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/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 (ACC). To study activation of AMPK in acute renal ischemia, the renal pedicle of anaesthetized Sprague-Dawley rats was cross-clamped for increasing time intervals. AMPK was strongly activated within 1 minute and remained so after 30 minutes. However, despite the robust activation of AMPK, acute renal ischemia did not increase phosphorylation of the AMPK phosphorylation sites eNOS-Ser¹¹⁷⁷ or ACC-Ser⁷⁹. Activation of AMPK in bovine aortic endothelial cells by the ATP depleting agent antimycin A and the anti-diabetic drug phenformin also did not increase phosphorylation of eNOS-Ser¹¹⁷⁷, 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-Ser¹¹⁷⁷ 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 to those in other organs such as the heart.