Caffeine-induced potentiation of twitches in frog single muscle fiber

To clarify the detailed mechanism of caffeine-induced twitch potentiation in single muscle fibers isolated from semitendinosus muscles of the frog, the fibers were stimulated by the different patterns of repetitive pulses. On rapid application of 1 mM caffeine to the fiber, progressive potentiation of the twitches were observed in all cases, and as the stimulation frequencies were increased (from 0.1 to 5Hz), the periods taken to the fully potenitated twitch were shortened from 23 sec to about 4 sec on the average. If the repetitive stimulations at 2 Hz were stopped for a while during the potentiating phase of an ordinarily stimulated fiber, the fully potentiated twitch occurred later than that observed in the ordinarily stimulated fiber. It was confirmed that facceine did not have any significant effects on both the features of the action potential and themagnitude of the resting potential of the single muscle fiber. These results indicate that the responses of the fiber themselves play an important role in the caffeine-induced twitch potentiation, and its mechanism was discussed with reference to the behavior of the fiber membrane and the triadic junction through which caffeine is assumed to act on the sarcoplasmic reticulum.