Electron Microscopic Studies of Skeletal and Cardiac Muscle of a Benthic Fish. I. Myofibrillar Structure in Resting and Contracted Muscle

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Abstract
SYNOPSIS. Electron microscopic studies are reported on glycerinated skeletal and cardiac muscle of a benthic fish, Coryphaenoides species. In white skeletal muscle, the sarcomeres have a resting length of approximately 1.8 μ, with thick filaments 1.4 μ and thin filaments 0.75 μ in length. These dimensions are somewhat shorter than filament lengths of oilier vertebrate muscles, possibly due to the elfect of volume increase during assembly of thick and thin filaments at high hydrostatic pressure. During ATP-induced contraction of Coryphaenoides muscle from sarcomere lengths of 1.8 μ to 1.6 μ, there is a characteristic interdigitation of thick and thin filaments, with decrease in I band length and no change in length of thick or thin filaments. However, in sarcomeres contracted to lengths of 1.5 μ. to 1.2 μ, there is a slight shortening of the A band, apparently due to shortening of thick filaments, that occurs despite the presence of residual I band in the same sarcomeres. There is no obvious crumpling or distortion of thick filaments during contraction to sarcomere lengths as low as 1.0 μ, but filament organization undergoes extensive disarray at sarcomere lengths approaching 0.7 μ. Although effects from heterogeneity of filament length cannot be excluded with certainty, the present evidence does suggest that contraction ot Coryphaenoides muscle from 1.6 μ to 1.0 μ sarcomere lengih is accompanied by shortening of thick filaments consequent to a structural change within the thick filament core.

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