Metabolic activity of the external intercostal muscle of patients with COPD

INTRODUCTION: The external intercostal muscle is a relevant contributor to ventilatory work in situations of overloading. Like other respiratory muscl es, the external intercostal muscle seems to undergo a process of structura l remodeling to adapt to a situation of functional disadvantage. However, f indings from published studies of morphology have differed to a certain deg ree. On the one hand, the proportion of fibers involved in anaerobic metabo lism increases; on the other hand, the number of capillaries also increases , an occurrence that would facilitate aerobic metabolism. OBJECTIVE: This study was designed to analyze the activity of several key e nzymes involved in the principal metabolic pathways in the external interco stal muscles of patients with COPD. METHODOLOGY: We studied 6 patients with COPD (65 +/- 8 years, BMI 23 +/- 3 kg/m(2), FEV1 51 +/- 9% ref, RV 184 +/- 38% ref, PaO2 81 +/- 10 mmHg) and 6 control subjects matched for age and anthropometric variables but with nor mal lung function. External intercostal muscle samples were taken from each patient (fifth intercostal space, non-dominant side). The samples were tre ated by conventional spectrophotometry to determine enzyme activity as foll ows: citrate synthase (CS, Krebs cycle), phosphofructokinase (PFK, by commo n glycolysis), lactate dehydrogenase (LDH, anaerobic glycolysis) and creati ne phosphokinase (CPK, use of energy reserves). RESULTS: Patients with COPD showed greater PFK enzyme activity (93 +/- 25 v ersus 44 +/- 9 mu mol/min/g of fresh weight; p = 0.001) and LDH (308 +/- 42 versus 231 +/- 29 mu mol/min/g; p < 0.01) than did control subjects. Howev er, CS and CPK activity was similar in both groups (82 +/- 31 versus 90 +/- 20 mu mol/min/g and 4017 +/- 1734 versus 3048 +/- 464 mu mol/min/g, respec tively), although the latter displayed noteworthy dispersion of values amon g COPD patients, with levels in some patients being three-fold greater than in controls. RV was directly related to glycolytic enzyme activity (with P FK, 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). CONCLUSIONS: 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 pr oportional to the severity of COPD. These findings are probably the express ion of a combination of adaptive structural factors.