IMMUNOLOCALIZATION OF 3-BETA-HYDROXYSTEROID DEHYDROGENASE, CYTOCHROME-P450 17-ALPHA-HYDROXYLASE 17,20-LYASE AND CYTOCHROME-P450 AROMATASE IN THE EQUINE CORPUS-LUTEUM OF DIESTRUS AND EARLY-PREGNANCY/

The onset of equine chorionic gonadotrophin (eCG) secretion in pregnan t mares is associated with an increase in luteal androgen and oestroge n production. The luteal cell type(s) responsible for the increased pr oduction of androgens and oestrogens has not been identified in the eq uine corpus luteum. In this study, we examined the pattern of expressi on of 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD), cytochrome P45 0 17 alpha-hydroxylase/17,20-lyase (P450(17 alpha)) and cytochrome P45 0 aromatase (P450(arom)) by immunohistochemistry in equine luteal tiss ue collected during dioestrus (days 7-10; n = 4) and early pregnancy, before (days 29-35; n = 4) and after (days 39-45; n = 4) the onset of endogenous eCG secretion. All luteal cells expressed 3 beta-HSD, P450( 17 alpha) and P450(arom). The distribution of 3 beta-HSD, P450(17 alph a) and P450(arom) did not differ with stage of the reproductive cycle. The intensity of immunohistochemical staining for 3 beta-HSD did not appear to differ with reproductive stage. In contrast, the intensity o f immunostaining for P450(17 alpha) increased after the onset of eCG s ecretion. The intensity of immunostaining for P450(arom) increased dur ing pregnancy before the onset of eCG secretion and diminished after t he onset of eCG secretion to the intensity seen in dioestrous corpora lutea. This finding suggests that androgen and oestrogen production is not compartmentalized within the equine corpus luteum. Both large and small luteal cells express the steroidogenic enzymes necessary for oe strogen production, and the intensity of immunostaining for P450(17 al pha) and P450(arom) appears to be stage-specific.