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Exercise, Muscle and Metabolism Unit, School of Health Sciences, Deakin University, Burwood, Victoria 3125, Australia
This study examined the effect of
epinephrine on glucose disposal during moderate exercise when
glycogenolytic flux was limited by low preexercise skeletal muscle
glycogen availability. Six male subjects cycled for 40 min at 59 ± 1% peak pulmonary O2 uptake on two occasions, either
without (CON) or with (EPI) epinephrine infusion starting after
20 min of exercise. On the day before each experimental trial, subjects
completed fatiguing exercise and then maintained a low carbohydrate
diet to lower muscle glycogen. Muscle samples were obtained after 20 and 40 min of exercise, and glucose kinetics were measured using
[6,6-2H]glucose. Exercise increased plasma epinephrine
above resting concentrations in both trials, and plasma epinephrine was
higher (P < 0.05) during the final 20 min in EPI
compared with CON. Muscle glycogen levels were low after 20 min of
exercise (CON, 117 ± 25; EPI, 122 ± 20 mmol/kg dry matter),
and net muscle glycogen breakdown and muscle glucose 6-phosphate levels
during the subsequent 20 min of exercise were unaffected by epinephrine
infusion. Plasma glucose increased with epinephrine infusion (i.e.,
20-40 min), and this was due to a decrease in glucose disposal
(Rd) (40 min: CON, 33.8 ± 3; EPI, 20.9 ± 4.9 µmol · kg
1 · min1,
P < 0.05), because the exercise-induced rise in
glucose rate of appearance was similar in the trials. These results
show that glucose Rd during exercise is reduced by elevated
plasma epinephrine, even when muscle glycogen availability and
utilization are low. This suggests that the effect of epinephrine does
not appear to be mediated by increased glucose 6-phosphate, secondary
to enhanced muscle glycogenolysis, but may be linked to a direct effect
of epinephrine on sarcolemmal glucose transport.
glucose transport; exercise
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