Assessing the physiological concentration and targets of nitric oxide in brain tissue.
Low nanomolar concentrations of nitric oxide activate guanylyl cyclase to produce cGMP, which has diverse physiological effects. Higher concentrations inhibit mitochondrial respiration at cytochrome c oxidase and this has been proposed to be important physiologically, increasing oxygen permeation into tissue (by reducing the oxygen use of cells near blood vessels), activating AMP kinase, and regulating the relationship between cerebral blood flow and oxygen use. It is unclear, however, whether nitric oxide can accumulate physiologically to concentrations at which inhibition of respiration occurs. In rat cerebellar slices, we activated nitric oxide production from each isoform of nitric oxide synthase. Only activation of inducible nitric oxide synthase, which is expressed pathologically, caused any significant inhibition of respiration. Modelling oxygen and nitric oxide concentrations predicted that, in vivo, physiological nitric oxide levels are too low to affect respiration. Even pathologically, the nitric oxide concentration may only rise to 2.5 nm, producing a 1.5% inhibition of respiration. Thus, under physiological conditions, nitric oxide signals do not inhibit respiration but are well-tuned to the dynamic range of guanylyl cyclase activation.
Department of Physiology, University College London, Gower St, London WC1E 6BT, UK. catherine.hall@ucl.ac.uk
Catherine N
CN
Hall
David
D
Attwell
D000818:Animals; D002531:Cerebellum / Q000187:drug effects / Q000378:metabolism*; D006162:Guanylate Cyclase / Q000037:antagonists & inhibitors; D007371:Interferon-gamma / Q000494:pharmacology; D008070:Lipopolysaccharides / Q000494:pharmacology; D009566:Nitrates / Q000494:pharmacology; D009569:Nitric Oxide / Q000378:metabolism*; D019001:Nitric Oxide Synthase / Q000378:metabolism; D010101:Oxygen Consumption / Q000502:physiology*; D020033:Protein Isoforms; D051381:Rats; D015398:Signal Transduction / Q000502:physiology; D013997:Time Factors
D008070:Lipopolysaccharides; D009566:Nitrates; D020033:Protein Isoforms; D009569:Nitric Oxide; D007371:Interferon-gamma; D019001:Nitric Oxide Synthase; D006162:Guanylate Cyclase