Electrophysiological effects of EGIS-7229, a new antiarrhythmic agent, in isolated mammalian and human cardiac tissues

The cellular electrophysiological effects of EGIS-7229 (5-chlor-4-[N-(3,4-dimethoxy-phenyl-ethyl)- amino-propylamino]-3(2H)-pyridazinone fumarate), a novel antiarrhythmic agent, was studied using conventional microelectrode techniques in canine cardiac Purkinje fibers and papillary muscle preparations obtained from man, rabbits and guinea pigs. Low concentration of EGIS-7229 (3 mu mol/l) selectively lengthened action potential duration (both APD(50) and APD(90)) in all preparations. The effect of higher concentrations (30-100 mu mol/l) of EGIS-7229 on action potential duration was variable depending on the preparation studied: in rabbit and human papillary muscles both APD(50) and APD(90) were lengthened, in canine Purkinje fibers APD(90) was lengthened but APD(50) was shortened, while in guinea pig papillary muscles both APD(50) and APD(90) were shortened by high concentrations of the drug. At these higher concentrations EGIS-7229 also decreased the maximum velocity of action potential upstroke (V-max) and depressed the plateau of action potentials without affecting the resting membrane potential or action potential amplitude. Both reduction of V-max and lengthening of APD were frequency dependent. The former effect was more prominent at higher pacing frequencies, while the latter was more pronounced at lower driving rates. In guinea pig papillary muscle, the time constant of recovery from V-max block was 719 +/- 33 ms (n = 18) and the rate of onset of the block was 1.81 +/- 0.06 AP(-1) (n = 16) in the presence of 100 mu mol/l EGIS-7229. EGIS-7229 had a complex action on refractoriness in guinea pig papillary muscles: ERP was lengthened at low concentrations (3 to 10 mu mol/l), unchanged at 30 mu mol/l and shortened at 100 mu mol/l. The ratio of ERP/APD(90) however, was significantly increased at concentrations higher than 3 mu mol/l. In canine Purkinje fiber when the delayed rectifier K current (I-K) was blocked by d-sotalol (60 mu mol/l) and APD was shortened back to its control value by additional application of nicorandil (15 mu mol/l), APD was not affected by 3 mu mol/l but was shortened by 30 mu mol/l of EGIS-7229. 100 mu mol/l EGIS-7229 shortened APD in guinea pig papillary muscle. This effect of EGIS-7229 was effectively prevented by nifedipine pretreatment (10 mu mol/l). In this preparation, EGIS-7229 also decreased the V-max of the slow action potential, evoked in the presence of 20 mmol/l external K+ plus 0.5 mmol/l Ba2+. It is likely that EGIS-7229 at low concentrations blocks I-K in human, canine, rabbit and guinea pig cardiac preparations, but at higher concentrations also inhibits Ca and Na currents. Therefore, EGIS-7229 appears to carry mixed class III, IV and IB antiarrhythmic properties.