Original Contributions Optical Measurements of Transmembrane Potential Changes During Electric Field Stimulation of

We evaluated transmembrane potential changes at the ends of isolated rabbit ventricular myocytes during defibrillation-strength shocks given in the cellular refractory period. The myocytes were stimulated (Si pulse) to produce an action potential. Then a constant-field shock (S2 pulse) with an electric field of 20 or 40 V/cm was given at an S1-S2 interval of 50 msec. The cells were stained with potentiometric dye (di-4-ANEPPS), and the cell end facing the S2 anode or cathode was illuminated with a laser while the fluorescence was recorded. During S2, the cell end facing the S2 cathode became more positive intracellu-larly, whereas the cell end facing the S2 anode became more negative intracellularly. The S2-induced transmembrane potential change at the cell end (AVm) was determined relative to the amplitude of the Si-induced action potential (APA) in each recording (i.e., AVm/APA). In Tyrode's solution containing 4.5 mM potassium, AVm/APA for 40-V/cm S2 was 1.36+0.34 at the cell end facing the S2 cathode and-1.65+0.61 at the cell end facing the S2 anode (n=9). For the 20-V/cm S2, AVMAPA was 0.61i0.33 at the cell end facing the S2 cathode and-0.71i0.33 at the cell end facing the S2 anode (n=6). The AVm/APA was not significantly influenced by 20 mM diacetyl monoxime. These results indicate that large AVm values occurred at the ends of the cells during S2. The calculated values of AVm, assuming a nominal APA of 130 mV, were 177 and-214 mV for the 40-V/cm S2 and 79 and-93 mV for the 20-V/cm S2. The AVm was correlated with cell size (r20.95) and agreed with values predicted by the S2 electric field strength multiplie