Characterization of Na V 1.6‐mediated Na + currents in smooth muscle cells isolated from mouse vas deferens

Patch‐clamp experiments were performed to investigate the behavior of voltage‐activated inward currents in vas deferens myocytes from Na V 1.6‐null mice (Na V 1.6 −/− ) lacking the expression of the Na + channel gene, Scn8a, and their wild‐type littermates (Na V 1.6 +/+ ). Immunohistochemistry confirmed expression of Na V 1.6 in the muscle of Na V 1.6 +/+ , but not Na V 1.6 −/− , vas deferens. PCR analysis revealed that the only β 1 ‐subunit gene expressed in Na V 1.6 +/+ vas deferens was Scn1b . In Na V 1.6 +/+ myocytes, the threshold for membrane currents evoked by 20 msec voltage ramps (−100 mV to 60 mV) was −38.5 ± 4.6 mV and this was shifted to a more positive potential (−31.2 ± 4.9 mV) by tetrodotoxin (TTX). In Na V 1.6 −/− myocytes, the threshold was −30.4 ± 3.4 mV and there was no TTX‐sensitive current. The Na + current (I Na ) in Na V 1.6 +/+ myocytes had a bell‐shaped current–voltage relationship that peaked at approximately −10 mV. Increasing the duration of the voltage ramps beyond 20 msec reduced the peak amplitude of I Na . I Na displayed both fast (τ ∼10 msec) and slow (τ ∼1 sec) recovery from inactivation, the magnitude of the slow component increasing with the duration of the conditioning pulse (5–40 msec). During repetitive activation (5–40 msec pulses), I Na declined at stimulation frequencies > 0.5 Hz and at 10 Hz ≤ 50% of the current remained. These findings indicate that I Na is due solely to Na V 1.6 in Na V 1.6 +/+ myocytes. The gating properties of these channels suggest they play a major role in regulating smooth muscle excitability, particularly in response to rapid depolarizing stimuli. J. Cell. Physiol. 223: 234–243, 2010. © 2010 Wiley‐Liss, Inc.