Trimethylapigenin Blocks Cardiac Potassium Channels And Increases Late Sodium Current

PartⅠEffects of the natural flavone trimethylapigenin on cardiac potassium currentsBackground and purpose:The natural flavones and polymethylflavone have been reported to have cardiovascular protective effects. In the present study, we determined whether quecertin, apigenin and their methylated compounds (3,7,3',4'-tetramethylquecertin,3,5,7,3',4'-pentamethylquecertin,7,4'-dimethylapigenin, and 5,7,4'-trimethylapigenin) would block the atrial specific potassium channel hKv1.5 using a whole-cell patch voltage-clamp technique.Results:We found that among the six flavonoids, trimethylapigenin showed the strongest inhibitory effect on hKv1.5 channel current. This compound suppressed hKv1.5 current in HEK 293 cell line (IC50=6.4μM), and the ultra-rapid delayed rectify K+current IKur in human atrial myocytes (IC50=8.0μM) by binding to the open channels. The mean value of blocking time constant was 126.1±15.4 ms for 3μM trimethylapigenin, and 59.6±4.7 ms for 10μM trimethylapigenin. The blockade showed a use-and frequency-dependent manner, the inhibition potent was increased with the elevation of the frequency, the IC50 on hKv1.5 was 6.2,5.1, and 4.2μM at 1,2, and 5 Hz, respectively. In addition, trimethylapigenin decreased transient outward potassium current (Ito) in human atrial myocytes, inhibited acetylcholine-activated K+current (IC50=6.8μM) in rat atrial myocytes. Interestingly, trimethylapigenin had a weak inhibition of hERG channel current (IC50=18-31μM) and rat ventricle IK1(IC50>30μM).Conclusion and implication:Our results indicate that trimethyapigenin significantly inhibits the atrial potassium currents hKv1.5/IKur and IKACh, which suggests that trimethylapigenin may be a potential candidate for anti-atrial fibrillation. Part II Trimethylapigenin increases inward sodium current on human atrial myocyte and cloned Navl.5 channelBackground and Purpose:Sodium channel is responsible for forming the 0 phase of cardiac action potential. It is also the target of Class I anti-arrythmia drugs. Trimethylapigenin has been proved to be functional inhibiting multipule potassium channels, especially those specially expressed only in atrial, but the effect of trimethylapigenin on sodium channe has not been well studied. We employed human atrial myocytes and HEK293 cell which stably expressed Navl.5 gene to study the potential influence.Results:We found that 1-10μM trimethylapigenin did not show much influence on the peak amplitude of sodium current on human atrial myocyte, interestingly, it could concentration dependently delay the "inactivation" procedure of sodium current:With the concentration elevated, it took much more time for the sodium current return to the basal line, meanwhile, the mono-exponential fitting for the inactivation was poor after trimethylapigenin was given but it fit much better to a bi-exponential function. Besides, this "delay" of inactivation showed no difference under 1,2,5 and 10 Hz stimulation, indicating the frequency was irrelevant. Trimethylapigenin 10μM could left shift the voltage-dependent available curve, the V0.5 was decresed from-94.3±0.27 mV to-99.3±0.2 mV but the slope was not affected. The time constant of recovery from inactivation was increased from 8.2±0.12 ms to 13.9±0.26 ms.The influence of trimethylapigenin of cloned Navl.5 was close to what we found on human atrial. Trimethylapigenin did not change the peak amplitude but obviously increase the total inward quantity of electricity and delay the inactivation procedure. The fitting parameter obtained from bi-exponential function taul and tau2 were both increased from 6.69±0.17 ms,1.58±0.02 ms to 11.39±0.16 ms,1.75±0.03 ms, respectively. Meanwhile, trimethylapigenin left shifted the voltage-dependent avalible curve and showed no influence on the voltage-dependent activation. Similar as human atrial, the recovery time constant from inactivation was prolonged from 13.6±0.50 ms to 25.2±1.25 ms.Conclusions:These results demonstrate that trimethylapigenin delay the inactivation procedure of INa without affecting the peak amplitude. The channel opening properties were similar between human atrial myocytes and cloned Navl.5 which indicated that the a subunit probable be the target of trimethylapigenin and the delay was not by directly blocking the pore of sodium channel. This effect was very alike the influence of ATX-II (a sea anemones toxin) and it is the first time that a flavonoid was proved to have a similar function.