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

Abstract

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 µM). 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 µM) had no effect on the inherent mouse PV rhythmicity. However, the Na + channel opener veratridine (10 and 50 µM) 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 µM) and completely abolished by nicardipine (1 µM). Furthermore, preincubation with the reverse-mode Na + -Ca 2+ exchange blocker KB-R7943 (10 µM) 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.

The cardiac sodium current Nav1.5 is functionally expressed in rabbit bronchial smooth muscle cells

“…Most commonly, the Na V 1.7 subtype has been found, especially in vascular tissue, but its functional role remains unclear (18,21,29). Several studies have pointed to the greater tendency for Na ϩ currents to be expressed in cultured than freshly dispersed cells, leading to suggestions that they participate in cell proliferation or migration (9,27).…”

“…In freshly dispersed smooth muscle cells, VGSC were previously regarded as a rarity, but evidence for their existence in this cell type has been gradually accumulating. In phasic tissues, such as uterine, lymphatic, vas deferens, portal vein, and gastrointestinal smooth muscle, they may play a part in the generation and/or propagation of spontaneous electrical activity underlying myogenic contractions (14,15,29,31,38). However, they have also been found in a variety of tonic arterial smooth muscles (7,9,21,27), although mostly only in cultured cells, suggesting that they are indicative of an altered phenotype (9,27).…”

Mechanism of the persistent sodium current activator veratridine‐evoked Ca2+ elevation: implication for epilepsy

“…The magnitude of [Na + ] i increase that we measured in our experiments was more than 30 mM, and thus should be sufficient for reversing the operation of the NCX. According to the calculation of Bouron and Reuter, based on data taken from the literature and not on their own measurements, a 5 mM increase in [Na + ] i is sufficient for the reversal (Bouron and Reuter 1996) Saleh et al 2005).…”

Cardiac ion channels

“…Expression of TTX-sensitive sodium channels has been described, 2 but the lack of cardiovascular effects of TTX in animals and human victims of accidental TTX poisoning suggests that these channels contribute little to normal cardiac function. 3 Expression of Nav1 channels in smooth muscle cells of the vasculature has been reported, 4 and the sodium channel activator veratridine induces contraction of several types of rodent blood vessels. The contractile effects of veratridine are partly mediated through actions on sympathetic nerves, but may also involve a direct effect on smooth muscle myocytes.…”