Augmented and slowed late Na + current (I NaL ) was implicated in action potential duration variability, early afterdepolarizations, and abnormal Ca 2+ handling in human and canine failing myocardium Objective was to study I NaL modulation by cytosolic Ca 2+ ([Ca 2+ ] i ) in normal and failing ventricular myocytes.Methods-Chronic heart failure was produced in 10 dogs by multiple sequential coronary artery microembolizations, 6 normal dogs served as a control. I NaL fine structure was measured by wholecell patch-clamp in ventricular myocytes and approximated by a sum of fast and slow exponentials produced by burst and late scattered modes of Na + channel gating, respectively.Results-I NaL greatly enhanced as [Ca 2+ ] i increased from "Ca 2+ free" to 1μM: its maximum density increased, decay of both exponentials slowed, and steady-state-inactivation curve (SSI) shifted towards more positive potentials. Testing inhibition of CaMKII and CaM revealed similarities and differences of I NaL modulation in failing vs. normal myocytes. Similarities: 1) CaMKII slows I NaL decay and decreases the amplitude of fast exponential; 2) Ca 2+ shifts SSI rightward. Differences: 1) slowing I NaL by CaMKII is greater; 2) CaM shifts SSI leftward; 3) Ca 2+ increases the amplitude of slow exponential.Conclusions-Ca 2+ /CaM/CaMKII signaling increases I NaL and Na + influx in both normal and failing myocytes by slowing inactivation kinetics and shifting SSI. This Na + influx provides a novel Ca 2+ positive feedback mechanism (via Na + /Ca 2+ exchanger), enhancing contractions at higher beating rates, but worsening cardiomyocytes contractile and electrical performance in conditions of poor Ca 2+ handling in heart failure.

We elucidate the role of late Na + current (I NaL ) for diastolic intracellular Ca 2+ (DCa)accumulation in chronic heart failure (HF). HF was induced in 19 dogs by multiple coronary artery microembolizations; 6 normal dogs served as control. Ca 2+ transients were recorded in field-paced (0.25 or 1.5Hz) fluo-4-loaded ventricular myocytes (VM). I NaL and action potentials were recorded by patch-clamp. Failing VM, but not normal VM, exhibited 1) prolonged action potentials and Ca 2+ transients at 0.25 Hz, 2) substantial DCa accumulation at 1.5Hz, 3) spontaneous Ca 2+ releases, which occurred after 1.5 Hz stimulation trains in ~31% cases. Selective I NaL blocker ranolazine (10μM) or the prototypical Na + channel blocker tetrodotoxin (2μM) reversibly improved function of failing VM. The DCa accumulation and the beneficial effect of I NaL blockade were reproduced in silico using an excitation-contraction coupling model. We conclude that I NaL contributes to diastolic Ca 2+ accumulation and spontaneous Ca 2+ release in HF.

Mishra S, Undrovinas NA, Maltsev VA, Reznikov V, Sabbah HN, Undrovinas A. Post-transcriptional silencing of SCN1B and SCN2B genes modulates late sodium current in cardiac myocytes from normal dogs and dogs with chronic heart failure. Am J Physiol Heart Circ Physiol 301: H1596 -H1605, 2011. First published June 24, 2011 doi:10.1152/ajpheart.00948.2009.-The emerging paradigm for Na ϩ current in heart failure (HF) is that its transient component (INaT) responsible for the action potential (AP) upstroke is decreased, whereas the late component (INaL) involved in AP plateau is augmented. Here we tested whether Nav␤1-and Nav␤2-subunits can modulate INaL 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. INa and APs were measured by the whole cell and perforated patch-clamp in freshly isolated and cultured VCMs, respectively. INaL was augmented with slower decay in HF VCMs compared with normal heart VCMs, and these properties remained unchanged within 5 days of culture. Posttranscriptional silencing SCN1B and SCN2B were achieved by virally delivered short interfering RNA (siRNA) specific to Nav␤1 and Nav␤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 Nav␤1 and Nav␤2 were reduced by Ͼ80%, but mRNA and protein of Nav1.5, as well as INaT, remained unchanged in HF VCMs. Nav␤1-siRNA reduced INaL density and accelerated INaL two-exponential decay, whereas Nav␤2-siRNA produced an opposite effect in VCMs from both normal and failing hearts. Physiological importance of the discovered INaL modulation to affect AP shape and duration was illustrated both experimentally and by numerical simulations of a VCM excitationcontraction coupling model. We conclude that in myocytes of normal and failing dog hearts Nav␤1 and Nav␤2 exhibit oppositely directed modulation of INaL. action potential; in silico simulation THE EMERGING PARADIGM FOR Na ϩ current (I Na ) in chronic heart failure (HF) is that its transient component (I NaT ) responsible for the action potential (AP) upstroke and excitation propagation is decreased, whereas the late component (I NaL ) involved in AP plateau is augmented (21,23,25,42,45,48). Molecular mechanisms of these HF-related I Na alterations are still understudied. Numerous studies indicate a possible role of Na v ␤ auxiliary subunits to modulate Na ϩ channel (NaCh) expression and function (27), but implications of Na v ␤ in I NaL modulation have not been studied in detail, especially in HF. Our previous studies in a canine chronic HF model showed that the protein level of the main NaCh isoform expressed in the heart, Na v 1.5, underlying I NaL (18), is reduced but remains unchanged for Na v ␤ 1 -and Na v ␤ 2 -subunits, making these ␤-subunits relatively upregulated (48). Thus an intriguing possibility...

Key pointsr Late Na + current (I NaL ) contributes to action potential remodelling and Ca 2+ /Na + changes in heart failure.r The molecular identity of I NaL remains unclear. r The contributions of different Na + channel isoforms, apart from the cardiac isoform, remain unknown.r We discovered and characterized a substantial contribution of neuronal isoform Na v 1.1 to I NaL . r This new component is physiologically relevant to the control of action potential shape and duration, as well as to cell Ca 2+ dynamics, especially in heart failure.Abstract Late Na + current (I NaL ) contributes to action potential (AP) duration and Ca 2+ handling in cardiac cells. Augmented I NaL was implicated in delayed repolarization and impaired Ca 2+ handling in heart failure (HF). We tested if Na + channel (Na v ) neuronal isoforms contribute to I NaL and Ca 2+ 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 Ca 2+

Background. Nitric oxide can successfully compete with oxygen for sites of electron-transport chain in conditions of myocardial hypoxia. These features may prevent excessive oxidative stress occurring in cardiomyocytes during sudden hypoxia-reoxygenation. Aim. To study the action of the potent stable NO donor dinitrosyl iron complex with glutathione (Oxacom®) on the recovery of myocardial contractile function and Ca2+ transients in cardiomyocytes during hypoxia-reoxygenation. Results. The isolated rat hearts were subjected to 30 min hypoxia followed by 30 min reoxygenation. The presence of 30 nM Oxacom in hypoxic perfusate reduced myocardial contracture and improved recovery of left ventricular developed pressure partly due to elimination of cardiac arrhythmias. The same Oxacom concentration limited reactive oxygen species generation in hypoxic cardiomyocytes and increased the viability of isolated cardiomyocytes during hypoxia from 12 to 52% and after reoxygenation from 0 to 40%. Oxacom prevented hypoxia-induced elevation of diastolic Ca2+ level and eliminated Ca2+ transport alterations manifested by slow Ca2+ removal from the sarcoplasm and delay in cardiomyocyte relaxation. Conclusion. The potent stable NO donor preserved cardiomyocyte integrity and improved functional recovery at hypoxia-reoxygenation both in the isolated heart and in cardiomyocytes mainly due to preservation of Ca2+ transport. Oxacom demonstrates potential for cardioprotection during hypoxia-reoxygenation.