Abstract

Mammalian voltage-gated Na⁺ channels were less sensitive to pyrethroids than their insect counterparts by 2 to 3 orders of magnitude. Deltamethrin at 10 μM elicited weak gating changes in rat skeletal muscle α-subunit Na⁺ channels (Nav1.4) after > 30 min of perfusion. About 10% of the peak current was maintained during the 8-ms, +50-mV pulse and, upon repolarization to −140 mV, the amplitude of the slow tail current corresponded to less than 3% of total Na⁺ channels modified by deltamethrin. A background mutation, Nav1.4-I687M (within D2:S4-S5 cytoplasmic linker), enhanced the deltamethrin-induced maintained current by ∼2.5-fold, whereas Nav1.4-I687T, a homologous superkdr mutation, reduced it by ∼2-fold. Repetitive pulses at 2 Hz further augmented the effects of deltamethrin on Nav1.4-I687M mutant channels so that ∼75% of peak currents were maintained. A second mutation, Nav1.4-I687M/F1278I at the middle of D3-S6, rendered the channel greatly resistant to deltamethrin. This double mutant channel remained sensitive to batrachotoxin (BTX), even though nearby residues S1276 and L1280 were critical for BTX action. We hypothesize that the deltamethrin receptor and the BTX receptor are situated at the middle but opposite surface of the D3-S6 α-helical structure. Another mutant, Nav1.4-I687M/N784K, exhibited a partial deltamethrin-resistant phenotype but was completely resistant to BTX. Consistent with the BTX-resistant phenotype of N784K and the known adjacentkdr mutation at position L785F, deltamethrin and BTX were probably situated next to each other upon binding at D2-S6. Evidently, distinct residues from multiple S6 segments were critical for deltamethrin and BTX actions.