[Metabolic activity of the external intercostal muscle of patients with COPD]

The external intercostal muscle is a relevant contributor to ventilatory work in situations of overloading. Like other respiratory muscles, the external intercostal muscle seems to undergo a process of structural remodeling to adapt to a situation of functional disadvantage. However, findings from published studies of morphology have differed to a certain degree. On the one hand, the proportion of fibers involved in anaerobic metabolism increases; on the other hand, the number of capillaries also increases, an occurrence that would facilitate aerobic metabolism.

This study was designed to analyze the activity of several key enzymes involved in the principal metabolic pathways in the external intercostal muscles of patients with COPD.

We studied 6 patients with COPD (65 +/- 8 years, BMI 23 +/- 3 kg/m2, FEV1 51 +/- 9% ref, RV 184 +/- 38% ref, PaO2 81 +/- 10 mmHg) and 6 control subjects matched for age and anthropometric variables but with normal lung function. External intercostal muscle samples were taken from each patient (fifth intercostal space, non-dominant side). The samples were treated by conventional spectrophotometry to determine enzyme activity as follows: citrate synthase (CS, Krebs cycle), phosphofructokinase (PFK, by common glycolysis), lactate dehydrogenase (LDH, anaerobic glycolysis) and creatine phosphokinase (CPK, use of energy reserves).

Patients with COPD showed greater PFK enzyme activity (93 +/- 25 versus 44 +/- 9 micromol/min/g of fresh weight; p = 0.001) and LDH (308 +/- 42 versus 231 +/- 29 micromol/min/g; p < 0.01) than did control subjects. However, CS and CPK activity was similar in both groups (82 +/- 31 versus 90 +/- 20 micromol/min/g and 4017 +/- 1734 versus 3048 +/- 464 micromol/min/g, respectively), although the latter displayed noteworthy dispersion of values among COPD patients, with levels in some patients being three-fold greater than in controls. RV was directly related to glycolytic enzyme activity (with PFK, r = 0.716, p < 0.01; with LDH r = 0.697, p < 0.05) and PFK and LDH also correlated with each other (r = 0.737, p < 0.01).

Based on the enzyme activity studied, oxidative activity seems to be conserved in the external intercostal muscle of patients with COPD. Activity in the glycolytic pathway seems to increase and the increase is proportional to the severity of COPD. These findings are probably the expression of a combination of adaptive structural factors.