O-04-7 CELLULAR MECHANISMS UNDERLYING THE GENESJS AND THE PHARMACOLOGICAL TREATMENT OF CARDiAC ARRHYTHMIAS Otto Hauswirth. Department of Physiology II, University of Bonn, D !X300 Bonn 1. FRG Cardiac rhythm disturbances may not only bs caused by a shortening of the action potential duration (APO) based on modifications of various membrane current components (Noble,l984) and by a low conduction velocity due to changes of extracellular ion concentrations or by an increase of the gap junction resistance: they may also be due to hypoxia, various toxins, Ca* +-overload and stretch leading to triggered activity, as well as to early and late afterpotentials and reentry based on unidirectional block. The CAST study triggered the search for arrhythmogenlc side effects, especially in class Ic. In the case of flecainlde, it was experimentally shown that the use dependent and tonic block of the fast sodium channel underlying antlarrhythmic as well as arrhythmogenic properties depend on heart rate and on concentradtion. Cardiac glycocides may cause a transient inward current (I) underlying transient depolarization (TO). Recent experimental data indicate that captopril like other ACE-inhibitors exert antiarrhythmic as well as positive inotropic effects.