INFLUENCE OF STRETCH ON EXCITATION THRESHOLD OF SINGLE FROG VENTRICULAR CELLS

The excitation threshold of enzymatically dissociated single frog vent ricular myocytes was measured as a function of cell length or sarcomer e length. Field stimulation with 3 ms duration rectangular current pul ses was applied via two platinum wire electrodes oriented either paral lel or perpendicular to the long axis of the cell. Excitation threshol d was measured as the amplitude of applied current, with an error of l ess than 1%. Single cells were held isometrically by two glass pipette s and wrapped around an optical fibre, which was used to control cell length. Sarcomere length was measured using video microscopy and a pha se-locked loop system. Changes in cell length and sarcomere length wer e induced by mechanical stretch and release of the whole cell. The exc itation threshold decreased with increasing resting length (preload), and increased with decrease of the cell length back to its original le ngth, regardless of the orientation of the electric field. The measure d change in excitation threshold with stretch was of the order of 1%/% strain. These results suggest that stretch affects the excitation thr eshold of cardiac cells by factors other than simple shape changes in the cell, perhaps through the activity of mechanosensitive ionic chann els.