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Effect of sodium and betaine in culture media on development and relative rates of protein synthesis in preimplantation mouse embryos in vitro

Results of recent experiments indicate that the improved development of mouse embryos in medium containing a low NaCl concentration (85 mM) or the inclusion of the organic osmolyte betaine in a medium containing a high NaCI concentration (125 mM) is correlated with the maintenance of intracellular sodium concentrations that more closely approximate those found in freshly isolated embryos (Biggers et al., 1993, Mol Reprod Dev 34:380–390). We examined the effect of these different culture media on the relative rates of protein synthesis since increased levels of intracellular sodium inhibit protein synthesis; a reduced rate of protein synthesis could therefore account for the differences in development in the different media, since cell division requires protein synthesis. We observe that the ability of these media to support development and to maintain more physiological concentrations of intracellular sodium is correlated with their ability to support increased relative rates of protein synthesis. Reducing the NaCI concentration from 125 mM to 85 mM leads to a greater fraction of the embryos developing from the 2‐cell stage to the 8‐cell stage after 1 day of culture and a substantially improves extent of development to the morula stage after 2 days of culture. This reduction in NaCI concentration also leads to a 2.4‐fold increase in the relative rate of protein synthesis in 4‐cell embryos. Moreover, addition of betaine to medium containing 125 mM NaCI increases the relative rate of protein synthesis. This finding provides an explanation, at least in part, for the increase in development to the blastocyst stage exhibited by mouse embryos cultured in these media. The improved development to the blastocyst is not observed when isolated morulae are cultured in medium containing either a low sodium concentration or in medium containing a high sodium concentration and betaine. These results indicate that the beneficial effect on development of media containing either a low sodium concentration or inclusion of betaine in media containing a high sodium concentration is restricted to stages prior to the morula. © 1993 Wiley‐Liss, Inc.