Enhanced Depolarization Drive in Failing Rabbit Ventricular Myocytes.

BACKGROUND: Heart failure (HF) is characterized by electrophysiological remodeling resulting in increased risk of cardiac arrhythmias. Previous reports suggest that elevated inward ionic currents in HF promote action potential (AP) prolongation, increased short-term variability of AP repolarization, and delayed afterdepolarizations. However, the underlying changes in late Na+ current (INaL ), L-type Ca2+ current, and NCX (Na+ /Ca2+ exchanger) current are often measured in nonphysiological conditions (square-pulse voltage clamp, slow pacing rates, exogenous Ca2+ buffers).

METHODS: We measured the major inward currents and their Ca2+ - and β-adrenergic dependence under physiological AP clamp in rabbit ventricular myocytes in chronic pressure/volume overload-induced HF (versus age-matched control).

RESULTS: AP duration and short-term variability of AP repolarization were increased in HF, and importantly, inhibition of INaL decreased both parameters to the control level. INaL was slightly increased in HF versus control even when intracellular Ca2+ was strongly buffered. But under physiological AP clamp with normal Ca2+ cycling, INaL was markedly upregulated in HF versus control (dependent largely on CaMKII [Ca2+ /calmodulin-dependent protein kinase II] activity). β-Adrenergic stimulation (often elevated in HF) further enhanced INaL . L-type Ca2+ current was decreased in HF when Ca2+ was buffered, but CaMKII-mediated Ca2+ -dependent facilitation upregulated physiological L-type Ca2+ current to the control level. Furthermore, L-type Ca2+ current response to β-adrenergic stimulation was significantly attenuated in HF. Inward NCX current was upregulated at phase 3 of AP in HF when assessed by combining experimental data and computational modeling.

CONCLUSIONS: Our results suggest that CaMKII-dependent upregulation of INaL in HF significantly contributes to AP prolongation and increased short-term variability of AP repolarization, which may lead to increased arrhythmia propensity, and is further exacerbated by adrenergic stress.