Numerical simulations of a propagating cardiac action potenttal utilizing Reeler-Reuter and Drouhard-Roberge physiological routines for the membrane current have been performed. These action potentials show increases in action potential duration when subjected to strong late stimuli of eiiher positive or negative polarity. The mechanism is the same as that reported in an earlier paper which utilized a different physiological approach: repolarizing stimuli can reset the fast sodium gates locally so that they can be retriggered by diffusive return of charge from surrounding tissue. This results in a large depolarizing transient that lengthens action potential duration. r 1992 Academic Prc~. ln~.
In this work we consider the response of a propagating cardiac action potential (AP) in a nearby region of tissue to a secondary stimulus applied late in the plateau phase of the AP at that particular site. We performed numerical simulations of the event utilizing physiological parameters now appropriate to ventricular myocardium. This brief paper is an extension of an earlier work (I) in which we applied the identical simulation methods and approach but with a different physiological routine. We now can demonstrate increased generality of the results reported originally. We performed standard implicit calculations of a propagating AP in a long enough cardiac fiber-and for a long enough time-so that a free-running AP free of end-effects could be studied. The fiber was then stimulated at a midway position by an injection of transmembrane current. The response of the tissue was noted over the whole region and, in particular, the dramatic voltage changes at the stimulus site were studied. We delineate the mechanism which produces an increase in the cardiac action potential duration (APD) in response to both depolarizing and repolarizing (positive membrane current) stimuli. Note that repolarizing stimuli reduce APD in space-clamped investigations. This effect is a feature of the propagating system and results in modest APD increases for late stimuli of either polarity and its generality indicates it may underlie observations of Dillon and Wit (2) and others.