Size and Composition Changes in Diaphragmatic Fibers in Rats Exposed to Chronic Hypercapnia

To test the hypothesis that chronichypercapnia changes the composition of the respiratory muscle bycontinuous augmentation of ventilation. Eighteen male Wistar rats were housed in 10% CO2 in air for 19 weeks, and their minute ventilation(▪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 adeninedinucleotide-tetrazolium reductase (NADH-TR) stain also were counted. The data were compared with those of rats kept in room air. The mean (± SD) Paco2after 19 weeks of sustained hypercapnia was 71.0 ± 4.7 mm Hg. The▪e remained at a high level until 12 weeks ofexposure, and then it significantly decreased at week 18. In acomparison with the control rats, a larger number of type I fibers anda smaller number of type IIb fibers were found in the diaphragm of thechronically hypercapnic rats. In addition, the latter group'scross-sectional area revealed fibers of a significantly smallerdiameter. Target-like fibers were observed in 5% of the, NADH-TR-stained fibers in the chronically hypercapnic rats but were notseen in the control rats. By increasingthe ratio of fatigue-resistant fibers, the diaphragm was able to adaptto a sustained load induced by hypercapnia. However, this adaptiveprocess was accompanied by a degenerative change in thetissue.