RATE-DEPENDENT EFFECTS OF AJMALINE AND PROPAFENONE ON ATRIOVENTRICULAR-CONDUCTION

The aim of the present study was to characterize the time dependence o f the depressant effects of ajmaline and propafenone on the Ca2+-chann el-dependent tissue of the atrioventricular node in isolated guinea pi g hearts perfused by the method of Langendorff Ajmaline at a concentra tion of 0.03 mu M and propafenone at a concentration of 0.3 mu M cause d a significant and comparable prolongation of the His bundle and atri oventricular conduction time (AVCT). When the pacing cycle length was abruptly shortened from 240 to 180 ms, the mean time constant (tau(on) ) of the rate-dependent AVCT prolongation was comparable for ajmaline and propafenone. In contrast, if the pacing cycle length was abruptly increased from 180 to 240 ms the mean time constant (tau(off)) for ajm aline was significantly higher than for propafenone. The rate-dependen t increase of the atrioventricular effective refractory period was sig nificantly more pronounced in the presence of ajmaline than of propafe none. Ajmaline and propafenone affect the Ca2+-channel-dependent tissu e of the myocardium. The more pronounced rate-dependent effect of ajma line on the atrioventricular effective refractory period may be explai ned by a slower dissociation kinetic from the channel.