The elasticity of relaxed insect fibrillar flight muscle.

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Abstract
The mechanical properties of glycerol-extracted fibers from the dorsal longitudinal muscle of Lethocerus [L. cordofanus, L. maximus and L. indicus] were determined by sinusoidal and transient analysis in the time range 1 ms-1000 s, and from rest length to 10% strain for fibers in relaxing and rigor solutions. The fibers behave reversibly up to strains of about 5%, but reach an elastic limit in the range 5-9% strain, depending upon the rate of strain. EM of fibers at different degrees of stretch, and after partial extraction of the contractile proteins, suggest that a connection between the end of the A filament and the Z line, named a C filament, is responsible for the high stiffness of the relaxed muscle. Estimates are made of the compliance of the A, I and C filaments. The mechanical response of the relaxed muscle, over the entire frequency range studied, is assignable to the C filaments. An analysis of the siffness of the fibers at different tensions in activating and relaxing solutions, and in fibers relaxed by orthovanadate, shows that the C filaments still exert their mechanical effect in the active muscle. The response of the active muscle consists of the contribution from the cross-bridges plus that of the C filaments, acting mechanically in parallel. This situation is incompatible with earlier explanations of the fully activated mechanical dynamics of fibrillar muscle. Alternative explanations at the cross-bridge level are described elsewhere.