Spatial insulin signalling in isolated skeletal muscle preparations

Area of Science:

• Biochemistry
• Cell Biology
• Physiology

Background:

• In vitro muscle incubation studies often observe glycogen depletion in the muscle core.
• Hypoxia and apoptosis markers increase in the muscle core during incubation.
• The role of insulin diffusion in these observations is unclear.

Purpose of the Study:

• To investigate if insulin diffuses sufficiently into isolated mouse skeletal muscle to activate signaling pathways.
• To determine if insulin promotes glucose uptake and glycogenesis throughout the muscle specimen.
• To explore the spatial resolution of insulin signaling within the muscle.

Main Methods:

• Phosphoprotein multiplex assay to measure insulin signaling pathway activation (insulin receptor, Akt, GSK3).
• Immunohistochemistry on muscle cryosections to assess spatial insulin signaling.
• In vitro incubation of mouse skeletal muscle (extensor digitorum longus, soleus) with and without insulin.

Main Results:

• Insulin sufficiently diffused into the muscle core, activating key signaling molecules (p-Tyr1146 insulin receptor, p-Ser473 Akt, p-Ser21/9 GSK3).
• Enhanced insulin signaling was detected in the muscle core, particularly in oxidative fibers.
• No evidence of restricted insulin action due to diffusion limitations was found.

Conclusions:

• Insulin effectively penetrates and signals within the entire mass of isolated mouse skeletal muscle under the tested conditions.
• The observed glycogen depletion in the muscle core is not attributable to insufficient insulin diffusion.
• Further research is needed to elucidate the cause of core glycogen depletion despite adequate insulin signaling.