The ability to react rapidly to dynamic changes in arterial blood gas composition is crucial for optimal delivery of molecular oxygen (O₂) to respiring tissues. The principal sensory components of this homeostatic mechanism are the carotid bodies. Ideally situated in the bifurcation of the common carotid artery, they respond muliplicatively to hypoxia, hypercapnia, pH and hypoglycaemia. When activated, secretion of a variety of transmitters by the carotid body glomus cells results in augmented input to the respiratory centres of the brain stem. Thus, during reduced O₂ availability, activation of the carotid bodies promotes increased rate and depth of ventilation as a compensatory response to systemic hypoxia. At the cellular level, hypoxia promotes inhibition of plasma membrane potassium channels of carotid body glomus cells which leads to calcium influx and transmitter release. In rat glomus cells, two potassium channels have been implicated in the hypoxia-dependent depolarization – a specific member of the tandem P-domain potassium channel family (almost certainly the TASK sub-type (Buckler et al., 2000) and the calcium-activated, large conductance potassium channel (BK_Ca – (Peers, 1990)). It appears likely that both BK_Ca and the TASK-like potassium channel contribute to carotid body O₂ sensing but, until recently, the identity of the O₂ sensor had remained elusive. In the search for a potential O₂ sensing mechanism which could account for rapid and reversible inhibition of BK_Ca channels, we carried out a proteomic screen for potential protein partners of the BK_Ca α-subunit using immunoprecipitation, two-dimensional electrophoresis and mass spectrometry. Of particular note was the protein partnership (verified by double-label immunocytochemistry) of BKα with an O₂-dependent enzyme, hemeoxygenase-2 (HO-2). In the presence of O₂ and NADPH, this enzyme oxidizes cellular heme to generate carbon monoxide (CO), iron and biliverdin. Downstream products of HO-dependent heme catalysis have been reported to play important roles in a wide variety of biological tissues including the immune, the central nervous and the cardiovascular systems. Of particular interest is the observation that HO-2 is expressed in the carotid body and that CO has a major impact on carotid body chemotransduction (Prabhakar et al., 1995). To define the molecular mechanism linking HO-2 activity to channel inhibition during hypoxia, we have employed single channel studies to show that BK_Ca channel (expressed in both HEK293 cells and natively in carotid body glomus cells) activity is robustly and reversibly activated by the downstream products of HO-2, biliverdin and CO (the latter via addition of the chemical CO-donor, [Ru(CO)₃Cl₂]). In the presence of O₂, addition the HO-2 co-substrates, heme and NADPH, evoke an increase in channel activity. Importantly, in their continued presence, hypoxia evokes a depression in channel activity which is much larger than that observed in the absence of HO-2 co-substrates. These observations suggest that HO-2 enzymatic activity confers a significant enhancement to the O2 sensing ability of the HO-2/BK_Ca protein complex. In support of this notion, selective knock-down of HO-2 protein by RNA interference dramatically depresses tonic channel activity and the NADPH/heme-dependent hypoxic channel suppression is absent. Crucially, CO rescues this loss-of-function (Williams et al., 2004). The mechanism of such gas/channel interactions is complex, and may involve interactions with heme (Jaggar et al., 2005). Using chemical and molecular modifications of the BKα subunit in combination with channel chimera studies, we are beginning to appreciate the kinetic and structural basis of the dynamic regulation of this potassium channel by endogenous production of CO, a gas which is emerging as an important second messenger. Together, such data have led to our proposal that significant O₂-sensing is conferred upon the BK_Ca channel by co-localization with HO-2. In normoxia, tonic HO-2 activity generates CO which maintains the open state of the channel. During hypoxic challenge, cellular CO levels are reduced and rapidly fall below the critical threshold for the maintenance of BK_Ca channel activity. In other words, HO-2 functions as a sensor of acute reduction in environmental O₂ by changing the balance between intracellular heme concentration and the production of CO.