METABOLIC RESPONSES OF RAT RESPIRATORY MUSCLES TO VOLUNTARY EXERCISE TRAINING

Voluntary wheel running for 4 or 8 wk was used to assess whether a vol itional training stimulus would induce adaptations in the oxidative ca pacity [citrate synthase activity (CS)], glucose phosphorylation capac ity [hexokinase activity (HK)], and glucose transporter protein level (GLUT-4) of rat respiratory muscles. Running distances averaged simila r to 10-13 km/day over the final 5 wk of training. Peak oxygen consump tion by the trained animals was 17% greater (P < 0.05) than by age-mat ched sedentary control animals after 8 wk. CS, HK, and GLUT-4 in soleu s and plantaris muscles all increased because of exercise training. CS increased in the rectus abdominis (+17%), external oblique (+28%), an d internal oblique (+17%) but not in the costal or crural diaphragm af ter 4 wk of training. However, after 8 wk, CS in the costal diaphragm was 59% greater than control but was unchanged in the crural diaphragm . Whereas HK was significantly greater than control in the costal diap hragm (+18%) and rectus abdominis (+54%) after 4 wk, 8 wk of running w ere required for increases in HK in the external oblique (+17%) and in ternal oblique (+14%). HK in the crural diaphragm was not significantl y altered by the exercise training. GLUT-4 did not change significantl y in any of the respiratory muscles studied. These results indicate th at significant adaptations in the glucose phosphorylation capacity and oxidative capacity of both inspiratory and expiratory muscles can tak e place in response to voluntary exercise. However, this same stimulus is not sufficient to cause an adaptive response in GLUT-4 protein lev el in these respiratory muscles.