Decreased contraction economy in mouse EDL muscle injured by eccentric contractions

The objective of this study was to find out whether basal and/or active energy metabolism are altered in isolated mouse extensor digitorum longus muscle injured by eccentric (Ecc) contractions. Measurements of basal O2 consumption and isometric tetanus O2 recovery cost were made at 25 degrees C on muscles that had done either 10 Ecc, 10 isometric (Iso), or no contractions (No). In parallel experiments, rates of lactate and pyruvate production were measured to estimate the anaerobic contribution. Basal O2 consumption was unaffected by the type of protocol performed (P = 0.07). However, the tetanus O2 cost per force-time integral was elevated by 30-36% for the Ecc protocol muscles over that for the Iso and No protocol muscles. When including the increased lactate production by the Ecc protocol muscles, the total energetic cost per force-time integral was 53% higher than that for the Iso protocol muscles [2.35 +/- 0.17 vs. 1.54 +/- 0.18 mumol O2/(N.m.s)]. The decreased economy was attributed to two factors. First, in skinned fibers isolated from the injured muscles, the ratio of maximal actomyosin adenosinetriphosphatase activity to force production was up by 37.5%, suggesting uncoupling of ATP hydrolysis from force production. Second, increased reliance on anaerobic metabolism along with the fluorescent microscopic study of mitochondrial membrane potential and histochemical study of ATP synthase suggested an uncoupling of oxidative phosphorylation in the injured muscles.