Fig. 4.

Propidium iodide (PI) staining. To assay for cell membrane disruption caused by OP, the heart was perfused with 30 µM of PI solution. Positive staining indicated permeation Out of the two hearts tested neither showed PI staining at the lowest stimulation threshold (6.3 J/cm²). One out of two hearts at the mid (7.9 J/cm²) and highest thresholds (9.7 J/cm²) exhibited a low level of PI staining. PI staining was found on the epicardial side of the atrium near the position of the optical fiber. The white arrows indicate the stimulation fiber direction. A: Positive control. B: Light staining was evident in one of the hearts when 8 ms pulses containing 7.9 J/cm² per pulse were directed to the heart for one minute (dashed circle). C: Pulses containing 6.3 J/cm² under similar conditions resulted in no apparent damage.