(Circulation. 1996;93:407-411.)
© 1996 American Heart Association, Inc.
Articles |
Extracellular Potassium Modulation of Drug Block of IKr
Implications for Torsade de Pointes and Reverse Use-Dependence
From Vanderbilt University School of Medicine, Departments of Medicine and Pharmacology, Nashville, Tenn.
Correspondence to Dan M. Roden, MD, Director, Division of Clinical Pharmacology, 532 Medical Research Bldg, Vanderbilt University School of Medicine, Nashville, TN 37232-6602.
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 (IKr). In this study, we evaluated the effect of changing extracellular potassium ([K+]o) on IKr block by the nonspecific agent quinidine and by the specific IKr blocker dofetilide.
Methods and Results IKr was measured in AT-1 cells, where contaminating outward currents are absent. The drug concentration producing 50% inhibition of IKr tails (IC50) was strikingly [K+]o-dependent. Elevating [K+]o from 1 to 8 mmol/L increased the IC50 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 IKr, with substantial clinical implications.
Key Words: potassium • torsade de pointes • ions
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