Structure of abdominal muscles in the hamster: effect of elastase-induced emphysema

The present study examined the effects of elastase-induced emphysema on the structure of the external oblique and transverse abdominis muscles and a non-respiratory muscle, the extensor digitorum longus. Muscle structure was assessed from the cross-sectional area (CSA) and percent of individual fiber types in histochemically stained sections and from the number of sarcomeres arranged in series along the length of individual fibers. Data were obtained in eight hamsters with emphysema and nine saline-injected controls. In the normal (control) animals the external oblique was thicker but contained fewer sarcomeres than the transverse abdominis. Fiber size was similar in the two muscles. In the transverse abdominis the percents of fast-glycolytic and fast-oxidative fibers were greater and smaller, respectively, than in the external oblique. Lung volume of emphysematous hamsters was 168% of control values (P less than 0.001). In emphysematous compared with control animals, the CSA of fast-twitch fibers in the external oblique and transverse abdominis was significantly reduced. Fiber length and sarcomere number were significantly decreased in the transverse abdominis but not in the external oblique in emphysematous hamsters. In contrast, fiber size and composition of the extensor digitorum longus was similar in emphysematous and control animals. These data indicate that cellular responses of the ventilatory muscles to chronic hyperinflation and altered thoracic geometry induced by emphysema are not present in limb skeletal muscle. We speculate that changes in fiber length and CSA of fast fibers in the abdominal expiratory muscles reflect responses to chronic alterations in the mechanics of breathing that may affect muscle load, length, or the pattern of activity.