ION FLUXES THROUGH THE PROGESTERONE-ACTIVATED CHANNEL OF THE SPERM PLASMA-MEMBRANE

We have characterized ionic changes triggered by progesterone in human spermatozoa. This steroid, which is a fast-acting stimulator of the a crosome reaction, triggered a rapid increase in the cytoplasmic Ca2+ c oncentration ([Ca2+]i) which was entirely due to influx across the pla sma membrane, as it was obliterated by chelation of extracellular Ca2. Ca2+ fluxes were insensitive to verapamil and pertussis toxin, thus suggesting that they did not occur via voltage-gated channels and did not involve a pertussis toxin-sensitive G protein, and were potentiate d in Na+-free, choline-containing or methylglucamine-containing medium . Progesterone also caused a depolarization of the plasma membrane in Na+-containing as well as in choline- or methyl-glucamine-containing s aline; depolarization was larger in the absence of extracellular Ca2+, suggesting that Na+ and Ca2+ fluxes occurred through the same channel . Progesterone was able to trigger the acrosome reaction in the three media investigated (Na+, choline and methylglucamine), provided that e xtracellular Ca2+ was also present. We conclude that progesterone acti vates a membrane ion channel that is permeable to monovalent cations a s well as to Ca2+.