Therapeutic clenbuterol treatment does not alter Ca²⁺ sensitivity of permeabilized fast muscle fibres from exercise trained or untrained horses

Abstract

Clenbuterol is a β₂-adrenoceptor agonist primarily used for treating bronchospasm and alleviating the symptoms of chronic obstructive pulmonary disease (COPD) in the horse. In other species (rats, mice, sheep, and cattle), chronic high doses of clenbuterol (typically in the milligram per kilogram body weight range) has been shown to cause a muscle directed protein anabolic response. Clenbuterol can also modify muscle fibre composition and therefore potentially affect muscle function. This has implications for the performance of exercising horses being treated with therapeutic doses of clenbuterol (typically in the microgram per kilogram body weight range) for bronchospasm or COPD. It is not known whether clenbuterol treatment affects muscle fibre function in horses. The purpose of this study was to examine the effects of a therapeutic dose of clenbuterol, with and without exercise, on the contractile activation characteristics of single membrane permeabilized fibres prepared from muscle biopsies. We tested the hypothesis that therapeutic treatment with clenbuterol would not affect muscle fibre function. Unfit Standardbred mares were treated for 8 weeks with; clenbuterol (2.4 μg/kg twice/day, 5 days/week) plus exercise (20 min at 50% $\dot V$O_2max 3 d/wk; CLENEX), clenbuterol only (CLEN), or exercise only (EX). Muscle biopsies were taken from the gluteus medius muscle before and after treatment and stored in a glycerol-based solution to prepare permeabilized muscle fibres. The force–pCa relationship for fibres from CLEN horses was steeper (P < 0.05) indicative of greater cooperative interactions within the thin filament, however, fibre sensitivity to Ca²⁺ was unchanged. In contrast, the steepness of the force–pCa relationship was not changed in fibres from EX and CLENEX horses and Ca²⁺ sensitivity was also unaffected. Rigor force, activation in the absence of ATP, was not affected by any treatment indicating an approximately equivalent number of participating cross-bridges during activation. The results indicate that a therapeutic dose of clenbuterol to Standardbred horses does not affect the Ca²⁺-activated contractile characteristics of isolated muscle fibres.