Differential expression of steroidogenic enzymes by the primary corpora lutea of pregnant mares during equine chorionic gonadotrophin secretion.

At the onset of equine chorionic gonadotrophin (eCG) secretion, eCG stimulates luteal androgen and oestrogen production. Although eCG concentrations increase exponentially from day 37 to day 60 of gestation and eCG is detectable in maternal serum until about day 120-150 of gestation, luteal androgen and oestrogen production peaks between 5 and 10 days after initial exposure to eCG and then decreases gradually. It is not clear how eCG regulates luteal androgen and oestrogen production. In the present study, the steady-state mRNA expression of 3beta-hydroxysteroid dehydrogenase (3beta-HSD), cytochrome P450 17alpha-hydroxylase/17,20-lyase (P450(17alpha)) and cytochrome P450 aromatase (P450arom) in primary corpora lutea before, during and after eCG secretion was determined by northern blotting. Expression of 3beta-HSD was similar at all the stages examined. Cytochrome P450(17alpha) expression increased at the onset of eCG secretion, decreased between days 42 and 46 of gestation and was constant for the remaining period of eCG secretion. Cytochrome P450arom expression was highest before and after eCG secretion and lowest during periods of peak eCG secretion. The differential expression of P45017alpha and P450arom indicates that production of luteal androgen and oestrogen is regulated by P450(17alpha), activity. The effect of eCG on luteal steroidogenic enzyme mRNA expression appears to be stage-specific.