Interaction between ventricular cells during the early part of excitation in the ferret heart

The source-load interaction during impulse propagation of excited and unexcited cells respectively was studied in the working myocardium. Velocity of propagation and the shape of the early part of the action potential was measured in ferret papillary muscles or trabeculae with the preparation surrounded by either a large volume or a thin layer of Tyrode solution, the latter situation being established by means of a silicone fluid bath technique. Quick changes between the two situations resulted in changes in the time constant of the foot of the action potential and maximal rate of depolarization of the action potential and in changes in excitation lag of the more deeply placed cells with respect to the surface cells. These results could be explained by the effect of changes in the shape of the wavefront of excitation. With a large Tyrode volume around the preparation, corresponding to physiological conditions, the curved configuration of the wavefront of excitation was so pronounced that one-dimensional cable theory was highly inadequate to describe the conditions for propagation.