Hypoxia and AMP independently regulate AMP-activated protein kinase activity in heart

The hypothesis was tested that hypoxia increases AMP-activated protein kinase (AMPK) activity independently of AMP concentration (AMP]) in heart. In isolated perfused rat hearts, cytosolic AMP] was changed from 0.2 to 16 microM using metabolic inhibitors during both normal oxygenation (95% O2-5% CO2, normoxia) and limited oxygenation (95% N2-5% CO2, hypoxia). Total AMPK activity measured in vitro ranged from 2 to 40 pmol.min(-1).mg protein(-1) in normoxic hearts and from 5 to 55 pmol.min(-1).mg protein(-1) in hypoxic hearts. The dependence of the in vitro total AMPK activity on the in vivo cytosolic AMP] was determined by fitting the measurements from individual hearts to a hyperbolic equation. The AMP] resulting in half-maximal total AMPK activity (A0.5) was 3 +/- 1 microM for hypoxic hearts and 28 +/- 13 microM for normoxic hearts. The A0.5 for alpha2-isoform AMPK activity was 2 +/- 1 microM for hypoxic hearts and 13 +/- 8 microM for normoxic hearts. Total AMPK activity correlated with the phosphorylation of the Thr172 residue of the AMPK alpha-subunit. In potassium-arrested hearts perfused with variable O2 content, alpha-subunit Thr172 phosphorylation increased at O2 < or = 21% even though AMP] was <0.3 microM. Thus hypoxia or O2 < or = 21% increased AMPK phosphorylation and activity independently of cytosolic AMP]. The hypoxic increase in AMPK activity may result from either direct phosphorylation of Thr172 by an upstream kinase or reduction in the A0.5 for AMP].