HUMAN SPERM PLASMA-MEMBRANE PROGESTERONE RECEPTOR(S) AND THE ACROSOMEREACTION

Progesterone initiation of the human sperm acrosome reaction (AR) incl udes stimulation of a transient Ca2+ influx and a transient Cl- efflux . A role for one or more plasma membrane receptors has been suggested, but, except for evidence supporting the involvement of a sperm GABA(A )-like receptor/Cl- channel, there is little information about possibl e receptor identity. Here, we attempt to identify plasma membrane prog esterone receptors in human sperm using a mouse monoclonal antibody (m Ab C-262) raised against the C-terminal steroid-binding domain of the human intracellular progesterone receptor, C-262 inhibited the progest erone-initiated AR in a dose-dependent manner. Maximum inhibition was 77% as detected by fluorescein isothiocyanate (FITC)-concanavalin A (C onA). Motility was unaffected. A control mouse mAb (h-151) raised agai nst the human estrogen receptor did not inhibit the progesterone-initi ated AR. Western blotting with C-262, but not with h-151, detected a m ajor sperm protein band of 50-52 kDa. In indirect immunofluorescence l ocalization studies, live and ethanol-fixed uncapacitated sperm and fi xed capacitated sperm incubated with C-262, but not with h-151, displa yed fluorescence at the equatorial segment region of the sperm head pl asma membrane. In spectrofluorometric studies using capacitated sperm loaded with the Ca2+ probe Fura-2 or the Cl- probe MEQ, 0-262 but not h-151 inhibited both Ca2+ influx and Cl- efflux. These ion fluxes coul d be due to the binding of progesterone to two different receptor/chan nels or to its binding to one and cross talk with the other. Our resul ts strongly support the involvement of sperm plasma membrane receptors in the progesterone-initiated AR and provide a candidate for one such receptor.