Biophysical mechanisms underlying tefluthrin-induced modulation of gating changes and resurgent current generation in the human Nav1.4 channel.

ABSTRACT

Human skeletal muscle contraction is triggered by activation of Nav1.4 channels. Nav1.4 channels can generate resurgent currents by channel reopening at hyperpolarized potentials through a gating transition dependent on the intracellular Navß4 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 Navß4 peptide, suggesting that muscle spasms may be caused by resurgent currents. Further, intracellular Navß4 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 Nav1.4 channels with intracellular Navß4 peptide, extracellular TEF, or both. TEF and Navß4 peptide induced a hyperpolarizing shift of activation and inactivation curves in the Nav1.4 channel. TEF also substantially prolonged the inactivation time constants, while simultaneous application of Navß4 peptide partially reversed this effect. Resurgent currents were enhanced by TEF and Navß4 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 Navß4 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 Navß4 peptide, which may explain the muscle spasms observed in TEF intoxication.