Abstract 5349: Dual Effects of Nifekalant on IKr
Purpose: Nifekalant, a potent and common antiarrhythmic drug, has been regarded to treat reentry arrhythmias through IKr block (antagonist action). But, recently nifekalant was reported to increase IKr at slight depolarizing level (agonist action) and details are still not known. In this study, profiles and molecular mechanisms of IKr antagonist and agonist actions by nifekalant were comparatively investigated.
Methods: IKr 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 IKr 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 IKr 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.