221 phosphorylation, free AMP content, and glucose clearance were more influenced by the absolute than by the relative exercise intensity, being greatest in 73% Normoxia with no difference between 51% Normoxia and 72% Hypoxia. In contrast to this, increases in muscle glycogen use, muscle lactate content, and plasma catecholamine concentration were more influenced by the relative than by the absolute exercise intensity, being similar in 72% Hypoxia and 73% Normoxia, with both trials higher than in 51% Normoxia. In conclusion, increases in muscle AMPK signaling, free AMP content, and glucose disposal during exercise are largely determined by the absolute exercise intensity, whereas increases in plasma catecholamine levels, muscle glycogen use, and muscle lactate levels are more closely associated with the relative exercise intensity. metabolic regulation; glucose kinetics; contraction AMP-ACTIVATED PROTEIN KINASE (AMPK) is a serine/threonine protein kinase that responds to cellular energy status by inhibiting ATP-consuming pathways and activating ATP-producing pathways (27). The catalytic ␣-subunits of AMPK (␣1 and ␣2) are expressed in skeletal muscle and are activated allosterically by increased AMP levels following phosphorylation of the upstream kinase LKB1 (17,27,48,53,58). Skeletal muscle AMPK activity is increased during contractions and exercise in rodents (18, 19, 38, 50) and humans (8, 9, 14, 57).It has been suggested that skeletal muscle glucose uptake during exercise is regulated by AMPK (53). This is largely based on studies using the nucleoside intermediate 5Ј-aminoimidazole-4-carboxamide ribonucleoside (AICAR). AICAR activates AMPK in rat skeletal muscle and increases glucose uptake (2, 19, 54) and GLUT4 translocation (32). Further support for AMPK in the regulation of glucose uptake during exercise is the finding that AICAR and contraction stimulation of glucose uptake are not additive (2, 19). However, in some circumstances, AMPK activation and glucose uptake during exercise are not tightly coupled. For example, during lowintensity exercise (9, 56) and during moderate-intensity exercise following short-term exercise training (35), skeletal muscle AMPK␣2 activity does not increase despite large increases in glucose disposal during exercise. In addition, AMPK dominant negative mice and AMPK␣2 null mice have partially reduced and normal contraction-stimulated glucose uptake, respectively (24, 37).Hypoxic exercise may be a useful experimental model to tease out the role of AMPK in skeletal muscle glucose uptake during exercise, as some studies suggest that hypoxia and contractions involve the same pathway to activate skeletal muscle glucose uptake (6). Indeed, both hypoxia and muscle contraction increase glucose uptake in isolated rat muscle, alter muscle energy status, and activate AMPK (18). However, others find that hypoxia has an additive effect on glucose uptake during contractions in perfused rat hindlimbs (12, 13). In addition, hypoxia-stimulated glucose uptake is absent in mice expressing a dom...

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