Cellular adaptations of the ventilatory muscles to a chronic increased respiratory load.

It is not known whether cellular adaptations of the ventilatory muscles are induced by increased respiratory loads. A chronic respiratory load was produced in rats by tracheal banding. Five weeks after the imposition of this increased load, biochemical and histochemical analyses were performed on the diaphragm and intercostal muscles. The oxidative capacity, as indicated by succinate dehydrogenase (SDH) activity, increased 38% in the diaphragm. The capacity for beta-oxidation fatty acids, as indicated by 3-hydroxy-acyl-CoA dehydrogenase (HADH) activity, increased 29%. The glycolytic capacity, as indicated by phosphofructokinase (PFK) activity, did not change. Similar enzymatic adaptations were observed in the intercostal muscles. The proportion of slow-twitch muscle fibers, as indicated by the myofibrillar adenosine triphosphatase (ATPase) stain, increased in the diaphragm, but not in the intercostal muscles. Thus, these ventilatory muscles responded with an increase in their oxidative capacity, and the diaphragm reponded with an increase in the proportion of muscle fibers having the myofibriller ATPase staining characteristic of slow-twich fibers. We conclude that cellular adaptations are induced in the ventilatory muscles by chronic increased respiratory loads.