Cat carotid body oxygen metabolism and chemoreception described by a two-cytochrome model

We have analyzed O2 disappearance curves (DCs) in cat carotid bodies (CBs) measured with our O2 microelectrode, after stopping flow of either blood (108 DCs in 12 cats) or a hemoglobin-free (Locke) perfusion solution (35 DCs in 6 cats). Prior to occlusion, the mean tissue PO2 levels were 74.5 +/- 2.8 (SE) Torr in blood-perfused CBs and 103.4 +/- 2.6 in Locke-perfused CBs. The O2 consumption rates (VO2) determined from the initial 3 s of the DCs were 1.46 +/- 0.08 and 1.50 +/- 0.10 (SE) ml O2 . 100 g-1 . min-1, respectively, for the blood-perfused and Locke-perfused CBs. The change in total sinus nerve activity from the CB was also measured following stopped flow. The nerve activity began to increase immediately, providing further evidence that classic hypoxia is not the mechanism of chemoreceptor discharge. However, about two-thirds of the increased activity in blood-perfused CBs occurred after tissue PO2 levels fell below 20 Torr. As the CB tissue PO2 decreased, the O2 disappearance rate (-dPO2/dt) also decreased for both experimental conditions, indicating that the CB VO2 varies with O2 concentration. The increase in nerve discharge and O2 disappearance rate can be interpreted by a two-cytochrome model for O2 metabolism, with both high and low affinities.