A fundamental aspect of acute renal
ischemia is energy depletion, manifest as a falling level of
ATP that is associated with a simultaneous rise in
AMP. The energy sensor
AMP-activated protein kinase (AMPK) is activated by a rising
AMP-to-
ATP ratio, but its role in acute renal
ischemia is unknown. AMPK is activated in the ischemic heart and is reported to phosphorylate both
endothelial nitric oxide synthase (eNOS) and
acetyl-CoA carboxylase. To study activation of AMPK in acute renal
ischemia, the renal pedicle of anesthetized Sprague-Dawley rats was cross-clamped for increasing time intervals. AMPK was strongly activated within 1 min and remained so after 30 min. However, despite the robust activation of AMPK, acute renal
ischemia did not increase phosphorylation of the AMPK phosphorylation sites eNOS-Ser(1177) or
acetyl-CoA carboxylase-Ser(79). Activation of AMPK in bovine aortic endothelial cells by the
ATP-depleting agent
antimycin A and the
antidiabetic drug phenformin also did not increase phosphorylation of eNOS-Ser(1177), confirming that AMPK activation and phosphorylation of eNOS are dissociated in some situations. Immunoprecipitation studies demonstrated that the dissociation between AMPK activation and phosphorylation of eNOS-Ser(1177) was not due to changes in the physical associations between AMPK, eNOS, or
heat shock protein 90. In conclusion, acute renal
ischemia rapidly activates the energy sensor AMPK, which is known to maintain
ATP reserves during energy stress. The substrates it phosphorylates, however, are different from those in other organs such as the heart.