Mechanisms Underlying Rate-Dependent Remodeling of Transient Outward Potassium Current in Canine Ventricular Myocytes

Abstract

Abstract-Transient outward Kϩ current (I to ) downregulation following sustained tachycardia in vivo is usually attributed to tachycardiomyopathy. This study assessed potential direct rate regulation of cardiac I to and underlying mechanisms. Cultured adult canine left ventricular cardiomyocytes (37°C) were paced continuously at 1 or 3 Hz for 24 hours. I to was recorded with whole-cell patch clamp. The 3-Hz pacing reduced I to by 44% (PϽ0.01). Kv4.3 mRNA and protein expression were significantly reduced (by Ϸ30% and Ϸ40%, respectively) in 3-Hz paced cells relative to 1-Hz cells, but KChIP2 expression was unchanged. Prevention of Ca 2ϩ loading with nimodipine or calmodulin inhibition with W-7, A-7, or W-13 eliminated 3-Hz pacing-induced I to downregulation, whereas downregulation was preserved in the presence of valsartan. Inhibition of Ca 2ϩ /calmodulin-dependent protein kinase (CaMK)II with KN93, or calcineurin with cyclosporin A, also prevented I to downregulation. CaMKII-mediated phospholamban phosphorylation at threonine 17 was increased in 3-Hz paced cells, compatible with enhanced CaMKII activity, with functional significance suggested by acceleration of the Ca 2ϩ i transient decay time constant (Indo 1-acetoxymethyl ester microfluorescence). Total phospholamban expression was unchanged, as was expression of Na ϩ /Ca 2ϩ exchange and sarcoplasmic reticulum Ca 2ϩ -ATPase proteins. Nuclear localization of the calcineurin-regulated nuclear factor of activated T cells (NFAT)c3 was increased in 3-Hz paced cells compared to 1-Hz (immunohistochemistry, immunoblot). INCA-6 inhibition of NFAT prevented I to reduction in 3-Hz paced cells. Calcineurin activity increased after 6 hours of 3-Hz pacing. CaMKII inhibition prevented calcineurin activation and NFATc3 nuclear translocation with 3-Hz pacing. We conclude that tachycardia downregulates I to expression, with the Ca 2ϩ /calmodulin-dependent CaMKII and calcineurin/NFAT systems playing key Ca 2ϩ -sensing and signal-transducing roles in rate-dependent I to control. Key Words: potassium channels Ⅲ calcium Ⅲ calmodulin Ⅲ remodeling Ⅲ arrhythmias S udden cardiac death caused by ventricular tachycardia or fibrillation is an important contributor to mortality in congestive heart failure (CHF) patients. 1 Rapid heart-rhythms can impair cardiac function and patients with "tachycardiomyopathy" are at risk of sudden cardiac death. 2 Chronic ventricular tachypacing in experimental animals produces a dilated cardiomyopathy that mimics clinical tachycardiomyopathy and is often used as an experimental model to study CHF-related cardiac remodeling. 3 Changes in cardiac ion channel transport are important components of this remodeling, and extensive evidence suggests that these ion transport changes are crucial contributors to the pathogenesis of CHFrelated ventricular tachyarrhythmias and sudden death. 3,4 Among the most ubiquitous changes are alterations in the transient outward K ϩ current (I to ), 3 which play potentially important roles in repolarization abnormalities, 5,6 c...

Ion Channel Subunit Expression Changes in Cardiac Purkinje Fibers

“…Rapid activation of ventricular cardiomyocytes modifies I to expression in vitro. 32 It is therefore possible that some of the changes we observed were attributable to rapid ventricular rates and not CHF per se; however, the ventricular ion channel function changes noted in animals with VTP-induced CHF do correspond closely to results obtained from explanted terminally failing human hearts. 6 We investigated modifications in transcript and protein expression of K ϩ channel, Na ϩ channel, and Cx subunits induced by CHF in canine cardiac PFs, based on a desire to understand previously observed functional K ϩ channel changes and to extend our understanding to ion channels governing cardiac conduction.…”

Exchange Protein Directly Activated by cAMP Mediates Slow Delayed-Rectifier Current Remodeling by Sustained β-Adrenergic Activation in Guinea Pig Hearts

“…Previous studies have identified a role for CaMKII in I to downregulation in tachycardia remodeling of canine ventricular cardiomyocytes. 48 Interestingly, in that work, like the present study, the primary signaling system involved was the Ca 2+ /calcineurin/ NFAT system, but intact CaMKII function was necessary for K + current downregulation. CaMKII activation also suppresses delayed-rectifier K + currents in neurons.…”

“…In mice with myocardial infarction, downregulation of I to and I Kslow1,2 was prevented by calcineurin inhibition or NFATc3 knockout. 46 Although calcineurin/NFAT signaling suppresses I to transcription in most studies, 47,48 I to upregulation occurs in neonatal rat cardiomyocytes.…”

Mechanisms of ventricular arrhythmias: a dynamical systems-based perspective

“…Studies in experimental models indicate that the downregulation of I to expression in heart failure may be mediated by the Ca 2ϩ /calmodulin-dependent protein kinase II (CaMKII) and calcineurin/nuclear factor of activated T-cells signaling cascades (157,228 CURRENT. Changes in L-type Ca 2ϩ current (I Ca-L ) density in diseased ventricles are variable, depending on the type of disease and its severity.…”