Assessment of mitochondrial function in cells grown in tissue culture

To assess mitochondrial function (pyruvate dehydrogenase [PDH] activity), cells were grown in the appropriate media to confluence, rinsed and incubated in glucose free media containing 25 microM L-lactate and [1-14C]-D,L-lactate. Lactate oxidation was measured as the amount of lactate oxidized in nmol of 14CO2 generated per mg of protein per minute. Basal activity varied with cell number and the cell type studied: fibroblast 2.26 +/- 0.01; Chinese hamster ovary (CHO) 42 +/- 0.4; BC3H-1 52 +/- 2.1 nmol per mg per minute. The CHO cells screened for PDH activity decreased their dependence on lactate as a substrate in the presence of 5mM glucose by 60 percent. Increasing the cold lactate concentration diluted the labelled lactate available for pyruvate oxidation in a dose dependent manner. The mitochondrial inhibitor rotenone (25 microM) decreased assay activity by > 75 percent in CHO and BC3H-1 cells. The lactate oxidation assay was shown to be sensitive enough to measure insulin stimulation of PDH in a dose dependent manner with maximum activity occurring at concentrations between 1 microU per ml and 100 microU per ml.