Abstract
The natural flavone acacetin has been demonstrated to inhibit transient outward potassium current (Ito) in human atrial myocytes. However, the molecular determinants of acacetin for blocking Ito are unknown. The present study was designed to investigate the properties and molecular determinants of this compound for blocking hKv4.3 channels (coding Ito) stably expressed in HEK 293 cells using the approaches of whole-cell patch voltage-clamp technique and mutagenesis. It was found that acacetin inhibited hKv4.3 current by binding to both the closed and open channels, and decreased the recovery from inactivation. The blockade of hKv4.3 channels by acacetin was use- and frequency-dependent, and IC50s of acacetin for inhibiting hKv4.3 were 7.9, 6.1, 3.9, and 3.2 µM, respectively, at 0.2, 0.5, 1, and 3.3 Hz. The mutagenesis study revealed that the hKv4.3 mutants T366A and T367A in the P-loop helix, and V392A, I395A and V399A in the S6-segment had a reduced channel blocking efficacy of acacetin (IC50, 44.5 µM for T366A, 25.8 µM for T367A, 17.6 µM for V392A, 16.2 µM for I395A, and 19.1 µM for V399A). These results demonstrate the novel information that acacetin may inhibit the closed channels and block the open state of the channels by binding to their P-loop filter helix and S6 domain. The use- and rate-dependent blocking of hKv4.3 by acacetin is likely beneficial for managing atrial fibrillation.
Highlights
It is well recognized that the 4-aminopyridine- (4-AP-) sensitive transient outward potassium current inhibit transient outward potassium current (Ito) is expressed in cardiomyocytes from mouse [1,2], rat [3], rabbit [4], ferret [5], cat [6], canine [7], and human [8], but not in cardiomyocytes from guinea pig [9] and pig hearts [10,11]
The present study demonstrates that the natural flavone acacetin inhibits hKv4.3 channels stably expressed in HEK 293 cells in a use- and frequency-dependent manner by binding to the open state of the channels, and the closed channels
The effect of acacetin for blocking hKv4.3 current was enhanced as the stimulus frequency was increased from 0.2 Hz (IC50 = 7.9 mM) to 3.3 Hz (IC50 = 3.2 mM)
Summary
It is well recognized that the 4-aminopyridine- (4-AP-) sensitive transient outward potassium current Ito is expressed in cardiomyocytes from mouse [1,2], rat [3], rabbit [4], ferret [5], cat [6], canine [7], and human [8], but not in cardiomyocytes from guinea pig [9] and pig hearts [10,11]. Ito is heterogeneously expressed in transmural ventricular wall of the hearts in human and dogs, determines the morphologies of cardiac action potentials, and generates the prominent phase 1 repolarization and ‘‘spike and dome’’ profile of ventricular epicardial and midmyocardial myocytes in these species [7,12]. Recent studies have demonstrated that Brugada syndrome-associated Ito gain-of-function mutations in KCND3-encoded Kv4.3 is believed to mediate an alteration of transmural voltage gradient Endocardium), and result in a net outward shift in current and heterogeneous loss of the action potential dome, ST segment elevation on electrocardiogram (ECG), and the development of potentially fatal polymorphic ventricular tachycardia or ventricular fibrillation via phase II reentry [15]. The present study was designed to investigate the properties and molecular determinants of acacetin for inhibiting hKv4.3 channels with whole-cell patch voltage-clamp and mutagenesis approaches
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