Fusion of prostasomes to human spermatozoa stimulates the acrosome reaction

OBJECTIVE: To determine the effect of the fusion of prostasomes to spermatozoa on the acrosome reaction. DESIGN: In vitro study of human spermatozoa. SETTING: Healthy volunteers in an academic research environment. PATIENT(S): Healthy volunteer men, 25 to 35 years old. INTERVENTION(S): Human semen was fractionated into spermatozoa and prostasomes. Fusion of prostasome to spermatozoa was performed at pH 5.5. Progesterone (1 microM) was added when required. MAIN OUTCOME MEASURE(S): Evaluation of the acrosome reaction by fluorescence microscopy. RESULTS(S): The percentage of spontaneously acrosome-reacted cells was very low unless the Ca(2+)-ionophore A 23187 was added. The treatment of spermatozoa with 1 microM of progesterone scarcely affected the acrosome reaction; a pretreatment in conditions permitting fusion increased it. The addition of progesterone to prostasome-fused spermatozoa further increased the extent of the acrosome reaction. CONCLUSION(S): The H(+)-dependent fusion with prostasomes makes spermatozoa more sensitive to the effect of progesterone on acrosome-reaction induction.