Oxygen sensitivity of mitochondrial function in rat arterial chemoreceptor cells
Buckler KJ., Turner PJ.
Key points: • Arterial chemoreceptors measure blood oxygen and are involved in the control of both breathing and the cardiovascular system. • Oxygen is mostly used by cells in their mitochondria to generate energy. • In this study we have investigated the effects of oxygen starvation (hypoxia) on the mitochondria of specialised oxygen sensing cells from arterial chemoreceptors. • Our data confirm that the mitochondria of these oxygen sensing cells are unusually sensitive to modest hypoxia. This effect seems to stem from a reduced affinity of the oxygen utilising enzyme cytochrome oxidase for oxygen. • These results are consistent with a functional adaptation of the mitochondria of oxygen sensing cells that may enable them to play a direct role in the oxygen sensing process itself. Abstract The mechanism of oxygen sensing in arterial chemoreceptors is unknown but has often been linked to mitochondrial function. A common criticism of this hypothesis is that mitochondrial function is insensitive to physiological levels of hypoxia. Here we investigate the effects of hypoxia (down to 0.5% O) on mitochondrial function in neonatal rat type-1 cells. The oxygen sensitivity of mitochondrial [NADH] was assessed by monitoring autofluorescence and increased in hypoxia with a P of 15 mm Hg (1 mm Hg = 133.3 Pa) in normal Tyrode or 46 mm Hg in Ca-free Tyrode. Hypoxia also depolarised mitochondrial membrane potential (ψ, measured using rhodamine 123) with a P of 3.1, 3.3 and 2.8 mm Hg in normal Tyrode, Ca-free Tyrode and Tyrode containing the Ca channel antagonist Ni, respectively. In the presence of oligomycin and low carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone (FCCP; 75 nm) ψ is maintained by electron transport working against an artificial proton leak. Under these conditions hypoxia depolarised ψ/inhibited electron transport with a P of 5.4 mm Hg. The effects of hypoxia upon cytochrome oxidase activity were investigated using rotenone, myxothiazol, antimycin A, oligomycin, ascorbate and the electron donor tetramethyl-p-phenylenediamine. Under these conditions ψ is maintained by complex IV activity alone. Hypoxia inhibited cytochrome oxidase activity (depolarised ψ) with a P of 2.6 mm Hg. In contrast hypoxia had little or no effect upon NADH (P= 0.3 mm Hg), electron transport or cytochrome oxidase activity in sympathetic neurons. In summary, type-1 cell mitochondria display extraordinary oxygen sensitivity commensurate with a role in oxygen sensing. The reasons for this highly unusual behaviour are as yet unexplained. © 2013 The Physiological Society.