Abstract
The effects of SCN on the twitch responses, the duration of the active state during a single twitch and the negative afterpotential were examined in relation to the exposure time of frog single fibers to SCN-Ringer solution. SCN potentiated the twitch tension and prolonged the duration of twitch. These effects of SCN depended on the exposure time and on the SCN concentration. However, the plateau tension of tetanus was not affected by SCN. The potentiated twitch tension completely recovered its control value, although it required a considerably longer time. SCN enhanced the amplitude of negative afterpotential and prolonged its duration, but these effects of SCN did not depend on the exposure time or on the SCN concentration. On the other hand, the duration of the active state was prolonged by lengthening of the exposure time and by increasing the SCN concentration. These results indicate that a close relation exists between twitch potentiation and the prolongation of the duration of the active state, but not the negative afterpotential. On the basis of these results, the sites of action of SCN and the mechanism of twitch potentiation by SCN are discussed.
