Abstract 5349: Dual Effects of Nifekalant on I Kr

Abstract

Purpose: Nifekalant, a potent and common antiarrhythmic drug, has been regarded to treat reentry arrhythmias through I Kr block (antagonist action). But, recently nifekalant was reported to increase I Kr at slight depolarizing level (agonist action) and details are still not known. In this study, profiles and molecular mechanisms of I Kr antagonist and agonist actions by nifekalant were comparatively investigated. Methods: I Kr was measured in rabbit ventricular myocytes. hERG currents were expressed in Xenopus oocytes . An Ala-scanning mutagenesis of hERG channel was performed in S4-S5 linker regions and S6 domain to seek for important amino acid residues to develop agonist action. Results: In I Kr of myocytes, nifekalant (0.3 μM) suppressed currents at all test voltages ranging from −60 mV to +40 mV and showed antagonist action. However, when a preceding depolarizing pulse (+20 mV for 2 sec) was applied, current amplitudes below −20 mV were transiently increased and showed agonist action. When repetitive short depolarizing pulses, mimic action potentials, were applied, agonist action was enhanced at higher frequency. When a preceding depolarizing pulses was prolonged, agonist action rapidly increased (+20 mV, time constant: 428±31 ms, n=4). When an interval at −90 mV between a preceding pulse and a tested depolarization to −50 mV was prolonged, currents were slowly decreased with time constant of 306±55 sec (n=4). These time constants were relatively similar to onset and offset time constant of current blocking by antagonist action (onset at 20 mV: 769±33 ms, offset at −90 mV: 218±44 sec, n=4). Nifekalant shifted activation curve to a hyperpolarizing direction by 28.0±2.5 mV (n=4), but did not affect inactivation curve. In 2 mutations made in the S4–S5 linker, which could affect activation gate, agonist action was abolished but antagonist action was maintained. Conclusions: Our results indicated that both antagonist and agonist actions on I Kr by nifekalant may exhibit continuously within normal range of heart rate, thereby contributing to antiarrhythmic effects. Agonist action may be derived by two factors; first, a binding to amino acid residues same as antagonist action and, second, an interaction with activation gate.