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Department of Physiology, University of Aarhus, DK-8000 Århus C, Denmark
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 45Ca uptake 34- and 21-fold and 22Na 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+ 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.
lactic acid dehydrogenase release; electrical stimulation; Na+ channels; muscle damage; extensor digitorum longus
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