piperidines and sematilide

Direct cardiac effects of sematilide, a new class III antiarrhythmic drug, were compared with those of E-4031 and MS-551 in canine isolated blood-perfused heart preparations. Doses of sematilide, E-4031, and MS-551 causing a 10% decrease in the spontaneous sinoatrial beating rate were 58 +/- 15, 9 +/- 5, and 84 +/- 10 micrograms (n = 5); those causing a 10% increase in developed tension of the papillary muscle were 485 +/- 49, 17 +/- 2, and 267 +/- 50 micrograms (n = 6); and those causing a 10% prolongation of effective refractory period (ERP) of the atrioventricular node were 68 +/- 10, 11 +/- 2, and 53 +/- 15 micrograms (n = 5), respectively. There were few effects on atrio-His or His-ventricular intervals. Also, in in situ open-chest dog hearts, the percent increases in ERP of the atrioventricular conduction system caused by 1 mg/kg of sematilide were 21 +/- 3, 16 +/- 2 and 9 +/- 1% at cycle lengths of 800, 600, and 400 ms, respectively (p < 0.01; n = 8). These results indicate that (a) sematilide, as well as E-4031 and MS-551, has direct negative chronotropic and positive inotropic effects and prolongs cardiac refractoriness without affecting conduction velocities; (b) quantitatively, the cardiac effects of sematilide were almost identical to those of MS-551 and five to ten times less potent than those of E-4031; (c) and prolongation of ERP of the atrioventricular conduction system by sematilide occurred in a reverse frequency-dependent manner.

Topics: Animals; Anti-Arrhythmia Agents; Atrioventricular Node; Dogs; Female; Heart Rate; Male; Papillary Muscles; Piperidines; Procainamide; Pyridines; Pyrimidinones; Sinoatrial Node

Saturation binding studies in guinea pig ventricular myocytes with 3H-dofetilide, a radioligand for the cardiac rapidly activating delayed rectifier K+ IKr channel, indicated specific high-affinity binding with a Kd of 83 nM and a Bmax of 0.18 pmol/mg cellular protein (1.36 x 10(6) sites/cell). Using displacement of high-affinity 3H-dofetilide binding as a measure of interaction with the IKr channel, potencies (Ki values) for binding to the IKr channel in guinea pig myocytes for six class III antiarrhythmic agents were characterized and compared to indices of functional electrophysiologic activity in isolated guinea pig papillary muscles [EC25 values, concentration required to increase effective refractory period (ERP) 25% above baseline]. Dofetilide, E-4031, sematilide, and d-sotalol, which have been characterized previously as selective IKr blockers, displayed good agreement between Ki values for displacement of 3H-dofetilide binding (47 +/- 7 nM, 38 +/- 8 nM, 12 +/- 5 microM, and approximately 100 microM, respectively) and EC25 values for increasing ERP in papillary muscles (45.0 nM, 76.9 nM, 20.2 microM and 63.5 microM, respectively). Ibutilide and RP58866, which have been reported to act via mechanisms other than IKr block, had Ki values for displacement of 3H-dofetilide binding (16 +/- 7 nM and 17 +/- 2 nM, respectively) that were approximately 10-fold lower than EC25 values for increasing ERP in papillary muscles (185.8 nM and 223.5 nM, respectively). The potent displacement of high-affinity 3H-dofetilide binding by ibutilide and RP58866 strongly suggest a role for interaction with IKr in their actions.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: Animals; Anti-Arrhythmia Agents; Binding, Competitive; Chromans; Electrophysiology; Guinea Pigs; Heart Ventricles; Papillary Muscles; Phenethylamines; Piperidines; Potassium Channel Blockers; Potassium Channels; Procainamide; Pyridines; Sotalol; Structure-Activity Relationship; Sulfonamides; Tritium

Voltage-clamp experiments were performed on isolated guinea pig ventricular myocytes to examine the voltage dependence of delayed rectifier block by methanesulfonamide channel blockers. Voltage-dependent channel block, in which block decreases as membrane potential is made more positive, could contribute to the phenomenon of reverse use dependence, in which the magnitude of the drug-induced prolongation in action potential duration (APD) is inversely proportional to stimulation rate. To determine whether such a voltage dependence exists, concentration-response curves were generated for dofetilide, E-4031, sematilide, and D,L-sotalol at test potentials ranging from 0 to 60 mV. All agents blocked current in a manner consistent with selective blockade of the rapidly activating component of delayed rectifier current, IKr. The rank order of potency was E-4031 approximately dofetilide > sematilide > sotalol. Block of tail currents by this class of compounds was more potent after test potentials to +60 mV than after those < or = 0-10 mV. These data are inconsistent with voltage-dependent channel block being a contributing factor to reverse use-dependence and suggest that other mechanisms must be responsible for this phenomenon.

Topics: Action Potentials; Animals; Anti-Arrhythmia Agents; Dose-Response Relationship, Drug; Electrophysiology; Guinea Pigs; Heart Ventricles; In Vitro Techniques; Phenethylamines; Piperidines; Procainamide; Pyridines; Sotalol; Stereoisomerism; Sulfonamides; Ventricular Function

This study compares the membrane activity of ibutilide, d-sotalol, sematilide, E-4031 and dofetilide on single ventricular cells under identical experimental conditions. We found that ibutilide and dofetilide produced a 'bell-shaped' concentration-dependent effect on action potential duration. Ionic current measurement showed that ibutilide, at 10(-8) M, increased a late inward current; the other compounds had either no effect or decreased it. Moreover, only ibutilide, at a high concentration of 10(-5) M, increased an outward current, as oppose to a uniform depression of IK by d-sotalol, sematilide, E-4031 and dofetilide, and the depression of IK by the latter compounds could be reversed by 10(-5) M ibutilide. Finally, low concentration of ibutilide could further prolong the action potential duration that had already been prolonged by a K+ channel blocker, but a high concentration of ibutilide did just the opposite by reversing the prolongation caused by K+ channel blockers. Therefore, action potentials agree well with the ionic current results. Possible mechanistic advantage of ibutilide over K+ channel blockers was discussed.

Topics: Action Potentials; Animals; Anti-Arrhythmia Agents; Guinea Pigs; Heart; In Vitro Techniques; Membrane Potentials; Phenethylamines; Piperidines; Potassium Channels; Procainamide; Pyridines; Sotalol; Sulfonamides