Contractile force of single heart cells compared with muscle strips of frog ventricle

Single heart cells were obtained from frog ventricle with an enzymatic dispersion technique. Isometric contractile force was measured in these cells by an ultrasensitive force transducer and compared with that generated by multicellular muscle strips under similar conditions. The shape of the single-cell twitch was qualitatively similar to that obtained in intact tissue; however, the time to peak was generally shorter, and the falling phase was prolonged in the single cell compared with the muscle strip. The single-cell contractile force was measured in response to alterations in stimulus rate, resting length, and extracellular Ca²⁺ concentration [Ca²⁺](o) or by addition of epinephrine; in all cases, the force response resembled the physiological response seen in the muscle strip. However, at a constant stimulation rate, the steady-state amplitude of the single-cell twitch exhibited beat-to-beat variations under all conditions tested, whereas that of the muscle strip was essentially constant. These results may prove to be useful in assessing the suitability of the single-cell preparation as a model for the intact tissue.