Ontogenetic, gravity-dependent development of rat soleus muscle

Area of Science:

• Muscle Physiology
• Developmental Biology
• Skeletal Muscle Adaptation

Background:

• Postnatal muscle development is crucial for establishing mature function.
• The role of mechanical loading in early muscle ontogeny requires further investigation.

Purpose of the Study:

• To test if rat soleus muscle growth and myosin phenotype changes are independent of weight-bearing during the postnatal, preweaning period.
• To investigate the impact of unloading on muscle development and the potential for recovery.

Main Methods:

• Intermittent hindlimb unloading in rat pups from postnatal day 4 to 21.
• Control groups included normal dam-reared pups and pups on an alternating feeding schedule.
• Muscle mass, myonuclear number, myonuclear domain, and myosin heavy chain isoform expression were analyzed.

Main Results:

• Hindlimb unloading inhibited muscle mass enlargement (3x), myonuclear number increase (1.6x), myonuclear domain expansion (2.6x), and slow fiber transformation (56% to 70% type I MHC).
• These inhibited developmental processes normalized or exceeded control levels within 1 month after reambulation.
• Unloading halted the ontogenetic progression of DNA transcription, cytoplasmic volume, and myosin isoform transitions.

Conclusions:

• Normal ontogenetic development of postural muscles, like the soleus, is highly dependent on the gravitational environment, even in early postnatal life.
• Chronic unloading during the preweaning period significantly impedes muscle development, but recovery is possible upon reambulation.