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

The effects of clenbuterol₂-agonist administration wereinvestigated in normal and atrophied [15-day hindlimb-unloaded(HU)] rat soleus muscles. We showed that clenbuterol had aspecific effect on muscle tissue, since it reduces soleus atrophyinduced by HU. The study ofCa²⁺ activation propertiesof single skinned fibers revealed that clenbuterol partly prevented thedecrease in maximal tension after HU, with a preferential effecton fast-twitch fibers. Clenbuterol improved theCa²⁺ sensitivity in slow- andfast-twitch fibers by shifting the tension-pCa relationship towardlower Ca²⁺ concentrations, butthis effect was more marked after HU than in normal conditions. Wholemuscle electrophoresis indicated slow-to-fast transitions of the myosinheavy chain isoforms for unloaded and for clenbuterol-treated soleus.The coupling of the two latter conditions did not, however, increasethese phenotypical transformations. Our findings indicated thatclenbuterol had an anabolic action and a₂-adrenergic effect onmuscle fibers and appeared to counteract some effects of unloadingdisuse conditions.