Fluorometric study of rabbit sperm head membrane phospholipid asymmetry during capacitation and acrosome reaction using Annexin-V FITC.

This study was conducted to evaluate phosphatidylserine translocation in head plasma membrane of Percoll-gradient purified of rabbit cauda epididymal sperm during capacitation and acrosome reaction (AR) using Annexin-V. Propidium iodide was used as control to reject dead or dying cells. The presence and distribution of Annexin-V binding sites were analyzed using flow fluorocytometry and confocal microscopy. After 6 h of incubation of sperm in capacitation medium, the number of cells positively stained with Annexin-V showed a small but significant increment. The Annexin-V binding sites produced during capacitation were found mainly in the post-acrosomal region of the sperm head plasma membrane. After AR induction with progesterone, the localization of phosphatidylserine was changed and the Annexin-V binding sites were found almost only in the acrosomal region, but with higher number of binding sites in the equatorial area. On the contrary, after AR induction with A23187, phosphatidylserine translocation, although predominant over the acrosomal region, was also observed in the post-acrosomal region. Plasma membrane destabilization during capacitation and AR may be important for sperm-oocyte fusion.