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.
Highlights
Light-responsive central clock genes in the suprachiasmatic nucleus (SCN) coordinate circadian rhythms through signaling of peripheral clock genes in other tissues [1]
We demonstrated circadian variation in intracellular protein synthesis signaling in rat skeletal muscles [4], similar to what has been observed in body temperature, locomotor activity, and hormone secretion [5, 6]
Twenty-four male Wistar rats were randomly assigned to two groups: Zeitgeber Time (ZT) 6 and ZT18 (ZT0: lights on at 18:00, ZT12: lights off at 6:00)
Summary
Light-responsive central clock genes in the suprachiasmatic nucleus (SCN) coordinate circadian rhythms through signaling of peripheral clock genes in other tissues [1]. We demonstrated circadian variation in intracellular protein synthesis signaling in rat skeletal muscles [4], similar to what has been observed in body temperature, locomotor activity, and hormone secretion [5, 6]. We observed diurnal variation in mechanistic (mammalian) target of rapamycin (mTOR)/70 kDa ribosomal protein S6 kinase 1 (p70S6K) signaling and extracellular signal-regulated kinase (ERK) signaling in skeletal and cardiac muscles. These pathways are key mechanisms for skeletal muscle hypertrophy under mechanical
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