Chronic Clenbuterol Administration Alters Myosin Heavy Chain Composition in Standardbred Mares

The purpose of this study was to examine changes in myosin heavy chain (MHC) composition due to chronic clenbuterol administration with or without exercise in mares. Unfit Standardbred mares (aged 10±3 years) were divided into four groups: clenbuterol (2.4 μ/kg BW twice daily) plus exercise (3 days/week for 20 min at 50% VO2max; CLENEX; n=6), clenbuterol only (CLEN; n=6), exercise only (EX; n=5), and control (CON; n=6). Muscle biopsies were obtained from gluteus medius muscle before and after the eight-week training/administration period. MHC composition was determined via SDS gel electrophoresis and quantified using a scanning and densometric system. and CLEN exhibited significant (P<0.05) MHC changes while EX and CON did not. MHC type IIA decreased (29.8±6.1 to 19.3±4.0%, CLENEX; and 36.8±12.4 to 26.4±7.9%, CLEN) and MHC type IIX increased (59.4±7.2 to 71.8±5.8%, CLENEX; and 50.5±12.5 to 62.0±9.3%, CLEN). Chronic clenbuterol administration with and without exercise resulted in a significant shift in MHC profile in Standardbred mares.