Luteogenic and luteotropic effects of eCG during pregnancy in the mare

The role of eCG during pregnancy was evaluated through the study of the temporal relationships between changes in eCG and progesterone concentrations and the formation of supplementary corpora lutea (SCL) in mares impregnated with donkey semen (mule pregnancies) or with horse semen (equine pregnancies). Concentrations of eCG were higher (p < 0.01) in equine than in mule pregnancies between weeks 6.5 and 13. Progesterone concentrations were higher in equine than in mule pregnancies between weeks 9 and 17. All animals developed at least one SCL, but more SCL accumulated during equine pregnancies than during mule pregnancies (1.9 ± 0.2 vs 1.2 ± 0.1; p < 0.01). In equine pregnancies, the mares that formed a second SCL had higher eCG concentrations (p < 0.05) during the two weeks preceding its formation than those mares remaining with only one SCL. Mares that formed a third SCL had higher (p < 0.5) eCG levels than those remaining with one or two SCL. Mares with equine pregnancies that formed three SCL had higher progesterone concentrations (p < 0.05) than those that formed only one or two SCL. No differences were found in progesterone or eCG concentrations between mares with mule pregnancies that accumulated different numbers of SCL during pregnancy (p > 0.05). It is concluded that eCG stimulates both the development of new SCL and the function of existing CL. While these effects are clearly expressed in mares impregnated by horses, the low eCG concentrations during mule pregnancies reduce the impact of this hormone on CL formation and function.