Intracellular Na+ and K+ shifts induced by contractile activities of rat skeletal muscles

The effects of direct and indirect electrical stimulation on intracellular potassium and sodium contents (K]_i and Na]_i, respectively) in rat soleus muscle (SOL) and extensor digitorum longus muscle (EDL) were investigated under in vivo conditions. The changes of K]_i and Na]_i contents in both muscles which were stimulated indirectly reached respective values at 30 min or 1 hr after the beginning of stimulation, whereas those of EDL stimulated with 60 Hz changed gradually through 2 hr stimulation. The shifts of K]_i and Na]_i in EDL occurred during the twitch contraction at considerably lower frequency stimulation (0.5–10 Hz), whereas those in SOL were observed during the tetanus contraction at high frequency stimulation (10–40 Hz). The difference of change in cationic shifts between EDL and SOL under low frequency stimulation was reduced by ouabain treatment, though the difference was still significant. When the muscles were indirectly stimulated 6000 times at 1,5,10 and 20 Hz, the cationic shifts in EDL were greater than those in SOL, extending over all frequencies. It was concluded that such a difference in ionic shift between contracting EDL and SOL may be primarily due to the difference in unidirectional ionic fluxes per stimulation and, secondly, to the difference in Na⁺-K⁺ pump activity.