Voltage-gated sodium channels isolated from mammalian tissue are composed of the pore-forming α- and auxiliary β-subunits. However, the composition of sodium channels in cardiac muscle has not been defined completely and uncertainty exists as to which α- and β-subunits co-localize at the cellular level in cardiac myocytes.

Using immunocytochemistry and confocal microscopy in isolated adult mouse cardiomyocytes (from humanely killed animals), we investigated the isoform-specific localization of sodium channel α- and auxiliary β-subunits in relation to marker proteins. Anti-Connexin 43 and anti-α-actinin antibodies were used to identify the region of the intercalated disks and the t-tubules, respectively. We find that, in addition to the cardiac α-isoform (Nav1.5), three major brain isoforms, Nav1.1, Nav1.3 and Nav1.6, are also present in ventricular myocytes (n > 10). These isoforms are localized differently within single cardiomyocytes: Nav1.5 is found at the intercalated discs, whereas staining for Nav1.1, Nav1.3 and Nav1.6 is striated, suggesting that it is within the transverse tubules at the z-lines. These locations were confirmed as α-actinin was colocalized with the expressed brain-type sodium channels, whereas connexin 43 staining pattern overlapped that of Nav1.5. The brain isoform, Nav1.2, could not be detected in our preparation (n > 10). All known auxiliary β-subunits (β1, β2, β3) were also found in adult mouse ventricular myocytes (n > 10), and these too displayed differential localisation on the myocytes. β1 and β3 colocalize in the t-tubules with α-actinin, whereas β2 is present at the intercalated disks, colocalized with connexin 43 (n > 10).

In summary, we show that the brain-type α-subunits colocalize with β1 and β3 in the transverse tubular system, and Nav1.5 with β2 in the intercalated disks. Our results suggest that the primary sodium channel complexes present in ventricular myocytes consist of Nav1.5 α associated with β2 subunits in the intercalated disks and Nav1.1, Nav1.3 and Nav1.6 associated with β1 or β3 subunits in the t-tubules.

All procedures accord with current National and local guidelines.