Abstract
The emerging paradigm for Na(+) current in heart failure (HF) is that its transient component (I(NaT)) responsible for the action potential (AP) upstroke is decreased, whereas the late component (I(NaL)) involved in AP plateau is augmented. Here we tested whether Na(v)β(1)- and Na(v)β(2)-subunits can modulate I(NaL) parameters in normal and failing ventricular cardiomyocytes (VCMs). Chronic HF was produced in nine dogs by multiple sequential coronary artery microembolizations, and six dogs served as a control. I(Na) and APs were measured by the whole cell and perforated patch-clamp in freshly isolated and cultured VCMs, respectively. I(NaL) was augmented with slower decay in HF VCMs compared with normal heart VCMs, and these properties remained unchanged within 5 days of culture. Post-transcriptional silencing SCN1B and SCN2B were achieved by virally delivered short interfering RNA (siRNA) specific to Na(v)β(1) and Na(v)β(2). The delivery and efficiency of siRNA were evaluated by green fluorescent protein expression, by the real-time RT-PCR, and Western blots, respectively. Five days after infection, the levels of mRNA and protein for Na(v)β(1) and Na(v)β(2) were reduced by >80%, but mRNA and protein of Na(v)1.5, as well as I(NaT), remained unchanged in HF VCMs. Na(v)β(1)-siRNA reduced I(NaL) density and accelerated I(NaL) two-exponential decay, whereas Na(v)β(2)-siRNA produced an opposite effect in VCMs from both normal and failing hearts. Physiological importance of the discovered I(NaL) modulation to affect AP shape and duration was illustrated both experimentally and by numerical simulations of a VCM excitation-contraction coupling model. We conclude that in myocytes of normal and failing dog hearts Na(v)β(1) and Na(v)β(2) exhibit oppositely directed modulation of I(NaL).