Oxygen diffusion and mitochondrial respiration in neuroblastoma cells

Suspensions of human neuroblastoma cells consume oxygen at a constant rate when the oxygen pressure is greater than approximately 11 Torr. The rate of oxygen consumption, however, becomes dependent on the oxygen pressure below this level. The falling respiratory rate at the lower pressures gives rise to an oxygen pressure for half-maximal respiration (P50) of approximately 0.8 Torr, which is consistent with the 0.5 Torr value for suspensions of isolated mitochondria in the presence of ATP (J. Biol. Chem. 263: 2712-2718, 1988). When the cellular metabolic energy state is lowered by addition of an uncoupler of mitochondrial oxidative phosphorylation, the respiratory rate increases up to fivefold, but the P50 decreases to approximately 0.6 Torr. In the cells treated with uncoupler, the P50 decreases further when the mitochondrial respiratory rate is inhibited with amobarbital (amytal), an inhibitor of the respiratory chain. The additional decrease in P50 is proportional to the decrease in respiratory rate. Thus, for cells treated with uncoupler, the P50 appears to be limited by oxygen diffusion from the external medium to the mitochondria. When the respiratory rate of the uncoupled cells is inhibited to the level of coupled cells, the P50 for the former is less than 0.15 Torr. This indicates that for coupled cells the difference in oxygen pressure from the external medium to the mitochondria is less than 0.15 Torr at half-maximal respiratory rate and does not significantly affect the P50 for oxygen that occurs at 0.8 Torr.(ABSTRACT TRUNCATED AT 250 WORDS)