Structure of parasternal intercostal muscles in the adult hamster: topographic effects

Parasternal intercostal muscle remodeling in severe chronic obstructive pulmonary disease

Studies in experimental animals indicate that chronic increases in neural drive to limb muscles elicit a fast-to-slow transformation of fiber-type proportions and myofibrillar proteins. Since neural drive to the parasternal intercostal muscles (parasternals) is chronically increased in patients with severe chronic obstructive pulmonary diseases (COPDs), we carried out the present study to test the hypothesis that the parasternals of COPD patients exhibit an increase in the proportions of both slow fibers and slow myosin heavy chains (MHCs). Accordingly, we obtained full thickness parasternal muscle biopsies from the third interspace of seven COPD patients (mean +/- SE age: 59 +/- 4 yr) and seven age-matched controls (AMCs). Fiber typing was done by immunohistochemistry, and MHC proportions were determined by SDS-PAGE followed by densitometry. COPD patients exhibited higher proportions of slow fibers than AMCs (73 +/- 4 vs. 51 +/- 3%; P < 0.01). Additionally, COPD patients exhibited higher proportions of slow MHC than AMCs (56 +/- 4 vs. 46 +/- 4%, P < 0.04). We conclude that the parasternal muscles of patients with severe COPD exhibit a fast-to-slow transformation in both fiber-type and MHC proportions. Previous workers have demonstrated that remodeling of the external intercostals, another rib cage inspiratory muscle, elicited by severe COPD is characterized by a slow-to-fast transformation in both fiber types and MHC isoform proportions. The physiological significance of this difference in remodeling between these two inspiratory rib cage muscles remains to be elucidated.

Progression of kyphosis in mdx mice

Spinal deformity in the form of kyphosis or kyphoscoliosis occurs in most patients with Duchenne muscular dystrophy (DMD), a fatal X-linked disorder caused by an absence of the subsarcolemmal protein dystrophin. Mdx mice, which also lack dystrophin, show thoracolumbar kyphosis that progresses with age. We hypothesize that paraspinal and respiratory muscle weakness and fibrosis are associated with the progression of spinal deformity in this mouse model, and similar to DMD patients there is evidence of altered thoracic conformation and area. We measured kyphosis in mdx and age-matched control mice by monthly radiographs and the application of a novel radiographic index, the kyphotic index, similar to that used in boys with DMD. Kyphotic index became significantly less in mdx at 9 mo of age (3.58 ± 0.12 compared with 4.27 ± 0.04 in the control strain; P ≤ 0.01), indicating more severe kyphosis, and remained less from 10 to 17 mo of age. Thoracic area in 17-mo-old mdx was reduced by 14% compared with control mice ( P ≤ 0.05). Peak tetanic tension was significantly lower in mdx and fell 47% in old mdx latissimus dorsi muscles, 44% in intercostal strips, and 73% in diaphragm strips ( P ≤ 0.05). Fibrosis of these muscles and the longissimus dorsi, measured by hydroxyproline analysis and histological grading of picrosirius red-stained sections, was greater in mdx ( P < 0.05). We conclude that kyphotic index is a useful measure in mdx and other kyphotic mouse strains, and assessment of paralumbar and accessory respiratory muscles enhance understanding of spinal deformity in muscular dystrophy.

Regional differences in serotonergic input to canine parasternal intercostal motoneurons

There is a mediolateral gradient in activation of the parasternal intercostal (PI) muscle during inspiration. In the present study, we tested the hypotheses that serotonergic [5-hydroxytryptamine (5-HT)] input from descending central drive and/or intrinsic size-related properties of PI motoneurons leads to the differential activation of PI muscles. In dogs, PI motoneurons innervating the medial and lateral regions of the PI muscles at the T(3)-T(5) interspaces were retrogradely labeled by intramuscular injection of cholera toxin B subunit. After a 10-day survival period, PI motoneurons and 5-HT terminals were visualized by using immunohistochemistry and confocal imaging. There were no differences in motoneuron morphology among motoneurons innervating the medial and lateral regions of the PI muscle. However, the number of 5-HT terminals and the 5-HT terminal density (normalized for surface area) were greater in motoneurons innervating the medial region of the PI muscle compared with the lateral region. These results suggest that differences in distribution of 5-HT input may contribute to regional differences in PI muscle activation during inspiration and that differences in PI motoneuron recruitment do not relate to size.