Extracellular adenosine has been implicated in vascular adaptation to hypoxia. Based on the observation that increases in intracellular adenosine can effectively elevate extracellular adenosine, we studied the contribution of adenosine kinase (AK, intracellular conversion of adenosine to adenosine monophosphate [AMP]) to vascular adenosine responses. Initial in vitro studies of ambient hypoxia revealed prominent repression of endothelial AK transcript (85% +/- 2% reduction), protein, and function. Transcription factor binding assays and hypoxia inducible factor 1-alpha (HIF-1alpha) loss- and gain-of-function studies suggested a role for HIF-1alpha in transcriptional repression of AK. Moreover, repression of AK by ambient hypoxia was abolished in conditional HIF-1alpha mutant mice in vivo. Studies of endothelial barrier function revealed that inhibition or siRNA repression of AK is associated with enhanced adenosine-dependent barrier responses in vitro. Moreover, in vivo studies of vascular barrier function demonstrated that AK inhibition with 5'-iodotubericidin (1 mg/kg prior to hypoxia) significantly attenuated hypoxia-induced vascular leakage in multiple organs and reduced hypoxia-associated increases in lung water. Taken together, our data reveal a critical role of AK in modulating vascular adenosine responses and suggest pharmacologic inhibitors of AK in the treatment of conditions associated with hypoxia-induced vascular leakage (eg, sepsis or acute lung injury).