IMPORTANCE OF SODIUM-ION TO THE PROGESTERONE-INITIATED ACROSOME REACTION IN HUMAN SPERM

Progesterone (P) has previously been shown to rapidly increase free in tracellular calcium concentration ([Ca2+](i)), and subsequently to ini tiate the acrosome reaction (AR) in capacitated human sperm. The prese nt study used cytochemical analysis of the AR, and spectrofluorometric determination of sperm [Ca2+](i) and intracellular pH (pH(i)) in Na+- containing and Na+-deficient bicarbonate/CO2-buffered media to investi gate the role of Na+ in these P-initiated changes. We found that P fai led to initiate the AR in Na+-deficient medium, and that the initial r ise in [Ca2+](i) following P(1 mu g/ml) stimulation was similar for bo th media; however, the [Ca2+](i) in the Nat-deficient medium regressed more rapidly and plateaued at a significantly lower [Ca2+](i). Moreov er, 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 i n 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 pH(i) regulation, as capacitated sperm that were incubate d in a Na+-deficient medium for greater than or equal to 30 min displa yed a significantly lower pH(i). (C) 1996 Wiley-Liss, Inc.