Subcellular heterogeneity of sodium current properties in adult cardiac ventricular myocytes

Background

Sodium channel α-subunits in ventricular myocytes (VMs) segregate either to the intercalated disc or to lateral membranes, where they associate with region-specific molecules.

Objective

To determine the functional properties of sodium channels as a function of their location in the cell.

Methods

Local sodium currents were recorded from adult rodent VMs and Purkinje cells by using the cell-attached macropatch configuration. Electrodes were placed either in the cell midsection (M) or at the cell end (area originally occupied by the intercalated disc [ID]). Channels were identified as tetrodotoxin (TTX)-sensitive (TTX-S) or TTX-resistant (TTX-R) by application of 100 nM of TTX.

Results

Average peak current amplitude was larger in ID than in M and largest at the site of contact between attached cells. TTX-S channels were found only in the M region of VMs and not in Purkinje myocytes. TTX-R channels were found in both M and ID regions, but their biophysical properties differed depending on recording location. Sodium current in rat VMs was upregulated by tumor necrosis factor-alpha. The magnitude of current increase was largest in the M region, but this difference was abolished by application of 100 nM of TTX.

Conclusions

Our data suggest that (a) a large fraction of TTX-R (likely Na_v1.5) channels in the M region of VMs are inactivated at normal resting potential, leaving most of the burden of excitation to TTX-R channels in the ID region; (b) cell–cell adhesion increases functional channel density at the ID; and (c) TTX-S (likely non-Na_v1.5) channels make a minimal contribution to sodium current under control conditions, but they represent a functional reserve that can be upregulated by exogenous factors.

Keywords: Sodium current, Intercalated disc, Nav1.5

Abbreviations: ID, intercalated disc, I_Na, sodium current, TNF-α, tumor necrosis factor-alpha, TTX, tetrodotoxin, TTX-R, TTX-resistant, TTX-S, TTX-sensitive, VGSCs, voltage-gated sodium channels