Molecular identity of the late sodium current in adult dog cardiomyocytes identified by Nav1.5-antisense inhibition

Late Na(+) current (INaL) is a major component of the action potential plateau in human and canine myocardium. Since INaL is increased in heart failure and ischemia, it represents a novel potential target for cardioprotection. However, the molecular identity of INaL remains unclear. Objective: We tested the hypothesis that the cardiac sodium channel (NaCh) isoform (Nav1.5) is a major contributor to INaL in adult dog ventricular cardiomyocytes (VCs). Methods: Cultured VCs were exposed to an antisense morpholino-based oligonucleotide (Nav1.5asOLIGO) targeting the region around the start codon of Nav1.5 mRNA or a control nonsense OLIGO (nsOLIGO). Both densities of transient Na(+) current (INaT) and late INaL (both in pA/pF) were monitored by whole cell patch clamp. Results: In HEK293 cells expressing Nav1.5 or Nav1.2, Nav1.5 asOLIGO silenced functional expression of Nav1.5 (up to 60 % of the initial INaT) but not Nav1.2. In both nsOLIGO-treated controls and untreated VCs, INaT and INaL remained unchanged for up to 5 days. However, both INaT and INaL exponentially decreased with a similar time course (tau=46 and 56h, respectively) after VCs were treated with Nav1.5 asOLIGO without changes in 1) decay kinetics, 2) steady-state activation and inactivation, and 3) the ratio of INaL to INaT. Four days after exposure to Nav1.5 asOLIGO, INaT and INaL amounted to 68+/-6% (n=20, mean+/-SE, P<0.01), and 60+/-7% (P<0.018, n=11) of those in VCs treated by nsOLIGO, respectively. Conclusion: In adult dog heart Nav1.5 sodium channels have a \"functional half-life\" of ~35h (0.69t) and make a major contribution to INaL. Key words: Late sodium current, molecular identity, sodium channel functional half-life, adult dog cardiomyocytes culture.