Plantaris muscle weakness in old mice: relative contributions of changes in specific force, muscle mass, myofiber cross-sectional area, and number

Abstract

The age-related decline in muscle function contributes to the movement limitations in daily life in old age. The age-related loss in muscle force is attributable to loss of myofibers, myofiber atrophy, and a reduction in specific force. The contribution of each of these determinants to muscle weakness in old age is, however, largely unknown. The objective of this study is to determine whether a loss in myofiber number, myofiber atrophy, and a reduction in specific muscle force contribute to the age-related loss of muscle force in 25-month-old mouse. Maximal isometric force of in situ m. plantaris of C57BL/6J male adult (9 months) and old (25 months) mice was determined and related to myofiber number, myofiber size, intramuscular connective tissue content, and proportion of denervated myofibers. Isometric maximal plantaris muscle force was 13 % lower in old than adult mice (0.97 ± 0.05 N vs. 0.84 ± 0.03 N; P < 0.05). M. plantaris mass of old mice was not significantly smaller than that of adult mice. There was also no significant myofiber atrophy or myofiber loss. Specific muscle force of old mice was 25 % lower than that of adult mice (0.55 ± 0.05 vs. 0.41 ± 0.03 N·mm(-2), P < 0.01). In addition, with age, the proportion of type IIB myofibers decreased (43.6 vs. 38.4 %, respectively), while the connective tissue content increased (11.6 vs. 16.4 %, respectively). The age-related reduction in maximal isometric plantaris muscle force in 25-month-old male C57BL/6J mice is mainly attributable to a reduction in specific force, which is for 5 % explicable by an age-related increase in connective tissue, rather than myofiber atrophy and myofiber loss.

Fig. 1

Experimental setup as used during all measurements. This is a modification of the setup used by de Haan et al. (1989). The anesthetized mouse was placed on a heated pad, while the m. plantaris was attached to the force transducer, which allowed to measure muscle force in situ

Fig. 2

Maximal isometric tetanus force (N) during a 150 Hz isometric tetanus plotted against time (ms) for adult and old m. plantaris of mice (a). Force (N) during a 400 Hz isometric tetanus plotted against time for adult and old m. plantaris of mice (b). Maximal force (mN) in adult mice was higher than that in old mice (*P < 0.05) (c). Specific force (N·mm⁻²) was less in old than adult m. plantaris (*P < 0.05). Values are mean ± SEM

Fig. 3

Examples of immunofluorescent staining for type IIB myosin in an adult (a) and old (b) m. plantaris. Bar represents 500 μm. Type IIB myofiber proportion (%) in adult and old mice was significantly different (P < 0.05). Type I, IIA, IIAX, IIX, and IIXB proportions did not change significantly (c). Myofiber CSA (μm²) did not significantly decline for any fiber type (P > 0.05) (d). Note that type I myofibers were not included in this analysis, because of the low presence of type I myofibers within the m. plantaris of C57Bl/6 mice. Values are mean ± SEM

Fig. 4

Collagen staining of m. plantaris of adult (a) and old mice (b). Bar represents 100 μm. Threshold analysis (c) revealed a significant age-related increase (*P < 0.05). Values are mean ± SEM

Fig. 5

Serial sections stained for type IIB myosin and PAS of adult (a and b) and old (c and d) muscles. Bar represents 100 μm. Corresponding denervated myofibers are specified with an asterisk. Analysis revealed no significant difference between adult and old denervated myofiber count expressed as percentage of all IIB myofibers (e). Values are mean ± SEM