title: Ionic mechanism of ibutilide in human atrium: evidence for a drug-induced Na+ current through a nifedipine inhibited inward channel.
abstract: This study examined the ionic mechanism of ibutilide, a class III antiarrhythmic in clinical use, on freshly isolated human atrial cells. Cells had resting potentials of -71.4 +/- 2.4 mV, action potentials with overshoot of 36.8 +/- 1.8 mV, duration of 265 +/- 89 msec at 90% repolarization and slow repolarization (n = 16). Ibutilide, at 10(-7) M, markedly increased action potential duration. Four types of outward currents were detected: Ito, Iso, a delayed rectifier and IK1. Ibutilide had no inhibitory effect on these outward currents at 10(-7) M (n = 28). In K(+)-free solutions and -40 mV holding potential, mean peak inward current at 20 mV was -1478 +/- 103 pA (n = 12). Ibutilide increased this current to -2347 +/- 75 pA at 10(-7) M, with half maximal effect (Kd) of 0.1 to 0.9 nM between -10 and +40 mV (n = 21). At similar concentrations, the drug increased APD, with Kd of 0.7 and 0.23 nM at 70 and 90% repolarization, respectively (n = 8). Ibutilide shifted the mid-point of the steady-state inactivation curve from -21 to -12.2 mV (n = 6), and reduced current decline during repetitive depolarization (n = 5). The drug induced inward current was carried by Na+o through a nifedipine inhibited inward channel because Na+o removal eliminated the effect, and nifedipine abolished the inward current and the drug induced APD prolongation. We propose that a Na+ current through the L-type Ca++ channel mediates ibutilide's potent clinical class III antiarrhythmic action.
authors.affiliation: Department of Pharmacology, Pharmacia And Upjohn, Kalamazoo, Michigan, USA.
authors.forename: K S
authors.initials: KS
authors.lastname: Lee
authors.forename: E W
authors.initials: EW
authors.lastname: Lee
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000889:Anti-Arrhythmia Agents / Q000494:pharmacology*; D016275:Atrial Function; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D015220:Calcium Channels / Q000187:drug effects*; D020746:Calcium Channels, L-Type; D004305:Dose-Response Relationship, Drug; D006325:Heart Atria / Q000187:drug effects*; D006801:Humans; D008875:Middle Aged; D009543:Nifedipine / Q000494:pharmacology*; D015222:Sodium Channels / Q000187:drug effects*; D013449:Sulfonamides / Q000494:pharmacology*
chemical_list: D000889:Anti-Arrhythmia Agents; D002121:Calcium Channel Blockers; D015220:Calcium Channels; D020746:Calcium Channels, L-Type; D015222:Sodium Channels; D013449:Sulfonamides; C067192:ibutilide; D009543:Nifedipine