FIGURE 4.

(A) Time course of the pH change in the 100-pL bath during a 10-min pulse train at field strength above the threshold for electrical stimulation of adult ventricular myocytes. The pH-sensitive fluorochrome BCECF was added to the saline buffered with 1 mM HEPES and excited every 200 ms for 5 ms to avoid bleaching. The change in emission was related to the initial emission and plotted against time. Pulses of 1-V amplitude applied to the stimulating electrode which was separated from the reference electrode by a 200-μm gap resulted in 50-V/cm field strength. Due to the limited diffusion in the pL volume, the pH did not recover from the drop caused by electrolysis at higher field strength (>75 V/cm). (B) The average field strength for suprathreshold stimulation was calculated to 27 V/cm ± 10 V/cm (n = 6). The shaded area indicates the range of field strengths which could possibly be used to continually pace myocytes of different length without impairment through electrolysis/electroporation.