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 Ca2+ current and was resistant to conventional Ca2+ channel blockers but was abolished by external Na+ replacement and tetrodotoxin (TTX, 1 microM). 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 (NaV1.6 and NaV1.7, 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 microM) had no effect on the inherent mouse PV rhythmicity. However, the Na+ channel opener veratridine (10 and 50 microM) 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 microM) and completely abolished by nicardipine (1 microM). Furthermore, preincubation with the reverse-mode Na+-Ca2+ exchange blocker KB-R7943 (10 microM) 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+-Ca2+ exchange.
Saleh
Sohag
S
Division of Basic Medical Sciences, St George's, University of London, London SW17 0RE, UK.
Yeung
Shuk Yin M
SY
Prestwich
Sally
S
Pucovsky
Vladimír
V
Greenwood
Iain
I
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002118:Calcium / Q000378:metabolism; D002121:Calcium Channel Blockers / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D005260:Female; D015640:Ion Channel Gating / Q000187:drug effects; D051379:Mice; D008807:Mice, Inbred BALB C; D009119:Muscle Contraction / Q000187:drug effects; D009131:Muscle, Smooth, Vascular / Q000187:drug effects / Q000378:metabolism*; D032389:Myocytes, Smooth Muscle / Q000187:drug effects / Q000378:metabolism*; D062557:NAV1.6 Voltage-Gated Sodium Channel; D062556:NAV1.7 Voltage-Gated Sodium Channel; D009419:Nerve Tissue Proteins / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D009529:Nicardipine / Q000494:pharmacology; D011169:Portal Vein / Q000187:drug effects / Q000502:physiology; D012964:Sodium / Q000378:metabolism; D015222:Sodium Channels / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D019831:Sodium-Calcium Exchanger / Q000037:antagonists & inhibitors / Q000378:metabolism; D013779:Tetrodotoxin / Q000494:pharmacology; D013890:Thiourea / Q000031:analogs & derivatives / Q000494:pharmacology; D014701:Veratridine / Q000494:pharmacology
C101670:2-(2-(4-(4-nitrobenzyloxy)phenyl)ethyl)isothiourea methanesulfonate; D002121:Calcium Channel Blockers; D062557:NAV1.6 Voltage-Gated Sodium Channel; D062556:NAV1.7 Voltage-Gated Sodium Channel; D009419:Nerve Tissue Proteins; C491046:Scn8a protein, mouse; C503479:Scn9a protein, mouse; D015222:Sodium Channels; D019831:Sodium-Calcium Exchanger; D013779:Tetrodotoxin; D014701:Veratridine; D012964:Sodium; D009529:Nicardipine; D013890:Thiourea; D002118:Calcium