Cellular mechanisms of oxygen sensing

O-2 sensing is a fundamental biological process necessary for adaptation of living organisms to variable habitats and physiological situations. Cellul ar responses to hypoxia can be acute or chronic. Acute responses rely mainl y on O-2-regulated ion channels, which mediate adaptive changes in cell exc itability, contractility, and secretory activity. Chronic responses depend on the modulation of hypoxia-inducible transcription factors, which determi ne the expression of numerous genes encoding enzymes, transporters and grow th factors. O-2-regulated ion channels and transcription factors are part o f a widely operating signaling system that helps provide sufficient O-2 to the tissues and protect the cells against damage due to O-2 deficiency. Des pite recent advances in the molecular characterization of O-2-regulated ion channels and hypoxia-inducible factors, several unanswered questions remai n regarding the nature of the O-2 sensor molecules and the mechanisms of in teraction between the sensors and the effectors. Current models of O-2 sens ing are based on either a heme protein capable of reversibly binding O-2 or the production of oxygen reactive species by NAD(P)H oxidases and mitochon dria. Complete molecular characterization of the hypoxia signaling pathways will help elucidate the differential sensitivity to hypoxia of the various cell types and the gradation of the cellular responses to variable levels of PO2. A deeper understanding of the cellular mechanisms of O-2 sensing wi ll facilitate the development of new pharmacological tools effective in the treatment of diseases such as stroke or myocardial ischemia caused by loca lized deficits of O-2.