Alterations in Early Action Potential Repolarization Causes Localized Failure of Sarcoplasmic Reticulum Ca 2+ Release

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Abstract
Depressed contractility of failing myocytes involves a decreased rate of rise of the Ca2+ transient. Synchronization of Ca2+ release from the junctional sarcoplasmic reticulum (SR) is responsible for the rapid rise of the normal Ca2+ transient. This study examined the idea that spatially and temporally dyssynchronous SR Ca2+ release slows the rise of the cytosolic Ca2+ transient in failing feline myocytes. Left ventricular hypertrophy (LVH) with and without heart failure (HF) was induced in felines by constricting the ascending aorta. Ca2+ transients were measured in ventricular myocytes using confocal line scan imaging. Ca2+ transients were induced by field stimulation, square wave voltage steps, or action potential (AP) voltage clamp. SR Ca2+ release was significantly less well spatially and temporally synchronized in field-stimulated HF versus control or LVH myocytes. Surprisingly, depolarization of HF cells to potentials where Ca2+ currents (ICa) were maximal resynchronized SR Ca2+ release. Correspondingly, decreases in the amplitude of ICa desynchronized SR Ca2+ release in control, LVH, and HF myocytes to the same extent. HF myocytes had significant loss of phase 1 AP repolarization and smaller ICa density, which should both reduce Ca2+ influx. When normal myocytes were voltage clamped with HF AP profiles SR Ca2+ release was desynchronized. SR Ca2+ release becomes dyssynchronized in failing feline ventricular myocytes because of reductions in Ca2+ influx induced in part by alterations in early repolarization of the AP. Therefore, therapies that restore normal early repolarization should improve the contractility of the failing heart.

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