At physiological concentrations nitric oxide (NO) inhibits mitochondrial cytochrome c oxidase in competition with oxygen. Using a technique that we have developed based on visible light spectroscopy we have demonstrated that endogenous NO enhances the reduction of the electron transport chain, thus enabling cells to maintain their VO₂ at low [O₂]. This favours the release of superoxide anion, which initiates the transcriptional activation of NF-kB as an early stress signalling response. We have recently used this technique to demonstrate that NO is inactivated by cytochrome c oxidase in its oxidised state and that cessation of such inactivation at low [O₂] may account for hypoxic vasodilatation.

Many cells respond to a decrease in oxygen availability via stabilisation of hypoxia-inducible factor-1a (HIF-1a), whose accumulation is normally prevented by the action of prolyl hydroxylases. We have found that inhibition of mitochondrial respiration by low concentrations of NO leads to inhibition of HIF-1a stabilisation. This prevents the cell from registering a state of hypoxia at low [O₂], which would otherwise result in upregulation of defensive genes associated with, for example, glycolysis and angiogenesis. Furthermore, upon inhibition of mitochondrial respiration in hypoxia, oxygen is redistributed toward non-respiratory oxygen-dependent targets.

Our results demonstrate that NO acts not only as a physiological regulator of cell respiration but also as a signalling agent in the mitochondria and a controller of the distribution of available oxygen. Such mechanisms may also be involved in the initiation of pathophysiology.