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1 Kinesiology, University of Wisconsin-Madison, Madison, WI, USA
* To whom correspondence should be addressed. E-mail: cartee{at}education.wisc.edu.
Exercise enhances insulin-stimulated glucose transport (GT) in skeletal muscle. Evidence suggests that 5'AMP-activated protein kinase (AMPK) and glycogen may be important for enhanced insulin sensitivity. Our goals were to investigate the effect of various in situ muscle contraction protocols on insulin-stimulated GT, and assess the relationship of contraction-induced changes in AMPK and glycogen with post-contraction improvement in insulin-stimulated GT. Rats were anesthetized, both ulnar nerves were exposed, and one nerve was electrically stimulated to contract forelimb muscles. We performed a series of five experiments, sequentially varying only one contraction parameter (train duration, train rate, pulse frequency, number of 5 min bouts, or pulse duration) while holding the others constant. Both epitrochlearis muscles were dissected out and incubated for 3.5 hr prior to measurement of GT. For each contraction parameter studied, we identified an apparent threshold value which did not induce a significant increase in insulin-stimulated GT, and an apparent peak value, above which there was a plateau or decline in insulin-stimulated GT. Using other rats, we evaluated muscle AMPK phosphorylation and glycogen concentration immediately post-contraction. AMPK phosphorylation and reduction in glycogen were increased compared to resting controls in each protocol which had previously been shown to increase insulin-stimulated GT, as well as in several protocols which did not significantly increase insulin-stimulated GT. These data suggest that contraction-induced AMPK phosphorylation and decrease in glycogen may be necessary, but are not sufficient for the post-contraction increase in insulin-stimulated GT in rat skeletal muscle. Keywords: AMPK; exercise; glucose transport
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