Taurine acts as an osmolyte in human and mouse oocytes and embryos

Taurine transport in mouse embryos has been shown to be osmotically regulated. We studied release of taurine from mouse and human oocytes and embryos when exposed to conditions that created osmotic imbalances, either by incubation in anisosmotic media or by inhibition of Na(+)-K(+)-ATPase with ouabain. Furthermore, we studied the effect of cleavage in mouse embryos on release of taurine. When human oocytes that remained unfertilized after in vitro fertilization, human embryos (2- to 8-cell), and mouse 2-cell embryos were loaded with [3H]taurine and subsequently incubated for 4 h in hyposmotic media (200 and 240 mOsm/kg), they showed significantly lower radioactivity as compared to those incubated in media of 280, 320, and 360 mOsm/kg and higher radioactivity of the incubation media. Incubation with 1.5 mM ouabain resulted in decreased radioactivity of mouse embryos and increased radioactivity of incubation medium. When mouse 2-cell embryos were cultured for 24 h after loading with [3H]taurine, radioactivity of embryos that cleaved to the 4-cell stage was significantly lower than that of uncleaved embryos. This finding is in accordance with the theory that cell division induces cell volume-regulatory mechanisms. In contrast, when 1-cell embryos were cultured for 24 h, radioactivity of embryos developing to the 2-cell stage was significantly higher than that of uncleaved embryos. These data support the theory that taurine is released by embryos when they have to adjust their cell volume because of either extracellularly induced or intracellularly occurring osmotic imbalances. When culture is performed without taurine, the resultant taurine depletion of embryos may be disadvantageous, either because the embryo has to rely more on its inorganic osmolytes for volume regulation or because taurine can no longer provide its other protective functions.