High-frequency fatigue of skeletal muscle: role of extracellular Ca²⁺

The present study evaluated whether Ca²⁺ entry operates during fatigue of skeletal muscle. The involvement of different skeletal muscle membrane calcium channels and of the Na⁺/Ca²⁺ exchanger (NCX) has been examined. The decline of force was analysed in vitro in mouse soleus and EDL muscles submitted to 60 and 110 Hz continuous stimulation, respectively. Stimulation with this high-frequency fatigue (HFF) protocol, in Ca²⁺-free conditions, caused in soleus muscle a dramatic increase of fatigue, while in the presence of high Ca²⁺ fatigue was reduced. In EDL muscle, HFF was not affected by external Ca²⁺ levels either way, suggesting that external Ca²⁺ plays a general protective role only in soleus. Calciseptine, a specific antagonist of the cardiac isoform (α1C) of the dihydropyridine receptor, gadolinium, a blocker of both stretch-activated and store-operated Ca²⁺ channels, as well as inhibitors of P2X receptors did not affect the development of HFF. Conversely, the Ca²⁺ ionophore A23187 increased the protective action of extracellular Ca²⁺. KB-R7943, a selective inhibitor of the reverse mode of NCX, produced an effect similar to that of Ca²⁺-free solution. These results indicate that a transmembrane Ca²⁺ influx, mainly through NCX, may play a protective role during HFF development in soleus muscle.