The Q270K mutation of the skeletal muscle Na(+) channel alpha subunit (Nav1.4) causes atypical paramyotonia with a striking sensitivity to cold. Attacks of paralysis and a drop in the compound muscle action potential (CMAP) are exclusively observed at cold. To understand the pathogenic process, we studied the consequences of this mutation on channel gating at different temperatures. WT or Q270K recombinant Nav1.4 channels fused at their C-terminal end to the enhanced green fluorescent protein (EGFP) were expressed in HEK-293 cells. Whole-cell Na(+) currents were recorded using the patch clamp technique to examine channel gating at 30 degrees C and after cooling the bathing solution to 20 degrees C. Mutant channel fast inactivation was impaired at both temperatures. Cooling slowed the kinetics and enhanced steady-state fast inactivation of both mutant and WT channels. Mutant channel slow inactivation was fairly comparable to that of the WT at 30 degrees C, but became clearly abnormal at 20 degrees C. Cooling enhanced slow inactivation in the WT by shifting the voltage dependence toward hyperpolarization, but induced the opposite effect in the mutant. Destabilization of mutant channel slow inactivation in combination with defective fast inactivation is expected to increase the susceptibility to prolonged membrane depolarization, and can ultimately lead to membrane inexcitability and paralysis at cold. Thus, abnormal temperature sensitivity of slow inactivation can be a determinant pathogenic factor, and should therefore be more widely considered in thermosensitive Na(+) channelopathies.
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