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1Department of Human Biology and Nutritional Sciences, University of Guelph, Guelph, Ontario N1G 2W1; and 2Department of Medicine, McMaster University, Hamilton, Ontario, Canada L8N 3Z5
Submitted 20 December 2002 ; accepted in final form 13 February 2003
Hormone-sensitive lipase (HSL) catalyzes the hydrolysis of intramuscular triacylglycerols (IMTGs), but HSL regulation is poorly understood in skeletal muscle. The present study measured human skeletal muscle HSL activity at rest and during 120 min of cycling at 60% of peak O2 uptake. Several putative HSL regulators were also measured, including muscle long-chain fatty acyl-CoA (LCFA CoA) and free AMP contents and plasma epinephrine and insulin concentrations. HSL activity increased from resting levels by 10 min of exercise (from 2.09 ± 0.19 to 2.56 ± 0.22 mmol · min-1 · kg dry mass-1, P < 0.05), increased further by 60 min (to 3.12 ± 0.27 mmol · min-1 · kg dry mass-1, P < 0.05), and decreased to near-resting rates after 120 min of cycling. Skeletal muscle LCFA CoA increased (P < 0.05) above rest by 60 min (from 15.9 ± 3.0 to 50.4 ± 7.9 µmol/kg dry mass) and increased further by 120 min. Estimated free AMP increased (P < 0.05) from rest to 60 min and was 
20-fold greater than that at rest by 120 min. Epinephrine was increased above rest (P < 0.05) at 60 (1.47 ± 0.15 nM) and 120 min (4.87 ± 0.76 nM) of exercise. Insulin concentrations decreased rapidly and were lower than resting levels by 10 min and continued to decrease throughout exercise. In summary, HSL activity was increased from resting levels by 10 min, increased further by 60 min, and decreased to near-resting values by 120 min. The increased HSL activity at 60 min was associated with the stimulating effect of increased epinephrine and decreased insulin levels. After 120 min, the decreased HSL activity was associated with the proposed inhibitory effects of increased free AMP. The accumulation of LCFA CoA in the 2nd h of exercise may also have reduced the flux through HSL and accounted for the reduction in IMTG utilization previously observed late in prolonged exercise.
fat oxidation; lipolysis; epinephrine; insulin; free AMP
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