Release of DEFB126 From Macaque Sperm and Completion of Capacitation Are Triggered by Conditions That Simulate Periovulatory Oviductal Fluid

Capacitation of macaque sperm in vitro has been achieved efficiently only with the addition of both cyclic nucleotides and methylxanthines. The use of these exogenous sperm activators clouds an understanding of the normal mechanisms underlying capacitation and may slow early embryo development following in vitro fertilization (IVF). We demonstrate that culture medium which simulates periovulatory oviductal fluid with respect to bicarbonate (HCO3-) and glucose concentration induces capacitation in a high percentage of macaque sperm as determined by the ability of sperm to undergo both the release of coating protein DEFB 126 and the zona pellucida-induced acrosome reaction (AR). Few sperm were able to undergo the AR following 6 hr incubation in medium containing either 35 mM HCO3- (similar to 7.2 pH) or 90 mM HCO3- (similar to pH 7.8) with 5 mM glucose. When glucose concentration was lowered to 0.5 mM to match levels reported for women at midcycle, the AR rate increased significantly in sperm incubated in both levels of HCO3-, indicating that glucose interferes with sperm responsiveness to increasing HCO3- concentration observed in the primate oviduct during ovulation. Even greater synchronization of capacitation could be achieved with nonphysiologic extremes of alkalinity or energy substrate deprivation. In the latter case, sperm achieved high rates of IVF. A shift in pH from 7.2 to 7.8 in a HEPES-buffered medium was sufficient to remove DEFB126 from the surface of most sperm after only 3 hr. The loss of DEFB126 from sperm under periovulaory fluid conditions has implications for the timing of release of sperm from the oviductal reservoir.