EFFECTS OF ACUTE EXERCISE ON CITRATE SYNTHASE ACTIVITY AS AN AEROBIC MARKER.

Abstract

1166 Citrate synthase (CS), a Kreb's Cycle enzyme, has been routinely used as a marker for aerobic capacity and mitochondrial density in skeletal muscle. However, reported CS activities have been notoriously variable even within similar experimental protocols; chronic exercise has produced increases in CS activity ranging from 0-50%. To better understand this variation in results we studied CS activity in human vastus lateralis muscle following both acute and chronic knee-extensor exercise (n=6). A total of four muscle biopsies were collected before and after eight weeks of training, both at rest and one hour after acute exercise (1: Untrained rested, 2: Untrained acutely exercised, 3: Trained rested, 4: Trained acutely exercised). Muscle samples were analyzed spectrophotometrically, by the method of Srere, for CS activity. Chronic exercise training measured in a rested state resulted in an 18.2% increase in CS activity (12.3±0.3 to 14.5±0.3 μmol/min/g tissue, p≤0.05). However, there were even greater increases due to acute exercise: 49.2% in the untrained state (12.3±0.3 to 18.3±0.5 μmol/min/g tissue, p≤0.05) and 50.8% in the trained state (14.5±0.3 to 21.8±0.4 μmol/min/g tissue, p≤0.05). As the half life of CS is 7-8 days, it is unlikely that the effect of acute exercise on CS activity is a result of transcriptional up-regulation and elevated CS concentration. However, it is more probable that these transient increases, occurring within one hour, are due to an allosteric modification of CS, or alternatively, an effect of exercised muscle on the assay itself. It is apparent from these data that acute exercise in man increases measured CS activity and may compound observed training responses producing artificially elevated CS values. Therefore, the timing of muscle sampling is critical when measuring CS activity and may explain some of the variations in CS activity already recorded.