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(Circulation Research. 2005;96:543.)
© 2005 American Heart Association, Inc.

Cellular Biology

Alterations in Early Action Potential Repolarization Causes Localized Failure of Sarcoplasmic Reticulum Ca²⁺ Release

David M. Harris, Geoffrey D. Mills, Xiongwen Chen, Hajime Kubo, Remus M. Berretta, V. Scott Votaw, Luis F. Santana, Steven R. Houser

From the Cardiovascular Research Center, Department of Physiology, Temple University, School of Medicine, Philadelphia, Pa.

Correspondence to Steven R. Houser, PhD, Laura H. Carnell Professor of Physiology and Medicine, Director, Cardiovascular Research Center, Department of Physiology, Temple University, School of Medicine, 3400 N Broad St, Philadelphia, PA 19140. E-mail Steven.houser{at}temple.edu

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 (I_Ca) were maximal resynchronized SR Ca²⁺ release. Correspondingly, decreases in the amplitude of I_Ca 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 I_Ca 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.

Key Words: heart failure • excitation contraction coupling • sarcoplasmic reticulum • calcium transients

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