Electrophysiological profile of the antiarrhythmic compound asocainol studied on perfused guinea-pig hearts and on isolated cardiac preparations

Sensitivity and specificity of isolated perfused guinea pig heart to test for drug-induced lengthening of QTc

INTRODUCTION

The purpose of this study was to determine the sensitivity and specificity for predicting the liability of a compound to lengthen QTc using isolated, perfused guinea pig hearts (Langendorff preparation).

METHODS

QTc (Fridericia correction) was calculated from bipolar transventricular electrograms. Hearts were exposed to escalating concentrations of 26 compounds thought to lengthen, and 13 compounds thought not to lengthen, QTc in humans.

RESULTS

In this preparation, QTc was found to lengthen in 26 of 26 compounds thought to be positive (sensitivity 1.00) and not to lengthen or to lengthen insignificantly in 13 of 13 compounds thought to be negative (specificity 1.0) in man. Probucol and ontazolast could not be studied because of limited solubility. Successful experiments were conducted on over 98% of guinea pigs anesthetized.

DISCUSSION

We believe that the isolated perfused guinea pig heart is an in vitro preparation that could be utilized early in preclinical testing for identifying a liability to lengthen QTc in humans, but we do not believe--as is true also for other in vitro methods--that the concentration at which the liability is demonstrated in vitro necessarily predicts the concentration at which a liability exists in man.

Assessment of the conduction of the cardiac impulse by a new epicardiac surface and stimulation technique (SST-ECG) in Langendorff perfused mammalian hearts

The assessment of the effects on the conduction of the cardiac impulse and of refractoriness simultaneously from multiple cardiac structures is important to evaluate the mode of action of new compounds, as well as to investigate undesired cardiac side effects. The measurements of intracardiac electrical activities of special structures have required catheter methods or averaging techniques to produce clear deflections of low-level potential wave forms. We have developed an epicardial surface electrocardiogram (S-ECG) recording technique to detect continuously sinus node and His-bundle activities in spontaneously beating Langendorff perfused heart preparations. A bipolar surface stimulation technique electrocardiogram (ST-ECG) could be combined with this S-ECG method to assess the effect of pacing at maximal rate on the refractoriness of the sino-atrial, AV-nodal, and His-bundle conduction, as well as that of the atrial and ventricular myocardium. Epicardial surface electrodes were also used for vector analysis of the signals, which include those of the sinus node and His-bundle potentials, in addition to the usual atrial and ventricular loops. The present results demonstrate that this new epicardial surface ECG recording technique, combined with the described SST-ECG, provided a way to improve or replace more elaborate intracardiac techniques used in isolated hearts or in animal experiments.

Continuous ECG measurements of intracardiac activity from the surface of Langendorff-perfused guinea pig hearts

The present paper describes a method by which it is possible to continuously detect early atrial and His-bundle activity from the surface of intact Langendorff-perfused guinea-pig hearts, by appropriate placement of two electrodes and the use of a custom designed instrumentation-amplifier. In some experiments the surface ECG recordings were compared with intracardiac ECG recordings. No difference in ECG durations could be observed between intracardiac and extracardiac measurements. In further experiments changes in ECG durations and heart rate were measured for 2 h. After 30 min equilibration time, no changes in heart rate and conduction time could be observed. In order to locate the best surface electrode positions to detect His-bundle activity, vector ECG recordings were taken at high gain. This vector ECG signal contained a His-loop which was split into a larger and a smaller part. The main and initial vector was directed to the left and the smaller to the right ventricle. The best recordings of the His-bundle activity could be observed when the electrodes were positioned as follows; one in a posterior position, near the valve plane and the other one in the opposite position near the initial part of the anterior interventricular artery and in the direction of the large His-loop. We conclude that the ECG surface recordings are a valuable tool for measuring impulse propagation through various segments of the cardiac conduction system in preparations of guinea-pig hearts, perfused by the Langendorff method.

Effects on transmembrane action potential, slow inward current and force of contraction in ventricular cardiac muscle of BRL 31660, a new antiarrhythmic drug with class I and class IV activity

The effects of 1-30 mumol l-1 BRL 31660 on transmembrane action potential and force of contraction were investigated in guinea-pig electrically driven papillary muscles. Lidocaine was studied for comparison. BRL 31660 depressed Vmax of the action potential without changing the resting potential, decreased the force of contraction and decreased the action potential duration. Similar effects were obtained with lidocaine. BRL 31660 inhibited the recovery of Vmax from inactivation, the time constant of which was estimated to be about 1,100 ms in the presence of 10 mumol l-1 BRL 31660. The depressive effect on Vmax was particularly pronounced at low (less negative) membrane potentials. BRL 31660 can thus be classified as a class I antiarrhythmic agent of the lidocaine type. Additional voltage-clamp experiments in cow ventricular trabeculae provided evidence that BRL 31660 also depressed the slow inward current at concentrations similar to those producing the effects on the transmembrane action potential. BRL 31660 thus exerted an additional class IV action. This effect was not shared by lidocaine. It is concluded that BRL 31660 is a new antiarrhythmic agent which depresses both the fast and slow inward current at similar concentrations. The dual effects of BRL 31660 conceivably contribute to its antiarrhythmic activity, but the clinical relevance of these results remains to be elucidated.