Biphasic effect of cardiac glycosides on action potential duration in isolated purkinje fibers

Despite the historical use of cardiac glycosides, the data describing the e lectrophysiological characteristics of this class of drug are not fully cle ar. The present study reported the biphasic effect of cardiac glycosides, d igoxin (1.25 muM) and acetylstrophanthidin (0.15 muM), on action potential duration in isolated Purkinje fibers by the conventional glass microelectro de technique. At the cycle lengths of 990, 690 and 490 msec., action potent ial duration lengthened within 10 min. and shortened after 10 min. of digox in and acetylstrophanthidin administration. The biphasic effect was observe d at a concentration of 4.0 mM [K+]o. However, at a higher [K+]o concentrat ion of 5.4 mM, only the shortening effect on action potential duration was recorded. These results suggest that the biphasic effect of cardiac glycosi des on action potential duration is related to the concentration of extrace llular potassium and is not related to the stimulating cycle lengths.