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. Surprisin gly, 28.5% of the fibers displayed coexpression of both MHC I and IIA, a ph enotype that is present in younger adults in very small percentages. Among these fibers coexpressing MHC I and IIA, the majority had a dominant expres sion of MHC I. Additionally, a small number of fibers coexpressing MHC I an d IIX without any MHC IIA, and fibers coexpressing all three isoforms were observed. Altogether, 52.6% of all fibers examined in these very old subjec ts coexpressed two or three MHC isoforms, The present study provides eviden ce that advanced age leads to a significant elevation of skeletal muscle fi bers displaying coexpression of two MHC isoforms and that a separation into slow and fast fibers in very old individuals may therefore be somewhat mis leading. The clinical significance of the elevated number of fibers coexpre ssing MHC I and IIA is uncertain. (C) 1999 John Wiley & Sons, Inc.