Influence of stretch on excitation threshold of single frog ventricular cells

The excitation threshold of enzymatically dissociated single frog ventricular myocytes was measured as a function of cell length or sarcomere length. Field stimulation with 3 ms duration rectangular current pulses was applied via two platinum wire electrodes oriented either parallel or perpendicular to the long axis of the cell. Excitation threshold was measured as the amplitude of applied current, with an error of less than 1%. Single cells were held isometrically by two glass pipettes and wrapped around an optical fibre, which was used to control cell length. Sarcomere length was measured using video microscopy and a phase-locked loop system. Changes in cell length and sarcomere length were induced by mechanical stretch and release of the whole cell. The excitation threshold decreased with increasing resting length (preload), and increased with decrease of the cell length back to its original length, regardless of the orientation of the electric field. The measured change in excitation threshold with stretch was of the order of 1%/% strain. These results suggest that stretch affects the excitation threshold of cardiac cells by factors other than simple shape changes in the cell, perhaps through the activity of mechanosensitive ionic channels.