Differential sensitivity of one-cell and two-cell rabbit embryos to sodium chloride and total osmolarity during culture into blastocysts

One-cell or two-cell rabbit embryos were cultured in protein-free media with various NaCl concentrations and osmolarity to determine relative sensitivity of embryos to changes in media composition. Embryos from replicates of donor rabbits were distributed randomly across treatments and cultured at 39°C. Zygotes were cultured in Expts 1, 2A and B, and 3, and two-cell embryos were cultured in Expts 4A and 4B. In Expt 1, blastocyst formation and number of cells were highest (P < 0.05) in the control medium with 93 mmol NaCl l^-1 (270 mosmols) compared with media containing 63 and 116 mmol NaCl l^-1 (220 and 316 mosmols). In Expt 2, embryos were cultured in media with 70 or 93 mmol NaCl l^-1, varying in osmolarity from 250 to 320 mosmols by adding sorbitol. In media with 70 mmol NaCl l^-1 and osmolarities of 250, 280 and 300 mosmols, there were 41, 56 and 50% expanded blastocysts, respectively (P < 0.05). With 93 mmol NaCl l^-1 and osmolarities of 270, 293 and 320 mosmols, embryos developed into 37, 53 and 27% expanded blastocysts, respectively, (P < 0.05). In Expts 3A and 3B and 4A and 4B, the osmolarity of the medium was maintained at 270 mosmols by adding sorbitol to media containing 40 or 60 mmol NaCl l^-1, and other components were reduced in media containing 100 and 116 mmol NaCl l^-1 to compensate for the higher NaCl. Zygote development into blastocysts was greatly suppressed (P < 0.05) in media with 40, 60, 100 and 116 mm NaCl l^-1, compared with the control (93 mmol NaCl l^-1), whereas development of two-cell embryos into blastocysts was much less affected. These results appear to reflect a direct sodium chloride as well as osmolarity effect on embryo development; and zygotes are much more sensitive to these effects than are two-cell embryos.