[Mechanical ventilation leads to remodeling of diaphragma and soleus in rats]

To investigate the structural response of diaphragm and soleus of the rat after mechanical ventilation (MV), and to explore the specific mechanism of the dysfunction of both muscles.

Sixteen male Sprague-Dawley (SD) rats were randomly divided into control group and MV group, with 8 rats in each group. Rats in MV group were treated with controlled ventilation and maintained anesthesia, and those in control group were only anesthetized without MV and maintained anesthesia. The diaphragm and soleus were harvested after MV for 18 hours, and the morphology changes were observed with light microscope. The cross section of muscle fiber was observed by immunofluorescence technique analysis, and the cross-sectional area of muscle fiber was calculated. The ultra structural changes in muscle fibers were observed under transmission electron microscope.

(1) Observed under light microscope, the cross section of the diaphragm and soleus muscle in the control group was regular, the nucleus was normal and the cytoplasm was homogeneous. The fibers in the diaphragm-biopsy specimens from MV subjects were smaller than those from control subjects, whereas these signs were not found in soleus. But fiber atrophy in MV specimens was not accompanied by an inflammatory-cell infiltrate. (2) Under the fluorescence microscope, the control group had a smaller cross-section of the slow-twitch muscle in diaphragm, while the fast-twitch muscle fibers were larger. As compared with diaphragm-biopsy specimens from control, specimens from MV subjects showed decreased cross-sectional areas of slow-twitch and fast-twitch fibers, respectively (μm²: 1 069.00±155.24 vs. 1 297.12±331.15, 2 279.66±442.31 vs. 3 031.80±596.11, both P < 0.05). The disproportionate decrease in fast-twitch fibers cross-sectional areas [(70.42±3.61)% vs. (75.63±2.48)%] resulted in an increase in the percentage of total area occupied by the slow-twitch fibers [(29.58±3.61)% vs. (24.35±2.48)%, both P < 0.01]. There were no significant differences in cross-sectional areas of slow-twitch and fast-twitch fibers in soleus between control group and MV group (μm²: 3 193.80±559.36 vs. 3 008.84±559.22, 3 392.86±514.56 vs. 3 594.35±651.67, both P > 0.05). (3) In the control group, the muscle fibers of the diaphragm and soleus were arranged orderly, the boundary of the light and dark bands and the "Z-line" were clear, and there was no autophagy in the visual field. The outer membrane of the mitochondria was complete, and the cristae were in the shape of clapboard. The signs of misalignment of myofibrils, disruption of "Z-line" and vacuolar mitochondria were found in diaphragm from MV group, whereas these signs were not found in soleus. Diaphragm from MV group exhibited an increase in autophagic vesicles visualized by transmission electron microscopy as compared with control group.

Controlled MV for 18 hours resulted in diaphragmatic inactivity and promoted muscle injury and atrophy, while autophagy and mitochondrial dysfunction were enhanced. Soleus immobilization for 18 hours was not associated with muscle atrophy. These facts suggest that the signaling associated with diaphragm atrophy during MV may involve different mechanisms compared with other models of muscle atrophy. Diaphragm appeared to be more susceptible to MV.