Excitation-induced Ca2+ influx in rat soleus and EDL muscle:: mechanisms and effects on cellular integrity

In rat skeletal muscle, electrical stimulation increases Ca2+ influx leading to progressive accumulation of calcium. Excitation-induced Ca2+ influx in extensor digitorum longus (EDL; fast-twitch fibers) and soleus muscle (slow-twitch fibers) is compared. In EDL and soleus, stimulation at 40 Hz increased Ca-45 uptake 34- and 21-fold and Na-22 uptake 17- and 7-fold, respectively. These differences may be related to the measured 70% higher concentration of Na+ channels in EDL. Repeated stimulation at 40 Hz elicited a delayed release of lactic acid dehydrogenase (LDH) from EDL (11-fold increase) and soleus (5-fold increase). Continuous stimulation at 1 Hz increased LDH release only from EDL (18-fold). This was associated with increased Ca2+ content and was augmented at high extracellular Ca2+ concentration ([Ca2+](o)) and suppressed at low [Ca2+](o). The data support the hypothesis that excitation-induced Ca2+ influx is mediated in part by Na 1 channels and that the ensuing increase in intracellular Ca2+ induces cellular damage. This is most pronounced in EDL, which may account for the repeated observation that prolonged exercise leads to preferential damage to fast-twitch fibers.