Effects of dihydrotestosterone administration with and without estradiol pretreatment on gonadotropin secretion in ovariectomized pony mares

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Twenty ovariectomized pony mares were used to determine if dihydrotestosterone propionate (DHTP) administration, with or without estradiol benzoate (EB) pretreatment, would have the same effects on follicle stimulating hormone (FSH) and luteinizing hormone (LH) secretion as testosterone propionate (TP) administration. All mares were given an initial injection of gonadotropin releasing hormone (GnRH) to characterize their LH and FSH response, and then two groups of mares (n = 4/group) were administered EB (22 μg/kg of body weight), two groups were administered vehicle (safflower oil) and a fifth group was administered TP (175 μg/kg of body weight) daily for 10 days. Following a second injection of GnRH, one group of EB-treated mares and one group of oil-treated mares were administered DHTP (175 μg/kg of body weight) daily for 10 days; the other EB- and oil-treated mares were administered oil and the TP-treated mares were continued on the same dose of TP for 10 days. A final injection of GnRH was then given. Treatment with EB increased (P<.01) concentrations of LH in daily blood samples and increased (P<.05) the LH response to exogenous GnRH. Administration of TP or DHTP reduced (P<.05) both daily LH concentrations and the LH response to exogenous GnRH. Concentrations of FSH in daily blood samples were reduced (P<.05) and the FSH response to exogenous GnRH was increased (P<.05) by administration of EB alone, DHTP alone or TP. Moreover, treatment with EB followed by DHTP resulted in a 12-fold increase in FSH response to exogenous GnRH that was greater (P<.05) than the individual treatments. We conclude that 1) unlike previous results with DHT benzoate, DHTP alone affects LH and FSH secretion in the ovariectomized mare in a manner similar to TP and 2) the stimulatory effects of EB and DHTP on FSH response to exogenous GnRH are additive. © 1987.

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Domestic Animal Endocrinology

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