Comparison between slow sodium channel inactivation in rat slow‐ and fast‐twitch muscle.

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Abstract
1. Voltage‐clamp Na+ currents (INa) were studied in rat soleus slow‐twitch muscle fibres at about 18 degrees C using the loose‐patch‐clamp technique. The maximum inward current density was produced by depolarizations to about ‐19 mV. 2. Fast inactivation was studied utilizing 20 ms conditioning potentials. INa was reduced by 50% with conditioning potentials to about ‐70 mV. 3. Changes in the conditioning membrane potential produced slow changes in the peak INa due to a slow inactivation process. INa was reduced by 50% at about ‐86 mV due to slow inactivation. 4. The mean maximum inward INa when slow inactivation was fully removed was 6.83 mA cm‐2. 5. Due to the slow inactivation process, slow‐twitch fibres were less susceptible to reduction in INa than fast‐twitch fibres.