METABOLIC CHARACTERISTICS OF PRIMARY INSPIRATORY AND EXPIRATORY MUSCLES IN THE DOG

These experiments examined the metabolic properties of the canine respiratory muscles. Because the costal diaphragm (COD), crural diaphragm (CRD), parasternal intercostals (PI), triangularis sterni (TS), and transversus abdominis (TA) are active during quiet breathing in the dog, we hypothesized that these muscles would have different metabolic profiles (i.e., higher oxidative and antioxidant enzyme activities) compared with ventilatory muscles recruited only at increased ventilatory requirements [e.g., scalene (SC) and external oblique (EO)] and locomotor muscles [e.g., deltoid (DEL)]. To test this hypothesis, muscle samples were removed from six healthy adult dogs and analyzed to determine the activities of citrate synthase (CS), phosphofructokinase (PFK), 3-hydroxyacyl-CoA dehydrogenase (HADH), and superoxide dismutase (SOD). The activities of these enzymes were interpreted as relative measures of metabolic capacities, and enzyme activity ratios were considered as representing relationships between different metabolic pathways. Analysis revealed that CS and HADH activities were significantly higher (P < 0.05) in the PI, COD, CRD, and TS compared with those in all other muscles. Muscles with the lowest CS, HADH, and SOD activities (i.e., SC, TA, EO, DEL) generally had the highest PFK activities. Furthermore, the PFK/CS ratio was significantly lower in the PI, COD, CRD, and TS compared with that in all other muscles studied. These data support the notion that the canine PI, COD, CRD, and TS are metabolically different from other key ventilatory muscles.