Block of hERG channels by berberine - Mechanisms of voltage- and state-dependence probed with site-directed mutant channels

Berberine prolongs the duration of cardiac action potentials without affecting resting membrane potential or action potential amplitude. Controversy exists regarding whether berberine exerts this action by preferential block of different components of the delayed rectifying potassium Current, I-Kr, and I-Ks. Here we have studied the effects of berberine on hERG (I-Kr) and KCNQ1/KCNE1 (I-Ks) channels expressed in HEK-293 cells and Xenopus oocytes. In HEK-293 cells, the IC50 for berberine was 3.1 +/- 0.5 mu M on hERG compared with 11 +/- 4% decreases on KCNQ1/KCNE1 channels by 100 mu M berberine. Likewise in oocytes, hERG channels were more sensitive to block by berberine (IC50 = 80 +/- 5 mu M) compared with KCNQ1/KCNiE1 channels (similar to 20% block at 300 mu M). hERG block was markedly increased by membrane depolarization. Mutation to Ala of Y652 or F656 located on the S6 domain, or V625 located at the base of the pore helix of hERG decreased sensitivity to block by herberine. An inactivation-deficient mutant hERG channel (G628C/S631C) was also blocked by berberine. Together these findings indicate that berberine preferentially blocks the open state of hERG channels by interacting with specific residues that were previously reported to be important for binding of more potent antagonists.