic compounds that were proven or probable substrates of either the Gly or the beta transport systems, mouse embryo development in vitro was protected from raised osmolarity. Zygotes developed past the "2-cell block," and with most Gly or beta substrates, to the blastocyst stage. The most effective osmoprotectants were glycine, glutamine, betaine, proline, beta-alanine, and hypotaurine; several others were partially effective. A model Gly substrate, glycine, was effective at a much lower concentration (EC50 = 50 microM) than was a model beta substrate, beta-alanine (EC50 = 1.3 mM). The protective effect of these two compounds was additive, indicating a common mode of action. The various effective compounds tested do not all share metabolic pathways or other such properties in common. Thus, it is likely that cleavage-stage mouse embryos utilize them, in large part, as organic osmolytes.