ACROSOME REACTION INDUCED IN A LIMITED POPULATION OF HUMAN SPERMATOZOA BY PROGESTERONE (CA2+-DEPENDENT) AND ATP (CA2+-INDEPENDENT)

We examined the kinetics of the acrosome reaction induced in human spermatozoa by progesterone (Ca2+-dependent) and adenosine 5'-triphosphate (ATP; Ca2+-independent), ATP and progesterone did not induce the acrosome reaction unless spermatozoa were incubated in a capacitation medium. ATP exhibited a constant induction of the acrosome reaction regardless of the incubation period, while progesterone began to induce the acrosome reaction after greater than or equal to 6 h of incubation. At 24 h of incubation, the percentages of spermatozoa in which the acrosome reaction had been induced by both progesterone and ATP were almost equal. To determine whether a limited population of human spermatozoa was reactive to both progesterone and ATP, we employed an MH61 bead binding method. When the acrosome-reacted spermatozoa were removed with MH61 beads, their percentage in the sperm suspension was decreased to <3%. At 24 h of incubation, progesterone and ATP induced the acrosome reaction in 12.0 and 10.0% of spermatozoa, respectively. After MH61 bead binding was performed, the remaining spermatozoa did not react to the other activator (progesterone --> ATP, ATP --> progesterone). These findings indicate that only a limited population of human spermatozoa has the potential to undergo the acrosome reaction when stimulated by both progesterone and ATP.