Voltage-gated sodium channels (VGSCs) generate the Na+ current (INa) responsible for the cardiac action potential upstroke largely due to the 'cardiac' isoform Nav1.5, but with ˜20% of the upstroke and a component of a sustained sodium current, attributed to other isoforms. This sustained component has been attributed with increasing arrhythmia risk and has been the target of development for new anti-arrhythmic drugs, targeted at ‘neuronal’ isoforms. Distinctive distribution patterns of Nav isoforms 1.1–1.6 have been demonstrated in human atrial myocardium,1 suggesting differing isoforms have spatially distinct roles; however little is known regarding the expression of the neuronal isoforms Nav1.7, Nav1.8 and Nav1.9 in the heart. Our study has investigated the subcellular distribution patterns of VGSC alpha-subunits Nav1.1–1.9 in the rat heart.
Rats ˜300 g were sacrificed and ventricular tissue or single myocytes isolated. Immunocytochemistry showed the location of VGSCs within single cardiac myocytes or tissue sections using primary VGSCs antibodies (Alomone, Israel) bound to secondary antibodies conjugated to Alexa 488 (Molecular probes, UK), and cellular membranes labelled by wheat germ agglutinin conjugated to rhodamine (Vector, UK), viewed by confocal microscopy (Zeiss 710, Germany).
The dominant cardiac isoform, Nav1.5, was expressed at the sarcolemma membrane, intercalated disk and t-tubular membrane, whereas the skeletal muscle isoform Nav1.4 was detected at intercalated disks as an intense punctate pattern. Nav1.3 was not detected.2 The neuronal-associated isoforms Nav1.1, Nav1.2 and Nav1.6 positively labelled rat brain sections; whereas in cardiac tissue Nav1.1 showed faint punctate labelling,1 Nav1.2 showed t-tubular staining, and Nav1.6 showed intercalated disk, faint t-tubular expression as well as nuclear labelling.
As reported by others,3,4 sensory neurons from rat dorsal root ganglia labelled positive for Nav1.7, 1.8 and 1.9. In heart tissue and cells; Nav1.8 protein expression was a striated pattern throughout the cell, whereas Nav1.9 protein was found at the intercalated disks, t-tubules and nuclei. Nav1.7 expression was not observed in cardiac tissue or myocytes.
We conclude that in addition to Nav1.5, other sodium channels are expressed in the rat heart (Nav1.1, Nav1.2, Nav1.4, Nav1.6, Nav1.7 and Nav1.8) and may play distinct roles in the cardiac action potential, however these are yet undetermined.
Westenbroek K, Lange M, Wischmeyer R, Muck B, Catterall E, Maier S. JMCC 2013;61:133–41
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- Sodium Channel
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