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) the onset of equine chorionic gonadotropin (eCG) secretion were incubated 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 production. Progesterone and oestrogen accumulation proceeded in a linear fashion during the 24-h incubation. No differences in rate of progesterone accumulation were observed between the three reproductive stages. A significant effect of reproductive stage on rate of oestrogen accumulation was observed, with oestrogen production being greatest in luteal cells from pregnant mares after the onset of eCG secretion and lowest in luteal cells from mares in dioestrus. Addition of gonadotropin to the medium resulted in a dose-dependent increase in progesterone accumulation during a 4-h incubation at most reproductive stages. However, a significant increase in progesterone accumulation was only observed at supraphysiological concentrations of gonadotropins. Addition of gonadotropin did not result in a change in oestrogen accumulation. The results of this study demonstrate that luteal oestrogen secretion increases with pregnancy and is highest during eCG secretion. These experiments did not demonstrate an effect of gonadotropin on luteal oestrogen secretion in vitro even though an effect on luteal progesterone secretion was observed. The lack of a gonadotropin-induced increase in oestrogen secretion is most likely due to the short duration of the incubation.