uk-66914 and Myocardial-Ischemia

1. Microelectrode recording techniques were used to study the effects of several potassium channel blockers which are considered to be Class III antiarrhythmic compounds. The effects of (+)-sotalol, UK-66,914, UK-68,798 and E-4031 on action potential duration (APD) were determined in guinea-pig isolated papillary muscles. The compounds were evaluated under normoxic or hypoxic/ischaemic conditions at 36.5 degrees C and compared to glibenclamide, which is considered to be a blocker of ATP-dependent potassium channels. Prolongation of action potential duration at 90% repolarization (APD90) was taken as an indirect measure of potassium channel blockade. 2. Under normoxic conditions, the Class III compounds prolonged APD in a concentration-dependent manner. According to EC15 values, the order of potency of the Class III compounds was found to be UK-68,798 > E-4031 > UK-66,914 > (+)-sotalol. Glibenclamide did not significantly prolong APD90 under normoxic conditions. 3. Perfusion with an experimental hypoxic or ischaemic bathing solution produced qualitatively similar effects on action potentials. Over a period of 20-25 min in either of the experimental solutions, there was a small decrease in action potential amplitude (APA) and a prominent shortening of APD. The ischaemic solution also depolarized the resting membrane potential by about 15 mV. 4. (+)-Sotalol and UK-66,914 did not reverse the shortening of APD induced by perfusion with hypoxic Krebs solution. High concentrations of glibenclamide (10 microM) and UK-68,798 (30 and 60 microM) partially reversed the hypoxia-shortened APD. Glibenclamide was more potent and exhibited a greater time-dependent action than UK-68,798. 5. During experimental ischaemia, the Class III compound E-4031 (10 microM, n = 7) produced small, but significant, increases in the APD90 (11 +/-3 ms after 20 min) which were not clearly time-dependent(14 +/- 4 ms after 30 min). UK-68,798 (10 microM) also produced a small, but insignificant, increase in APD90(12 =/-6 ms at 20 min, n = 4). Higher concentrations of UK-68,798 (30 and 60 microM, n = 4) did not produce a consistently significant increase in APD90 during ischaemia: significance was only attained after 20 min in the presence of 60 microM UK-68,798 (24 +/- 12 ms). However, in marked contrast to the effects of the Class III compounds, glibenclamide (10 microM) produced large time-dependent increases in ischaemic APD90 (34 +/- 11 ms at 7 min, n = 9) which were significant 15 min or

Topics: Action Potentials; Animals; Anti-Arrhythmia Agents; Electrophysiology; Glyburide; Guinea Pigs; Heart; Hypoxia; In Vitro Techniques; Microelectrodes; Myocardial Ischemia; Papillary Muscles; Piperidines; Potassium Channels; Pyrazines; Pyridines; Sotalol

The effective refractory period was measured in paced (4 Hz) perfused guinea-pig hearts in vitro. The effective refractory period was linearly correlated with temperature of the perfusing solution: as temperature was reduced the effective refractory period was increased. Reduction of the coronary flow rate to 10% of control resulted in a marked reduction in the effective refractory period. UK-66,914, dofetilide, ibutilide and phentolamine caused a prolongation in the effective refractory period, but during ischaemia the effective refractory period was reduced by the same degree as in vehicle-treated hearts. Glibenclamide had no effect on the effective refractory period prior to ischemia but it abolished the ischaemia-induced shortening. These results suggest that the opening of KATP channels may be responsible for the ischaemia-induced shortening of the effective refractory period in perfused guinea-pig hearts and that the class III effects of UK-66,914, dofetilide and ibutilide are attenuated during ischaemia.

Topics: Analysis of Variance; Animals; Anti-Arrhythmia Agents; Coronary Circulation; Electrocardiography; Glyburide; Guinea Pigs; Heart; Male; Myocardial Ischemia; Phenethylamines; Phentolamine; Pyrazines; Pyridines; Refractory Period, Electrophysiological; Sulfonamides; Temperature

1. UK66,914 is a specific and selective blocker of the delayed rectifying potassium current (IK). The effectiveness of IK block as a mechanism for prevention of ischaemia- and reperfusion-induced arrhythmias was tested by use of UK66,914: its actions in rat, a species deficient in cardiac IK were compared with its actions in rabbit, a species possessing functional cardiac IK. Antiarrhythmic actions in rabbit but none in rat is the only outcome possible if selective IK blockade is responsible for the antiarrhythmic actions of the drug during ischaemia and/or reperfusion. 2. During 30 min regional ischaemia, 0.3 and 1 microM UK66,914 had no influence on the incidence of ventricular fibrillation (VF) in rat (n = 9/group), values being 78% in controls, 100% in 0.3 microM-treated hearts and 78% in 1.0 microM-treated hearts (NS). UK66,914 also had no effect on reperfusion-induced VF incidence (100% in each group), nor on the latency to onset of ischaemia- or reperfusion-induced arrhythmias. In contrast, in rabbit (n = 13/group), similar concentrations of drug reduced the incidence of reperfusion-induced VF from 77% in controls, to 38% and 31% (P < 0.05) respectively. The incidence of ischaemia-induced arrhythmias was too low in controls to permit detection of an antiarrhythmic effect in rabbit; however no drug-induced proarrhythmia was seen.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: Animals; Anti-Arrhythmia Agents; Arrhythmias, Cardiac; Electrocardiography; Heart; Hemodynamics; In Vitro Techniques; Male; Myocardial Ischemia; Myocardial Reperfusion Injury; Potassium; Pyrazines; Pyridines; Rabbits; Rats; Rats, Wistar; Ventricular Fibrillation

In a perfused guinea-pig heart model of myocardial ischaemia, reducing coronary flow by 95% for four successive 6 min periods caused a reproducible net loss of K+ into the coronary perfusate. This was reduced in a concentration-dependent manner by ATP dependent K+ channel blockers (glibenclamide and glipizide) and calcium channel blockers (verapamil and nifedipine). Other K+ channel blockers (UK-66,914, 4-aminopyridine, R56865 and phentolamine) and beta 1-adrenoceptor and beta 2-adrenoceptor antagonists (betaxolol and ICI118551) did not reduce this loss significantly. A single 30 min low-flow period reliably induced K+ release and ventricular fibrillation in control hearts. Glibenclamide, glipizide and phentolamine suppressed ventricular fibrillation but not ischaemic K+ loss in this model. R56865 and 4-aminopyridine and coadministration of betaxolol and ICI118551 yielded similar results while UK-66,914 suppressed neither. In our model, modulation of ischemic K+ loss and suppression of ventricular fibrillation were not closely associated and appeared to occur via separate mechanisms.

Topics: 4-Aminopyridine; Animals; Anti-Arrhythmia Agents; Benzothiazoles; Calcium Channel Blockers; Disease Models, Animal; Glipizide; Glyburide; Guinea Pigs; In Vitro Techniques; Male; Myocardial Ischemia; Piperidines; Potassium; Potassium Channels; Pyrazines; Pyridines; Sympatholytics; Thiazoles; Ventricular Fibrillation