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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 CO 2  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 O 2  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 CO 2  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.

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