Contractile and biochemical properties of diaphragm: effects of exercise training and fatigue.

The effect of high-intensity trained (6 X 4.5 min at 40 m/min, 15% grade, 2.5-min rest between bouts, 5 days/wk, for 6 wk) on contractile, biochemical, and fatigue properties of the rat diaphragm were examined. The exercise program produced significant elevations in the mitochondrial marker enzyme citrate synthase (mumol X g-1 X min-1) in the soleus (SOL) (27.2 +/- 1.5 vs. 46.7 +/- 2.4; mean +/- SE), deep vastus lateralis (DVL) (40.8 +/- 2.6 vs. 58.3 +/- 2.8), and superficial vastus lateralis (SVL) (8.5 +/- 0.6 vs. 11.4 +/- 0.7). No significant differences were observed in the crural (CRU) (45.9 +/- 2.0 vs. 44.0 +/- 2.3) or ventral costal (VEN) (41.5 +/- 2.0 vs. 45.8 +/- 2.6) diaphragmatic regions. Phosphofructokinase, the rate-limiting enzyme of glycolysis, significantly increased in the SOL (19.0 +/- 0.8 vs. 23.3 +/- 1.3 mumol X g-1 X min-1) and DVL (69.3 +/- 6.0 vs. 86.6 +/- 5.0), but no alterations were seen in the SVL (98.6 +/- 5.7 vs. 106.1 +/- 9.0), CRU (54.4 +/- 2.8 vs. 53.8 +/- 1.5), or VEN (44.7 +/- 2.4 vs. 46.4 +/- 1.4) posttraining. Diaphragm contractile properties, with the exception of an increased rate of fall in twitch tension, remained unchanged after training. Glycogen values were significantly higher in trained diaphragms at rest (6.54 +/- 0.39 vs. 4.86 +/- 0.41 mg/g) and during 1, 5, and 10 min of fatiguing stimulation. During fatigue no differences were observed in force, rate of rise in force, rate of fall in force, muscle lactate, ATP, or creatine phosphate in trained vs. control.(ABSTRACT TRUNCATED AT 250 WORDS)