The effect of elastase-induced emphysema on diaphragmatic muscle structure in hamsters

The structure of the diaphragm was examined in 46 adult Golden hamsters in which emphysema was induced by intracheal instillation of elastase (25 units per 100 g body weight) and compared with 51 control animals injected with saline. Diaphragmatic muscle mass, thickness, fiber length, composition, and cross-sectional area, as well as the number of sarcomeres in series along the length of the muscle, were examined. The volume of excised lungs (25 cm H₂O inflation pressure) was significantly increased in emphysematous animals (163% of control; p < 0.001). Muscle mass, which was assessed from the weight (dry and wet), was not significantly different in the 2 groups, but in both groups, whole muscle mass correlated directly with body weight (r = 0.87). Muscle thickness increased and fiber length decreased in emphysematous hamsters compared with that in control animals (p < 0.001 for each comparison). Reduction in fiber length in emphysema appeared to be due at least in part to a reduction in the number of sarcomeres in series (sarcomere number = 87% of control; p < 0.01). The percentage of slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG) fibers assessed using conventional histochemical techniques was the same in emphysematous as in control animals. However, the cross-sectional area of all fiber types was significantly increased in emphysematous animals (SO = 143%, FO = 132%, and FG = 131% of control, respectively; p < 0.01 for each comparison). These data indicate that emphysema causes a remodeling of diaphragm structure without affecting its mass. A decrease in sarcomere number contributes to a reduction in diaphragm fiber length, whereas hypertrophy of all fiber types explains the increase in muscle thickness. The structural alterations present in elastase-induced emphysema resemble those present in limb muscle subjected to chronic decreases in length and increases in load. We speculate that these structural changes act to augment diaphragmatic force production.