Equine chorionic gonadotropin regulates luteal steroidogenesis in pregnant mares.

The study examines the relationship between equine chorionic gonadotropin (eCG) and luteal steroid production in pregnant horses. The results indicate that the increase in steroid production that happens with the onset of eCG secretion is indeed induced by eCG and leads to an increase in androgen and estrogen synthesis, supporting the idea that eCG performs a luteotropic action in pregnant mares.

Experiment Design

• The researchers observed the effect of eCG on the corpus luteum (CL), a part of the horse’s reproductive system during pregnancy. A group of pregnant mares were supplemented with 44 mg of altrenogest, a progestogen, each day between Days 18-50.
• The mares were segregated into groups: four with a functional CL and four without, with eCG doses administered on Days 26-28, before the natural onset of eCG secretion. Four mares with a functional CL received no extra eCG. This strategy allowed the researchers to compare the effects of eCG on mares with and without a functional CL.

Results

• In eCG-treated mares without a functional CL, there were no significant increases in progestin, androstenedione, and estrogen concentrations after eCG administration or the onset of natural eCG secretion.
• In contrast, eCG-treated mares with a functional CL showed significant increases in progestin and estrogen production after both exogenous eCG administration and the onset of endogenous eCG secretion. Androstenedione concentrations also seemed to increase following exogenous eCG and significantly increased after natural eCG secretion.
• In mares with a functional CL that did not receive exogenous eCG, progestin and estrogen concentrations increased, and androstenedione concentrations slightly increased only after the onset of endogenous eCG secretion.

Conclusions

• The findings of this study demonstrate that eCG does induce an increase in luteal steroidogenesis, leading to an escalation in luteal androgen and estrogen synthesis. This conclusion is based on the significant variations in hormone concentrations in mares with functional CL after eCG administration, compared with mares without active CL.
• In the context of equine physiology and reproduction, this research underscores the importance of eCG in the luteotropic action in pregnant mares. A luteotropic action refers to factors that promote the maintenance or growth of the corpus luteum. Hence, these findings also have implications for managing and understanding equine pregnancy.