TRICARBOXYLIC-ACID CYCLE INTERMEDIATES IN HUMAN MUSCLE AT REST AND DURING PROLONGED CYCLING

Previous studies have used the muscle concentration of citrate + malat e + fumarate to estimate tricarboxylic acid (TCA) cycle pool size in h umans [e.g., Am. J. Physiol. 259 (Cell Physiol. 28): C834-C841, 1990]. Our purpose was to quantify changes in individual TCA cycle intermedi ates (TCAI) and total pool size by measuring the concentrations of the eight TCAI in human muscle. Eight males cycled to exhaustion (Exh) at similar to 70% of their maximal oxygen uptake, and biopsies were obta ined from the vastus lateralis at rest and during exercise. Succinyl-C oA was not consistently detectable, but the sum of the other seven TCA I was 1.23 +/- 0.04 mmol/kg dry wt at rest, 4.80 +/- 0.25 and 4.87 +/- 0.30 mmol/kg after 5 and 15 min of exercise, respectively, and 3.08 /- 0.15 mmol/kg at Exh. Pool size during exercise was similar to 50% h igher than that seen in rodent muscle after intense electrical stimula tion (Eur. J. Biochem. 110: 371-377, 1980). Relative changes in indivi dual TCAI were not uniform, and no one intermediate was ''representati ve'' of the changes in total pool size. We conclude that changes in sp ecific intermediates or total pool size cannot be used as indicators o f cycle flux and that the apparent species differences in total pool s ize may reflect differences in fiber type composition, recruitment pat tern, or relative intensity of contraction.