Macroscopic current from the μI skeletal muscle sodium channel expressed in Xenopus oocytes shows inactivation with two exponential components. The major, slower component's amplitude decreases with rapid pulsing. When μI cRNA is coinjected with rat skeletal muscle or brain mRNA the faster component becomes predominant. Individual μI channels switch between two principal gating modes, opening either only once per depolarization, or repeatedly in long bursts. These two modes differ in both activation and inactivation kinetics. There is also evidence for additional gating modes. It appears that the equilibrium among gating modes is influenced by a modulating factor encoded in rat rat skeletal muscle and brain mRNA. The modal gating is similar to that observed in hyperkalemic periodic paralysis.
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