In isolated rat extensor digitorum longus (EDL) muscle mounted for isometric contractions, chronic low-frequency electrical stimulation was found to lead to an increased uptake of ⁴⁵Ca (154% above control after 240 min) and a progressive accumulation of Ca²⁺ (85% above control after 240 min). In soleus, however, this treatment led to a small, but significant, increase in ⁴⁵Ca uptake (30% above control after 180 min) but no significant accumulation of Ca²⁺. In muscles mounted for isotonic contractions without any external load, electrical stimulation gave rise to a larger ⁴⁵Ca uptake and accumulation of Ca²⁺ in both EDL and soleus. These uptakes of Ca²⁺ coincided with an accumulation of Na⁺. During isometric or isotonic contractions, stimulation at 40 Hz increased the initial (60 s) rate of ⁴⁵Ca uptake in soleus muscle 15- and 30-fold, respectively. The stimulation-induced increase in ⁴⁵Ca uptake was only reduced by 17% by the Ca²⁺-channel blockers nifedipine and verapamil but was blocked by tetrodotoxin. The initial rate of stimulation-induced ²²Na and ⁴⁵Ca uptake was correlated (r = 0.80; P < 0.003). Stimulation of Na⁺ channels with veratridine increased ⁴⁵Ca uptake by 93 and 139% in soleus and EDL, respectively (P < 0.001), effects that were abolished by tetrodotoxin. The results indicate that in skeletal muscle, excitation induces a considerable influx of Ca²⁺, mediated by Na⁺ channels.

soleus; extensor digitorum longus; ⁴⁵Ca uptake; electrical stimulation; Na⁺ channels

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