Abstract
1. Contractures were evoked in isolated muscle fibres of the extensor carpopoditi muscle of the crayfish (Astacus fluviatilis) by increasing [K](o) at constant [Cl](o) or at constant [K](o)[Cl](o) product.2. The relation between tension and log [K](o) is S-shaped with a less steep slope if [K](o) was increased at constant [Cl](o). This is due to a smaller drop in membrane potential for a given change in [K](o) in the latter case.3. The curves relating the tension to the membrane potential overlap in either case. In the linear part of the curve, the slope is around 0.3 kg cm(-2)m V(-1).4. The mechanical threshold of contracture is about -55 mV and mechanical saturation is at -20 mV.5. Fibres exert the greatest tension when stretched to 1.25 l(o) (8 kg/cm(2)), where l(o) is the length at which the fibres are just taut in the solution. Tension falls on either side of this optimal length. Tension vanishes when the fibre is stretched to 1.95 l(o).6. Sarcomere length at optimal fibre length is 10.5 +/- 0.3 mu. The A band is 3.95 +/- 0.8 mu long and does not alter during stretch.7. The crayfish muscle fibres were of the phasic type, since they relaxed spontaneously at maintained high [K](o).8. At [K](o) near saturation point, contractures attain the maximum tension in 5 +/- 1.3 sec and the time to half decay is 8.1 +/- 0.5 sec.9. If the contracture is allowed to relax spontaneously, it is not possible to obtain initial tension until after 20-30 min. When the contracture is terminated by a return to low [K](o) after reaching its maximum, but before spontaneous relaxation appears, the fibre is capable of repeatedly exerting the initial tension.10. The rate of recovery after a spontaneously relaxed contracture depends on [K](o) in the solution, in which the fibre lies before evoking the test contracture. The relation of recovery upon log [K](o) is S-shaped and the tension is the greater, the lower the [K](o) in the solution in which recovery is taking place.