Electrophysiological characteristics of ischemic cardiac cells in hypoxic and hyperkalemic conditions

The difference in the electrophysiological properties between the subepicardial cells and the subendocardial cells (Purkinje fibers and ordinary myocardial cells) was examined in dogs using a microelectrode technique. The preparations were obtained one hour after coronary occlusion. Immediately after exposure to a hypoxic solution, spontaneous activities could be recorded in neither the subepicardial nor the subendocardial cells. The electrical activities induced by electrical stimulation disappeared 5-10 min after exposure to the hypoxic solution in the subepicardial cells, while they remained in the subendocardial cells. Vmax, action potential amplitude, action potential duration and resting membrane potential of the subendocardial cells were all reduced significantly in the hypoxic solution containing 7 mK K+ in contrast to those in the hypoxic solution containing 4 mM K+ (p less than 0.001). In the hypoxic solution containing 15 mM K+, all cardiac cells depolarized partially and became electrically quiescent in both normal and ischemic cardiac cells. These findings support the idea that during regional hypoxia and hyperkalemia resulting from acute coronary occlusion, heterogeneous changes in electrical activities occur between the subepicardium and the subendocardium in the ischemic regions.