Acute oxygen sensing in cellular models: relevance to the physiology of pulmonary neuroepithelial and carotid bodies.

The combination of studies in native tissues and immortalised model systems during the last decade has made possible a deeper understanding of the physiology and functional morphology of arterial and airway oxygen sensors. Complementary and overlapping information from these earlier studies has allowed a detailed description of the cellular events that link decreased environmental oxygen to the release of physiologically important vasoactive transmitters. Since these basic pathways have now been defined functionally, what remains to be determined is the molecular identity of the specific proteins involved in the signal transduction pathways, and how these proteins interact to produce a full physiological response. With these goals clearly in sight, we have embarked upon a strategy that is a novel combination of proteomics and functional genomics. It is hoped this strategy will enable us to develop and refine the initial models in order to understand more completely the process of oxygen sensing in health and disease.

Lack of contribution of mitochondrial electron transport to acute O(2) sensing in model airway chemoreceptors.

We have recently reported that the model airway chemoreceptors, H146 cells, exhibit a significant component of their oxygen-sensing transduction pathway which cannot be explained by activity of NADPH oxidase. Using patch-clamp, we have studied the transduction system linking reduced O(2) to k(+) channel inhibition and report that, in complete contrast to recent suggestions in pulmonary vasculature, O(2) sensing by the model airway chemoreceptors, H146 cells, does not require functional mitochondria. These data show, for the first time, that mitochondrial production of reactive O(2) species is not the unifying mechanism in O(2) sensing.

Post-transcriptional control of human maxiK potassium channel activity and acute oxygen sensitivity by chronic hypoxia.

Various cardiorespiratory diseases (e.g. congestive heart failure, emphysema) result in systemic hypoxia and patients consequently demonstrate adaptive cellular responses which predispose them to conditions such as pulmonary hypertension and stroke. Central to many affected excitable tissues is activity of large conductance, Ca2+-activated K+ (maxiK) channels. We have studied maxiK channel activity in HEK293 cells stably co-expressing the most widely distributed of the human alpha- and beta-subunits that constitute these channel following maneuvers which mimic severe hypoxia. At all [Ca2+]i, chronic hypoxia (approximately 18 mm Hg, 72 h) increased K+ current density, most markedly at physiological [Ca2+]i K+ currents in cells cultured in normoxia showed a [Ca2+]i-dependent sensitivity to acute hypoxic inhibition ( approximately 25 mm Hg, 3 min). However, chronic hypoxia dramatically changed the Ca2+ sensitivity of this acute hypoxic inhibitory profile such that low [Ca2+]i could sustain an acute hypoxic inhibitory response. Chronic hypoxia caused no change in alpha-subunit immunoreactivity with Western blotting but evoked a 3-fold increase in beta-subunit expression. These observations were fully supported by immunocytochemistry, which also suggested that chronic hypoxia augmented alpha/beta-subunit co-localization at the plasma membrane. Using a novel nuclear run-on assay and RNase protection we found that chronic hypoxia did not alter mRNA production rates or steady-state levels, which suggests that this important environmental cue modulates maxiK channel function via post-transcriptional mechanisms.

Airway chemotransduction: from oxygen sensor to cellular effector.

The process of sensing, transducing, and acting on environmental cues is critical to normal physiologic function. Furthermore, dysfunction of this process can lead to the development of disease. This is especially true of the homeostatic mechanisms that have evolved to maintain the carriage of O2 to respiring tissues during acute hypoxic challenge. During periods of reduced O2 availability, three major mechanisms act conjointly to increase ventilation and optimize the ventilation-perfusion ratio throughout the lung by directing pulmonary blood flow to better ventilated areas of the lung. These mechanisms are as follows: (1) increased carotid sinus nerve discharge rate to the respiratory centers of the brain, (2) intrinsic hypoxic vasoconstriction of pulmonary resistance vessels, and (3) potential local and central modulation via stimulation of neuroepithelial bodies of the lung. The key to the rapid response to the O2 signal is the ability of each of these tissues to sense acutely the changes in PO2, to transduce the signal, and for cellular effectors to initiate compensatory mechanisms that will offset rapidly the reduction in PO2 before O2 availability to tissues is compromised. This review concentrates on the signal transduction mechanism that links altered PO2 to depolarization in the recently proposed airway chemosensory element, the neuroepithelial body (and its immortalized cellular counterpart, the H146 cell line), and discusses the pertinent similarities and differences that exist between airway, carotid body, and pulmonary arteriolar O2 sensing.