Elsevier

Heart Rhythm

Volume 18, Issue 12, December 2021, Pages 2197-2209
Heart Rhythm

Experimental
Inhibitory G-protein–mediated modulation of slow delayed rectifier potassium channels contributes to increased susceptibility to arrhythmogenesis in aging heart

https://doi.org/10.1016/j.hrthm.2021.09.014Get rights and content

Background

Slow delayed rectifier potassium current (IKs) is an important component of repolarization reserve during sympathetic nerve excitement. However, little is known about age-related functional changes of IKs and its involvement in age-dependent arrhythmogenesis.

Objective

The purpose of this study was to investigate age-related alteration of the IKs response to β-adrenergic receptor (βAR) activation.

Methods

Dunkin–Hartley guinea pigs were used. Whole-cell patch-clamp recording was used to record K+ currents. Optical mapping of membrane potential was performed in ex vivo heart.

Results

There was no difference in IKs density in ventricular cardiomyocytes between young and old guinea pigs. However, in contrast to IKs potentiation in young hearts, isoproterenol (ISO) evoked an acute inhibition on IKs in a concentration-dependent manner in old guinea pig hearts. The β2AR antagonist, but not β1AR antagonist, reversed the inhibitory response. Preincubation of cardiomyocytes with the inhibitory G protein (Gi) inhibitor pertussis toxin (PTX) also reversed the inhibitory response. In HEK293 cells cotransfected with cloned IKs channel and β2AR, ISO enhanced the current but reduced it when cells were cotransfected with Gi2, and PTX restored the ISO-induced excitatory response. Moreover, in aging cardiomyocytes, Gβγ inhibitor gallein, PLC inhibitor U73122, or protein kinase C inhibitor Bis-1 prevented the reduction of IKs by ISO. Furthermore, cardiac-specific Gi2 overexpression in young guinea pigs predisposed the heart to ventricular tachyarrhythmias. PTX pretreatment protected the hearts from ventricular arrhythmias.

Conclusion

βAR activation acutely induces an inhibitory IKs response in aging guinea pig hearts through β2AR-Gi signaling, which contributes to increased susceptibility to arrhythmogenesis in aging hearts.

Introduction

Cardiac arrhythmias, particularly severe ventricular tachyarrhythmia, are the primary causes of death in patients with cardiovascular diseases.1 Aging is an independent risk factor for arrhythmogenesis; thus, mortality due to cardiovascular diseases mainly affects older individuals.2,3 However, the ion channel mechanism that underlies the increased predisposition to arrhythmogenesis in older individuals is not completely understood. Delayed rectifier K+ channels are the key molecular basis for determining the morphology and duration of the ventricular action potential (AP). Dysfunction of these channels is associated with an increased risk for torsades des pointes arrhythmias and sudden death.4,5 The repolarizing delayed rectifier potassium current (IK) consists of a slowly activating component (IKs) and a rapidly activating component (IKr), which are indispensable to AP phase 2 and 3 repolarization in humans. Previous studies have shown that IKr plays a dominant role in AP repolarization under resting physiological conditions, whereas when exercise and stress lead to increased sympathetic nerve excitement, IKs is rapidly enhanced by β-adrenergic receptor (βAR) stimulation, resulting in an acceleration of AP repolarization and ventricular repolarization biases to rely on IKs.6 Therefore, IKs constitutes an important cardiac repolarization reserve that is invoked especially during elevated sympathetic tone.7

The channel conducting IKs is co-assembled by pore-forming subunits KCNQ1 (Kv7.1) and auxiliary subunits KCNE1.8 βAR activation enhances IKs through coupling excitatory G protein (Gs) to activate adenylate cyclase (AC) and increase the level of cyclic adenosine monophosphate (cAMP) and further activate protein kinase A (PKA). PKA phosphorylates the N-terminal residues S27 and S92 of the KCNQ1 subunit, resulting in the acceleration of channel activation.9 Disruption of IKs regulation via the Gs-cAMP-PKA signaling pathway elevates the risk for arrhythmias in congenital mutations within the KCNQ1/KCNE1 genes and acquired channelopathies.4,10 However, limited information is available regarding age-dependent functional changes in IKs. A study reported a markedly elevated incidence of cardiac arrhythmias in aging fruit flies and a concomitant decrease in the expression of the Drosophila homolog of human KCNQ1.11 This finding hints at a possible involvement of IKs dysfunction in the increased susceptibility to arrhythmias in the aging heart. However, to date, there is no evidence on age-related alterations in IKs in the mammalian heart. Therefore, we performed this study to evaluate age-related functional changes in IKs in guinea pig hearts and investigate the underlying mechanisms to understand the molecular bases for the age-dependent cardiac vulnerability to arrhythmias.

Section snippets

Methods

A detailed methods section is given in the Supplemental Methods.

Age-related differences in susceptibility to arrhythmogenesis

Langendorff-perfused hearts were used, thereby precluding the effects of hormone/neurotransmitter levels. Representative ex vivo electrocardiographic (ECG) waves in the presence of the βAR agonist isoproterenol (ISO) (1 and 3 μM) are shown in Figure 1A. ISO typically shortened the heart rate–corrected QT (QTc) interval in a concentration-dependent manner in young animals (Figure 1B). However, in aging guinea pig hearts, the extent of QTc interval shortening was much smaller than that in young

βAR activation acutely induces age-dependent inhibition of IKs

The concept of “repolarization reserve” was first introduced by Roden and Yang.14 The pivotal role of IKs in the repolarization reserve, especially during an elevated sympathetic tone, was experimentally proven first in dogs and subsequently in humans.7,15 In this study, we did observe a significant increase in IKs and a shortening of QT interval on ex vivo ECG recordings following βAR activation in young guinea pig hearts. Conversely, we uncovered an opposite response (decrease) of IKs to βAR

Conclusion

Our findings demonstrate that βAR activation induces an acute reduction in cardiac IKs in aging guinea pig hearts, which is mediated through the β2AR-Gi signaling pathway. The IKs inhibitory response to βAR stimulation might contribute to the increased susceptibility to arrhythmogenesis in aging hearts. Increased sympathetic nerve excitability is an important cause of ventricular arrhythmias under various stress conditions or combined with cardiovascular diseases in older individuals.27 Our

Acknowledgment

We acknowledge Core Facilities and Centers at Hebei Medical University for the use of the optical mapping system.

References (27)

  • M. Hayashi et al.

    The spectrum of epidemiology underlying sudden cardiac death

    Circ Res

    (2015)
  • E. Marban

    Cardiac channelopathies

    Nature

    (2002)
  • T. Banyasz et al.

    Beta-adrenergic stimulation reverses the IKr-IKs dominant pattern during cardiac action potential

    Pflugers Arch

    (2014)
  • Cited by (0)

    Funding sources: This work was supported by the National Natural Science Foundation of China (32071105); Science Fund for Creative Research Groups of Natural Science Foundation of Hebei Province (H2020206474); and Science and Technology Foundation of Hebei Province (GCC2014024). Disclosures: The authors have no conflicts of interest to disclose.

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