Fibre type-specific satellite cell response to aerobic training in sedentary adults

Abstract

In the present study, we sought to determine the effect of a traditional, 12 week aerobic training protocol on skeletal muscle fibre type distribution and satellite cell content in sedentary subjects. Muscle biopsies were obtained from the vastus lateralis [n = 23 subjects (six male and 17 female); body mass index 30.7 ± 1.2 kg m(-2)] before and after 12 weeks of aerobic training performed on a cycle ergometer. Immunohistochemical analyses were used to quantify myosin heavy chain (MyHC) isoform expression, cross-sectional area and satellite cell and myonuclear content. Following training, a decrease in MyHC hybrid type IIa/IIx fibre frequency occurred, with a concomitant increase in pure MyHC type IIa fibres. Pretraining fibre type correlated with body mass index, and the change in fibre type following training was associated with improvements in maximal oxygen consumption. Twelve weeks of aerobic training also induced increases in mean cross-sectional area in both MyHC type I and type IIa fibres. Satellite cell content was also increased following training, specifically in MyHC type I fibres, with no change in the number of satellite cells associated with MyHC type II fibres. With the increased satellite cell content following training, an increase in myonuclear number per fibre also occurred in MyHC type I fibres. Hypertrophy of MyHC type II fibres occurred without detectable myonuclear addition, suggesting that the mechanisms underlying growth in fast and slow fibres differ. These data provide intriguing evidence for a fibre type-specific role of satellite cells in muscle adaptation following aerobic training.

Figure 1. Study design

Subjects underwent pretraining testing prior to collection of a muscle biopsy from the vastus lateralis before undergoing a 12 week aerobic training programme. Following training, a second muscle biopsy was collected, and testing was repeated.

Figure 2. Representative immunohistochemical image of myosin heavy chain (MyHC) fibre type identification

Panels are MyHC type I (pink; A), MyHC type IIa (green; B), MyHC type IIx (red; C) and the fused image (D). Scale bar represents 100 μm.

Figure 3. Twelve weeks of aerobic training yields a decrease in MyHC hybrid IIa/IIx fibre frequency and an increase in MyHC type IIa fibre frequency

A, quantification of MyHC fibre type from immunohistochemical analysis, presented as mean fibre type frequency + SEM. B, correlation of the change in relative frequency of MyHC type IIa fibres with the change in maximal oxygen consumption () following 12 weeks of aerobic training. C, correlation of pretraining MyHC hybrid IIa/IIx fibre frequency and body mass index (BMI). *Significantly different from pretraining value (P < 0.05).

Figure 4. Twelve weeks of aerobic training yields an increase in mean fibre cross-sectional area (CSA) and in mean fibre CSA of MyHC types I and IIa

A, mean fibre CSA (in square micrometres) of MyHC type I, IIa and IIa/IIx fibres presented as means + SEM. B, pooled mean fibre CSA presented as means + SEM. C, histogram distributions of pooled fibre CSA. *Significantly different from pretraining value (P < 0.05).

Figure 5. Histogram distributions of individual MyHC fibre CSA following 12 weeks of aerobic training demonstrate rightward shifts

Myosin heavy chain type I CSA presented as a binned histogram (A), MyHC type IIa CSA presented as a binned histogram (B) and MyHC type IIa/IIx CSA presented as a binned histogram (C).

Figure 6. Representative immunohistochemical image of fibre type-specific satellite cell identification

A, fused image demonstrating laminin (green), Pax7 (yellow), MyHC type I (pink) and 4′,6-diamidino-2-phenylindole (DAPI; blue) with MyHC type I satellite cells denoted by white arrowheads and a MyHC type II satellite cell denoted by a white arrow. Scale bar represents 50 μm. B, fused image demonstrating satellite cell location within laminin (green), costained with DAPI (blue), and MyHC type I satellite cells denoted by white arrowheads and a MyHC type II satellite cell denoted by a white arrow. C, single-channel image demonstrating MyHC type I (pink). D, single-channel image demonstrating laminin (green). E, single-channel image demonstrating Pax7 (yellow). F, single-channel image demonstrating DAPI (blue).

Figure 7. Following 12 weeks of aerobic training, an increase in satellite cell content associated with MyHC type I fibres occurred

A, quantification of satellite cell content, expressed as mean satellite cells per fibre + SEM. B, quantification of fibre type-specific satellite cell content, expressed as mean satellite cells per fibre + SE. C, relative frequency of satellite cells associated with MyHC type I and II fibres, expressed as mean frequency + SEM. D, relative frequency of satellite cells associated with MyHC type IIa and IIa/IIx fibres in a subset of subjects (n = 9), expressed as mean frequency + SEM. *Significantly different from pretraining value (P < 0.05). †Significantly different from MyHC type IIa value (P < 0.05).

Figure 8. Following 12 weeks of aerobic training, an increase in myonuclear number in MyHC type I fibres occurred

A, representative image demonstrating MyHC type I (pink), laminin (green), Pax7 (yellow) and DAPI (blue). Myonuclei associated with MyHC type I fibres are denoted by white arrowheads; myonuclei associated with MyHC type II fibres are denoted by white arrows; a satellite cell is denoted by a yellow arrowhead; and an interstitial nucleus is denoted by a yellow arrow. Scale bar: 100 μm. B, quantification of myonuclei in MyHC type I and II fibres, expressed as mean myonuclei per fibre + SEM. *Significantly different from pretraining value (P < 0.05).