Oxygen Sensing in Neuroepithelial and Adrenal Chromaffin Cells
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Oxygen sensing and initiation of appropriate physiological responses to hypoxia are crucial for survival. The molecular identity of the sensor has generally sparked considerable interest and controversy in O2-sensitive cells. In mammals, pulmonary neuroepithelial bodies (NEBs) and adrenal chromaffin cells (AMCs) are O2 sensitive, particularly during the transition from intrauterine to air-breathing life. In NEBs, there is good evidence that the O2 sensor is a plasma membrane-bound NADPH oxidase which during hypoxia, signals K+ channel inhibition, membrane depolarization and neurosecretion via changes in reactive oxygen species (ROS) (e.g. H2O2). Accordingly, hypoxic sensitivity is lost in NEBs from transgenic mice deficient in the gp91(phox) subunit of NADPH oxidase; it is, however, retained in neonatal AMCs from these transgenic mice. A search for the O2 sensor in neonatal rat AMCs suggests a role for the mitochondrial electron transport chain. For example, the complex I blocker, rotenone (1 microM), mimics hypoxia in causing K+ channel inhibition and ATP secretion, and occludes hypoxic sensitivity. The evidence is consistent with hypoxia and rotenone acting via a decrease in ROS. In contrast, the complex IV blocker cyanide (2 mM) did not mimic the effects of hypoxia. We propose thatchanges in ROS serve as a common link between the O2 sensor and secretion in perinatal NEBs and chromaffin cells. However, the subcellular localization of the O2 sensor appears to be different between these two cell types.
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