Luteoprotective Role of Equine Chorionic Gonadotropin (eCG) During Pregnancy in the Mare

Abstract

The effects of repeated cloprostenol administration were compared in mares impregnated by horses and mares impregnated by donkeys in order to assess the role of eCG on the development of pregnancy-associated resistance to the luteolytic and abortifacient effects of PGF2α. Eleven mares impregnated by donkey (mule pregnancy) and 9 mares impregnated by horse (horse pregnancy) were used. Six mares with mule pregnancy and four with horse pregnancy were injected with cloprostenol (0.25 mg) when they were between day 65 and day 75 of pregnancy, and the treatment was repeated 48, 72 and 96 h latter. The rest of the mares remained as controls. Concentrations of eCG were 10 times higher (p < 0.001) in mares impregnated by horses than in mares impregnated by donkeys, and they were not affected by cloprostenol treatment. Luteolysis was completed 30 h after the first cloprostenol injection in mule pregnancies, while mares with horse pregnancies required 96 h and three cloprostenol injections to complete luteolysis. Regression analysis revealed significant associations between eCG concentrations at time 0 and the time required for completion of luteolysis (p < 0.001), foetal death (p < 0.01) and foetal expulsion (p < 0.05). It is concluded that high eCG concentrations in mares impregnated by horses protect the corpora lutea of pregnancy against the luteolytic effects of PGF2α. Low eCG concentrations in mares carrying mule foetuses afford them less protection against the luteolytic effect of PGF2α, and this may be a cause of the increased foetal mortality that occurs between days 60 and 90 of pregnancy in these mares.

Review of the reproductive endocrinology of the pregnant and parturient mare

“…Equine chorionic gonadotropin is secreted by the endometrial cups, explants of binucleate trophoblast that detach from the chorionic girdle of the conceptus and colonize the endometrium. Secretion begins in the sixth week of equine pregnancies long after maternal recognition of pregnancy has occurred (prevention of programmed luteolysis), providing direct luteotrophic support [79,80] but at a much later stage of pregnancy than does the CG of primates. Primate CG is secreted at the late blastocyst stage within a day or two of endometrial attachment, preventing programmed luteal regression that otherwise maintains nonpregnant females in a cyclic state [81,82], the primate maternal recognition of pregnancy in other words.…”

Roles of N-linked and O-linked glycosylation sites in the activity of equine chorionic gonadotropin in cells expressing rat luteinizing hormone/chorionic gonadotropin receptor and follicle-stimulating hormone receptor

“…CG was reported to be expressed only in primate and Equidae species during early pregnancy. In particular, eCG is secreted from the endometrial cups that detach from the chorionic girdle of the conceptus between days 37 and 120 of pregnancy [ 6 – 8 ]. The administration of eCG increases the ovulation rate [ 9 , 10 ], especially in early post-partum cows [ 11 ] and cows under prolonged anestrus or seasonal heat stress [ 9 ].…”

Internalization of Rat FSH and LH/CG Receptors by rec-eCG in CHO-K1 Cells

“…In mares, eCG is secreted from binucleate trophoblastic cells, called endometrial cups, in maternal plasma during the first half of equine gestation. These cells detach from the chorionic girdle of the conceptus between days 37–120 of pregnancy ( Boeta and Zarco, 2012 ; Flores-Flores et al, 2014 ; Conley, 2016 ). Progesterone has not been detected in the second half of gestation in mares ( Conley, 2016 ).…”

“…Along with the FSHR, LH/CGR is a member of the rhodopsin-like subfamily of G-protein-coupled receptors (GP CRs), which has been shown to mediate the internalization of its agonist ( Segaloff and Ascoli, 1993 ; Ascoli et al, 2002 ; Flores-Flores et al, 2014 ). The internalized receptors are either degraded in lysosomes ( Ghinea et al, 1992 ) or undergo resensitization and reinsertion into the plasma membrane ( Ascoli and Segaloff, 1987 ; Lazari et al, 1998 ).…”