Oxidative phosphorylation of isolated rat skeletal muscle mitochondria after exposure to
lactic acidosis in either phosphorylating or nonphosphorylating states has been evaluated. Mitochondrial respiration and transmembrane potential (DeltaPsi(m)) were measured with
pyruvate and
malate as the substrates. The addition of
lactic acid decreased the pH of the reaction medium from 7.5 to 6.4. When
lactic acid was added to nonphosphorylating mitochondria, the subsequent maximal
ADP-stimulated respiration decreased by 27% compared with that under control conditions (P < 0.05), and the apparent Michaelis-Menten constant (K(m)) for
ADP decreased to 10 microM vs. 20 microM (P < 0.05) in controls. In contrast, maximal respiration and
ADP sensitivity were not affected when mitochondria were exposed to
acidosis during active phosphorylation in state 3.
Acidosis significantly increased mitochondrial oxygen consumption in state 4 (post-state 3), irrespective of when
acidosis was induced. This effect of
acidosis was attenuated in the presence of
oligomycin. The addition of
lactic acid during state 4 respiration decreased DeltaPsi(m) by 19%. The ratio between added
ADP and consumed
oxygen (P/O) was close to the theoretical value of 3 in all conditions. The addition of
potassium lactate during state 3 (i.e., medium pH unchanged) had no effect on the parameters measured. It is concluded that
lactic acidosis has different effects when induced on nonphosphorylating vs. actively phosphorylating mitochondria. On the basis of these results, we suggest that the influence of
lactic acidosis on muscle aerobic energy production depends on the physiological conditions at the onset of acidity.