We compared in human
skeletal muscle the effect of absolute vs. relative exercise intensity on
AMP-activated protein kinase (AMPK) signaling and substrate metabolism
under normoxic and hypoxic conditions. Eight untrained
males cycled for 30 min under hypoxic conditions (11.5% O2, 111
12 W, 72 3% hypoxia V˙ O2 peak; 72% Hypoxia) or under normoxic
conditions (20.9% O2) matched to the same absolute (111 12 W,
51 1% normoxia V˙ O2 peak; 51% Normoxia) or relative (to V˙ O2 peak)
intensity (171 18 W, 73 1% normoxia V˙ O2 peak; 73% Normoxia).
Increases (P 0.05) in AMPK activity, AMPK Thr172 phosphorylation,
ACC Ser221 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.