Comparison of three beta-amino anilides: IQB-M-81, lidocaine and tocainide, on isolated rat atria.

The effects of a new beta-amino anilide, IQB-M-81, on electrical and mechanical activity of isolated rat atria were studied and compared with those of lidocaine and tocainide. In spontaneously beating right atria IQB-M-81 produced a dose-dependent decrease in rate, contractile force and maximum following frequency and prolonged the sinus node recovery time and the effective refractory period of atrial fibers. The negative inotropic effect was not modified by pretreating the atria with atropine or practolol. In isolated right atria IQB-M-81 did not block the positive chronotropic and inotropic responses to isoprenaline but shifted the dose-response curve to the right. IQB-M-81 and lidocaine, but not tocainide, reduced in a dose-dependent manner the slow contractions induced by isoprenaline in K-depolarized right atria. These results suggest that in contrast to other beta-amino anilides, IQB-M-81 exerted a negative chronotropic effect and that it presented a class I antiarrhythmic effect on isolated rat atria. A possible class IV antiarrhythmic effect is also discussed.