Received May 07, 2007
Accepted September 26, 2007

The objective of these studies was to assess the relationships between capacitation time and conditions, plasma membrane translocation of hosphatidylserine (PS) and protein tyrosine phosphorylation, and the spontaneous and agonist-induced acrosome reaction of human spermatozoa. Purified populations of highly motile sperm were incubated in human tubal fluid (HTF) supplemented with varying doses of human serum albumin (HSA) and examined at 0, 3, 6, 12 and 24 hours. Acrosome reaction of live sperm was detected by fluorescein isothiocyanate-labelled Pisum Sativum Agglutinin/Hoechst; annexin V binding was used to monitor PS translocation; tyrosine phosphorylation was examined by immunoblotting. The percentage of live, acrosome reacted sperm and the percentage of live sperm depicting PS translocation increased significantly over time, both under spontaneous- and calcium ionophoretreated conditions. Under spontaneous conditions there was a significant positive correlation between the percentage of live, acrosome reacted sperm and the percentage of live sperm with PS translocation. Tyrosine phosphorylation increased over time and with higher doses of HSA, and was associated with increased PS externalization. Taken together, these results suggest that the spontaneous acrosome reaction of human spermatozoa incubated under conditions that support capacitation is time dependent, is positively associated with changes in plasma membrane lipid distribution resulting in externalization of PS, and is accompanied by tyrosine phosphorylation. The responses to the agonistic effect of a calcium ionophore appear to result in different membrane dynamics.