Strontium supports human sperm capacitation but not follicular fluid-induced acrosome reaction

The ability of strontium (Sr2+) to replace calcium (Ca2+) in maintaining human sperm function has still not been completely characterized. In the present study, acrosome reaction (AR) inducibility in response to human follicular fluid (hFF) was compared in spermatozoa incubated in either Ca2+- or Sr2+-containing media. Other events related to sperm capacitation, such as protein tyrosine phosphorylation and hyperactivation as well as zona pellucida (ZP) recognition under both conditions, were also analyzed. Spermatozoa incubated overnight in the presence of Sr2+ were unable to undergo the AR when exposed to hFF. Nevertheless, when spermatozoa were incubated under this condition and then transferred to medium with Ca2+, sperm response to hFF was similar to that of cells incubated throughout in the presence of Ca2+. The sperm protein tyrosine phosphorylation patterns and the percentages of sperm motility and hyperactivation were similar after incubation in Ca2+- or Sr2+-containing media. Under both conditions, the same binding capacity to homologous ZP was observed. Similar results were obtained when EGTA was added in order to chelate traces of Ca2+ present in Sr2+ medium. From these results, it can be concluded that Sr2+ can replace Ca2+ in supporting capacitation-related events and ZP binding, but not hFF-induced AR of human spermatozoa.