The effect of ageing and immobilization on structure and function of human skeletal muscle fibres

Biopsy samples were taken from vastus lateralis muscle of seven young (YO, age 30.2 +/- 2.2 years), and seven elderly (EL, age 72.7 +/- 2.3 years) subjects and two elderly subjects whose right leg had been immobilized for 3.5 months (EL-IMM, ages 70 and 75). The following main parameters were studied: (1) myosin heavy chain (MHC) isoform distribution of the samples, determined by SDS-PAGE; (2) cross-sectional area (CSA), specific force (P-o/CSA) and maximum shortening velocity (V-o) of a large population (n = 593) of single skinned muscle fibres, classified on the basis of MHC isoform composition determined by SDS-PAGE; (3) actin sliding velocity (V-f) on pure myosin isoforms determined by in vitro motility assays; (4) myosin concentration in single fibres determined by quantitative SDS-PAGE. MHC isoform distribution was shifted towards fast isoforms in EL and to a larger extent in EL-IMM. In EL and, more consistently, in EL-IMM we observed a higher percentage of hybrid fibres than in YO, and noted the presence of MHC-neonatal and of unusual hybrid fibres containing more than two MHC isoforms. P-o/CSA significantly decreased in type 1 and 2A fibres in the order YO --> EL --> EL-IMM. V-o of type I and 2A fibres was significantly lower in EL and higher in EL-IMM than in YO, i.e. immobilization more than counteracted the age-dependent decrease in V-o. The latter phenomenon was not observed for V-f. V-f on myosin 1 was lower in both EL and EL-IMM than in YO. V-f on myosin 2X was lower in EL than in YO, and a similar trend was observed for myosin 2A. Myosin concentration decreased in type I and 2A fibres in the order YO --> EL --> EL-IMM and was linearly related to the PO/CSA values of corresponding fibre types from the same subjects. The experiments suggest that (1) myosin concentration is a major determinant of the lower P-o/CSA of single fibres in ageing and especially following immobilization and (2) ageing is associated with lower V-o of single fibres due to changes in the properties of myosin itself, whereas immobilization is associated with higher V-o in the absence of a change in myosin function.