The force—velocity relation of isolated twitch and slow muscle fibres of Xenopus laevis

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Abstract
The relation between force and speed of shortening, or lengthening, in isolated twitch and slow muscle fibers, dissected from the iliofibularis muscle of X. laevis was studied. Both after-loaded and quick-release contractions were studied. Twitch fibers were stimulated electrically to give tetanic contractions (5-20.degree. C). Slow fibers were activated by a rapid change to solutions with high K concentration (30-75 mM; experiments at 21-24.degree. C). The velocity of slow fibers was constant during shortening over 10% length change in after-loaded contractions, except at forces exceeding about 0.8 of isometric tension, Po. In quick-release experiments, shortening velocity was found to depend not only on the relative load, P/Po, but also on the instant when the release was made. With increasing time after onset of contraction the initial rate of shortening decreased. A progressive fall in speed during shortening became more marked. The relation between the relative force, P/Po, and shortening velocity, V, in after-loaded contractions (75 mM-K) and quick-release contractions (45 mM-K, early releases) in slow fibers could be represented by a hyperbola with the constants a = 0.10Po, b = 0.11 lengths/s; extrapolated Vmax was 1.10 lengths/s. a/Po for twitch fibers was independent of temperature in the range 5-20.degree. C. Q10 for b was 2.24 (10-20.degree. C) and 2.86 (5-10.degree. C). Vmax for twitch fibers was 6.34 lengths/s at 22.5.degree. C, the average temperature in the slow fiber experiments. The maximum shortening velocity in twitch fibers is 6 .times. higher than in slow fibers. Factors affecting the shape of the force-velocity curve are discussed. Calculations based on A.F. Huxley''s model for muscle contraction indicated that cross-bridge turnover rate is about 15 .times. lower in slow than in twitch fibers.