Disproportionate loss of thin filaments in human soleus muscle after 17-day bed rest

• Published in: Muscle & nerve Vol. 21; no. 10; pp. 1280 - 1289
• Main Authors: Riley, Danny A., Bain, James L.W., Thompson, Joyce L., Fitts, Robert H., Widrick, Jeffrey J., Trappe, Scott W., Trappe, Todd A., Costill, David L.
• Format: Journal Article
• Language: English
• Published: New York John Wiley & Sons, Inc 01.10.1998; Wiley
• Subjects: actin; Actin Cytoskeleton - ultrastructure; Adult; Atrophy; Bed Rest; Biological and medical sciences; Biopsy; electron microscopy; Fundamental and applied biological sciences. Psychology; Humans; Life Sciences (General); Male; Middle Aged; Muscle Fibers, Skeletal - ultrastructure; Muscle, Skeletal - pathology; Muscle, Skeletal - ultrastructure; myofilaments; myosin; skeletal muscle; Space life sciences; Striated muscle. Tendons; Time Factors; unloading; Vertebrates: osteoarticular system, musculoskeletal system; Non-Nasa Center; Nasa Discipline Musculoskeletal; Atrophy; Human; Myofibril; Actin; Ultrastructure; Muscular fiber type I; Myosin; Weightlessness; Morphometry; Soleus muscle; Bed rest; NASA Discipline Musculoskeletal; Non-NASA Center
• ISSN: 0148-639X; 1097-4598
• DOI: 10.1002/(SICI)1097-4598(199810)21:10<1280::AID-MUS6>3.0.CO;2-7

Previously we reported that, after 17-day bed rest unloading of 8 humans, soleus slow fibers atrophied and exhibited increased velocity of shortening without fast myosin expression. The present ultrastructural study examined fibers from the same muscle biopsies to determine whether decreased myofilament packing density accounted for the observed speeding. Quantitation was by computer-assisted morphometry of electron micrographs. Filament densities were normalized for sarcomere length, because density depends directly on length. Thick filament density was unchanged by bed rest. Thin filaments/microm2 decreased 16-23%. Glycogen filled the I band sites vacated by filaments. The percentage decrease in thin filaments (Y) correlated significantly (P < 0.05) with the percentage increase in velocity (X), (Y = 0.1X + 20%, R2 = 0.62). An interpretation is that fewer filaments increases thick to thin filament spacing and causes earlier cross-bridge detachment and faster cycling. Increased velocity helps maintain power (force x velocity) as atrophy lowers force. Atrophic muscles may be prone to sarcomere reloading damage because force/microm2 was near normal, and force per thin filament increased an estimated 30%.