IN-VITRO REGULATION OF LUTEAL FUNCTION IN MARES

Dispersed equine luteal cells collected during dioestrus (days 7-10), before (days 30-35 of pregnancy) and after (days 40-45 of pregnancy) t he onset of equine chorionic gonadotropin (eCG) secretion were incubat ed in defined medium for 24 h to determine steroid production at these specific stages of the reproductive cycle. Luteal cells collected at these three reproductive stages were also incubated for 4 h in defined medium with increasing amounts of eLH, eCG and hCG added to determine the effect of gonadotropins on luteal progesterone and oestrogen prod uction. Progesterone and oestrogen accumulation proceeded in a linear fashion during the 24-h incubation. No differences in rate of progeste rone accumulation were observed between the three reproductive stages. A significant effect of reproductive stage on rate of oestrogen accum ulation was observed, with oestrogen production being greatest in lute al cells from pregnant mares after the onset of eCG secretion and lowe st in luteal cells from mares in dioestrus. Addition of gonadotropin t o the medium resulted in a dose-dependent increase in progesterone acc umulation during a 4-h incubation at most reproductive stages. However , a significant increase in progesterone accumulation was only observe d at supraphysiological concentrations of gonadotropins. Addition of g onadotropin did not result in a change in oestrogen accumulation. The results of this study demonstrate that luteal oestrogen secretion incr eases with pregnancy and is highest during eCG secretion. These experi ments did not demonstrate an effect of gonadotropin on luteal oestroge n secretion in vitro even though an effect on luteal progesterone secr etion was observed. The lack of a gonadotropin-induced increase in oes trogen secretion is most likely due to the short duration of the incub ation.