Long-term effects of clenbuterol on diaphragm morphology and contractile properties in emphysematous hamsters

The aim of thepresent study was to investigate the effect of chronic long-termclenbuterol treatment (1 mg/kg subcutaneously twice a day for 12 wk) ondiaphragm morphology and function in emphysematous (EH) and normalhamsters (NH). Clenbuterol increased body weight, diaphragm weight, andskeletal muscle weight in both EH and NH to a similarextent. In the diaphragm, clenbuterol significantly increased myosin heavy chain type I, IIa, and IIx muscle fiber cross-sectional areas by ~35-55% in both EH and NH. Thisresponse to clenbuterol treatment was not significantly differentbetween EH and NH diaphragm. In EH, twitch force(P_t), maximal tetanic force, andforce-frequency curve were significantly reduced compared with NH. InEH, clenbuterol increased P_t by~10%, restoring P_t to NH level.A similar improvement was observed in the force-frequency characteristics. Clenbuterol did not alter contractile properties inNH. In conclusion, long-term clenbuterol treatment resulted in anincreased size of all diaphragm muscle fiber types in both NH and EH.Clenbuterol completely abolished the reduced force generation inducedby emphysema.