Progesterone elicits a rapid, transient calcium influx in sperm that is a prerequisite for the progesterone-induced acrosome reaction. The possibility that the GABA(A) receptor/chloride Channel was the receptor that mediated the progesterone-induced calcium influx in human sperm was examined. A-ring reduced 3 alpha-hydroxy pregnane steroids (e.g. alfaxalone, allopregnanolone, pregnanolone), which are active on the GABA(A) receptor/chloride channel, were found to be much weaker than progesterone at stimulating Ca2+ influx in sperm. The effects of a variety of progesterone metabolites and analogs and other steroids were compared for their ability to (i) stimulate GABA-induced Cl-36(-) uptake in synaptoneurosomes, (ii) stimulate GABA-induced Cl- currents in HEK-293 cells transfected with alpha 1, beta 2, and gamma 2 subunits of the GABA(A) receptor/chloride complex, and (iii) elicit a rapid Ca2+ influx in sperm. No correlation was observed between the ability of a given steroid to stimulate Ca2+ influx and efficacy in eliciting either Cl-36(-) uptake or chloride currents. Importantly, the action of progesterone to stimulate Ca2+ influx was not modified by GABA, diazepam, picrotoxin and pentobarbitol (known regulators of the GABA(A) receptor/chloride channel). It is concluded from these studies that the cell surface progesterone binding site on human sperm that mediates progesterone-induced changes in [Ca2+](i) is unlike the steroid binding site on the GABA(A) receptor/chloride channel. Because picrotoxin, a highly selective inhibitor of GABA(A) receptors, inhibits the progesterone-induced acrosome reaction without preventing the progesterone-stimulated Ca2+ influx it is possible that progesterone interacts with two cell surface receptors in sperm: one that mediates the rapid Ca2+ influx and another which is GABA(A)-like and mediates Cl- flux. Activation of both receptors may be required for the progesterone-induced acrosome reaction. Alternatively, because the progesterone-induced acrosome reaction was demonstrated to require a functional GABA(A) receptor/chloride channel, but not necessarily a progesterone induced Cl- flux, it is also possible that only the interaction of progesterone with the receptor mediating Ca2+ influx is required to elicit the acrosome reaction.
