Effect of Active Immunization against Estrogen on Gonadotropin Response to Testosterone Propionate Treatment in Ovariectomized Pony Mares

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Abstract
An experiment was conducted to determine whether partial neutralization of estrogens via active immunization alters testosterone propionate (TP)-induced increases in FSH secretion after GnRH administration in ovariectomized pony mares. Twenty mares were used in a 2 × 2 factorial arrangement of treatments (n = 5/group). Factor 1 was long-term active immunization against either bovine serum albumin (BSA) or estrone-17-oxime-BSA. Factor 2 was 11-d administration of either vehicle (vegetable oil) or TP (175 µg/kg BW). Plasma concentrations of FSH were not affected (P > .1) by either factor. As expected, the FSH response to exogenous GnRH was threefold greater (P < .05) in BSA-immunized mares treated with TP than in BSA-immunized mares receiving oil. However, immunization against estrogens reduced (P < .05) this TP-induced increase in FSH response by 52%. Plasma concentrations of LH were decreased (P < .08) by TP; this effect was not altered (P > .1) by immunization against estrogen. The LH response to exogenous GnRH was not affected (P > . 1) by either factor. We conclude that aromatization of testosterone to estrogen is partially responsible for the increased FSH response to exogenous GnRH in TP-treated mares. In contrast, suppression of LH concentrations by TP appears to involve only the androgenic effect of TP. Copyright © 1989. American Society of Animal Science . Copyright 1989 by American Society of Animal Science

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