Effects of temperature on slow and fast inactivation of rat skeletal muscle Na1 channels

Ruff, Robert L. Effects of temperature on slow and fast inactivation of rat skeletal muscle Na1 channels. Am. J. Physiol. 277 (Cell Physiol. 46): C937–C947, 1999.—Patchclamp studies of mammalian skeletal muscle Na1 channels are commonly done at subphysiological temperatures, usually room temperature. However, at subphysiological temperatures, most Na1 channels are inactivated at the cell resting potential. This study examined the effects of temperature on fast and slow inactivation of Na1 channels to determine if temperature changed the fraction of Na1 channels that were excitable at resting potential. The loose patch voltage clamp recorded Na1 currents (INa) in vitro at 19, 25, 31, and 37°C from the sarcolemma of rat type IIb fast-twitch omohyoid skeletal muscle fibers. Temperature affected the fraction of Na1 channels that were excitable at the resting potential. At 19°C, only 30% of channels were excitable at the resting potential. In contrast, at 37°C, 93% of Na1 channels were excitable at the resting potential. Temperature did not alter the resting potential or the voltage dependencies of activation or fast inactivation. INa available at the resting potential increased with temperature because the steady-state voltage dependence of slow inactivation shifted in a depolarizing direction with increasing temperature. The membrane potential at which half of the Na1 channels were in the slow inactivated state was shifted by 116 mV at 37°C compared with 19°C. Consequently, the low availability of excitable Na1 channels at subphysiological temperatures resulted from channels being in the slow, inactivated state at the resting potential.