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 IG F-I and insulinlike growth factor binding protein (IGFBP) concentratio ns due to administration of a combined trenbolone acetate (TBA) and es tradiol (Ea) implant. This study was part of a larger serial slaughter study in which 64 large-framed (394.1 kg) crossbred steers were rando mly assigned to one of four pens. Pens were assigned to one of two tre atments: implanted (120 mg of TBA and 24 mg of E(2)) and nonimplanted. After d 2, 24 steers/treatment remained on the study. These steers we re 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 re maining steers. Serum was harvested and analyzed for IGF-I, IGFBP, and mitogenic activity. Glycyl-glycine (GG) extraction of serum was perfo rmed to reduce IGFBP interference in the IGF-I RIA. Implantation with TBA+E(2) 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 +E(2) had 16 and 22%, respectively, greater (P < .001) circulating IGF -I concentrations than nonimplanted steers. For steers in the study fo r at least 115 d, TBA+E(2) increased (P < .05) IGF-I concentrations 9, 13, and 19% on d 21, 40, and 115, respectively, compared with nonimpl anted steers. Implantation with TBA+E(2) resulted in greater (P < .05) serum concentration of a 43/39-kDa IGFBP (IGFBP-3) on d 21 and 40 aft er implantation. Sera from steers implanted with TBA+E(2) stimulated p roliferation of cultured muscle satellite cells to a greater extent (P < .05) than did sera from nonimplanted steers on d 21, 40, 115, and 1 43 after implantation. In summary, TBA+E(2) increased serum concentrat ions of both IGF-I and IGFBP-3. Additionally, implantation increased m itogenic activity of sera from implanted as compared to nonimplanted s teers. These alterations may be partially responsible for the positive effects of TBA+E(2) implants on feedlot performance and rate of prote in accretion in steers.