Functional expression of Na_V1.7 channels in freshly dispersed mouse bronchial smooth muscle cells

Isolated smooth muscle cells (SMCs) from mouse bronchus were studied using the whole cell patch-clamp technique at ∼21°C. Stepping from -100 mV to -20 mV evoked inward currents of mean amplitude -275 pA. These inactivated (tau = 1.1 ms) and were abolished when external Na⁺ was substituted with N-Methyl-d-glucamine. In current-voltage protocols, current peaked at -10 mV and reversed between +20 and +30 mV. The V_1/2s of activation and inactivation were -25 and -86 mV, respectively. The current was highly sensitive to tetrodotoxin (IC₅₀ = 1.5 nM) and the Na_V1.7 subtype-selective blocker, PF-05089771 (IC₅₀ = 8.6 nM), consistent with Na_V1.7 as the underlying pore-forming α subunit. Two Na_V1.7-selective antibodies caused membrane-delineated staining of isolated SMC, as did a nonselective pan-Na_V antibody. RT-PCR, performed on groups of ∼15 isolated SMCs, revealed transcripts for Na_V1.7 in 7/8 samples. Veratridine (30 µM), a nonselective Na_V channel activator, reduced peak current evoked by depolarization but induced a sustained current of 40 pA. Both effects were reversed by tetrodotoxin (100 nM). In tension experiments, veratridine (10 µM) induced contractions that were entirely blocked by atropine (1 µM). However, in the presence of atropine, veratridine was able to modulate the pattern of activity induced by a combination of U-46619 (a thromboxane A2 mimetic) and PGE₂ (prostaglandin E₂), by eliminating bursts in favor of sustained phasic contractions. These effects were readily reversed to control-like activity by tetrodotoxin (100 nM). In conclusion, mouse bronchial SMCs functionally express Na_V1.7 channels that are capable of modulating contractile activity, at least under experimental conditions.