Stimulation of circulating insulin-like growth factor I (IGF-I) and insulin-like growth factor binding proteins (IGFBP) due to administration of a combined trenbolone acetate and estradiol implant in feedlot cattle.

Objectives of this study were to analyze alterations in circulating IGF-I and insulin-like growth factor binding protein (IGFBP) concentrations due to administration of a combined trenbolone acetate (TBA) and estradiol (E2) implant. This study was part of a larger serial slaughter study in which 64 large-framed (394.1 kg) crossbred steers were randomly assigned to one of four pens. Pens were assigned to one of two treatments: implanted (120 mg of TBA and 24 mg of E2) and nonimplanted. After d 2, 24 steers/treatment remained on the study. These steers were assigned to one of three serial slaughter dates (d 40, 115, and 143). Blood samples were obtained on d 0, 2, 21, 40, 115, and 143 from remaining steers. Serum was harvested and analyzed for IGF-I, IGFBP, and mitogenic activity. Glycyl-glycine (GG) extraction of serum was performed to reduce IGFBP interference in the IGF-I RIA. Implantation with TBA+E2 interference in the IGF-I RIA. Implantation with TBA+E2 increased (P < .001) circulating IGF-I concentrations during the period from d 0 to d 40. On d 21 and 40, steers implanted with TBA+E2 had 16 and 22%, respectively, greater (P < .001) circulating IGF-I concentrations than nonimplanted steers. For steers in the study for at least 115 d, TBA+E2 increased (P < .05) IGF-I concentrations 9, 13, and 19% on d 21, 40, and 115, respectively, compared with nonimplanted steers. Implantation with TBA+E2 resulted in greater (P < .05) serum concentration of a 49/39-kDa IGFBP (IGFBP-3) on d 21 and 40 after implantation. Sera from steers implanted with TBA+E2 stimulated proliferation of cultured muscle satellite cells to a greater extent (P < .05) than did sera from nonimplanted steers on d 21, 40, 115, and 143 after implantation. In summary, TBA+E2 increased serum concentrations of both IGF-I and IGFBP-3. Additionally, implantation increased mitogenic activity of sera from implanted as compared to nonimplanted steers. These alterations may be partially responsible for the positive effects of TBA+E2 implants on feedlot performance and rate of protein accretion in steers.