Hormone sensitive lipase (HSL) catalyzes the hydrolysis of intramuscular triacylglycerol (IMTG) but HSL regulation is poorly understood in skeletal muscle.
To measure human skeletal muscle HSL activity and several putative regulators of HSL at rest and during prolonged moderate intensity exercise.
Healthy male volunteers (n = 7; 21 ± years, 77 ± kg) cycled for 120 min at 60% peak pulmonary oxygen uptake. Muscle, venous blood and respiratory gas samples were obtained at rest and 10, 60 and 120 min of exercise.
HSL activity increased from resting levels by 10 min of exercise (2.09 ± 0.19 to 2.56 ± 0.22 mmol/min/kg dm, P < 0.05), increased further by 60 min (3.1227 ± 0. mmol/min/kg dm, 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 (15.9 ± 3.0 to 50.4 ± 7.9 mmol/kg dm) and increased further by 120 min. Estimated free AMP increased (P < 0.05) from rest to 60 min, and was ∼20-fold greater than 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 rest by 10 min and continued to decrease throughout exercise.
HSL activity was increased above rest by 10 min, increased further by 60 min and decreased to near-resting values by 120 min. The increased HSL activity from 10–60 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 LCFA CoA accumulation in the second hour of exercise may have reduced flux through HSL and accounted for the reduction in IMTG utilization previously observed late in prolonged exercise. Supported by NSERC and CIHR, Canada.