Effect of acute exercise on citrate synthase activity in untrained and trained human skeletal muscle

Maximal citrate synthase activity (CS) is routinely used as a marker of aer obic capacity and mitochondrial density in skeletal muscle. However, report ed CS has been notoriously variable, even with similar experimental protoco ls and sampling from the same muscles. Exercise training has resulted in in creases in CS ranging from 0 to 100%. Previously, it has been reported that acute exercise may significantly affect CS. To investigate the hypothesis that the large variation in CS that occurs with training is influenced by a lterations during the exercise itself, we studied CS in human vastus latera lis both in the rested and acutely exercised state while trained and untrai ned (n = 6). Tissues obtained from four biopsies (untrained rested, untrain ed acutely exercised, trained rested, and trained acutely exercised) were a nalyzed spectrophotometrically for maximal CS. Exercise training measured i n a rested state resulted in an 18.2% increase in CS (12.3 +/- 0.3 to 14.5 +/- 0.3 mu mol.min(-1).g tissue(-1), P less than or equal to 0.05). However , even greater increases were recorded 1 h after acute exercise: 49.4% in t he untrained state (12.3 +/- 0.3 to 18.3 +/- 0.5 mu mol.min(-1).g tissue(-1 ), P less than or equal to 0.05) and 50.8% in the trained state (14.5 +/- 0 .3 to 21.8 +/- 0.4 mu mol.min(-1).g tissue(-1), P less than or equal to 0.0 5). Ultrastructural analysis, by electron microscopy, supported an effect o f acute exercise with the finding of numerous swollen mitochondria 1 h afte r exercise that may result in greater access to the CS itself in the CS ass ay. In conclusion, although unexplained, the increased CS with acute exerci se can clearly confound training responses and artificially elevate CS valu es. Therefore, the timing of muscle sampling relative to the last exercise session is critical when measuring CS and offers an explanation for the lar ge variation in CS previously reported.