Effects of beta(2)-agonist clenbuterol on biochemical and contractile properties of unloaded soleus fibers of rat

The effects of clenbuterol beta(2)-agonist administration were investigated in normal and atrophied [15-day hindlimb-unloaded (HU)] rat soleus muscles . We showed that clenbuterol had a specific effect on muscle tissue, since it reduces soleus atrophy induced by HU. The study of Ca2+ activation prope rties of single skinned fibers revealed that clenbuterol partly prevented t he decrease in maximal tension after I-IU, with a preferential effect on fa st-twitch fibers. Clenbuterol improved the Ca2+ sensitivity in slow- and fa st-twitch fibers by shifting the tension-pCa relationship toward lower Ca2 concentrations, but this effect was more marked after HU than in normal co nditions. Whole muscle electrophoresis indicated slow-to-fast transitions o f the myosin heavy chain isoforms for unloaded and for clenbuterol-treated soleus. The coupling of the two latter conditions did not, however, increas e these phenotypical transformations. Our findings indicated that clenbuter ol had an anabolic action and a beta(2)-adrenergic effect on muscle fibers and appeared to counteract some effects of unloading disuse conditions.