Effects of twitch train on the tetanic contractility of the frog skeletal muscle.

The effects of twitch trains on the contractility of succeeding tetani were investigated in the frog toe muscle. The changes in isometric tension and its first derivative were analyzed. Tetani of 1 sec duration were induced every 5 min and trains of twitches (less than 250) with 0.1 to 3 Hz were interposed between two successive tetani. A twitch train which clearly shows an ascending staircase exerted at least three different effects on the following tetani. These were a rapidly decaying potentiation (P1), a slowly decaying potentiation (P2), and an inhibitory effect (I) which was regarded as fatigue. These after-effects were modified by various interventions. Increasing of the twitch frequency at a constant number of stimuli augmented both the potentiating and the inhibiting effects. When the bath temperature was lowered to 4 degrees C, the potentiation was masked by a marked inhibition. At a higher temperature (28 degrees C) the potentiating effect was facilitated. Prolonged perfusion of low concentration (0.5 mM) of caffeine mimicked the effects of low temperature. Effects of twitch train on the contracture induced by 5 mM caffeine were examined and it was found that the repetitive twitches had no effect on the following caffeine contracture. Although the exact mechanism for these after-effects was not clear, it was assumed that the intracellular calcium turnover may play some role in the observed phenomena.