Activation State of Muscle Satellite Cells Isolated from Steers Implanted with a Combined Trenbolone Acetate and Estradiol Implant

Muscle satellite cells were isolated from seven yearling steers implanted for 31 d with a combined implant that contained 120 mg of trenbolone acetate (TEA) and 24 mg of estradiol (E₂) and from seven nonimplanted, control steers. Implanted steers had a 28% greater ADG and a 23% greater feed efficiency than did nonimplanted steers. Implanted steers had increased (P < .001) circulating IGF-I concentrations on d 6, 14, and 31 after implantation, and circulating IGF-I concentrations in control steers remained constant or decreased (P < .05) at these times. Maximum fusion percentage was greater ( P < .005) in satellite cell cultures isolated from implanted steers (ISC cultures) than in satellite cell cultures isolated from control steers (NSC cultures) (72.8% vs 54.8%, respectively). Satellite cell cultures isolated from implanted steers (ISC cultures) also contained a greater ( P < .001) number of myotube nuclei than did NSC cultures (7,998 nuclei/cm² vs 5,150 nuclei/cm², respectively). After 72 h in culture, the number of cells (corrected for plating density) was 43% greater (P < .05) in ISC cultures than in NSC cultures. [³H]Thymidine incorporation rates per 10⁵ cells at 24 and 34 h after plating were greater (P < .05) in ISC cultures than in NSC cultures; however, incorporation rates did not differ at 72 h. These data indicate that TBA + E₂ implantation may result in an in vivo activation of muscle satellite cell proliferation that can be detected in cell culture. This activation may play an important role in TBA + E₂-enhanced muscle growth.