A force transducer and a length-ramp generator for mechanical investigations of frog-heart myocytes

An apparatus for studying the mechanics of isolated frog heart myocytes is described. The cells are held horizontal in a through of Ringer solution by means of two suction micropipettes. Myocyte force is measured with an opto-electronic system recording the deflection of the tip of one micropipette, which acts as a cantilever force probe. The force probes are selected for compliance according to the force a myocyte is expected to develop in a given condition, so as to limit myocyte shortening during force development to no more than 1% of the slack cellular length (l₀). The other micropipette, which is stiff relative to the forces measured, is mounted on an electromagnetic-loudspeaker motor by which controlled-velocity length changes, of preset size and in either direction, are imposed on myocytes. The force transducer has a sensitivity of 5–10 mV/nN, with a frequency response of 700–900 Hz in Ringer solution and a resolution of 0.5–1 nN. The motor with a suction micropipette can complete controlled-velocity length ramps within 1.5–2.0 ms, across a range of ±100 m at a resolution of 8.0 nm. These values correspond, for frog-heart myocytes 200 m and 400 m long, to 25%–50% l₀ and 0.002%–0.004% l₀ respectively.