Importance of bicarbonate to the progesterone-initiated human sperm acrosome reaction.

Progesterone, a putative in vivo initiator of the human sperm acrosome reaction (AR), has previously been shown to act at the sperm plasma membrane to initiate the AR in vitro. Here, we have investigated whether bicarbonate (HCO3-) was required for the progesterone-initiated human AR and whether HCO3(-)-dependent cAMP activation might be involved. Capacitated human sperm were suspended in the presence of high (25 mM) or low (1 mM) HCO3- media. The AR was assayed using fluorescein isothiocyanate (FITC)-conconavalin A with sperm fixed 5 minutes after progesterone or solvent control addition. Progesterone initiated the AR in both high and low HCO3- media, but the percentage of AR was significantly lower in the latter medium. In the presence of high HCO3-, 20-minute preincubation with 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), a blocker of HCO3- transport, inhibited the progesterone-initiated AR in a dose-dependent manner. The maximum inhibition (85%) was obtained with 18 microM DIDS. Inhibition by DIDS was reversed by washing sperm after treatment. Preincubation of sperm with dibutyryl cAMP (0.1 microM-1 mM) plus DIDS almost completely eliminated the inhibition of the progesterone-initiated AR by DIDS. Dibutyryl cAMP alone did not have a stimulatory effect on the progesterone-initiated AR, when high HCO3- was present, but it was able to partially overcome the reduction of AR by low HCO3-. These results are the first to demonstrate the importance of HCO3- to: 1) mammalian AR initiation by the putative physiological initiator progesterone, and 2) the human sperm AR.(ABSTRACT TRUNCATED AT 250 WORDS)