Evidence for 5' AMP-activated protein kinase mediation of the effect of muscle contraction on glucose transport.

The intracellular signaling proteins that lead to exercise-stimulated glucose transport in skeletal muscle have not been identified, although it is clear that there are separate signaling mechanisms for exercise- and insulin-stimulated glucose transport. We have hypothesized that the 5'AMP-activated protein kinase (AMPK) functions as a signaling intermediary in exercise-stimulated glucose uptake. This hypothesis was based on recent studies showing the following: 1) muscle contraction increases AMPK activity and 2) perfusion of rat hindlimb skeletal muscles with 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR), a compound that results in increased AMPK activity, increased insulin-stimulated glucose uptake. In the current study, isolated rat epitrochlearis muscles were treated to contract in vitro (via electrical stimulation for 10 min) and/or incubated in the absence or presence of AICAR (2 mmol/l), insulin (1 micromol/l), or wortmannin (100 nmol/l). Both contraction and AICAR significantly increased AMPK activity, while the enzyme was not activated by insulin. AICAR, contraction, and insulin all increased 3-O-methylglucose (3MG) transport by threefold to fivefold above basal. The phosphatidylinositol 3-kinase (PI 3-kinase) inhibitor wortmannin completely blocked insulin-stimulated transport, but did not inhibit AICAR- or contraction-stimulated transport. The increase in glucose transport with the combination of maximal AICAR plus maximal insulin treatments was partially additive, suggesting that these stimuli increase glucose transport by different mechanisms. In contrast, there was no additive effect on glucose transport with the combination of AICAR plus contraction. These data suggest that AICAR and contraction stimulate glucose transport by a similar insulin-independent signaling mechanism and are consistent with the hypothesis that AMPK is involved in exercise-stimulated glucose uptake.