Cryopreservation alters the levels of the bull sperm surface protein P25b

Fertility of frozen-thawed bull sperm is reduced by cryopreservation. Freez ing-thawing procedures can result in as much as a sevenfold fertility decre ase. Sperm mortality and loss of motility do not fully explain the reduced fertility of cryopreserved semen; they may be partially explained by the lo ss of sperm surface proteins, which are necessary for fertilization. We hav e previously identified P25b, a sperm surface protein, which is associated with the fertility index of bulls used for artificial insemination. Using W estern blotting techniques, we have evaluated P25b levels before and after cryopreservation of bull spermatozoa in extenders based on either egg yolk or milk. Long storage periods (28 days) in liquid nitrogen results in a thr eefold decrease of P25b levels associated with cryopreserved versus fresh s permatozoa. Over a short storage period (3-7 days), a stable P25b level was observed on spermatozoa cryopreserved in extender containing either egg yo lk or milk. A decrease in P25b levels associated with spermatozoa was obser ved after 5 days of storage in egg yolk extender, whereas a significant dec rease was observed after 14 days of sperm storage in milk extender (P < .05 ). Therefore, the loss of P25b may be responsible, at least in part, for th e decrease in fertility following the freezing-thawing procedure of bull se men. Moreover, the cryopreservation extender used may have different effect s on the loss of sperm surface proteins after even brief storage periods in liquid nitrogen. Considering that a sperm protein similar to P25b exists i n humans (P34H), these results may have significant clinical applications i n which frozen semen is used.