Abstract
AbstractContractures were induced in single slow muscle fibres of Xenopus laevis by step increases in external potassium concentration or by application of solutions containing acetylcholine or choline. [K]o needed to evoke a contracture was lower in slow fibres (about 15 mM‐K) than in twitch fibres (about 25 mM‐K). In [K]o near the mechanical threshold tension developed rapidly after a long initial delay. The relation between [K]o and peak tension was extremely steep. Evidence was obtained that the steepness is not due to a regenerative process. In contrast to contractures caused by an increase in [K]o ACh‐ and choline‐induced contractures started without a delay and they were smoothly graded with concentration of the stimulating agent. The differences in the response can be explained if it is assumed that ACh and choline cause a longitudinal potential gradient whereas potassium depolarizes the cell membrane uniformely.
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