Sodium channel-blocking properties of spiradoline, a kappa receptor agonist, are responsible for its antiarrhythmic action in the rat.
ABSTRACT Spiradoline (U-62,066E), a selective kappa (kappa) receptor agonist, was examined for actions on the cardiovascular system and on myocardial ionic currents in rats. We initially characterized cardiac, hemodynamic, and antiarrhythmic actions of spiradoline in isolated perfused rat hearts and pentobarbital-anesthetized rats. Electrophysiologic studies in isolated myocytes were used to elucidate the mechanism for changes observed in vivo in the ECG, as well as for antiarrhythmic actions against electrical and ischemia-induced arrhythmias. In isolated rat hearts, spiradoline reduced heart rate and cardiac contractility and increased the PR interval and QRS width of the ECG in a concentration-dependent manner. In anesthetized rats, spiradoline dose-dependently reduced blood pressure and heart rate and prolonged the PR interval and QRS width. At slightly higher doses, it increased the QaT interval of the ECG. RSh, an index of sodium channel blockade in the rat, also was dose-dependently increased. Electrical stimulation of the left ventricle suggested that spiradoline may exert its antiarrhythmic action by blockade of myocardial sodium currents. The electrophysiologic actions of spiradoline on sodium currents, the transient outward (i(to)) and sustained plateau potassium (ik(sus)) currents were studied in isolated cardiac rat myocytes by whole-cell patch-clamp techniques. Spiradoline (15-500 microM) reduced peak sodium current in a rapid, reversible, and concentration-dependent manner; it also increased the rate of decay of I(to) and reduced the amplitude of Ik(sus). At a concentration of 150 microM, spiradoline produced a 24 +/- 2 mV hyperpolarizing shift in sodium current inactivation kinetics but did not alter activation processes. Spiradoline showed both tonic and frequency-dependent components of sodium current block. Thus spiradoline produced its antiarrhythmic actions via sodium channel blockade in myocardial tissue, although higher doses also block potassium currents. This combined ion channel-blocking property may be of added clinical benefit in the setting of myocardial ischemia.