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1 University of Castilla-la Mancha at Toledo, Toledo 45071, Spain; and 2 The Human Performance Laboratory, Department of Kinesiology and Health Education, The University of Texas at Austin, Austin, Texas 78712
This study
determined the effects of elevated plasma epinephrine on fat metabolism
during exercise. On four occasions, seven moderately trained subjects
cycled at 25% of peak oxygen consumption (
O2 peak) for 60 min.
After 15 min of exercise, subjects were intravenously infused with low
(0.96 ± 0.10 nM), moderate (1.92 ± 0.24 nM), or high
(3.44 ± 0.50 nM) levels (all P < 0.05) of epinephrine
to increase plasma epinephrine above control (Con; 0.59 ± 0.10 nM).
During the interval between 35 and 55 min of exercise, lipolysis
[i.e., rate of appearance of glycerol] increased above Con
(4.9 ± 0.5
µmol · kg
1 · min1)
with low, moderate, and high (6.5 ± 0.5, 7.1 ± 0.8, and
10.6 ± 1.2
µmol · kg1 · min1,
respectively; all P < 0.05) levels of epinephrine despite
simultaneous increases in plasma insulin. The release of fatty acid
into plasma also increased progressively with the graded epinephrine
infusions. However, fatty acid oxidation was lower than Con
(11.1 ± 0.8
µmol · kg1 · min1)
during moderate and high levels (8.7 ± 0.7 and 8.1 ± 0.9
µmol · kg1 · min1,
respectively; P < 0.05). In one additional trial, the same
subjects exercised at 45%
O2 peak without
epinephrine infusion, which produced a plasma epinephrine concentration
identical to low levels. However, lipolysis was lower (i.e.,
5.5 ± 0.6 vs. 6.5 ± 0.5
µmol · kg1 · min1;
P < 0.05). In conclusion, elevations in plasma epinephrine
concentration during exercise at 25% of
O2 peak progressively
increase whole body lipolysis but decrease fatty acid oxidation. Last, increasing exercise intensity from 25 to 45%
O2 peak attenuates the
lipolytic actions of epinephrine.
stable isotopes; indirect calorimetry; lipolysis; free fatty acid kinetics
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