Circadian rhythms modulate the effect of eccentric exercise on rat soleus muscles

Abstract

We investigated whether time-of-day dependent changes in the rat soleus (SOL) muscle size, after eccentric exercises, operate via the mechanistic target of rapamycin (mTOR) signaling pathway. For our first experiment, we assigned 9-week-old male Wistar rats randomly into four groups: light phase (zeitgeber time; ZT6) non-trained control, dark phase (ZT18) non-trained control, light phase-trained, and dark phase-trained. Trained animals performed 90 min of downhill running once every 3 d for 8 weeks. The second experiment involved dividing 9-week-old male Wistar rats to control and exercise groups. The latter were subjected to 15 min of downhill running at ZT6 and ZT18. The absolute (+12.8%) and relative (+9.4%) SOL muscle weights were higher in the light phase-trained group. p70S6K phosphorylation ratio was 42.6% higher in the SOL muscle of rats that had exercised only in light (non-trained ZT6). Collectively, the degree of muscle hypertrophy in SOL is time-of-day dependent, perhaps via the mTOR/p70S6K signaling.

Fig 1. Body weight, fat weight, and food intake.

Body weight (a), fat weight (epididymal white adipose tissue) (b), average food intake (c), and weekly food intake (each value represents the mean of 6 days) (d). L, light phase; D, dark phase; C, control; TR, trained group. Values are the mean ± standard error (SE); n = 6 per group. The results of the two-way analysis of variance (ANOVA) are displayed in a, b, c; *P < 0.05, **P < 0.01. The results of the one-way analysis of variance (ANOVA) are displayed in d; ^✝LC versus LTR, P < 0.05; ^♯DC versus DTR, P < 0.05.

Fig 2. Effect of eccentric exercise on soleus muscle at different times of the day for 8 weeks.

Soleus muscle weight (a), soleus muscle weight relative to body weight (b), soleus cross-sectional area (CSA) (c), hematoxylin and eosin (H&E) staining of rat soleus muscle sections (10× magnification, scale bar = 50 μm) (d), centronucleated fibers of the soleus muscle (e). L, light phase; D, dark phase; C, control; TR, trained group. Values are the mean ± standard error (SE); n = 6 per group. The results of the two-way analysis of variance (ANOVA) are displayed. *P < 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001.

Fig 3. Effect of eccentric exercise on mTOR signaling in soleus muscle at different times of the day.

Phosphorylation rates of mTOR (a), p70S6K (b), 4EBP1(c), and ERK1/2 (d) in the soleus muscle. Samples were collected before (Pre), immediately after (Pt0), and 1 h after (Pt1) eccentric exercise. L, light phase; D, dark phase. Values are the mean ± standard error (SE); n = 7 per group. The results of the two-way analysis of variance (ANOVA) are displayed. **P < 0.01, ****P < 0.0001.

Fig 4. Increased rate of serum corticosterone immediately after eccentric exercise.

Serum corticosterone concentration (a), rate of increase in serum corticosterone (b). L, light phase; D, dark phase. Values are the mean ± standard error (SE); n = 7 per group. The results of the two-way analysis of variance (ANOVA) are displayed in a. The results of the unpaired t-test are displayed in b. *P < 0.05, **P < 0.01, ****P < 0.0001.