Respiratory muscle response to load and glycogen content in type I and II fibers

To study the relation between the response of respiratory muscle to inspiratory loads and glycogen content, we subjected unanesthetized sheep to moderate and severe inspiratory flow resistive (IFR) loads. Only severe IFR loads eventually led to a decrease in transdiaphragmatic pressure (Pdi) and a concomitant rise in PaCO2. Respiratory and nonrespiratory skeletal muscle samples were obtained at necropsy. Glycogen content was determined biochemically in muscle homogenates. Frozen sections were stained with periodic acid-Schiff (PAS) for glycogen and fibers were typed using myosin ATPase stain. Fibers were categorized as full, intermediate, or devoid of glycogen by a subjective scoring system of PAS staining intensity. We found that glycogen content decreased in the costal and crural diaphragm and in the intercostal muscles as the duration of moderate IFR loaded breathing was increased. With severe loads glycogen content decreased significantly, reaching about 40 and 22% of control levels in the costal and crural diaphragm, respectively (P less than 0.01). In addition, with severe IFR loads, a statistically significant proportion of both type I and type II muscle fibers was depleted of glycogen when compared with that of controls (P less than 0.05), but more type II fibers were depleted than type I fibers (50 vs 23%). These data indicate that in sheep subjected to IFR loads: (1) glycogen content in the respiratory muscles decreases as the severity and duration of loaded breathing increases and (2) respiratory muscle fatigue occurs at a time when considerable glycogen is still present in type I fibers in the diaphragm.