The effect of higher ATP cost of contraction on the metabolic response to graded exercise in patients with chronic obstructive pulmonary disease
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To better understand the metabolic implications of a higher ATP cost of contraction in chronic obstructive pulmonary disease (COPD), we used (31)P-magnetic resonance spectroscopy ((31)P-MRS) to examine muscle energetics and pH in response to graded exercise. Specifically, in six patients and six well-matched healthy controls, we determined the intracellular threshold for pH (T(pH)) and inorganic phosphate-to-phosphocreatine ratio (T(Pi/PCr)) during progressive dynamic plantar flexion exercise with work rate expressed as both absolute and relative intensity. Patients with COPD displayed a lower peak power output (WRmax) compared with controls (controls 25 ᠴ W, COPD 15 ᠵ W, P = 0.01) while end-exercise pH (controls 6.79 ᠰ.15, COPD 6.76 ᠰ.21, P = 0.87) and PCr consumption (controls 82 ᠱ0%, COPD 70 ᠱ8%, P = 0.26) were similar between groups. Both T(pH) and T(Pi/PCr) occurred at a significantly lower absolute work rate in patients with COPD compared with controls (controls: 14.7 ᠲ.4 W for T(pH) and 15.3 ᠲ.4 W for T(Pi/PCr); COPD: 9.7 ᠴ.5 W for T(pH) and 10.0 ᠴ.6 W for T(Pi/PCr), P < 0.05), but these thresholds occurred at the same percentage of WRmax (controls: 63 ᠱ1% WRmax for T(pH) and 67 ᠱ8% WRmax for T(Pi/PCr); COPD: 59 ᠹ% WRmax for T(pH) and 61 ᠱ2% WRmax for T(Pi/PCr), P > 0.05). Indexes of mitochondrial function, the PCr recovery time constant (controls 42 ᠷ s, COPD 45 ᠱ1 s, P = 0.66) and the PCr resynthesis rate (controls 105 ᠲ1%/min, COPD 91 ᠳ1%/min, P = 0.43) were similar between groups. In combination, these results reveal that when energy demand is normalized to WRmax, as a consequence of higher ATP cost of contraction, patients with COPD display the same metabolic pattern as healthy subjects, suggesting that skeletal muscle energy production is well preserved in these patients.
Journal of Applied Physiology
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