LONG-TERM EFFECTS OF CLENBUTEROL ON DIAPHRAGM MORPHOLOGY AND CONTRACTILE PROPERTIES IN EMPHYSEMATOUS HAMSTERS

The aim of the present study was to investigate the effect of chronic long-term clenbuterol treatment (1 mg/kg subcutaneously twice a day fo r 12 wk) on diaphragm morphology and function in emphysematous (EH) an d normal hamsters (NH). Clenbuterol increased body weight, diaphragm w eight, and skeletal muscle weight in both EH and NH to a similar exten t. In the diaphragm, clenbuterol significantly increased myosin heavy chain type I, IIa, and IIx muscle fiber cross-sectional areas by simil ar to 35-55% in both EH and NH. This response to clenbuterol treatment was not significantly different between EH and NH diaphragm. In EH, t witch force (P-t), maximal tetanic force, and force-frequency curve we re significantly reduced compared with NH. In EH, clenbuterol increase d P-t by similar to 10%, restoring P-t to NH level. A similar improvem ent was observed in the force-frequency characteristics. Clenbuterol d id not alter contractile properties in NH. In conclusion, long-term cl enbuterol treatment resulted in an increased size of all diaphragm mus cle fiber types in both NH and EH. Clenbuterol completely abolished th e reduced force generation induced by emphysema.