An experimental model of the production of early after depolarizations by injury current from an ischemic region

An ischemic myocardial region contains cells with a depolarized resting membrane potential. This depolarization leads to an intercellular current flow between the ischemic region and the surrounding normal myocardial cells which has been termed an injury current. We have devised an experimental model system in which an isolated guinea pig ventricular cell is electrically coupled to a model depolarized cell in order to evaluate the effects of this injury current on the electrical properties of a normal ventricular cell exposed to drugs which increase calcium current or decrease potassium current. Using low doses of isoproterenol, forskolin, or Bay K 8644 (or 8-bromo-cyclic adenosine monophosphate in the pipette) we found that the action potential duration of the isolated cell was lengthened, but that early after depolarizations (EADs) were not produced unless the cell was also coupled to a depolarized cell model representing an adjacent ischemic region. A similar prolongation of the action potential was produced by low doses of quinidine, but EADs were not produced unless coupling to a depolarized cell model was added. EADs could not be produced in any cells in the absence of the drugs even though the coupling to the depolarized cell model was increased up to the level at which the action potential was indefinitely prolonged. At higher isoproterenol concentrations, EADs or spontaneous activity were produced without coupling to the depolarized cell model. Under these conditions, coupling of the cell to a cell model with normal resting membrane potential stopped the spontaneous activity and prevented the occurrence of EADs even with high levels of resistive coupling. These findings suggest that the electrotonic influences of a localized depolarized region can produce EADs if the calcium current magnitude is increased, which would be the case for sympathetic innervation.