Diaphragm muscle fiber function and structure in humans with hemidiaphragm paralysis

Area of Science:

• Physiology
• Muscle Biology
• Respiratory Medicine

Background:

• Mechanical ventilation can lead to diaphragm inactivity, raising concerns about atrophy and weakness.
• Conclusive human evidence linking mechanical inactivity directly to diaphragm weakness is limited.

Purpose of the Study:

• To investigate the effects of hemidiaphragm paralysis on human diaphragm muscle fiber structure and function.
• To determine the time course of diaphragm muscle atrophy and weakness following paralysis.

Main Methods:

• Biopsies from paralyzed hemidiaphragm (n=8) and control subjects (n=3) were analyzed.
• Muscle fiber contractile performance, ultrastructure, and morphology were assessed.
• Expression of E3 ligases (MAFbx, MuRF-1) and proteasome activity were measured.

Main Results:

• Diaphragm muscle fiber force-generating capacity and myofibrillar ultrastructure remained preserved up to 8 weeks post-paralysis.
• Slow muscle fibers showed reduced cross-sectional area after 2 weeks, while fast fibers were preserved up to 8 weeks.
• E3 ligase expression and proteasome activity were not upregulated, even with marked atrophy after prolonged paralysis (72-88 weeks).

Conclusions:

• Human diaphragm muscle fiber atrophy and weakness following hemidiaphragm paralysis develop gradually over months.
• The ubiquitin-proteasome pathway does not appear to be significantly activated in the early stages of paralysis-induced diaphragm atrophy.