Chronic probucol treatment decreases the slow component of the delayed-rectifier potassium current in CHO cells transfected with KCNQ1 and KCNE1: a novel mechanism of QT prolongation.

Indirect effects of drugs on ion channel expression levels on plasma membrane are focused as the cause of QT prolongation, and we explored the chronic effects of QT-prolonging drugs on the slow component of the delayed-rectifier potassium current (IKs). Chinese Hamster Ovary cells expressing IKs channels were constructed by transfecting KCNQ1/KCNE1 genes, and the IKs values were measured using IonWorks Quattro in the population patch-clamp mode. After 24 hours of treatment with IKs blockers (HMR1556, L-768673, or chromanol 293B) or human Ether-à-go-go related gene channel trafficking inhibitors (amiodarone,17-AAG, brefeldin A, pentamidine, thioridazine, or probucol), brefeldin A, pentamidine, and probucol decreased IKs. Probucol, which is a cholesterol-lowering drug and clinically reported to cause QT prolongation, potently inhibited the IKs in a concentration-dependent manner, with a half maximal inhibitory concentration of 149.1 nM. A reduction in the IKs by 1 µM of probucol was observed beginning 2 hours after treatment, and the current was reduced by about 80% at 24 hours. The activation and deactivation time constants of residual IKs currents became faster compared with that in the vehicle-treatment group. Acute application of probucol did not directly inhibit IKs channels at concentrations of up to 10 µM. Western blotting analysis indicated the reduction of multimeric complex of KCNQ1 proteins by probucol treatment but not monomeric form. These results suggest that chronic probucol treatment may contribute to QT prolongation in humans by decreasing the functional IKs channel complexes.