Chronic clenbuterol administration alters myosin heavy chain composition in standardbred mares.

This research article investigates the impact of long-term clenbuterol administration, both on its own and combined with exercise, on the myosin heavy chain (MHC) composition within the muscles of unfit standardbred mares.

Objective and Method

• The main goal of this research was to explore how long-term administration of clenbuterol (a drug commonly used to treat respiratory conditions in horses), either by itself or in conjunction with exercise, can cause alterations in the MHC composition within the muscle tissues of unfit standardbred mares.
• To study this, researchers separated unfit Standardbred mares, aged around 10, into four groups: those given clenbuterol twice daily and exercise thrice a week (CLENEX), those given only clenbuterol (CLEN), those with exercise only (EX), and a control group (CON).
• Muscle biopsies were obtained from the gluteus medius muscle – a large muscle in the horse’s hip area – both before and after an eight-week training and/or drug administration period.

Outcomes and Results

• The composition of MHC in the muscle tissues was determined by SDS gel electrophoresis, a commonly used method in biochemistry to separate proteins, and then quantified using a scanning and densometric system.
• The two clenbuterol groups, CLENEX and CLEN, showed significant (p<0.05) MHC changes whereas the exercise only (EX) and control (CON) groups did not.
• Specifically in the clenbuterol groups, the percentage of MHC type IIA (fast-twitch, fatigue-resistant fibres) decreased, and the percentage of MHC type IIX (fast-twitch, highly fatigable fibres) increased.
• These results indicate that chronic administration of clenbuterol, both with or without exercise, resulted in a substantial shift in MHC profile within the muscles of Standardbred mares.
• This suggests that long-term clenbuterol use could potentially remodel muscle fibre types in horses, which could have considerable implications for the performance and health of such animals.