In cardiac ventricular myocytes, Na current is generated mainly by the cardiac Na(V)1.5 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. I-Na was recorded using the patch clamp technique in control and detubulated myocytes. Detubulation reduced cell capacitance (by similar to 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 I-Na 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: similar to 11% of total sarcolemmal current, similar to 3% of cell surface, and similar to 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/mu m(2) : similar to 11 in total sarcolemma, similar to 13 at the cell surface, similar to 10 at the t-tubules). In contrast, neuronal Na channel isoforms are concentrated at the t-tubules (in channels/mu m(2) : similar to 1 in total sarcolernma, similar to 0.3 at the cell surface, similar to 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 (similar to 13 channels/mu m(2)). (c) 2006 Elsevier Inc. All rights reserved.