Biophysical mechanisms underlying tefluthrin-induced modulation of gating changes and resurgent current generation in...

Human skeletal muscle contraction is triggered by activation of Na_v1.4 channels. Na_v1.4 channels can generate resurgent currents by channel reopening at hyperpolarized potentials through a gating transition dependent on the intracellular Na_vβ4 peptide in the physiological conditions. Tefluthrin (TEF) is a pyrethroid insecticide that can disrupt electrical signaling in nerves and skeletal muscle, resulting in seizures, muscle spasms, fasciculations, and mental confusion. TEF can also induce tail currents through other voltage-gated sodium channels in the absence of Na_vβ4 peptide, suggesting that muscle spasms may be caused by resurgent currents. Further, intracellular Na_vβ4 peptide and extracellular TEF may show competitive or synergistic effects; however, their binding sites are still unknown. To address these issues, electrophysiological recordings were performed on CHO-K1 cells expressing Na_v1.4 channels with intracellular Na_vβ4 peptide, extracellular TEF, or both. TEF and Na_vβ4 peptide induced a hyperpolarizing shift of activation and inactivation curves in the Na_v1.4 channel. TEF also substantially prolonged the inactivation time constants, while simultaneous application of Na_vβ4 peptide partially reversed this effect. Resurgent currents were enhanced by TEF and Na_vβ4 peptide at negative potentials, but TEF more potently enhances resurgent currents and dampens decay of resurgent currents. With longer depolarization, peak resurgent currents decay was fastest with the TEF alone. Molecular docking suggested that TEF and Na_vβ4 peptide binding site(s) are not in the narrowest part of the channel pore, but rather in the bundle-crossing regions and in the domain linkers, respectively. TEF can induce resurgent currents independently and synergistically with Na_vβ4 peptide, which may explain the muscle spasms observed in TEF intoxication.