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, althoug h it is clear that there are separate signaling mechanisms for exercis e- and insulin-stimulated glucose transport, mie have hypothesized tha t the 5'AMP-activated protein kinase (AMPK) functions as a signaling i ntermediary 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 musc les with 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR), a comp ound that results in increased AMPK activity, increased insulin-stimul ated 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 m mol/l), insulin (1 mu mol/l), or wortmannin (100 nmol/l). Both contrac tion and AICAR significantly increased AMPK activity, while the enzyme was not activated by insulin. AICAR, contraction, and insulin all inc reased 3-O-methylglucose (3MG) transport by threefold to fivefold abov e basal. The phosphatidylinositol 3-kinase (PI 3-kinase) inhibitor wor tmannin completely blocked insulin-stimulated transport, but did not i nhibit AICAR- or contraction-stimulated transport. The increase in glu cose transport with the combination of maximal AICAR plus maximal insu lin treatments was partially additive, suggesting that these stimuli i ncrease glucose transport by different mechanisms. In contrast, there was no additive effect on glucose transport with the combination of AI CAR plus contraction. These data suggest that AICAR and contraction st imulate glucose transport by a similar insulin-independent signaling m echanism and are consistent with the hypothesis that AMPK is involved in exercise-stimulated glucose uptake.