ELECTRO-MECHANICAL COUPLING IN CRAYFISH MUSCLE FIBERS EXAMINED BY THE VOLTAGE CLAMP METHOD

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1. Electro-mechanical coupling in the crayfish short muscle fibers was investigated by the voltage clamp method using intracellular microelectrodes.2. The change in membrane potential was uniformly distributed when depolarization was more negative than -20mV and longer than 100 msec.3. The mechanical threshold potential was -50 to -58mV. The curve relating the tension to the membrane potential was nearly linear in the range between -45 and -20mV for every duration of depolarization. The slope of the curve was 0.3 of the tension at 0mV for 10mV change in potential. The tension-membrane potential curve obtained in the potassium contracture experiments was quite similar to that in the voltage clamp experiments.4. The rising phase of isometric contraction, except for the initial phase within 0.1 sec, was explained by two exponential components. The time constants were 0.4-0.6 sec and 0.08-0.12 sec at 20-25°C, and they were independent of the membrane potential.5. The relaxation phase after the end of depolarization was also explained by three exponential components. The time constants were 0.4-0.65 sec, 0.10-0.12 sec, and 0.05-0.08 sec at 20-27°C, and they were independent of the level and duration of the clamped membrane potential.6. The mechanical threshold potential was unaffected by the change in the speed of depolarization. The maximum rate of tension rise did not decrease until the speed of depolarization was reduced to the critical value of 40-80 mV/sec.