Mechanisms by which dichloroacetate lowers lactic acid levels: the kinetic interrelationships between lactate, pyruvate, alanine, and glucose

Mechanism of dichloroacetate-induced hypolactatemia in humans with or without cirrhosis

Dichloroacetate (DCA) has been used as an experimental treatment for lactic acidosis because it lowers plasma lactic acid concentration. Three potential mechanisms could underlie the hypolactatemic action of DCA, but the dominant mechanism in vivo remains unclear. This study tested whether DCA-induced hypolactatemia occurs via decreased lactate production, increased lactate clearance, or decreased rate of glycolysis in healthy humans and in patients with end-stage cirrhosis. Cirrhosis is associated with decreased hepatic pyruvate dehydrogenase (PDH) content. Six healthy volunteers and 7 cirrhotic patients received a primed, constant infusion of 1-13C-pyruvate and 15N-alanine for 5 hours. DCA (35 mg/kg intravenously) was administered at 2 hours. Plasma isotopic enrichment was measured by gas chromatography/mass spectrometry (GC/MS), and exhaled CO2 enrichment by isotope ratio mass spectrometry. Pyruvate and alanine production rates (Ra) were determined by isotope dilution, and pyruvate oxidation calculated as 13CO2 production from 13C-pyruvate. Ra lactate was calculated as the difference between Ra pyruvate and its disposal by oxidation to CO2 and conversion to alanine. Baseline plasma lactate kinetics in cirrhotic patients did not differ from controls. DCA decreased lactate concentration in both groups by approximately 53%. DCA decreased glycolysis (Ra pyruvate) by 24%, increased the fraction of pyruvate oxidized to CO2 by 26%, and decreased pyruvate transamination to alanine by 25%. DCA also inhibited lactate production by 85%, but decreased plasma lactate clearance by 60% in both groups. DCA reduces plasma lactic acid concentration by inhibiting production, via stimulating pyruvate oxidation and inhibiting glycolysis, rather than increasing clearance. In addition, end-stage cirrhosis does not alter either the mechanism or the magnitude of the metabolic response to DCA.

Biochemical and neurotoxicological effects of L-2-chloropropionic acid on rodent brain

L-2-Chloropropionic acid (L-CPA) is selectively toxic to cerebellar granule cells; necrosis is first observed in rats 36 h after L-CPA administration (750 mg/kg p.o.) and becomes marked by 48 h. L-CPA has also been shown to activate the mitochondrial pyruvate dehydrogenase (PDH) complex in fasted adult rats, resulting in reduced blood glucose and lactate levels. This study aimed to investigate the biochemical and neurotoxicological effects of L-CPA on the brain. Extracts, prepared from guinea-pig cerebellar and cerebral cortex slices incubated in the presence of L-CPA, were analysed using 1H magnetic resonance spectroscopy, 31P magnetic resonance spectroscopy, and amino acid analysis. Glucose metabolism was studied by monitoring the metabolism of [1-(13)C]glucose using gas chromatography/mass spectrometry. Increased glucose metabolism and decreases in the pool sizes of lactate and alanine were observed in both tissues, demonstrating activation of the PDH complex. Extracts were also prepared from the forebrain and cerebellum of animals that had been treated in vivo with L-CPA and analysed as described for the in vitro studies. Similar evidence for PDH activation was demonstrated at 2 and 24 h after dosing in both tissues. At 48 h after dosing, when signs of toxicity are observed, an increase in the lactate concentration and a decrease in N-acetylaspartate in the cerebellum but not in the forebrain confirmed the selective neurotoxic action of L-CPA. These results suggest that activation of the PDH complex does not directly lead to the delayed selective neurotoxicity of L-CPA.

Hepatic pyruvate dehydrogenase activity in humans: effect of cirrhosis, transplantation, and dichloroacetate

The liver is the major site for lactate clearance, and liver disease exacerbates lactic acidosis during orthotopic liver transplantation (OLT). This study assessed pyruvate dehydrogenase (PDH) activity in control, cirrhotic, and graft liver to test the hypotheses that 1) liver disease decreases hepatic PDH activity, 2) graft PDH activity is inhibited due to protracted ischemia, and 3) dichloroacetate (DCA) reverses functional PDH inhibition in cirrhotic and graft liver. After having given their informed consent, 43 patients received either DCA (80 mg/kg) or aqueous 5% glucose during OLT. Six patients without apparent liver dysfunction that were undergoing subtotal hepatic resection served as controls. Liver biopsy PDH activity was assayed by measuring [14C]citrate synthesis from [14C]oxaloacetate and PDH-derived acetyl-CoA. PDH in the active form (PDHa) in cirrhotic and control liver was 5.6 +/- 1.3 (SE) and 57 +/- 10 nmol.g wet wt-1.min-1, respectively (P < 0.001). Total PDH activity (PDHt) was 21.5 +/- 3.6 and 264 +/- 27 nmol.g wet wt-1.min-1, respectively (P < 0.001). DCA increased PDHa in cirrhotic liver to 22.3 +/- 4.1 nmol.g wet wt-1.min-1 (P < 0.05 vs. no DCA) without altering PDHt. Graft liver PDHa was 166 +/- 19 nmol.g wet wt-1.min-1, which was not altered by DCA. We conclude that decreased hepatic PDH activity secondary to decreased content may underlie lactic acidosis during OLT, which can be partially compensated by DCA administration. There is no apparent inhibition of graft liver PDH activity after reperfusion.

In vivo NMR investigation of intramuscular glucose metabolism in conscious rats

In vivo 13C nuclear magnetic resonance (NMR) spectroscopy was used to determine quantitatively the flux of muscle glycolysis, glycogen synthesis, pyruvate dehydrogenase, and pyruvate carboxylation in the hindlimb of conscious rats. 13C NMR spectroscopy was used to observe [1-13C]glucose label precursor incorporation into intramuscular [1-13C]glycogen, [3-13C]lactate, and [3-13C]alanine during a hyperglycemic (approximately 11 mM)-hyperinsulinemic (10 mU.kg-1.min-1) clamp. The glycogen synthesis rate was calculated to be 224 +/- 23 nmol.g-1.min-1. The kinetic data obtained from the label turnover in the intramuscular C-3 lactate and C-3 alanine metabolite pools, as well as in plasma C-3 lactate and C-3 alanine, were combined with a steady-state rate analysis to determine the glycolytic flux (67.4 +/- 10.1 nmol.g-1.min-1). Steady-state isotopomer analysis of glutamate and pyruvate in skeletal muscle tissue extracts was used to determine the anaplerotic contribution of substrate via pyruvate carboxylation (Vpc). The pyruvate dehydrogenase flux (Vpdh) was calculated after a steady-state flux correction for Vpc. Calculated values of Vpc and Vpdh were 24.8 +/- 4.3 and 110.0 +/- 18.7 nmol.g-1.min-1, respectively. In addition, [2-13C]acetate was used in a separate study to determine that pyruvate carboxylation was the major pathway for anaplerosis in skeletal muscle under conditions of hyperglycemia-hyperinsulinemia.