Changes in equine follicular aromatase activity during transition from winter anoestrus

Follicular aromatase activity during sexual resurgence after the winter anoestrus was investigated in 3 groups of 5 Pony mares. Group ET was studied during the early transition period, Group LT in late transition and Group C in full breeding condition. Granulosa and theca cells were incubated for 3 h with 3H-labelled androstenedione or progesterone. Analysis of the free oestrogenic products was by high performance liquid chromatography (HPLC) and recrystallization revealed highly variable oestrogen production in both cell types from mares in all 3 groups. Only oestrone and oestradiol peaks were found in the chromatographs. Conversion of androstenedione to oestrogens was greater than that of progesterone. Granulosa cells were more active than were thecal cells in converting precursors to oestrogens. The oestrone: oestradiol ratio varied between groups; Group ET mares had a lower ratio, possibly due to increased oestradiol production. The 3H activity in the water-soluble phase after initial extraction of ether-soluble materials varied during early transition and thereafter increased as transition progressed. This activity was significantly higher in incubations of granulosa cells with androstenedione. Although individual follicle variation in oestrogen production rates prevented the comparison of absolute conversion rates among the 3 groups, the change in oestrone: oestradiol ratio and the change in water-soluble materials suggest a qualitative change in follicular oestrogen production and perhaps overall steroid metabolism during the transition period.