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Muscle Metabolism Laboratory, Department of Physiology, University of Arizona College of Medicine, Tucson, Arizona 85721-0093
Exercise training
(ET) or the antioxidant R(+)-
-lipoic acid (R-ALA)
individually increases insulin action in the insulin-resistant obese
Zucker rat. The purpose of the present study was to determine the
interactions of ET and R-ALA on insulin action and oxidative stress in
skeletal muscle of the obese Zucker rat. Animals either remained
sedentary, received R-ALA (30 mg · kg body
wt
1 · day
1), performed ET
(treadmill running), or underwent both R-ALA treatment and ET for 6 wk.
During an oral glucose tolerance test, ET alone or in combination with
R-ALA resulted in a significant lowering of the glucose (26-32%)
and insulin (29-30%) responses compared with sedentary controls.
R-ALA alone decreased (19%) the glucose-insulin index (indicative of
increased insulin sensitivity), and this parameter was reduced
(48-52%) to the greatest extent in the ET and combined treatment
groups. ET or R-ALA individually increased insulin-mediated glucose
transport activity in isolated epitrochlearis (44-48%) and soleus
(37-57%) muscles. The greatest increases in insulin action in
these muscles (80 and 99%, respectively) were observed in the combined
treatment group. Whereas the improvement in insulin-mediated glucose
transport in soleus due to R-ALA was associated with decreased protein
carbonyl levels (an index of oxidative stress), improvement because of
ET was associated with decreased protein carbonyls as well as enhanced
GLUT-4 protein. However, there was no interactive effect of ET and
R-ALA on GLUT-4 protein or protein carbonyl levels. These results
indicate that ET and R-ALA interact in an additive fashion to improve
insulin action in insulin-resistant skeletal muscle. Because the
further improvement in muscle glucose transport in the combined group was not associated with additional upregulation of GLUT-4 protein or a
further reduction in oxidative stress, the mechanism for this
interaction must be due to additional, as yet unidentified, factors.
insulin resistance; oxidative stress; glucose tolerance; GLUT-4 protein; protein carbonyls
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