QUERCETIN ACTIVATES HUMAN Kv1.5 CHANNELS BY A RESIDUE I502 IN THE S6 SEGMENT

The aims of the present study were to investigate the pharmacological effects of quercetin on wild-type (WT) and mutant (I502A) human (h) Kv1.5 channel currents (I-kur) and to identify whether mutation in the S6 segment is critical to activation of I-kur by quercetin. Experiments were performed on WT and site-directed mutant hKv1.5 channels, which were stably expressed in Xenopus oocytes using the two-microelectrode voltage-clamp technique. Quercetin increased WT hKv1.5 channel current in a concentration-, voltage- and time-dependent manner, with an EC50 of 37.8 mu mol/L and a negative shift in the steady state activation and inactivation curves. Quercetin accelerated channel activation and inactivation, significantly decreasing activation and inactivation time constants. However, mutating the I502 residue to Ala abolished the activating effect of quercetin. Quercetin did not modify the activation and inactivation kinetics of I502A channels. As an anti-oxidant, tanshinone IIA (4 mmol/L) inhibited the H2O2-induced activation of WT hKv1.5 channels. In contrast, quercetin had no significant effect. We conclude that: (i) quercetin preferentially binds to and increases the current amplitude of WT hKv1.5 channels; (ii) Ile502, an aliphatic and neutral amino acid residue residing in the S6 segment, is important in quercetin binding; and (iii) quercetin-induced changes in the properties of WT hKv1.5 channels may be foreign to its own anti-oxidant action.