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The inability of some synthetic progestagens to maintain pregnancy in the mare
Author(s) -
Mckin A. O.,
Lescun T. B.,
Walker J. H.,
Vasey J. R.,
Allen W. R.
Publication year - 2000
Publication title -
equine veterinary journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.82
H-Index - 87
eISSN - 2042-3306
pISSN - 0425-1644
DOI - 10.2746/042516400777612035
Subject(s) - fertility , unit (ring theory) , demography , medicine , mathematics , sociology , population , mathematics education
Progesterone or synthetic progestagens are administered frequently by equine veterinary clinicians to pregnant mares with a history of pregnancy loss (Neely 1988) in the belief that such therapy is necessary to supplement inadequate production of endogenous progesterone by the corpus luteum (CL) or placenta (Allen 1984). Low serum progesterone concentrations have been associated with early embryonic loss in some mares (Douglas et al. 1985; Ginther 1985; Bergfelt et al. 1992), but no evidence exists to support a conclusion that primary luteal deficiency is a significant cause of early embryonic death (Allen 1984; Ginther 1992). Indeed, Irvine et al. (1990) showed convincingly that serum progesterone concentrations were coincidentally low in only 1 of 17 instances of spontaneous early fetal death which occurred in a large herd of Standardbred mares. Inadequate luteal function has been observed (Bergfelt and Ginther 1992) in seasonally anoestrous mares stimulated to ovulate by treatment with gonadotrophin releasing hormone (GnRH) but the resulting increase in early embryonic loss rate was apparently related to a lack of gonadotrophic support for luteal function that would not be likely to exist in normal, cycling mares in the breeding season. Shideler et al. (1982) demonstrated that pregnancy could be maintained in ovariectomised mares by administering high doses of progesterone or the orally active synthetic progestagen, altrenogest, at a dose rate of 0.044 mg/kg bwt daily. However, we showed subsequently that another synthetic progestagen, hydroxyprogesterone caproate, would not similarly maintain pregnancy in ovariectomised mares when administered as a single intramuscular (i.m.) injection of 1000 mg according to the manufacturer’s instructions (McKinnon et al. 1 9 9 3 ) . Furthermore, in a parallel study, Nobelius (1992) demonstrated that hydroxyprogesterone caproate exhibits very low binding a ffinity for equine endometrial progesterone receptors. We have extended this line of enquiry in the present study by examining the abilities of 5 commercially available synthetic progestagen preparations, medroxyprogesterone acetate, hydroxyprogesterone hexanoate, norgestomet, megesterol acetate and altrenogest, to maintain pregnancy in mares following treatment with prostaglandin F2α to induce luteolysis and so remove the endogenous source of progesterone.