RESUMO
Equine blastocyst fluid has a lower [Na+], a higher [K+] and a lower osmolality than does normal blood serum. Based on the assumptions that the sodium pump is primarily responsible for fluid accumulation and that ions transported actively into the blastocyst increase blastocyst osmolality above that of the external medium, we hypothesized that the [Na+] and the osmolality of mare uterine fluid are lower than those of blastocyst fluid. Microdialysis and ion chromatography were used to estimate [Na+] and [K+] of uterine fluid. Mares (n=10) were used for in vivo measurements at different stages of the oestrous cycle and during early pregnancy. On the basis of the results of studies in vitro, the mean +/- SD intrauterine [Na+] and [K+] were 110.0 +/- 15.1 and 12.3 +/- 6.8 mmol l(-1), respectively. These results indicate that uterine fluid of mares has a lower [Na+] and a higher [K+] than does normal serum. However, the [Na+] in uterine fluid is considerably higher than that of blastocyst fluid. Thus, these results do not support our hypothesis and the mechanism of production of hypotonic blastocyst fluid in horses remains unclear.