Cardiac glycoside toxicity in small laboratory animals

Cardiovascular parameters in anaesthetized guinea pigs: A safety pharmacology screening model

INTRODUCTION

Assessment of cardiovascular functions in vivo is part of the core battery of guideline ICH S7A and is thereby required by regulatory authorities. The haemodynamic effects of repeated intravenous administrations of reference compounds were analyzed in order to validate the guinea pig model for safety pharmacology studies under GLP conditions.

METHODS

Male guinea pigs (n=54, weighing 565-762 g) were anaesthetized using 1.5 g/kg, i.p., urethane. Systolic arterial blood pressure (SAP), diastolic arterial blood pressure (DAP), heart rate (HR), left ventricular pressure (LVP), cardiac contractility (dp/dt(max)), and ECG (RR, QT, and QTc intervals) were recorded continuously. Animals received vehicle i.v. followed by cumulative doses of reference compounds.

RESULTS

Vehicle did not produce any relevant changes, either in cardiovascular or ECG parameters. Isoproterenol caused a rapid and significant increase in HR, LVP, and dp/dt(max), in contrast to a dose-dependent decrease in SAP and DAP. Epinephrine led to a potent increase in all cardiovascular parameters. Nifedipine produced a slight decrease in HR and LVP, and a potent decrease in blood pressure and dp/dt(max). Verapamil caused a dose-dependent decrease in all cardiovascular parameters. Ouabain resulted in a significant increase in SAP, DAP, LVP, and dp/dt(max); ECG showed an atrioventricular block and arrhythmia. Terfenadine, cisapride, and sotalol prolonged QT and QTc intervals, whereas vehicle and the other tested compounds did not produce any prolongation of the QTc interval.

DISCUSSION

Our results on HR, blood pressure, and ECG obtained after i.v. administration of reference compounds show the usefulness of the guinea pig in assessing cardiovascular safety. The anaesthetized guinea pig allows the measurement of cardiac contractility and the use of doses higher than in conscious animals. Using this animal model, several cardiovascular parameters can be recorded simultaneously at a modest cost in terms of test compound and the number of animals required.

Three-dimensional electrocardiography in the rat

Three-dimensional electrocardiography with computerized analysis provides a means to rapidly collect and quantitatively analyze electrocardiographic data from rats. Electrical activity at each point in time can be expressed quantitatively as a vector with magnitude and direction. Rats are fitted with plate and needle electrodes in three mutually perpendicular axes: X (right to left), Y (rostral to caudal), and Z (ventral to dorsal). Electrocardiographic signals are recorded on paper and also collected at 1000/sec using an IBM-compatible computer equipped with a fast A/D convertor. Signals are analyzed using an interactive program written in Turbo PASCAL which transforms data into an array consisting of vector magnitude and direction at each time point. The data are further used to produce both a tabular report and graphic output. This method retains simplicity while extending the accuracy of electrocardiographic (ECG) measurement. It literally adds another dimension of information to the three-lead ECG most commonly measured in rats.