The effect of low temperature on the excitation-contraction coupling phenomena of frog single muscle fibres.

1. Potassium contractures are affected by low temperature: the maximum contracture tension is diminished by about 15% at 3 degrees C, while the response time course is greatly prolonged.2. The contractile threshold for potassium contractures is lowered by about 10 mV at 3 degrees C.3. The fibre's membrane is depolarized by approximately the same amount when exposed to solutions with increased potassium concentrations at 20 or 3 degrees C.4. The repriming process, that is, the process by which the fibres recover their contractile ability following a potassium contracture, proceeds about six times slower at 3 degrees C. This effect is not due to failure of the fibres to repolarize in the cold when transferred from a high potassium to a low potassium medium.5. At low temperature repolarization occurs, even though it is somewhat slower. Following the solution change, from 190 mM potassium to a low potassium solution, the initial rate of repolarization is 8.5 mV/sec at 20 degrees C, and 3.4 mV/sec at 3 degrees C. This effect is not sufficient to account for the delay in the repriming process.6. After a potassium contracture, recovery of the fibre's twitching ability at 3 degrees C is also delayed. At a time when twitches have not yet been recovered, membrane potentials of -90 mV and almost normal action potentials can be recorded.