Oxygen-sensing by ion channels and mitochondrial function in carotid body glomus cells

Carotid body glomus cells release transmitters in response to hypoxia due to the increase of excitability resulting from inhibition of O2-regulated K+ channels. The mechanisms involved in the detection of changes of O2 tension are unknown. Inhibition of the mitochondrial electron transport chain (ETC) at proximal and distal complexes induces external Ca(2+)-dependent catecholamine secretion. At saturating concentration of the ETC inhibitors, the cellular response to hypoxia is maintained. However, rotenone, a complex I blocker, selectively occludes the responsiveness to hypoxia of glomus cells in a dose-dependent manner. The effect of rotenone is not mimicked by complex I inhibitors acting on different sites. We have also generated a knock-out mouse lacking SDHD, the small membrane-anchoring protein of the succinate dehydrogenase (complex II) of the mitochondrial electron transport chain. Homozygous Sdhd(-/-) animals die at early embryonic stages. Heterozygous Sdhd(+/-) mice show a general, non-compensated, deficiency of complex II activity, and abnormal enhancement of resting carotid body secretion rate due to decrease of K+ conductance and persistent Ca2+ influx into glomus cells. However, responsiveness to hypoxia of carotid bodies from Sdhd(+/-) mice remains intact. These data strongly suggest that sensitivity to hypoxia of carotid body glomus cells is not linked in a simple way to mitochondrial electron flow. Nevertheless, it is possible that a rotenone-sensitive molecule critically participates in acute carotid body oxygen sensing.