Patch-clamp experiments were performed to investigate the behavior of voltage-activated inward currents in vas deferens myocytes from Na V1.6-null mice (NaV1.6-/-) lacking the expression of the Na+ channel gene, Scn8a, and their wild-type littermates (NaV1.6+/+). Immunohistochemistry confirmed expression of NaV1.6 in the muscle of NaV1.6 +/+, but not NaV1.6-/-, vas deferens. PCR analysis revealed that the only β1-subunit gene expressed in NaV1.6+/+ vas deferens was Scn / b. In Na V1.6+/+ myocytes, the threshold for membrane currents evoked by 20 msec voltage ramps (-100mV to 60 mV) was -38.5±4.6mV and this was shifted to a more positive potential (-31.2±4.9 mV) by tetrodotoxin (TTX). In NaV1.6-/- myocytes, the threshold was -30.4±3.4mV and there was no TTX-sensitive current. The Na + current (INa) in NaV1.6+/+ 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 INa. INa 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), INa declined at stimulation frequencies > 0.5 Hz and at 10 Hz≤50% of the current remained. These findings indicate that INa is due solely to NaV1.6 in NaV1.6+/+ 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.
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