Apparatus for recording the small tensions developed by electrically stimulated single intact myocytes of frog heart is described. A laser-light optoelectronic transducer was used. The compliance of the force probes was 10-20 nm/nN, with a frequency response of 600-900 Hz in Ringer solution. The myocyte shortening during an ordinary twitch contraction was no greater than 1% of the slack length. The overall sensitivity of the transducer system was 5-10 mV/nN, with a total noise of 0.5-1 nN peak to peak. The experiments were performed at 20-23 degrees C on either atrial or ventricular myocytes at 2.15-2.2 microns sarcomere length, in 1 mM-Ca2+ Ringer solution. 2. Isoprenaline (5 microM), increases in external Ca2+ concentration ([Ca2+]o), and shortening of stimulus interval potentiated the myocyte twitch tension. The dependence of twitch characteristics on these inotropic interventions for all the atrial and ventricular myocytes tested was comparable to that of multicellular preparations under similar experimental conditions. This implies that the enzymatic isolation procedure had not altered the physiological properties of the myocytes. 3. The stimulus interval-tension relation for premature twitches of atrial and ventricular myocytes showed (i) a very steep rising phase in the region of intervals just longer than 0.52 and 0.66 s (the duration of the mechanical refractoriness in atrial or ventricular cells), (ii) a peak, at intervals of 0.7-0.8 s, where the twitch tension was strongly potentiated compared to that of the controls, and (iii) as the stimulus interval was further increased, a progressive return to the control level. The stimulus interval-tension relation for steady-state conditions exhibited similar characteristics. 4. The degree of tension potentiation by isoprenaline was greater in the controls than in the earliest test twitches. The result was that the stimulus interval-tension relations for isoprenaline-treated myocytes showed a gentler rise and a lower peak than for untreated cells. 5. The stimulus interval-tension relation of the heart is a basic property of its cells. It is related to changes in the activation level. The higher the activation level reached in control twitches, the lower the stimulus interval-dependent potentiation capability.
