Osmo-sensitivity of the human sperm acrosome reaction

The osmo-sensitivity of the human sperm acrosome reaction was investigated by determining the effect of hyper- and hypo-osmolal conditions on the ionophore A23187- and dbcAMP-induced reaction of both capacitated and non-capacitated spermatozoa. Hyper-osmolal conditions inhibited the agonist-induced reactions of both types of spermatozoa. Hypo-osmolal conditions caused a spontaneous loss of acrosomes from capacitated but not from non-capacitated spermatozoa. The loss of acrosomes under hypo-osmolal conditions was further enhanced by dbcAMP but not by ionophore A23187. Although significant decreases in sperm viability were occasionally observed at the high and low osmolalities, these decreases were not consistent and could not account for the observed loss of acrosomes. It is concluded that the human sperm acrosome reaction is osmo-sensitive. The acrosome reaction stimulated by ionophore A23187 (raises intracellular Ca2+) and dbcAMP (activates protein kinase A which causes protein phosphorylation) appears to involve water entry downstream from the action of these agonists. Preincubation in albumin (capacitation) causes human spermatozoa to lose their acrosomes under hypo-osmolal conditions. Finally, capacitation is not an essential prerequisite to the acrosome reaction as long as agonists are used that by-pass certain membrane-related events.