Stuart has got the PoWeR! Skeletal muscle adaptations to a novel heavy progressive weighted wheel running exercise model in C57BL/6 mice

Abstract

Murine exercise models are developed to study the molecular and cellular mechanisms regulating muscle mass. A progressive weighted wheel running model, named 'PoWeR', was previously developed to serve as a more translatable alternative to involuntary resistance-type exercise models in rodents, such as synergist ablation. However, mice still run great distances despite the added resistance as evidenced by a large glycolytic-to-oxidative shift in muscle fibre type. Thus, PoWeR reflects a blended resistance/endurance model. In an attempt to bias PoWeR further towards resistance-type exercise, we developed a novel heavy PoWeR model (hPoWeR) utilizing higher wheel loads (max of 12.5 g vs 6 g). Adult male C57BL/6 mice voluntarily performed an 8-week progressive loading protocol (PoWeR or hPoWeR). Running distance peaked at ∼5-6 km day^-1 in both treatments and was maintained by PoWeR mice, but declined in the hPoWeR mice as load increased beyond 7.5 g. Peak isometric force of the gastrocnemius-soleus-plantaris complex tended to increase in wheel running treatments. Soleus mass increased by 19% and 24% in PoWeR and hPoWeR treatments, respectively, and plantaris fibre cross-sectional area was greater in hPoWeR, compared to PoWeR. There were fewer glycolytic and more oxidative fibres in the soleus and plantaris muscles in the PoWeR treatment, but not hPoWeR. Collectively, these data suggest hPoWeR may modestly alter skeletal muscle supporting the aim of better reflecting typical resistance training adaptations, in line with decreased running volume and exposure to higher resistance. Regardless, PoWeR remains an effective hypertrophic concurrent training model in mice.

Keywords: murine exercise models; muscle hypertrophy; resistance exercise.

FIGURE 1

Running distance, running speed, body mass, food consumption and peak GPS force following PoWeR and hPoWeR. Sedentary (SED; white), PoWeR (cardinal red), hPoWeR (gold). (a,b) PoWeR and hPoWeR average daily running distance (a) and average daily running speed (b) after the acclimation week. (c) Body mass of SED, PoWeR, and hPoWeR before and after treatment. (d) Mean daily food consumption of SED, PoWeR and hPoWeR mice. (e) Peak GPS force of SED, PoWeR and hPoWeR mice measured by in situ muscle function test. *Significantly different from SED; #significantly different from PoWeR; n ≥ 10 per treatment for each analysis.

FIGURE 2

Soleus muscle mass, fibre type (FT) and fibre size following PoWeR and hPoWeR. Sedentary (SED; white), PoWeR (cardinal red), hPoWeR (gold). (a) SED, PoWeR and hPoWeR soleus wet tissue mass normalized to body mass. (b) Soleus fibre type distribution. (c) Soleus mean fibre cross‐sectional area (CSA). (d) Soleus fibre type‐specific CSA (MyHC I, MyHC IIA, MyHC IIX+IIB). (e) Representative images of fibre type and laminin staining of entire soleus muscle cross sections from SED, PoWeR and hPoWeR mice; scale bar, 200 µm; red, type I; magenta, type IIA; unstained/black, type IIX; green, type IIB. *Different from SED; #different from PoWeR; n ≥ 8 per treatment for each analysis.

FIGURE 3

Plantaris muscle mass, fibre type (FT) and fibre size following PoWeR and hPoWeR. Sedentary (SED; white), PoWeR (cardinal red), hPoWeR (gold). (a) SED, PoWeR and hPoWeR plantaris wet tissue mass normalized to body mass. (b) Plantaris fibre type distribution. (c) Plantaris mean fibre cross‐sectional area (CSA). (d) Plantaris fibre type‐specific CSA (MyHC I, MyHC IIA, MyHC IIX+IIB). (e) Representative images of fibre type and laminin staining of entire plantaris muscle cross sections from SED, PoWeR and hPoWeR mice; scale bar, 200 µm; red, type I; magenta, type IIA; unstained/black, type IIX; green, type IIB. *Different from SED; #different from PoWeR; n ≥ 8 per treatment for each analysis.

FIGURE 4

Western blot analysis of the phosphorylation status of ribosomal protein S6 (rpS6) in soleus (SOL) and plantaris (PLT) muscles as measured by ratio of phosphorylated rps6 to total rps6. Sedentary (SED; white), PoWeR (cardinal red), hPoWeR (gold). (a) Representative image of TGX stain‐free blot demonstrating equal protein loading across lanes. (b) Images of preliminary western blots indicating the specificity of the phosphorylated rpS6 antibody (left) and the total rpS6 antibody (right) in different skeletal muscle tissue homogenates, including negative controls (empty well). (c) Phosphorylated‐rps6: total rps6 in soleus muscle of SED, PoWeR and hPoWeR. Upper: representative images of western blots. Lower: quantification of western blot data. (d) Phosphorylated rps6: total rps6 in plantaris muscle of SED, PoWeR and hPoWeR. Upper: representative images of western blots. Lower: quantification of western blot data. n ≥ 10 per treatment for all analyses. AU, arbitrary units; GAS, gastrocnemius muscle; PLT, plantaris muscle; SOL, soleus muscle; total, total protein; TRI, triceps muscle; phospho, phosphorylated protein.