Contribution of sodium channel neuronal isoform Na(v)1.1 to late sodium current in ventricular myocytes from failing hearts

Late Na+ current (I-NaL) contributes to action potential (AP) duration and Ca2+ handling in cardiac cells. Augmented I-NaL was implicated in delayed repolarization and impaired Ca2+ handling in heart failure (HF). We tested if Na+ channel (Na-v) neuronal isoforms contribute to I-NaL and Ca2+ cycling defects in HF in 17 dogs in which HF was achieved via sequential coronary artery embolizations. Six normal dogs served as control. Transient Na+ current (I-NaT) and I-NaL in left ventricular cardiomyocytes (VCMs) were recorded by patch clamp while Ca2+ dynamics was monitored using Fluo-4. Virally delivered short interfering RNA (siRNA) ensured Na(v)1.1 and Na(v)1.5 post-transcriptional silencing. The expression of six Na(v)s was observed in failing VCMs as follows: Na(v)1.5 (57.3%) > Na(v)1.2 (15.3%) > Na(v)1.1 (11.6%) > Na(v)2.1 (10.7%) >Na(v)1.3 (4.6%) > Na(v)1.6 (0.5%). Failing VCMs showed up-regulation of Na(v)1.1 expression, but reduction of Na(v)1.6 mRNA. A similar Na-v expression pattern was found in samples from human hearts with ischaemic HF. VCMs with silenced Na(v)1.5 exhibited residual I-NaT and I-NaL (approximate to 30% of control) with rightwardly shifted steady-state activation and inactivation. These currents were tetrodotoxin sensitive but resistant to MTSEA, a specific Na(v)1.5 blocker. The amplitude of the tetrodotoxin-sensitive I-NaL was 0.1709 +/- 0.0299pApF(-1) (n=7 cells) and the decay time constant was =790 +/- 76ms (n=5). This I-NaL component was lacking in VCMs with a silenced Na(v)1.1 gene, indicating that, among neuronal isoforms, Na(v)1.1 provides the largest contribution to I-NaL. At -10mV this contribution is approximate to 60% of total I-NaL. Our further experimental and in silico examinations showed that this new Na(v)1.1 I-NaL component contributes to Ca(2+)accumulation in failing VCMs and modulates AP shape and duration. In conclusion, we have discovered an Na(v)1.1-originated I-NaL component in dog heart ventricular cells. This component is physiologically relevant to controlling AP shape and duration, as well as to cell Ca2+ dynamics.