The role of osmotic resistance on equine spermatozoal function.

Cryopreservation requires exposure of sperm to extreme variations in temperature and osmolality. The goal of this experiment was to determine the osmotic tolerance levels of equine sperm by analyzing motility, viability, mitochondrial membrane potential (MMP), and mean cell volume (MCV). Spermatozoa were incubated at 22 degrees C for 10 min in isosmolal TALP (300 mOsm/kg), or a range of anisosmolal TALP solutions (75-900 mOsm/kg), for initial analysis, and then returned to isosmolal conditions for 10 min for further analysis. Total sperm motility was lower (P < 0.05) in anisosmolal conditions compared to sperm motility in control medium. When cells were returned to isosmolal conditions, only sperm previously incubated in 450 mOsm/kg TALP were able to recover to control levels of motility. Sperm viability and MMP were lower (P < 0.05) when exposed to hypotonic solutions in comparison to control solutions. Sperm suspensions that were returned to isosmolal conditions from 75, 150, and 900 mOsm/kg had lower (P < 0.05) percentages of viable sperm than control suspensions (300 mOsm/kg). MMP was lower (P < 0.05) in cells previously incubated in 75 and 900 mOsm/kg when returned to isosmolal, as compared to control cells. MCV differed (P < 0.05) from control cell volume in all anisosmolal solutions. Cells in all treatments were able to recover initial volume when returned to isosmolal medium. Although most spermatozoa are able to recover initial volume after osmotic stress, irreversible damage to cell membranes may render some sperm incapable of fertilizing an oocyte following cryopreservation.