Isometric and isovelocity contractile performance of red muscle fibres from the dogfish Scyliorhinus canicula
Maximum isometric tetanic force produced by bundles of red muscle fibres from dogfish, Scyliorhinus canicula (L.), was 142.4+/-10.3 kN m(-2) (N=35 fibre bundles); this was significantly less than that produced by white fibres 289.2+/-8.4 kN m(-2) (N=25 fibre bundles) (means +/- S.E.M.). Part, but not all, of the difference is due to mitochondrial content. The maximum unloaded shortening velocity, 1.693+/-0.108 L(0) s(-1) (N=6 fibre bundles), was measured by the slack-test method. L(0) is the length giving maximum isometric force. The force/velocity relationship was investigated using a step-and-ramp protocol in seven red fibre bundles. The following equation was fitted to the data: [(P/P(0))+(a/P(0))](V+b)=[(P(0)(*)/P(0))+(a/P(0))]b, where P is force during shortening at velocity V, P(0) is the isometric force before shortening, and a, b and P(0)(*) are fitted constants. The fitted values were P(0)(*)/P(0)=1.228+/-0.053, V(max)=1.814+/-0.071 L(0) s(-1), a/P(0)=0.269+/-0.024 and b=0.404+/-0.041 L(0) s(-1) (N=7 for all values). The maximum power was 0.107+/-0.005P(0)V(max) and was produced during shortening at 0.297+/-0.012V(max). Compared with white fibres from dogfish, the red fibres have a lower P(0) (49%) and V(max) (48%), but the shapes of the force/velocity curves are similar. Thus, the white and red fibres have equal capacities to produce power within the limits set by the isometric force and maximum velocity of shortening of each fibre type. A step shortening of 0.050+/-0.003L(0) (N=7) reduced the maximum isometric force in the red fibres' series elasticity to zero. The series elasticity includes all elastic structures acting in series with the attached cross-bridges. Three red fibre bundles were stretched at a constant velocity, and force (measured when length reached L(0)) was 1.519+/-0.032P(0). In the range of velocities used here, -0.28 to -0.63V(max), force varied little with the velocity.