Increase in the degree of coexpression of myosin heavy chain isoforms in skeletal muscle fibers of the very old.

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.