Background The rapidly activating delayed rectifier potassium current (/Kr),whose pore-forming alpha subunit is encoded by the human ether-a-go-go-related gene (hERG),is a key contributor to the third phase of action potential repolarization.The aim of this study was to investigate the effect and mechanism of aracoline hydrobromide induced inhibition of hERG K+ current (/hERG).Methods Transient transfection of hERG channel cDNA plasmid pcDNA3.1 into the cultured HEK293 cells was performed using Lipofectamine.A standard whole-cell patch-clamp technique was used to record the /hERG before and after the exposure to arecoline.Results Arecoline decreased the amplitude and the density of the /hERG in a concentration-dependent manner (ICs0=9.55 μmol/L).At test potential of +60 mV,the magnitude of /hERG tail at test pulse of -40 mV was reduced from (151.7±6.2) pNpF to (84.4±7.6) pNpF (P ＜0.01,n=20) and the magnitude of /hERG tail at test pulse of -110 mV was reduced from (-187.5±9.8) pNpF to (-97.6±12.6) pA/pF (P ＜0.01,n=20).The blockade of arecoline in the open and inactivated state was significant in a state-dependent manner.The maximal blockade was achieved in the inactivated state.Studies of gating mechanism showed that the steady-state activation curve of /hERG was significantly negatively shifted by arecoline.Time constants of activation were shortened.Steady-state inactivation curve and time constants of fast inactivation were not significantly affected by arecoline.Furthermore,the inhibition of /hERG by arecoline was characterized markedly by a frequency-dependent manner from 0.03 to 1.00 Hz pulse.Conclusion Arecoline could potently block/hERG in both frequency and state-dependent manner.