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Background: N588K-KCNH2 and V307L-KCNQ1 mutations lead to a gain-of-function of IKr and IKs thus causing short-QT syndromes (SQT1, SQT2). Combined pharmacotherapies using K(+) -channel-blockers and β-blockers are effective in SQTS. Since β-blockers can block IKr and IKs , we aimed at determining carvedilol's and metoprolol's electrophysiological effects on N588K-KCNH2 and V307L-KCNQ1 channels., Methods: Wild-type (WT)-KCNH2, WT-KCNQ1 and mutant N588K-KCNH2 and V307L-KCNQ1 channels were expressed in CHO-K1 or HEK-293T cells and IKs and IKr were recorded at baseline and during β-blocker exposure., Results: Carvedilol (10 μM) reduced IKs tail in WT- and V307L-KCNQ1 by 36.5 ± 5% and 18.6 ± 9% (P < 0.05). IC50 values were 16.3 μM (WT) and 46.1 μM (V307L), indicating a 2.8-fold decrease in carvedilol's IKs -blocking potency in V307L-KCNQ1. Carvedilol's (1 μM) inhibition of the IKr tail was attenuated in N588K-KCNH2 (4.5 ± 3% vs 50.3 ± 4%, WT, P < 0.001) with IC50 values of 2.8 μM (WT) and 25.4 μM (N588K). Carvedilol's IKr end-pulse inhibition, however, was increased in N588K-KCNH2 (10 μM, 60.7 ± 6% vs 36.5 ± 5%, WT, P < 0.01). Metoprolol (100 μM) reduced IKr end-pulse by 0.23 ± 3% (WT) and 74.1 ± 7% (N588K, P < 0.05), IKr tail by 32.9 ± 10% (WT) and 68.8 ± 7% (N588K, P < 0.05), and reduced IKs end-pulse by 18.3 ± 5% (WT) and 57.1 ± 11% (V307L, P < 0.05) and IKs tail by 3.3 ± 1% (WT) and 45.1 ± 13 % (V307L, P < 0.05), indicating an increased sensitivity to metoprolol in SQT mutated channels., Conclusions: N588K-KCNH2 and V307L-KCNQ1 mutations decrease carvedilol's inhibition of the IKs or IKr tail but increase carvedilol's IKr end-pulse inhibition and metoprolol's inhibition of tail and end-pulse currents. These different effects on SQT1 and SQT2 mutated channels should be considered when using β-blocker therapy in SQTS patients., (© 2013 Wiley Periodicals, Inc.)