Acta Physiol Scand 1982 Apr 01;1144:523-35.

Contractile properties of two varieties of twitch muscle fibres in Xenopus laevis.

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The contractile properties of twitch muscle fibres in the iliofibularis muscle of Xenopus laevis with characteristic differences in light-microscopical appearance have been studied under isometric and isotonic conditions. Type 1 fibres (large, pale) have a short contraction time (tc), a shoulder in the relaxation phase, and a high twitch-tetanus ratio. In type 2 fibres (medium-to-large with abundant mitochondria and lipid droplets) tc is longer, half-relaxation time equals tc, and the twitch-tetanus is lower. The capillary density is about 4 times higher for type 2 than for type 1. Computer-fitted hyperbolic force-velocity curves gave the following Hill constants (20 degrees C) for type 1 (2) fibres: P0*/P0 1.03 (1.15), a/P0* 0.48 (0.26), b 3.51 (1.53) 10/s; extrapolated Vmax (0-0.8P0) was 7.60 (6.27) 10/s. Lowered temperature (10 degrees C) increases the curvature of the P-V relation in type 1 fibres, little effect was seen in type 2. Increased PCO2 depressed the isometric tension in both types; in type 1 fibres the P-V relation became less curved, in type 2 fibres no change in curvature was observed. The results suggest that type 1 and type 2 fibres might contain myosin isozymes with slightly different temperature- and pH-sensitivities.

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