The effect of temperature in the range 0-35 degrees C on the resting tension of frog's muscle.

1. It has long been known that a resting muscle shows a reversible ;rubber-like' response to a change of temperature: it develops tension when warmed and this reverses on cooling. It was recently shown (Hill, 1968) that part of the resting tension is due to a weak interaction between the sliding filaments, and it was suggested that the rubber-like tension might be caused by the same process rather than by some inert structural component. Further information on this point has now been obtained from some observations on the effect of temperature on isometric tension at temperatures not far below the level (28-35 degrees C) at which a heat contracture is produced.2. The temperature of a frog's sartorius was changed by moving it up and down in a vessel containing Ringer solution at two different temperatures. The temperature in one part was maintained constant in the range 0-20 degrees C and in the other at 5-35 degrees C. The changes was 90% complete in 12 sec.3. From 0 degrees up to about 23 degrees C the tension developed per 1 degrees C rise of temperature (DeltaP/DeltaT) is almost constant, but as the temperature approaches the level at which a heat contracture appears the value of DeltaP/DeltaT rises steeply. This behaviour indicates that the tension in question has an ;active' rather than a ;passive' origin, and this view is supported by other evidence from experiments with muscles in hypertonic solutions, described in the next paper.4. It was found that the temperature at which a muscle goes into a heat contracture depends on the length at which it is set; the critical temperature is lower in an extended muscle. A shortened muscle at a temperature not far below the critical value will therefore produce a contracture when it is stretched.