Scaling of intrinsic contractile properties and myofibrillar protein composition of fast muscle in the fish Myoxocephalus scorpius L

Contractile properties were determined for fast muscle fibres from short-horn sculpin (Myoxocephalus scorpius L.) ranging from 5 to 35 cm total body length (L) and from 2.0 to 758g body mass. Maximum unloaded shortening velocity (V₀) decreased with increasing body size, scaling as 19.5L^-0.³⁴ and isometric twitch activation and tetanus relaxation times became longer, scaling as 12.0L⁰.³¹ and 19.5L⁰.⁴² respectively. Myofibrillar Mg²⁺/Ca²⁺-ATPase activity scaled as 2.51L^-0.²⁸. In order to investigate the mechanisms underlying the scaling of contractile properties, myofibrillar protein composition and I filament lengths were determined. One-dimensional SDS-PAGE and two-dimensional isoelectric focusing/non-equilibrium isoelectric focusing-PAGE revealed no differences in the myofibrillar protein isoforms of myosin light chains, actin, tropomyosin, troponin-T and troponin-C in fish of differing body size. Peptide maps of purified myosin heavy chains digested with eight different proteolytic enzymes were also similar in all fish examined. Three isoforms of troponin-I were present in fish less than 20 cm in total length with relative molecular masses of 17 (TnI(f3)), 22 (TnI(f2)) and 23 (TnI(fl)). The ratio of TnI isoforms varied with body length, and only TnI(f3) was present in fish greater than 28 cm total length. The length of I filaments was independent of body length. Thus, although the expression of TnI isoforms changes during growth, the underlying mechanism responsible for increased shortening velocity with decreased body size remains unknown.