Modulation And Underlying Mechanisms Of β-adrenergic Receptor Subtypes On IKr In Ventricular Myocytes Of Chronic Heart Failure

Background Chronic heart failure (CHF) has been a momentous problem of social public health. Approximately half of the mortality in patients with CHF are sudden and unexpected, and presumably the consequence of lethal ventricular arrhythmias. Human ether-a-go-go-related gene (hERG) encodes theαsubunit of the channel underlying the rapid component of the delayed rectifier potassium current (IKr). IKr is the major outward current involved in phase 3 rapid repolarization of cardiac action potential duration. Inhibition of IKr may lead to action potential duration prolongation. A reduction in hERG currents due to either genetic mutations, or adverse drug effects can result in congenital long QT syndrome (LQTS-2) or acquired long QT syndrome (aLQTS). The both forms of LQTS are characterized by action potential prolongation, lengthening of the QT interval on the surface ECG, and an increased risk for"torsade de pointes"ventricular tachycardia and sudden cardiac death.The electrophysiological remodeling renders the failing heart more vulnerable to ventricular arrhythmias. Prolongation of the cardiac action potential duration is a hallmark of CHF, predisposing patients to early afterdepolarizations and"torsade de pointes"ventricular tachycardia. Some of the changes in CHF mimic congenital ion channelopathies that cause long QT syndromes, and CHF can be viewed as a special form of acquired LQTS. The reduction in IKr has crucial role in prolonging action potential in CHF; however, the remodeling of IKr/hERG channels in CHF exist discrepancies. The relation of the IKr/hERG channels remodeling with ventricular arrhythmias need further study. Moreover, recent studies have revealed that hERG channels are modulated by G protein-coupled receptors includingα- andβ-adrenergic receptors through the intracellular second messengers such as cAMP, PKA and PKC. These studies were almost based on normal cardiac myocytes or transfected cells. So far, limited work has been done onβ-AR subtype-specific modulation of the IKr in CHF.The present study was designed to establish a valid CHF model of guinea pigs, examine the effect of CHF on the remodeling of ERG channels, and assess modulation ofβ-adrenergic receptor subtypes on IKr and underlying mechanisms in ventricular myocytes of guinea pigs with CHF.Part 1 Establishment of a chronic heart failure model of guinea pigAim: To establish a valid chronic heart failure (CHF) model of guinea pigs.Methods: The CHF model of guinea pigs was established by subtotal descending thoracic aortic banding. The left ventricular end-diastolic diameter (LVEDd), LV end-systolic diameter (LVESd), and heart function were measured by two-dimensional echocardiography; the heart rate, QT interval and corrected QTc were detected by electrocardiogram.Results: Following 12 weeks of descending thoracic aortic banding, LV ejection fraction (EF) and LV fractional shortening (FS) were significantly decreased; LVEDd, LVSDd, QT interval,and QTc were significantly increased. All guinea pigs with CHF had ascites and pleural effusion.Conclusion: The CHF model of guinea pigs was successfully established by pressure overload. Part2 The remodeling of IKr/ERG channel in CHF guinea pigsAim: To observe the remodeling of IKr/ERG channel and the effect of selective IKr blocker dofetilide on the QTc and the incidence of ventricular arrhythmia in guinea pigs with CHF.Methods: Real time-PCR and Western blot were used to detect hERG mRNA and protein in LV of guinea pigs with CHF. Whole-cell patch-clamp technique was used to record IKr in LV myocytes. The effect of treatment with dofetilide on the incidence of ventricular tachycardia in CHF was observed.Results: ERG mRNA was not significantly altered; however, a trend towards reduced expression in CHF was observed. ERG protein expression remained unchanged. IKr tail current was reduced in CHF ventricular myocytes compared with normal ventricular myocytes (at 40mV, 0.21±0.01 pA/pF, n=33, versus 0.43±0.03 pA/pF, n=24; p<0.001). IKr blockers significantly prolonged QTc and increased the incidence of ventricular tachycardia in guinea pigs with CHF.Conclusion: IKr was downregulated in guinea pigs with CHF, which may play an important role in CHF-related arrhythmogenesis.Part 3 Modulation and underlying mechanism ofβ-adrenergic receptor subtypes on IKr in ventricular myocytes of chronic heart failure guinea pigsAim: To assess the effect ofβ-adrenergic receptor subtypes on IKr in ventricular myocytes of guinea pigs with CHF.Methods: Whole-cell patch-clamp technique was used to record IKr in ventricular myocytes isolated from guinea pigs. The effect of isoproterenol, xamoterol, and fenoterol on IKr in CHF ventricular myocytes was detected.Results: Following treatment with the nonselectiveβ-AR agonist isoproterenol (10 μM), the amplitude of Ikr tail current was decreased by 66% and 54% in CHF and normal ventricular myocytes, respectively. The response toβ2-AR activation on IKr was increased in CHF ventricular myocytes. The predominant inhibitory response toβ-AR activation on Ikr in CHF was mediated byβ1-AR andβ2-AR, whereas this response was due toβ1-AR activation alone in normal ventricular myocytes. Specific PKA inhibition significantly attenuatedβ1-AR-induced inhibition of Ikr; however, KN93, an inhibitor of CaMKⅡ, did not attenuate the inhibitory effect in CHF ventricular myocytes. KT5720 also partly prevented the inhibitory effect ofβ2-AR activation on IKr in CHF ventricular myocytes.Conclusions: IKr exhibited an increased response toβ2-AR stimulation, and the inhibitory effect of IKr byβ-AR activation is mediated byβ1-andβ2-AR in CHF ventricular myocytes. IKr is inhibited byβ1-AR activation via a PKA pathway, and the inhibition of IKr induced byβ2-AR stimulation may be partly mediated through the activation of PKA in CHF.