Electrotonic influences on action potentials from isolated ventricular cells. This work combines a theoretical study of electrical interactions between two excitable heart cells, using a variable coupling resistance, with experimental studies on isolated rabbit ventricular cells coupled with a variable coupling resistance to a passive resistance and capacitance circuit. The theoretical results show that the response of an isolated cell to an increased frequency of stimulation is strongly altered by the presence of a coupling resistance to another cell. As the coupling resistance gradually is decreased, the stimulated cell becomes able to respond successfully to more rapid stimulation, and then, at levels of coupling resistance that allow conduction between the two cells, the coupled pair of cells exhibits arrhythmic interactions not predicted by the intrinsic properties of either cell. The experimental results show that the isolated rabbit ventricular cell is extremely sensitive to even a very small electrical load, with shortening of the action potential by 50% with electrical coupling to a model cell (of similar input resistance and capacitance to the ventricular cell) as high as 1,000 M omega, even though the action potential amplitude and current threshold are very insensitive to the electrical load. 