PERSPECTIVES (2003)

The ability to sense oxygen tension is a key function of several tissues, including the carotid body and pulmonary vasculature. There is little consensus concerning the mechanisms underlying oxygen sensing, although recently there has been a resurgence of interest in the role of mitochondria. Numerous studies over many years have shown that inhibitors of mitochondrial function mimic hypoxia in the carotid body glomus cell, and similar, though less consistent, results have been reported for the pulmonary vasculature (see López-Barneo et al. 2001). This has led to the long standing hypothesis that oxygen sensing is related to disruption of electron flow through the mitochondrial electron transport chain (ETC), although there are significant differences of opinion concerning the details and sequellae of this disruption (e.g. Chandel & Schumacker, 2000; Michelakis et al. 2002). In this issue of the Journal of Physiology, the paper by Ortega-Sáenz et al. (2003) casts doubt on the above hypothesis, at least for the carotid body. Elegant experiments were performed on a thin-slice preparation of rat carotid body, designed to minimise tissue disruption but allow amperometric detection of catecholamine release from single glomus cells. The authors report that both hypoxia and inhibition at any point in the ETC (Fig. 1) caused similar increases in extracellular Ca2+-dependent catecholamine secretion. However, with the exception of the complex I inhibitors rotenone and MPP+, no inhibitor prevented further increases in secretion during hypoxia, including the other complex I inhibitors myxothiazol, rhein and DPI. Rotenone and MPP+ act at a single but different binding site from these latter, and they alone abolished the response to hypoxia. Moreover, this could not be reversed by succinate, which should bypass block of complex I by providing electrons via complex II (Fig. 1; Leach et al. 2001). Cartoon representing the electron transport chain in the mitochondrial inner membrane, showing the point of interaction of key inhibitors used by Ortega-Sáenz et al. (2003) (boxes), possible locations for oxygen sensing and production of putative signalling intermediates (SO, superoxide) (Chandel & Schumacker, 2000; Michelakis et al. 2002), and a highly simplified representation of the enzyme Q cycle (Q). Inhibition at any point in the chain will cause proximal reduction and distal oxidation. CytC, cytochrome C; uQ, ubisemiquinone. These intriguing observations are inconsistent with the widely held hypothesis that oxygen sensing is mediated via changes in electron flow through the mitochondrial ETC. In consequence they are also inconsistent with the main hypotheses regarding distal signalling pathways, namely changes in redox state due to reduced consumption of reducing equiva lents (NADH) by the ETC, or production of ETC-derived reactive oxygen species (ROS) (Fig. 1; Chandel & Schumacker, 2000; Weir et al. 2002; Michelakis et al. 2002). They also imply that although both hypoxia and disruption of the ETC cause secretion via Ca2+ influx, different signalling pathways must be involved. This provides a salutary reminder that procedures that mimic physio logical stimuli do not necessarily involve the same mechanisms, and subsequent ablation of the response to such stimuli might merely reflect saturation of the endpoint effector. It also reiterates the fact that perturbations to mitochondrial function can influence membrane ionic permeabilities independently of any oxygen sensing mechanism, possibly via redox-ROS pathways, although the fact that Ortega-Sáenz et al. (2003) find that TTFA, a blocker of complex II, increases secretion to the same extent as the other inhibitors is perplexing, as it would not be expected to alter cell redox state in the same way, at least in terms of NADH (see Fig. 1). This suggests yet another complexity. The results reported by Ortega-Sáenz et al. (2003) may not be applicable to other systems. In the pulmonary circulation Michelakis et al. (2002) have reported that inhibition at any point of the ETC not only mimics hypoxia but also blocks oxygen sensing. In contrast, others have shown that inhibition proximal to ubisemiquinone in complex III blocks oxygen sensing without mimicking hypoxia, whereas distal inhibitors potentiate the hypoxic response, suggesting a key role for complex III in several tissues (Fig. 1; Chandel & Schumacker, 2000), including pulmonary artery (Leach et al. 2001). In the latter case, inhibition by rotenone was reversed by succinate, indicating that here at least the effects of rotenone are mediated via restriction of electron flow through the ETC. There is also good evidence that oxygen sensing in tissues such as neuroepithelial cells involves a non-mitochondrial NAD(P)H oxidase (see López-Barneo et al. 2001). The above may well reflect true differences between tissues. However, the spectre raised by Ortega-Sáenz et al. (2003) of an oxygen sensor that is blocked by rotenoids and yet independent of mitochondrial electron trans port, coupled with implicit limitations concerning the use of ETC inhibitors generally, should elicit pause for thought in all those interested in mitochondrial oxygen sensing, and instigate re-evaluation of some previous studies. Ortega-Sáenz et al. (2003) speculate that this putative sensor may involve either a flavoprotein attached to a K+ channel sub-unit, or a more complex arrangement involving mitochondria in microdomains closely adjacent to K+ channels. Whatever the final outcome, this intriguing paper raises the stakes yet again in an already highly controversial field.