Density and sub-cellular distribution of cardiac and neuronal sodium channel isoforms in rat ventricular myocytes

In cardiac ventricular myocytes, Na current is generated mainly by the cardiac NaV1.5 isoform, but the presence of “neuronal” Na channel isoforms in the heart has been demonstrated recently. In this study, we quantified the density and sub-cellular distribution of cardiac and neuronal channel isoforms in rat ventricular myocytes. INa was recorded using the patch clamp technique in control and detubulated myocytes. Detubulation reduced cell capacitance (by ∼29%) but maximum conductance was not altered (1.94 ± 0.15, 14 control vs 1.98 ± 0.19 nS/pF, 17 detubulated myocytes). The kinetic properties of INa were similar in both cell types suggesting good voltage control of surface and t-tubule membranes. We calculated Na channel densities assuming the sub-cellular current localization we recently provided (neuronal isoform: ∼11% of total sarcolemmal current, ∼3% of cell surface, and ∼31% of t-tubule current). Single channel conductances were assumed to be 2.2 and 2.5 pS for the cardiac and neuronal isoforms, respectively, after accounting for the use of low Na concentration. We calculated that the density of the cardiac Na channel isoform is relatively constant (in channels/μm2: ∼11 in total sarcolemma, ∼13 at the cell surface, ∼10 at the t-tubules). In contrast, neuronal Na channel isoforms are concentrated at the t-tubules (in channels/μm2: ∼1 in total sarcolemma, ∼0.3 at the cell surface, ∼2.5 at the t-tubules). We conclude that, in contrast to skeletal muscle in which Na channel density is higher at the cell surface than the t-tubules, in ventricular cardiac myocytes the sub-cellular distribution of Na channel density is relatively homogeneous (∼13 channels/μm2).