Myosin heavy chain, myogenic regulatory factors, and steroid hormone receptor responses in serially slaughtered implanted or nonimplanted beef steers

Objective

The objective of this study was to quantify changes in beef steers in myosin-heavy-chain fiber type, myogenic regulatory factors, and steroid hormone receptors over time and in response to implantation in the longissimus dorsi (LD), semitendinosus (ST), and diaphragm (D) muscles.

Materials and Methods

Eighty beef steers were paired based on estimated final BW, and individuals received no implant (CON) or Revalor-XS (Merck Animal Health) administered at d 0 and 190 (IMP). Four pairs of steers were randomly assigned to slaughter group at 42-d intervals from d 0 to 378. At slaughter, samples were collected from the LD, ST, and D muscles of each animal for immunohistochemical analysis.

Results and Discussion

Proportions of cross-sectional area (CSA) occupied by each myosin-heavy-chain type were not different between implant treatments. Increased days on feed led to increased CSA in all muscles. Total nuclei density tended to be greater in CON steers. Myogenic regulatory factors expressed as a proportion of total nuclei led to no difference due to implant treatment in all variables, except a tendency for increased proportion of myonuclei in the ST of IMP steers. Day influenced all variables in the LD with positive quadratic effects on proportion of myonuclei and MYF5-positive nuclei. In the ST, day affected proportion of myonuclei and PAX7- and MYF5/ PAX7-dual-positive nuclei. All myogenic factors were affected by day in the D. Steroid receptor expression was unaffected by implant status regardless of muscle. Day tended to affect steroid receptor variables in the LD, ST, and D, with varying trends by muscle and variable.

Implications and Applications

Implant treatment did not alter the proportion of muscle fiber types, myogenic outcomes relative to total nuclei, or steroid hormone receptors. However, implant administration increased overall CSA by 9% to 16%, suggesting enhanced muscle growth without affecting muscle composition or regulatory mechanisms.