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

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 (t_e), 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) t_e is longer, half-relaxation time equals t_e, 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°C) for type 1 (2) fibres: P_o*/P_o 1.03 (l.15), a/P_o* 0.48 (0.26), b 3.51 (133) I_o/s; extrapolated V_max (0–0.8P_o) was 7.60 (6.27) I_o/s. Lowered temperature (10°C) increases the curvature of the P-V relation in type 1 fibres, little effect was seen in type 2. Increased P_co2 depressed the isometric tension in both types; in type I 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.