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Department of Physiology, University of Cape Town Medical School, Observatory 7925, South Africa
Euglycemia was maintained in 13 subjects with low
muscle glycogen [low glycogen, euglycemic (LGE),
n = 8; low glycogen, euglycemic, hyperinsulinemic (LGEI), n = 5]
and 6 subjects with normal muscle glycogen (NGE), whereas hyperglycemia
was maintained in 8 low muscle glycogen subjects (LGH). All subjects
cycled for 145 min at 70% of maximal oxygen uptake during the
infusions. Insulin was infused in LGEI at 0.2 mU · kg
1 · min
1.
During exercise, respiratory exchange ratio (RER) was lower and
norepinephrine higher in LGE than in NGE. In LGEI and LGH, RER at the
start of exercise was the same as in LGE but did not decrease as in
LGE. Free fatty acids (FFA) were higher and plasma insulin
concentrations lower in LGE than NGE, LGEI, or LGH over the first 45 min of exercise. Rate of glucose infusion
(Ri) and rate of glucose
oxidation (Rox) were higher in
LGH and LGEI than in NGE or LGE, and
Ri matched
Rox in all groups except LGH, in which Ri was greater than
Rox. Muscle glycogen disappearance
was greater in NGE than LGE, LGEI, or LGH, but the latter three groups did not differ. In conclusion, this study showed that low muscle glycogen content results in a decrease in RER, an increase in FFA, fat
oxidation, and norepinephrine both at rest and during exercise, and
does not affect Rox when
euglycemia is maintained by infusion of glucose alone.
Rox was increased only during
insulin and hyperglycemia.
hyperglycemia; euglycemia; muscle glycogen; respiratory exchange ratio
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