Cryopreservation of ram sperm alters the dynamic changes associated with in vitro capacitation.

The aim of this study was to investigate the dynamic changes that ram sperm experience during in vitro capacitation before and after cryopreservation. Using flow cytometry and computer assisted sperm analysis system (CASA), protein tyrosine phosphorylation and several functional parameters were evaluated in fresh and cryopreserved ram sperm incubated under capacitating and non-capacitating conditions at 0, 1, 5, 15, 30, 60, 120, 180 and 240 min. A short incubation period (5-30 min) under capacitating conditions was enough to increase mitochondrial activity and tyrosine phosphorylation in cryopreserved sperm, inducing also changes in the motility pattern, which could be related to hyperactivation. However, fresh sperm required a longer incubation (180-240 min) under capacitating conditions to undergo similar modifications. In both types of samples, tyrosine phosphorylation increased in a sequential manner in the midpiece, principal piece and tail at specific time points during in vitro capacitation. Moreover, the proportion of viable sperm with intact acrosome begun to decrease during capacitation, occurring before in cryopreserved sperm. Our findings suggest that cryopreserved ram sperm become competent for fertilization after a short exposure to capacitating conditions as a result of drastic changes inflicted by the freezing-thawing procedure, while prolonged incubations after cryopreservation severely impair sperm quality.