Although vascular endothelial cells are crowded with mitochondria, these organelles are not the main ATP generators. Vascular endothelial cells are instead highly glycolytic, despite their close proximity to O2 in the blood. Moncada's group had previously found that nitric oxide (NO), which is abundant in vascular endothelial cells, efficiently competes with O2 for the binding to the cytochrome c oxidase, the terminal enzyme in the mitochondrial respiratory chain.
In the new report, the authors show that, at relatively low O2 concentrations, this competition diverts O2 away from the mitochondria into the cytosol, thereby reducing the response of the cells to hypoxia. Much lower concentrations of O2 (0.5%) were required in endothelial cells to stabilize hypoxia-inducible factor (HIF)1α when compared with other cells. The diversion of O2 and the decrease in HIF1α stabilization might help to maintain endothelial cells in a nonangiogenic state for proper vascular smooth muscle oxygenation.
In addition, the authors show that the competition between NO and O2 generates reactive oxygen species (ROS) as a result of a backlog of electrons in the respiratory chain. The ROS activated AMP-activated protein kinase (AMPK), an enzyme that promotes better use of energy by activating catabolic pathways and repressing anabolic ones. Moncada has not yet shown the physiological outcome of AMP kinase activation, but it might turn on defensive responses.