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Brigham Young University, Provo, Utah 84602
Fatty acid oxidation
in muscle has been reported to be diminished when insulin and glucose
levels are elevated. This study was designed to determine whether
activation of AMP-activated protein kinase (AMPK) will prevent
inhibitory effects of insulin and glucose on the rate of fatty
acid oxidation. Rat hindlimbs were perfused with medium
containing 0, 0.3, or 60 nM insulin with or without 2 mM
5-aminoimidazole-4-carboxamide-1-
-D-ribofuranoside (AICAR). Glucose uptake was stimulated four- to fivefold by inclusion of insulin in the medium. Insulin attenuated the increase in AMPK caused by AICAR both in perfused hindlimbs and in isolated
epitrochlearis muscles. The activation constant for citrate activation
of acetyl-CoA carboxylase (ACC) was significantly increased in response
to AICAR, and the increase was slightly attenuated if insulin was
present in the perfusion medium. Insulin stimulated an increase in
malonyl-CoA content of the muscles in the absence of AICAR. Malonyl-CoA
was decreased to approximately the same value in AICAR-perfused muscle, regardless of insulin concentration. Muscle glucose 6-phosphate and
citrate were significantly increased in response to AICAR and insulin.
The rate of palmitate oxidation tended to decrease in response to
insulin and in the absence of AICAR. AICAR increased palmitate
oxidation to approximately the same level regardless of the insulin
concentration or the rate of glucose uptake into the muscle. The rate
of palmitate oxidation showed a curvilinear relationship as a function
of muscle malonyl-CoA content, with half-maximal inhibition at ~0.6
nmol/g. We conclude that AMPK activation can prevent high rates of
glucose uptake and glycolytic flux from inhibiting palmitate oxidation
in predominantly fast-twitch muscle under these conditions.
AMP-activated protein kinase; acetyl-coenzyme A carboxylase; malonyl-coenzyme A; muscle citrate
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