OXYGEN SENSORS IN THE ORGANISM - EXAMPLES OF REGULATION UNDER ALTITUDE HYPOXIA IN MAMMALS

Oxygen sensing is a determinant function of mammals, especially humans , to maintain their activity under acute or chronic exposure to hypoxi a. True O-2 sensors (chemoreceptors, erythropoietin secreting cells) a re involved in regulation loops, which aim to restore O-2 availability to the cells. Pseudo O-2 sensors are cells activated by the lack of o xygen but not clearly involved in regulation processes. Potassium chan nels in the carotid bodies have been suspected to be O-2 sensitive and could mediate the chemosensitivity to hypoxia. Na,K-ATPase related io n transport in alveolar pneumocytes could be sensitive to O-2 availabi lity and regulate the flux of water and sodium in the alveolar space. Signal transduction in G-protein-dependent receptor systems is modifie d in hypoxia, such as in cardiac beta-receptors and adenosinergic and muscarinic receptors. Recent studies have provided some evidence to th e possible role of hypoxia inducible factors (HIF-1) in the regulation of protein synthesis at the transcriptional level. Similarities betwe en O-2 sensing mechanisms in erythropoiesis and in the synthesis of va scular endothelial growth factor were recently evidenced. Both genes a re upregulated in hypoxia. However, the precise structure (heme-linked enzyme?) of all these O-2-sensitive sites is not known, either in the erythropoietic system or in the chemoreceptor function. An adequate b alance between hypoxia-induced upregulation and downregulation process es is necessary for optimal survival in a hypoxic environment. (C) 199 7 Elsevier Science Inc.