Electrophysiological and molecular identification of voltage‐gated sodium channels in murine vascular myocytes

A voltage‐gated Na + current was characterised in freshly dissociated mouse portal vein (PV) smooth muscle myocytes. The current was found superimposed upon the relatively slow L‐type Ca 2 + current and was resistant to conventional Ca 2 + channel blockers but was abolished by external Na + replacement and tetrodotoxin (TTX, 1 μ m ). The molecular identity of the channel responsible for this conductance was determined by RT‐PCR where only the transcripts for Na + channel genes SCN7a , 8a and 9a were detected. The presence of the protein counterparts to the SCN8a and 9a genes (NaV 1.6 and NaV 1.7 , respectively) on the individual smooth muscle myocytes were confirmed in immunocytochemistry, which showed diffuse staining around a predominantly plasmalemmal location. TTX inhibited the action potential in individual myocytes generated in the current clamp mode but isometric tissue tension experiments revealed that TTX (1 and 5 μ m ) had no effect on the inherent mouse PV rhythmicity. However, the Na + channel opener veratridine (10 and 50 μ m ) significantly increased the length of contraction and the interval between contractions. This effect was not influenced by pre‐incubation with atropine, prazosin and propranolol, but was reversed by TTX (1 μ m ) and completely abolished by nicardipine (1 μ m ). Furthermore, preincubation with the reverse‐mode Na + –Ca 2 + exchange blocker KB‐R7943 (10 μ m ) also inhibited the veratridine response. We have established for the first time the molecular identity of the voltage‐gated Na + channel in freshly dispersed smooth muscle cells and have shown that these channels can modulate contractility through a novel mechanism of action possibly involving reverse mode Na + –Ca 2 + exchange.