The effects of equine somatotropin on pituitary and testicular function in the stallion during the nonbreeding season

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An experiment was conducted to determine the effects of equine somatotropin on the reproductive axis of the stallion during the nonbreeding season. Adult stallions were treated with equine somatotropin (20 μg/kg body weight [BW]; n = 5) or saline (n = 4) daily for 21 days starting in January. During the last week of treatment, stallions were subjected to low- and high-dose injections of luteinizing hormone (LH), as well as low- and high-dose injections of gonadotropin-releasing hormone (GnRH) and thyrotropin-releasing hormone (TRH). Two months after the onset of somatotropin treatment, semen was collected from all stallions every other day for 14 days. Treatment with equine somatotropin increased (P < .001) daily IGF-1 concentrations but had no effect (P > .1) on concentrations of LH, follicle-stimulating hormone (FSH), or testosterone. The testosterone responses to injections of LH were similar (P > .1) between treatments. Likewise, the LH, FSH, prolactin, and testosterone responses to the injections of GnRH/TRH were similar (P > .1) between groups. At seminal collections, stallions treated with somatotropin exhibited greater volumes of gel-free semen (P < .01) and gel (P < .05) and had decreased time until ejaculation (P < .05). In conclusion, somatotropin treatment for 21 days may alter the long-term accessory gland contribution to seminal volume but does not appear to alter pituitary gonadotrope function or testicular testosterone secretion. © 2005 Elsevier Inc. All rights reserved.

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Journal of Equine Veterinary Science

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