Intrinsic transient tracheal occlusion training and myogenic remodeling of rodent parasternal intercostal fibers.

It is recognized that diaphragm muscle plasticity occurs with mechanical overloads, yet less is known about synergistic parasternal intercostal muscle fiber remodeling. We conducted overload training with intrinsic transient tracheal occlusion (ITTO) exercises in conscious animals. We hypothesized that ITTO would yield significant fiber hypertrophy and myogenic activation that would parallel diaphragm fiber remodeling. Sprague-Dawley rats underwent placement of a tracheal cuff and were randomly assigned to receive daily 10 min sessions of conscious ITTO or observation (sham) over 2 wk. After training, fiber morphology, myosin heavy chain (MHC) isoform composition, cross-sectional area, proportion of Pax7-positive nuclei, and presence of embryonic MHC (eMHC) were quantified. Type IIx/b fibers were 20% larger after ITTO training than with sham training (ITTO: 4,431 +/­ 676 µm2, sham: 3,689 +/­ 400 µm2, p < 0.05), and type I fibers were more prevalent after ITTO (p < 0.01). Expression of Pax7 was increased in ITTO parasternals and diaphragm (p < 0.05). In contrast, the proportion of eMHC-positive fibers was increased only in ITTO parasternals (1.2% [3.4%­0.6%], sham: 0% [0.6%­0%], p < 0.05). Although diaphragm and parasternal type II fibers hypertrophy to a similar degree, myogenic remodeling appears to differ between the two muscles.

Chronic intrinsic transient tracheal occlusion elicits diaphragmatic muscle fiber remodeling in conscious rodents.

BACKGROUND: Although the prevalence of inspiratory muscle strength training has increased in clinical medicine, its effect on diaphragm fiber remodeling is not well-understood and no relevant animal respiratory muscle strength training-rehabilitation experimental models exist. We tested the postulate that intrinsic transient tracheal occlusion (ITTO) conditioning in conscious animals would provide a novel experimental model of respiratory muscle strength training, and used significant increases in diaphragmatic fiber cross-sectional area (CSA) as the primary outcome measure. We hypothesized that ITTO would increase costal diaphragm fiber CSA and further hypothesized a greater duration and magnitude of occlusions would amplify remodeling. METHODOLOGY/PRINCIPAL FINDINGS: Sprague-Dawley rats underwent surgical placement of a tracheal cuff and were randomly assigned to receive daily either 10-minute sessions of ITTO, extended-duration, 20-minute ITTO (ITTO-20), partial obstruction with 50% of cuff inflation pressure (ITTO-PAR) or observation (SHAM) over two weeks. After the interventions, fiber morphology, myosin heavy chain composition and CSA were examined in the crural and ventral, medial, and dorsal costal regions. In the medial costal diaphragm, with ITTO, type IIx/b fibers were 26% larger in the medial costal diaphragm (p<0.01) and 24% larger in the crural diaphragm (p<0.05). No significant changes in fiber composition or morphology were detected. ITTO-20 sessions also yielded significant increases in medial costal fiber cross-sectional area, but the effects were not greater than those elicited by 10-minute sessions. On the other hand, ITTO-PAR resulted in partial airway obstruction and did not generate fiber hypertrophy. CONCLUSIONS/SIGNIFICANCE: The results suggest that the magnitude of the load was more influential in altering fiber cross-sectional area than extended-duration conditioning sessions. The results also indicated that ITTO was associated with type II fiber hypertrophy in the medial costal region of the diaphragm and may be an advantageous experimental model of clinical respiratory muscle strength training.