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

Abstract

BACKGROUND\nSlow 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.\n\n\nOBJECTIVE\nTo investigate age-related alteration of the IKs response to β-adrenergic receptor (βAR) activation.\n\n\nMETHODS\nDunkin-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.\n\n\nRESULTS\nThere 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 co-transfected with cloned IKs channel and β2AR, ISO enhanced the current but reduced it when cells were co-transfected 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.\n\n\nCONCLUSION\nβAR 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.