Effects of the beta(2)-agonist clenbuterol on respiratory and limb musclesof weaning rats

The aim of this study was to analyze the effects of chronic administration of the beta (2)-agonist clenbuterol (1.5 mg . kg(-1) . day(-1) for 4 wk in the drinking water) on respiratory (diaphragm and parasternal intercostal) and hindlimb (tibialis and soleus) muscles in young rats during postnatal d evelopment (21 to 49 postnatal days). The treatment resulted in very little stimulation of muscle growth. Significant slow-to-fast transitions in the expression of myosin heavy chain isoforms and significant increases in the myofibrillar ATPase activity were found in the diaphragm and soleus, wherea s tibialis anterior and intercostal muscles did not show any significant fi ber-type alteration. Decrease of oxidative enzyme activities and increase o f glycolytic enzyme activities were also observed. It is concluded that whe reas the growth stimulation is age dependent and only detectable in adult r ats, the fiber-type transformation is also present in weaning rats and part icularly evident in the soleus and diaphragm. The fiber-type transformation caused by clenbuterol might lead to an enhancement of contractile performa nce and also to a reduced resistance to fatigue.