PROGESTERONE-INDUCED ACROSOME REACTION IN STALLION SPERMATOZOA IS MEDIATED BY A PLASMA-MEMBRANE PROGESTERONE-RECEPTOR

The aim of the present study was to investigate whether the induction of stallion sperm acrosome reaction (AR) by progesterone is mediated b y binding of progesterone to a receptor on the sperm plasma membrane o r to an intracellular progesterone receptor. Progesterone-BSA conjugat e labeled with fluorescein isothiocyanate (P-BSA-FITC) in combination with a vital stain, ethidium homodimer, was applied to visualize the p resence of the progesterone receptor on living spermatozoa. Alternativ ely, an indirect immunofluorescence technique employing a monoclonal a ntibody (C-262) against human intracellular progesterone receptor was conducted to validate the presence of the progesterone receptor. Immun ogold labeling techniques enabled ultrastructural localization of P-BS A-FITC or C-262 with transmission electron microscopy. The dynamic cha nges in labeling patterns were monitored for sperm cells, using fluore scence microscopy and flow cytometry during a 5-h capacitation period. An increasing number of viable cells showed affinity for P-BSA-FITC o r C-262 at the acrosomal plasma membrane region of the sperm head, whi le a decreasing number of viable cells were not labeled. In contrast, almost all deteriorated cells were labeled in the cytosol of the poste quatorial region of the sperm head. Incubation with P-BSA-FITC resulte d in the induction of AR but to a lesser extent than that for sperm in cubated with free progesterone. Therefore, coupling of progesterone to its receptor on the sperm plasma membrane appears to be an important step in the induction of the AR.