Relationship between acrosome reactions and hamster egg penetration after ionophore challenge in absence of teratozoospermia.

Objective: To determine the relationship between acrosome reactions and hamster egg penetration after ionophore challenge in nonteratozoospermic semen samples.
Setting: A tertiary care center, the Andrologic Clinic, Department of Internal Medicine, University of L'Aquila.
Patients: Twenty-five nonteratozoospermic patients with infertile marriages and nine fertile men.
Interventions: The hamster egg penetration assay and the acrosome reaction assessment were performed on capacitated sperm suspensions in basal conditions and after ionophore challenge with ionomycin.
Main Outcome Measure: The relationship between the acrosome reactions and the hamster egg penetration was analyzed.
Results: Although in basal conditions the spontaneous acrosome reaction rate was not correlated with the hamster egg penetration, after ionophore challenge both the induced acrosome reaction rate and the acrosome reaction increase (induced minus spontaneous acrosome reaction rate) correlated with the hamster egg penetration. The highest correlation was observed between the induced acrosome reaction rate and the penetration index. Considering a penetration index = 2 as the threshold of a good response of the hamster egg penetration assay to the ionophore challenge, significantly lower acrosome reaction rates and increases were associated with poor hamster egg penetration. However, different penetration indexes occurred at the same level of induced acrosome reaction rate.
Conclusions: The capability of sperm to react after ionophore challenge reflects to some extent the capability of the acrosome-reacted sperm to undergo the changes of the plasma membrane that are necessary for sperm-oocyte fusion. The simple evaluation of the acrosome reactions may represent a useful complement rather than a substitute for the hamster egg penetration assay in monitoring the responses of human sperm to the ionophore challenge. An impaired inducibility of the acrosome reactions may account for poor hamster egg penetrations exhibited by nonteratozoospermic semen samples.