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Substantial working muscle glycerol turnover during two-legged cycle ergometry

¹Exercise Physiology Laboratory, Department of Integrative Biology, University of California, Berkeley; ²Exercise Physiology Laboratory, Clinical Studies Unit, Veterans Affairs Palo Alto Health Care System, Palo Alto, California; ³Department of Exercise and Sports Sciences, University of Miami, Miami, Florida; ⁴University of Colorado Health Science Center, Division of Cardiology, Denver, Colorado; and ⁵Cardiovascular Research Group, Departments of Pediatrics and Pharmacology, Faculty of Medicine, University of Alberta, Edmonton, Alberta, Canada

Submitted 13 February 2007 ; accepted in final form 2 July 2007

We combined tracer and arteriovenous (a-v) balance techniques to evaluate the effects of exercise and endurance training on leg triacylglyceride turnover as assessed by glycerol exchange. Measurements on an exercising leg were taken to be a surrogate for working skeletal muscle. Eight men completed 9 wk of endurance training [5 days/wk, 1 h/day, 75% peak oxygen consumption (O_2peak)], with leg glycerol turnover determined during two pretraining trials [45 and 65% O_2peak (45% Pre and 65% Pre, respectively)] and two posttraining trials [65% of pretraining O_2peak (ABT) and 65% of posttraining O_2peak (RLT)] using [²H₅]glycerol infusion, femoral a-v sampling, and measurement of leg blood flow. Endurance training increased O_2peak by 15% (45.2 ± 1.2 to 52.0 ± 1.8 ml·kg^–1·min^–1, P < 0.05). At rest, there was tracer-measured leg glycerol uptake (41 ± 8 and 52 ± 15 µmol/min for pre- and posttraining, respectively) even in the presence of small, but significant, net leg glycerol release (–68 ± 19 and –50 ± 13 µmol/min, respectively; P < 0.05 vs. zero). Furthermore, while there was no significant net leg glycerol exchange during any of the exercise bouts, there was substantial tracer-measured leg glycerol turnover during exercise (i.e., simultaneous leg muscle uptake and leg release) (uptake, release: 45% Pre, 194 ± 41, 214 ± 33; 65% Pre, 217 ± 79, 201 ± 84; ABT, 275 ± 76, 312 ± 87; RLT, 282 ± 83, 424 ± 75 µmol/min; all P < 0.05 vs. corresponding rest). Leg glycerol turnover was unaffected by exercise intensity or endurance training. In summary, simultaneous leg glycerol uptake and release (indicative of leg triacylglyceride turnover) occurs despite small or negligible net leg glycerol exchange, and furthermore, leg glycerol turnover can be substantially augmented during exercise.

exercise; crossover concept; stable isotopes; lipid metabolism