Influence of in vitro Lactic Acidosis and Hypercapnia on Respiratory Activity of Isolated Rat Brain Mitochondria

Respiratory activity and the ADP/O ratio of isolated rat brain mitochondria were measured following incubation with varying concentrations of lactic acid in reaction media buffered either with bicarbonate and CO₂ or with phosphate alone, at a pH of 7.1. Increasing lactic acid levels caused a progressive decrease in substrate-, phosphate-, and ADP-stimulated (State 3) respiration and ADP/O ratios. Fifteen millimolar lactic acid, pH 6.4, caused ∼50% inhibition of State 3 respiration (with malate + glutamate as substrate). At lower pH values (5.3–6.1), addition of ADP caused little or no increase in O₂ consumption; i.e., ATP formation ceased. Addition of lactic acid at constant pH moderately affected respiratory control ratios but did not change State 3 respiration or ADP/O ratios. Thus, the effect of lactic acid was related to the pH change. Increasing CO₂ concentrations in the reaction medium had similar effects on mitochondrial respiration, indicating that changes in extramitochondrial pH rather than in transmembrane H⁺ gradients determined the respiratory alterations. Following a 5-min incubation of mitochondria with lactic acid, pH 6.1, there was an incomplete recovery of State 3 respiration and respiratory control ratios. It is concluded that mitochondrial respiration is inhibited by a decrease in pH which, if excessive, may lead to a permanent suppression of ATP production. These results may, at least partly, explain the deleterious effects of enhanced lactic acidosis in brain ischemia.