ENDURANCE-TRAINING-INDUCED CELLULAR ADAPTATIONS IN RESPIRATORY MUSCLES

Controversy exists concerning the adaptability of mammalian respiratory muscles in response to endurance training. We examined the effects of 8 wk of progressive treadmill exercise (45 min/day 5 days/wk) on the biochemical adaptations of rat diaphragm and intercostal muscles. Female Sprague-Dawley rats were randomly assigned to a sedentary control (n = 10) or an exercise-training group (n = 10). Endurance training resulted in an enhanced oxidative capacity in the anterior costal diaphragm as evidenced by a 29% increase (P < 0.05) in the activity of succinate dehydrogenase (SDH) in trained animals compared with controls (4.15 ± 0.13 vs. 3.21 ± 0.17 μmol · g-1 · min-1). Similarly, SDH activity in the intercostal muscles was 32% greater (P < 0.05) in the trained animals than in the untrained animals (1.72 ± 0.11 vs. 1.30 ± 0.06 μmol · g-1 · min-1). In contrast, the crural region of the diaphragm showed no significant increase (P > 0.05) in oxidative capacity as a result of the training program (3.28 ± 0.12 vs. 3.13 ± 0.18). Furthermore, training did not alter (P > 0.05) lactate dehydrogenase activity in the intercostals or in the crural or the costal diaphragm. These data demonstrate that the oxidative capacity of the costal diaphragm and the intercostal muscles can be enhanced by increasing respiratory loads via regular endurance exercise. We speculate that the lack of metabolic adaptation in the crural region of the diaphragm was not due to limited plasticity of the fibers in this area but to failure of the exercise-training program to provide the appropriate stimulus for cellular adaptation.