Electromechanical characterization of cinnamophilin, a natural thromboxane A2 receptor antagonist with anti-arrhythmic activity, in guinea-pig heart.

BACKGROUND AND PURPOSE: Cinnamophilin, a thromboxane A(2) receptor antagonist, has been identified as a prominent anti-arrhythmic agent in rat heart. This study aimed to determine its electromechanical and anti-arrhythmic effects in guinea-pig hearts. EXPERIMENTAL APPROACH: Microelectrodes were used to study action potentials in ventricular papillary muscles. Fluo-3 fluorimetric ratio and whole-cell voltage-clamp techniques were used to record calcium transients and membrane currents in single ventricular myocytes, respectively. Intracardiac electrocardiograms were obtained and the anti-arrhythmic efficacy was determined from isolated perfused hearts. KEY RESULTS: In papillary muscles, cinnamophilin decreased the maximal rate of upstroke (V(max)) and duration of action potential, and reduced the contractile force. In single ventricular myocytes, cinnamophilin reduced Ca(2+) transient amplitude. Cinnamophilin decreased the L-type Ca(2+) current (I(Ca,L))(IC(50)=7.5 microM) with use-dependency, induced a negative shift of the voltage-dependent inactivation and retarded recovery from inactivation. Cinnamophilin also decreased the Na(+) current (I(Na)) (IC(50)=2.7 microM) and to a lesser extent, the delayed outward (I(K)), inward rectifier (I(K1)), and ATP-sensitive (I(K,ATP)) K(+) currents. In isolated perfused hearts, cinnamophilin prolonged the AV nodal conduction interval and Wenckebach cycle length and the refractory periods of the AV node, His-Purkinje system and ventricle, while shortening the ventricular repolarization time. Additionally, cinnamophilin reduced the occurrence of reperfusion-induced ventricular fibrillation. CONCLUSIONS AND IMPLICATIONS: These results suggest that the promising anti-arrhythmic effect and the changes in the electromechanical function induced by cinnamophilin in guinea-pig heart can be chiefly accounted for by inhibition of I(Ca,L) and I(Na).