Abstract
This report concerns structural forces in resting muscle and proposes three additions to the sliding filament model to account for these mechanical properties. The proposal includes: connecting filaments (C-filaments) which connect the ends of each thick filament to the neighboring Z-lines, core filaments which support the myosin of the thick filament and which attach to the C-filaments, and side-struts which bind the thick filaments together along their length and restrict their radial movement. C-filaments would act as the parallel elastic element and transmit the passive tension to the thick filaments.
Isolated myofibrils (mechanically-skinned and detergent-treated frog semitendinosus fibers) when stretched progressively showed exponentially-increasing passive tension which did not disappear when filament overlap was exceeded, but continued to rise. SL was monitored with a HeNe laser. Passive tension phasically exceeded 3x105 N/M2.
Electron microscopy (thin-sectioned and freeze-fracture/deep-etch specimens) of non-overlap fibers showed orderly fibril structure with clear separation of A- and I-bands. In the gap between them could be seen filaments, 4.0-50 Å in diameter, connected to the thick filament ends. Unlike actin, these filaments did not become decorated by myosin S-1.
Equatorial X-ray measurements showed that stretching relaxed skinned muscles squeezed the thick filaments closer; this radial compression continued beyond filament overlap.
Extreme stretch of fibers caused the thick filaments to strain several-fold.
Treatment of non-overlap fibers with a high ionic strength pyrophosphate myosin solvent caused a large drop in passive tension and stiffness, but no change in SL was detected nor was myofibril continuity detectably affected.
Non-overlap fibrils, when treated with elastase, released A-segments which retain three-dimensional coherency. Deep-etch EM’s of non-overlap fibers disclosed abundant structures (about 75 Å) wide attaching adjacent thick filaments.
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Magid, A., Ting-Beall, H.P., Carvell, M., Kontis, T., Lucaveche, C. (1984). Connecting Filaments, Core Filaments, and Side-Struts: A Proposal to Add Three New Load-Bearing Structures to the Sliding Filament Model. In: Pollack, G.H., Sugi, H. (eds) Contractile Mechanisms in Muscle. Advances in Experimental Medicine and Biology, vol 37. Springer, Boston, MA. https://doi.org/10.1007/978-1-4684-4703-3_26
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DOI: https://doi.org/10.1007/978-1-4684-4703-3_26
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