Progesterone induced a rapid influx of calcium in capacitated human sperm, followed by a long-lasting, dose-dependent increase of intracellular free calcium. Thereafter, progesterone increased the fraction of hyperactivated sperm and the acrosome reaction. On the contrary, the progesterone antagonist RU486 (mifepristone) induced an immediate and transient, dose-dependent decrease of intracellular free calcium and a drop in the values of sperm movement parameters related to hyperactivation. Moreover, RU486 counteracted the effects of progesterone on calcium influx, lateral sperm head displacement, and the acrosome reaction. Therefore, RU486 effects were opposite to those of progesterone. The nature of the membrane receptor(s) involved is unknown. Several steroids bearing 11(-phenyl substitutions, with different pharmacological profiles, were also investigated. It was concluded that the steroid structure and chemical groups added to the 11(-phenyl influence effects on calcium influx.Capacitation leads to functional changes ofthe sperm that are characterized by modified movement parameters designated as hyperactivation and by exocytotic events (the acrosome reaction) (1-3). The acrosome reaction is an essential step in the complex series of processes implied in Eutherian fertilization, necessary for sperm penetration through the zona pellucida and fusion with the oocyte plasma membrane. It can be induced by follicular fluid, cumulus cells (4), or zona pellucida (5). Progesterone (PROG) was identified as a major component of the follicular fluid for inducing the acrosome reaction in human sperm (6), preceded by an immediate, transient calcium influx into spermatozoa (7,8). These rapid biological effects suggest a nongenomic mechanism and a cell surface receptor for PROG like the one responsible for the meiotic maturation of Xenopus laevis oocyte (9).The antiprogestin RU486 {RU38486, mifepristone, 17(-hydroxy-11(8-[4-(dimethylamino)phenyl]-17a-propynylestra-4,9-dien-3-one} binds with high affinity to the intracellular PROG receptor in most vertebrate species (10). It has been reported that this potent antiprogestin has either a small or negligible inhibitory effect on PROG-mediated calcium influx into human sperm (11,12). In this study, we evaluated the effect of RU486, alone or in combination with PROG, on intracellular free calcium concentration ([Ca2l]i) and the movement of human sperm and acrosome reaction, two calcium-related events (3, 13).MATERIALS AND METHODS Materials. PROG, human serum albumin (HSA) fraction 5, Fura2-AM, Fluo3-AM, Hoechst 33258, fluorescein-labeled Pisum sativum agglutinin, verapamil, EGTA, Triton X-100, sodium pyruvate, and Hepes were purchased from Sigma.The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.RU486, 17p-hydroxy-11p-{4-[2-dimethylamino)ethoxy]phenyl}-17a-propynylestra-4,9-dien-3-one(RU39009), 11-[4-(di-phenyl}estra...
