Importance of sodium ion to the progesterone-initiated acrosome reaction in human sperm.

Abstract

Progesterone (P) has previously been shown to rapidly increase free intracellular calcium concentration ([Ca2+]i), and subsequently to initiate the acrosome reaction (AR) in capacitated human sperm. The present study used cytochemical analysis of the AR, and spectrofluorometric determination of sperm [Ca2+]i and intracellular pH (pHi) in Na(+)-containing and Na(+)-deficient bicarbonate/CO2-buffered media to investigate the role of Na+ in these P-initiated changes. We found that P failed to initiate the AR in Na(+)-deficient medium, and that the initial rise in [Ca2+]i following P (1 microgram/ml) stimulation was similar for both media; however, the [Ca2+]i in the Na(+)-deficient medium regressed more rapidly and plateaued at a significantly lower [Ca2+]i. Moreover, the differences in plateau [Ca2+]i were directly related to the percentage of acrosome reactions, suggesting that the plateau phase is not due to [Ca2+]i, but rather to the release of intracellular fura-2 into the medium during the AR. These [Ca2+]i, and AR results are in contrast to those reported previously by others for human sperm and suggest that a Na(+)-dependent mechanism is important in the P-initiated human sperm AR. Such a Na+ requirement may reflect the involvement of this ion in pHi regulation, as capacitated sperm that were incubated in a Na(+)-deficient medium for > or = 30 min displayed a significantly lower pHi.