Temperature dependence of enhancement and diminution of tension evoked by staircase and by tetanus in rat muscle.

1. The effect of temperature (20-37.5 degrees C) on the potentiation of twitch tension was examined during and after the staircase (250 stimuli, 5/sec) and after the tetanus (188 stimuli, 125/sec) in the extensor digitorum longus muscle of adult Lewis rats.2. During the staircase at 20 degrees C the twitch tension decreased (negative staircase) by 10-20%. At 25-30 degrees C the staircase was initially negative and later positive. At 37.5 degrees C the staircase was positive throughout the train. Both at the end of the staircase and 2 sec after the tetanus the potentiation increased linearly with increasing temperature.3. After the staircase and the tetanus at 20-30 degrees C the twitch tension increased initially rapidly and later after the staircase at a slower rate. Maximal potentiation at 20 degrees C was attained 3 min after the staircase (+ 30 +/- 3%, n = 10, s.e. of mean) and 1 min after the tetanus (+ 16 +/- 1%, n = 10, s.e. of mean). At 37.5 degrees C the potentiation decayed rapidly after the staircase and the tetanus.4. During the staircase the time course of the twitch was shortened twice as much at 20 as at 37.5 degrees C. At the end of the staircase and 2 sec after the tetanus the contraction time was the more prolonged the greater the potentiation. At maximal potentiation the contraction time was prolonged three times as much at 20 degrees C (+ 19 +/- 3%, n = 10, s.e. of mean) as at 37.5 degrees C (P < 0.005). The half-relaxation time at the end of the staircase was prolonged 10 times more at 20 than at 37.5 degrees C (P < 0.02).5. When extrapolated to time zero after the staircase and the tetanus the potentiation at 20 degrees C was still marked (20-50%). The rate of decay of potentiation (time constant, 20 degrees C, 561.2 +/- 37.4 sec, n = 20, s.e. of mean) increased with increasing temperature (Q(10) = 2.6). The event of potentiation with a fast rate of decay, present after the tetanus but not after the staircase at 37.5 degrees C, was abolished below 30 degrees C.6. The increase in twitch tension after the staircase and the tetanus at 20-30 degrees C was taken to indicate the recovery of events that diminished the twitch, occurring simultaneously with potentiation.7. (i) One process of diminution, present after the staircase but not after the tetanus, increased on cooling and was assumed to be due to fatigue. The rate of recovery of the process (time constant, 20 degrees C, 79.6 +/- 7.4 sec, n = 10, s.e. of mean) increased with increasing temperature (Q(10) = 1.9). The half-relaxation time of the last twitch in the staircase was the more prolonged the greater the process. (ii) Another process causing diminution was present after the staircase and the tetanus at 20-30 degrees C. It recovered at 20 degrees C with a time constant of 14.9 +/- 2.2 sec (n = 10, s.e. of mean). This process, possibly responsible for the initially negative staircase, was not thought to be due to fatigue. It may reflect a diminished depolarization of the transverse tubules by repetitive stimuli.