Effect of clenbuterol on sarcoplasmic reticulum function in single skinned mammalian skeletal muscle fibers

We examined the effect of the beta(2)-agonist clenbuterol (50 mu M) on depolarization-induced force responses and sarcoplasmic reticulum (SR) function in muscle fibers of the rat (Rattus norvegicus; killed by halothane overdose) that had been mechanically skinned, rendering the beta 2-agonist pathway inoperable. Clenbuterol decreased the peak of depolarization-induced force responses in the extensor digitorum longus (EDL) and soleus fibers to 77.2 +/- 9.0 and 55.6 +/- 5.4%, respectively of controls. The soleus fibers did not recover. Clenbuterol significantly and reversibly reduced SR Ca2+-loading in EDL and soleus fibers to 81.5 +/- 2.8 and 78.7 +/- 4.0%, respectively, of controls. Clenbuterol also produced an similar to 25% increase in passive leak of Ca2+ from the SR of the EDL and soleus fibers. These results indicate that clenbuterol has direct effects on fast-and slow-twitch skeletal muscle, in the absence of the beta(2)-agonist pathway. The increased Ca2+ leak in the triad region may lead to excitation-contraction coupling damage in the soleus fibers and could also contribute to the anabolic effect of clenbuterol in vivo.