Extracellular Potassium Modulation of Drug Block of I Kr

Background Torsade de pointes often occurs with underlying hypokalemia and bradycardia. A common effect of many drugs producing torsade de pointes is block of the rapidly activating component of the cardiac delayed rectifier (I Kr ). In this study, we evaluated the effect of changing extracellular potassium ([K + ] o ) on I Kr block by the nonspecific agent quinidine and by the specific I Kr blocker dofetilide. Methods and Results I Kr was measured in AT-1 cells, where contaminating outward currents are absent. The drug concentration producing 50% inhibition of I Kr tails (IC 50 ) was strikingly [K + ] o -dependent. Elevating [K + ] o from 1 to 8 mmol/L increased the IC 50 for dofetilide block from 2.7±0.9 to 79±32 nmol/L and for quinidine block from 0.4±0.1 to 3.8±1.2 μmol/L. Conclusions (1) The increase in drug block with low [K + ] o provides a mechanism to explain the link between hypokalemia and torsade de pointes. (2) Elevations in [K + ] o occur with myocardial ischemia and with rapid pacing. Possible consequences of blunted drug block with high [K + ] o include loss of drug efficacy with ischemia and with rapid pacing; the latter may contribute to “reverse use-dependent” action potential prolongation. Extracellular potassium is a critical determinant of drug block of I Kr , with substantial clinical implications.