| The QT interval (time from the beginning of the QRS complex to the end of the T wave) of the electrocardiogram (ECG) is a measure of the duration of ventricular depolarization and repolarization. When ventricular depolarization is delayed and the QT interval is prolonged, there is an incresed risk of ventricular tachyarrhythmias, which include torsade de pointes (TdP), even sudden cardiac death (SCD). In recent years, more and more pharmaceutical compounds have been found to be associated with QT prolongation by suppressing the rapidly activating component of the delayed-rectifier potassium current, IKr, (encoded by human ether-a-go-go-related gene). The spectrum of such drugs involves antiarrhythmic drugs and non-antiarrhythmic drugs, such as antihistamine, antibiotics and psychotropic drugs. Although incidence of drug-induced QT interval prolongation is low, it is life-threatening since TdP or even SCD was so frequently associated with QT interval prolongation. To detect drug-induced arrhythmogenic potentials and thus manage proarrhythmic pharmaceuticals have attracted international attention. The International Committee for Harmonisation (ICH) issued the guideline:"Safety pharmacology studies for assessing the potential for delayed ventriculat repolarization: QT interval prolongation by human pharmaceuticals"in 2005. The guideline requires that the non-clinical safety assessment for druginduced QT interval prolongation should include following 4 different functional levels: 1) Ionic currents measured in isolated animal or human cardiac myocytes, cultured cardiac cell lines, or heterologous expression systems for cloned human ion channels. 2) Action potential parameters in isolated cardiac preparations or specific electrophysilology parameters indicative of action potential duration in anesthetized animals. 3) ECG parameters in conscious or anesthetized animals. 4) Proarrhythmic effects measured in isolated cardiac preparations or animals. In vitro and in vivo assays are complementary approaches and should be conducted to get general risk estimation.Based on the fact that,up to now, little has been done on the safty assessment on drug-induced QT interval prolongation in China, we are trying to set up an experimental system for cardiac-safty assessment on proarrhythmic risk of potential drugs. The aim of this study was to find proarrhythmic predictive parameters for action potential recordings in isolated papillary muscle of guinea pig by using the conventional glass electrode technique. In addition, the effect of tripterine and aconitine on the action potential parameter we assessed. The study will be of importance for forecasting the adverse effect of drug and reasonably evaluating the foreground of new pharmaceutical compounds in drug research and development.Part.1 Choosing of action potential parameters predictive of arrhythmiasAim: To find action potential parameters highly -associated with drug induced torsade de points.Methods: The conventional glass electrode technique was used to record the action potential of papillary muscle in guinea pig. The effects of four drugs, sparfloxacin, sotalol, amiodarone and phenobarbital sodium were evaluated and compared on action potential parameters, which included APD90, APD90-30, APD90/50. Four drugs were chosen bsaed on the reports that all of them can prolong the QT interval and sparfloxacin and sotalol are torsadogenic, but amiodarone and phenobarbital sodium are not torsadogenic. The effect of these drugs on action potential parameters were also observed at different stimulation frequencies.Results: (1) At the stimulatory frequency of 1Hz, sparfloxacin, sotalol, and amiodarone prolonged the APD90 in a concentration-dependent manner. However, phenobarbital sodium concentration-dependent shortened action potential durations. (2) Sparfloxacin and sotalol significantly prolonged APD90-30, while amiodarone and phenobarbital sodium have no effect on APD90-30. Sotalol and phenobarbital sodium significantly prolonged APD90/50, while sparfloxacin and amiodarone had no effect on APD90/50. The effect of these drugs on action potential parameters were further observed at another two stimulation frequencies of 0.5 Hz and 2 Hz. The results showed that at the stimulatory frequency of 0.5 Hz, sparfloxacin and sotalol prolonged APD90-30, while amiodarone and phenobarbital sodium had no effect on APD90-30. Phenobarbital sodium significantly prolonged APD90/50, while sparfloxacin, sotalol, and amiodarone had no effect on this parameter. At the stimulatory frequency of 2 Hz, sparfloxacin significantly prolonged APD90-30, while sotalol, amiodarone and phenobarbital sodium have no effect on it. Sparfloxacin, sotalol, phenobarbital sodium prolonged APD90/50, while amiodarone had no effect on this parameter. (3) At different stimulatory frequencies, the effects of sparfloxacin and sotalol on APD90 exhibited a marked reverse use-dependence, while the effects of amiodarone on APD90 showed a use-dependence property.Conclusion: The prolongation of APD90-30 at the stimulatory frequency of 1 Hz or reverse- frequency-dependence prolongation of APD90 was highly -associated with drug induced torsade de points.Part 2 The effect of tripterine and aconitine on action potentialsAim: To investigate the effect of tripterine and aconitine on action potential in the isolated papillary muscle of guinea pig.Methods: The conventional glass electrode technique was used to record the action potential of papillary muscle in guinea pig. Action potential parameters APD90, APD90-30, or reverse use-dependence in the presence of tripterine, aconitine under different stimulation frequency (0.5 Hz,1 Hz,2 Hz) were observed.Results: Tripterine (10-6M -10-1M) had no effect on action potential durations (APD90) and APD90-30. Aconitine significantly prlonged APD90-30 although it shortened action potential in a concentration-dependent manner (0.01μmol/L-1μmol/L), with IC50 0.3μmol/L.Conclusion: Tripterine had no effect on action potential in isolated level. Aconitine significantly prolonged late repolarization although it shortened action potential duration, suggesting of its proarrhythmic potential. |
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