Taking the first steps in contraction mechanics of single myocytes from frog heart

Intact or skinned atrial and ventricular myocytes from frog heart were mounted horizontally between the lever arms of a force transducer and a servo-controlled electromagnetic loud-speaker "motor" in a trough filled with Ringer or relaxing solution. The myocyte length-sarcomere length relation for intact preparations at rest is linear at least in the range from l0 (sarcomere length about 2.1 microns, resting force zero) to 1.6 l0 (resting force about 100 nN). The peak force value for control twitches (21-23 degrees C, stimulus interval 10 s, [Ca2+]o 1 mM) varies from 20 to 100 nN in atrial and ventricular intact myocytes. The effects induced by isoprenaline or changes in [Ca2+]o, stimulation pattern and bath temperature on twitch characteristics are comparable to those observed in multicellular preparations. The steady force produced by maximally Ca(2+)-activated skinned myocytes is much greater than that developed in control twitches and varies from 0.5 to 3.5 microN in different cells. The saturating pCa in the activating solution is around 5.50. The force response of a resting myocyte to slow ramp stretches shows an initial velocity- and length-dependent component during the stretch itself and, after completion of the length change, a gradual recovery towards a steady level which only depends on the stretch extent. The force response of a stimulated myocyte to length steps complete in 2 ms consists of an apparently elastic change during the step itself and then of a rapid partial recovery followed by slowering of recovery. Whether or not the force recovery includes different phases as reported for skeletal muscle remains unclear.