title: Size and composition changes in diaphragmatic fibers in rats exposed to chronic hypercapnia.
abstract: OBJECTIVE
  To test the hypothesis that chronic hypercapnia changes the composition of the respiratory muscle by continuous augmentation of ventilation.
  
  
  METHODS
  Eighteen male Wistar rats were housed in 10% CO(2) in air for 19 weeks, and their minute ventilation V(E) was measured every 6 weeks. The diaphragm, excited at 19 weeks of exposure, was classified as fiber type I, IIa, or IIb. Cross-sectional areas of individual fibers were measured. Fibers with a target-like appearance on reduced nicotinamide adenine dinucleotide-tetrazolium reductase (NADH-TR) stain also were counted. The data were compared with those of rats kept in room air.
  
  
  RESULTS
  The mean (+/- SD) PaCO(2) after 19 weeks of sustained hypercapnia was 71.0 +/- 4.7 mm Hg. The V(E) remained at a high level until 12 weeks of exposure, and then it significantly decreased at week 18. In a comparison with the control rats, a larger number of type I fibers and a smaller number of type IIb fibers were found in the diaphragm of the chronically hypercapnic rats. In addition, the latter group's cross-sectional area revealed fibers of a significantly smaller diameter. Target-like fibers were observed in 5% of the NADH-TR-stained fibers in the chronically hypercapnic rats but were not seen in the control rats.
  
  
  CONCLUSIONS
  By increasing the ratio of fatigue-resistant fibers, the diaphragm was able to adapt to a sustained load induced by hypercapnia. However, this adaptive process was accompanied by a degenerative change in the tissue.
authors.affiliation: Department of Medicine, Tokai University School of Medicine, Isehara, Japan.
authors.forename: M
authors.initials: M
authors.lastname: Kumagai
authors.forename: T
authors.initials: T
authors.lastname: Kondo
authors.forename: Y
authors.initials: Y
authors.lastname: Ohta
authors.forename: T
authors.initials: T
authors.lastname: Ishihara
mesh_terms: D000818:Animals; D002908:Chronic Disease; D003964:Diaphragm / Q000473:pathology*; D006935:Hypercapnia / Q000473:pathology*; D008297:Male; D018485:Muscle Fibers, Skeletal / Q000473:pathology*; D051381:Rats; D017208:Rats, Wistar