title: Increase in the degree of coexpression of myosin heavy chain isoforms in skeletal muscle fibers of the very old.
abstract: Myosin heavy chain (MHC) isoform composition was determined in 2264 single skeletal muscle fibers from vastus lateralis muscle of a group (n = 12) of very old subjects (average age, 88 years). The number of fibers containing only MHC I, IIA, or IIX was 19.9%, 27.2%, and 0.3%, respectively. Surprisingly, 28.5% of the fibers displayed coexpression of both MHC I and IIA, a phenotype that is present in younger adults in very small percentages. Among these fibers coexpressing MHC I and IIA, the majority had a dominant expression of MHC I. Additionally, a small number of fibers coexpressing MHC I and IIX without any MHC IIA, and fibers co-expressing all three isoforms were observed. Altogether, 52.6% of all fibers examined in these very old subjects coexpressed two or three MHC isoforms. The present study provides evidence that advanced age leads to a significant elevation of skeletal muscle fibers displaying coexpression of two MHC isoforms and that a separation into slow and fast fibers in very old individuals may therefore be somewhat misleading. The clinical significance of the elevated number of fibers coexpressing MHC I and IIA is uncertain.
authors: Department of Human Physiology, Copenhagen, Muscle Research Centre, August Krogh Institute, University of Copenhagen, Denmark.
authors: J L
authors: JL
authors: Andersen
authors: G
authors: G
authors: Terzis
authors: A
authors: A
authors: Kryger
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D000375:Aging / Q000378:metabolism*; D005260:Female; D006801:Humans; D008297:Male; D018485:Muscle Fibers, Skeletal / Q000201:enzymology*; D018482:Muscle, Skeletal / Q000201:enzymology*; D018995:Myosin Heavy Chains / Q000096:biosynthesis*; D009218:Myosins / Q000096:biosynthesis*
chemical_list: D018995:Myosin Heavy Chains; D009218:Myosins