Evidence for second metabolic pathway for O2 from PtiO2 measurements in denervated cat carotid body

O2 microelectrode studies were conducted in the cat carotid body (CB) to investigate the hypothesis that there is a second, low affinity metabolic pathway for O2 in addition to classical oxidative metabolism. Tissue PO2 (PtiO2) and O2 disappearance rates (dPO2/dt) after brief blood flow occlusion were measured with recessed cathode microelectrodes (tip diameter less than 5 microns) at 150 sites in 15 normal cats (controls) and at 154 sites in 5 cats in which one CB had been denervated 2 or 3 days before the experiments. Mean PtiO2 was slightly higher in denervated CBs: 79.6 +/- 1.6 (SE) Torr compared with 76.4 +/- 2.0 Torr for controls (P = not significant). Mean dPO2/dt was 8.4% faster: -8.42 +/- 0.28 Torr/s compared with -7.77 +/- 0.43 Torr/s for controls (P less than 0.05). The O2 consumption rate (VO2), calculated from dPO2/dt correcting for cat oxyhemoglobin, was 7.5% higher: 1.62 and 1.51 ml.100 g-1.min-1, respectively, for denervated and control CBs (P less than 0.05). The apparent Michaelis-Menten constant, Kmapp (defined as the PtiO2 where dPO2/dt decreased by 50% from the initial rate during the first 3 s after occlusion) was determined for each O2 disappearance curve. After denervation, Kmapp decreased significantly by -47%: 12.0 +/- 1.3 Torr compared with 22.6 +/- 2.5 Torr for controls (P less than 0.01). The data provide evidence for a second metabolic pathway for O2 in the CB that loses its influence on VO2 after denervation.