No apparent requirement for neuronal sodium channels in excitation-contraction coupling in rat ventricular myocytes

The majority of Na channels in the heart are composed of the tetrodotoxin (TTX)-resistant (K-D, 2 to 6 mu mol/L) "cardiac" Na(V)1.5 isoform; however, TTX-sensitive (K-D, 1 to 25 nmol/L) "neuronal" Na channel isoforms have recently been detected in several cardiac preparations. In the present study, we determined the functional subcellular localization of Na channel isoforms (according to their TTX sensitivity) in rat ventricular myocytes by recording I-Na in control and detubulated myocytes. We found that TTX-sensitive I-Na (K-D, approximate to 8.8 nmol/L) makes up 14 +/- 3% of total I-Na in control and <= 4% in detubulated myocytes and calculated that approximate to 80% of TTX-sensitive I-Na is located in the t-tubules, where it generates approximate to 1/3 of t-tubular I-Na. In contrast, TTX-resistant I-Na is located predominantly (approximate to 78%) at the surface membrane. We also investigated the possible contribution of TTX-sensitive I-Na to excitation-contraction coupling, using 200 nmol/L TTX to selectively block TTX-sensitive I-Na. TTX decreased the rate of depolarization of the action potential by 10% but did not delay the rise of systolic Ca2+ in the center of the cell (transverse confocal line scan), suggesting that TTX-sensitive I-Na does not play a role in synchronizing Ca2+ release at the t-tubules; the amplitude of the Ca2+ transient and contraction were also unchanged by 200 nmol/L TTX. The quantity of charge entering via I-Ca elicited by control or TTX action potential waveforms was similar, suggesting that the trigger for Ca2+ release is not altered by blocking TTX-sensitive I-Na. We conclude that neuronal I-Na is concentrated at the t-tubules, but there is no evidence of a requirement for these channels in normal excitation-contraction coupling in ventricular myocytes.