Optical recordings of ventricular excitability of frog heart by an extracellular stimulating point electrode

To enhance understanding of the excitability of cardiac muscle during rest, an optical technique using the fluorescent voltage sensitive dye di-4-ANEPPS was used. Unlike conventional electrical recordings, optical recordings are free from electrical artifacts and, therefore, allow the observation of the transmembrane potential not only following the stimulation pulse, but also during the pulse itself. Transmembrane potentials (Vm) were recorded optically from frog ventricular epicardium in calcium containing Ringer's solution directly under an extracellular stimulating point electrode. Anodal and cathodal S1 stimuli were applied at rest. As observed by previous investigators, the post-pulse excitatory responses for cathodal pulses, compared with anodal pulses were greater. Changes in transmembrane potential (delta Vm) during the pulse were as expected for a passive cable only for low intensity pulses (< 4 x the cathodal threshold of excitation in diastole, CTE). However, at the higher intensities necessary to produce an excitatory response (> 6-8 x CTE), an "irregular" response in Vm was observed--a reversal of the hyperpolarization during an anodal stimulus pulse and a reversal of the depolarization during a cathodal stimulus pulse. To elucidate further the biophysical basis for this behavior, delta Vm was mapped around the stimulating electrode. During stimulation, regions could be observed having a response with opposite polarity to that under the electrode (i.e., depolarization for an anodal pulse and hyperpolarization for a cathodal pulse). Removal of the bath solution or the addition of channel blockers did not eliminate the occurrence of these regions. These regions appear to be the basis for the irregular behavior of delta Vm directly under the electrode as well as for anodal excitation.