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

Area of Science:

• Exercise physiology
• Molecular biology
• Chronobiology

Background:

• Muscle hypertrophy is influenced by various factors, including exercise stimulus and signaling pathways.
• The mechanistic target of rapamycin (mTOR) pathway is a key regulator of muscle protein synthesis and growth.
• Circadian rhythms can affect physiological processes, but their role in exercise-induced muscle adaptation is not fully understood.

Purpose of the Study:

• To investigate the time-of-day dependency of muscle hypertrophy in the rat soleus (SOL) muscle following eccentric exercise.
• To determine if the mechanistic target of rapamycin (mTOR) signaling pathway mediates these time-of-day effects.

Main Methods:

• Wistar rats were divided into light phase (ZT6) and dark phase (ZT18) trained groups, performing downhill running for 8 weeks.
• A separate experiment involved shorter exercise bouts at ZT6 and ZT18 to assess acute signaling responses.
• Muscle weights and p70S6K phosphorylation (a marker of mTOR activity) were analyzed.

Main Results:

• The light phase-trained group exhibited significantly higher absolute (+12.8%) and relative (+9.4%) SOL muscle weights compared to controls.
• p70S6K phosphorylation was notably higher (42.6%) in the SOL muscle of rats exercising during the light phase.
• These findings suggest a time-of-day effect on exercise-induced muscle hypertrophy.

Conclusions:

• Muscle hypertrophy in the rat soleus is dependent on the time of day eccentric exercise is performed.
• The mTOR/p70S6K signaling pathway likely plays a role in mediating these time-of-day dependent adaptations.
• Optimizing exercise timing may enhance muscle growth outcomes.