Structural Studies of Glycerinated Skeletal Muscle. I. A-Band Length and Cross-Bridge Period in ATP-Contracted Fibers
An electron microscope study is reported of structural changes during ATP-induced contraction of glycerinated rabbit psoas. In the absence of ATP, A-band length is constant at sarcomere lengths above 1.9 μm, with average length of 1.54 μ. In ATP-treated fibers, A-band length is also constant at sarcomere lengths above 2.0 μm, but the apparent length of A-band decreases to approximately 1.3 μm, as sarcomere length decreases from 1.9 μm to 1.5 μ. The occurrence of short A-bands cannot be attributed to crumpling of thick filaments against Z-lines, since I-bands remain patent; nor to the presence of heterogeneous filaments, since resting muscle does not show comparable heterogeneity, nor to compressive artifacts, which are minor when knife edge is oriented parallel with fiber axis during microtomy. The decrease of A-band length appears related, at least in part, to disarray of terminal cross-bridges as the thick filaments encroach upon the N-line, a structure which becomes evident within the I-band during contraction of glycerinated fibers. In preliminary studies, optical transforms of A-bands from individual sarcomeres reveal a characteristic myosin layer-line pattern as low as 1.5 μm sarcomere length. A cross-bridge repeat of 143 Å is obtained for sarcomeres above 1.6 μm length; however, an appreciable proportion of sarcomeres in the range from 1.5 μm to 1.9 μ length generate meridional reflections less than 143 Å, and as low as 130 Å.
KeywordsThin Filament Fiber Axis Sarcomere Length Thick Filament Knife Edge
Unable to display preview. Download preview PDF.
- Hanson, J. and Huxley, H.E. (1955). The structural basis of contraction in striated muscle. Symp. Soc. Expt. Biology. 9: 228–264.Google Scholar
- Herman, L. and Dreizen, P. (1971). Electron microscopic studies of skeletal and cardiac muscle of a benthic fish. I. Myofibrillar structure in resting and contracted muscle. Amer. Zoologist. 11: 543–557.Google Scholar
- Huxley, H.E. (1980). Muscle Cells. In: The Cell, Vol. 4, pp. 365–481, ed. Brachet, J. and Mirsky, A.R. New York, Academic Press.Google Scholar
- Huxley, H.E. (1965). Structural evidence concerning the mechanism of contraction in striated muscle. In: Muscle. pp. 3–28. Paul, W.M., Daniel, E.E., Kay, C.M., and Monkton, G. Oxford, Pergamon Press.Google Scholar
- Samosudova, N.V. and Frank, G.M. (1971). Change in the ultrastructure of contractile apparatus of striated muscle under toxic contraction. Biophysika. 16: 244.Google Scholar
- Samosudova, N.V., Lyudkovskaya, R.G. and Frank, G.M. (1972). Ultrastructural studies of slow and intermediate isolated frog muscle fibers under toxic contraction. Biophysika. 17: 1055.Google Scholar