Chronic heart failure slows late sodium current in human and canine ventricular myocytes: implications for repolarization variability.

Background: Late Na(+) current (I(NaL)) in human and dog hearts has been implicated in abnormal repolarization associated with heart failure (HF). HF slows inactivation gating of late Na(+) channels, which could contribute to these abnormalities.

Aims: To test how altered gating affects I(NaL) time course, Na(+) influx, and action potential (AP) repolarization.

Methods: I(NaL) and AP were measured by patch clamp in left ventricular cardiomyocytes from normal and failing hearts of humans and dogs. Canine HF was induced by coronary microembolization.

Results: I(NaL) decay was slower and I(NaL) density was greater in failing hearts than in normal hearts at 24 degrees C (human hearts: tau=659+/-16 vs. 529+/-21 ms; n=16 and 4 hearts, respectively; mean+/-SEM; p<0.002; dog hearts: 561+/-13 vs. 420+/-17 ms; and 0.307+/-0.014 vs. 0.235+/-0.019 pA/pF; n=25 and 14 hearts, respectively; p<0.005) and at 37 degrees C this difference tended to increase. These I(NaL) changes resulted in much greater (53.6%) total Na(+) influx in failing cardiomyocytes. I(NaL) was sensitive to cadmium but not to cyanide and exhibited low sensitivity to saxitoxin (IC(50)=62 nM) or tetrodotoxin (IC(50)=1.2 muM), tested in dogs. A 50% I(NaL) inhibition by toxins or passing current opposite to I(NaL), decreased beat-to-beat AP variability and eliminated early afterdepolarizations in failing cardiomyocytes.

Conclusions: Chronic HF leads to larger and slower I(NaL) generated mainly by the cardiac-type Na(+) channel isoform, contributing to larger Na(+) influx and AP duration variability. Interventions designed to reduce/normalize I(NaL) represent a potential cardioprotective mechanism in HF via reduction of related Na(+) and Ca(2+) overload and improvement of repolarization.