Evidence suggesting involvement of a unique human sperm steroid receptor/Cl- channel complex in the progesterone-initiated acrosome reaction.

Progesterone (4-pregnen-3-20-dione) initiates the acrosome reaction (AR) of human sperm in vitro. Anesthetic progestins (e.g., progesterone) potentiate gamma-aminobutyric acid (GABA) activation of neuronal GABAA receptor/Cl- channel complexes (GBRCs), and some can activate neuronal GBRCs in the absence of GABA. Here, we study whether human sperm acrosome reaction initiation by progesterone and several other progestins involves steroid interaction with a sperm GBRC. Anesthetically active 3 alpha-OH isomers of two progesterone metabolites, as well as the synthetic 3 alpha-OH anesthetic steroid Alphaxalone, their 3 beta-OH isomers (inactive as anesthetics), or progesterone, were added to capacitated human sperm. The 3 alpha-OH isomers initiated the AR, and the 3 beta-OH isomers were without effect. Moreover, the 3 alpha-OH isomers were less effective than progesterone, a progestin that does not activate neuronal GBRCs in the absence of GABA. Also, alphaxalone, a very potent activator of neuronal GBRCs, was the least active of the progestins tested. GABA was not detected in human sperm. Preincubation of capacitated sperm with GBRC Cl- channel blockers dramatically reduced the progesterone-initiated AR, and sperm were unable to undergo the progesterone-initiated AR in Cl(-)-deficient medium. AR initiation by the calcium ionophore ionomycin was not inhibited in Cl(-)-deficient medium or by GBRC Cl- channel blockers. Indirect immunofluorescence, using a monoclonal antibody to the bovine cerebral cortex GABAA receptor alpha-subunit, localized immunoreactivity in live and fixed sperm as a fluorescent band in the sperm plasma membrane overlying or near the narrow equatorial segment region of the acrosome. Immunoblotting using this antibody detected two major bands with apparent molecular weights of 50 kDa (as reported in other cell types) and 75 kDa (not reported in other cells). Our data suggest that (1) progesterone exerts its effect on capacitated human sperm at least partially by interaction with a unique sperm steroid receptor/Cl- channel complex resembling a neuronal GBRC but apparently possessing a different sensitivity to progestins; and that (2) this interaction results in an increased Cl- flux essential to AR initiation.