Selective increase of brain lactate synthesis in experimental acute liver failure: Results of a [1H-13C] nuclear magnetic resonance study

Abstract

Acute liver failure (ALF) results in alterations of energy metabolites and of glucose-derived amino acid neurotransmitters in brain. However, the dynamics of changes in glucose metabolism remain unclear. The present study was undertaken using 1 H and 13 C nuclear magnetic resonance (NMR) spectroscopy to determine the rates of incorporation of glucose into amino acids and lactate via cell-specific pathways in relation to the severity of encephalopathy and brain edema in rats with ALF because of hepatic devascularization. Early (precoma) stages of encephalopathy were accompanied by significant 2-to 4.5-fold (P < .001) increases of total brain glutamine and lactate concentrations. More severe (coma) stages of encephalopathy and brain edema led to a further significant increase in brain lactate but no such increase in glutamine. Furthermore, 13 C isotopomer analysis showed a selective increase of de novo synthesis of lactate from [1-13 C]glucose resulting in 2.5-fold increased fractional 13 C enrichments in lactate at coma stages. [2-13 C]glutamine, synthesized through the astrocytic enzyme pyruvate carboxylase, increased 10-fold at precoma stages but showed no further increase at coma stages of encephalopathy. 13 C-label incorporation into [4-13 C]glutamate, synthesized mainly through neuronal pyruvate dehydrogenase, was selectively reduced at coma stages, whereas brain GABA synthesis was unchanged at all time points. In conclusion, increased brain lactate synthesis and impaired glucose oxidative pathways rather than intracellular glutamine accumulation are the major cause of brain edema in ALF. Future NMR spectroscopic studies using stable isotopes and real-time measurements of metabolic rates could be valuable in the elucidation of the cerebral metabolic consequences of ALF in humans. (HEPATOLOGY 2003;37:420-428.) H epatic encephalopathy and brain edema are serious central nervous system complications of acute liver failure (ALF), which are accompanied by significant alterations of brain metabolites. For example, biochemical studies in brain tissue from animals with ALF resulting from hepatectomy, 1 hepatic devascularization, 2-4 or toxic liver injury 5 as well as autopsied brain tissue from patients with ALF 6 reveal significant increases in concentrations of glutamine and a concomitant reduction of glutamate. Several of these studies also show that brain lactate concentrations are elevated as a function of the deterioration of central nervous system function in ALF, a finding that has been attributed to decreased pyruvate oxidation because of ammonia exposure. 7 Although these biochemical studies provide important hints on metabolic derangements in ALF, they do not permit the elucidation of rates of synthesis/degradation of these metabolites, nor do they provide information on the cellular localization of the altered metabolism. The use of nuclear magnetic resonance (NMR) spectroscopy and stable isotope precursors provides an effective approach to the study of metabolic turnover in the brain. In particular, ...

Brain Alanine Formation as an Ammonia-Scavenging Pathway during Hyperammonemia: Effects of Glutamine Synthetase Inhibition in Rats and Astrocyte—Neuron Co-Cultures

“…15 This indicates that a concerted action of GDH and ALAT constitutes an alternative scavenging pathway when GS is inhibited. Furthermore, increased activity of glycolysis and decreased flux through pyruvate dehydrogenase in brain during acute liver failure 16 support the notion of an augmented capacity to synthesize alanine owing to an increased availability of pyruvate, the precursor of alanine. This pathway is likely localized to the astrocytes that possess both high glycolytic and GDH activity.…”

Pathogenesis of Hepatic Encephalopathy: Role of Ammonia and Systemic Inflammation

“…53 The implications of energy failure in ALF have largely been disregarded despite the presence of higher lactate levels in patients with ALF, which is a consequence of energy failure. 54 In an experimental rodent model of ALF, 55 in the early (pre-coma) stages of HE there was a significant 2-4.5-fold increase in total brain glutamine and lactate but in the severe (coma) stages of HE and brain edema, there was a further significant increase in brain lactate but no such increase in glutamine implying that impaired glucose oxidative pathways rather than intracellular glutamine accumulation per se may play a more dominant role. 56,57 …”

“…83 This removal of a substrate in the Tricarboxylic Acid Cycle (TCA), as well as ammonia being an inhibitor to enzymes required for TCA cycle activity (such as pyruvate dehydrogenase and a-ketoglutarate dehydrogenase), is likely to explain the high levels of pyruvate and lactate seen in brains of HE patients. 55 One critical consequence of oxidative and nitrosative stress is the induction of mitochondrial permeability transition (MPT). 84 The MPT usually develops in response to an increase in mitochondrial calcium levels and results in a sudden opening of the permeability transition pore (PTP), a large non-selective permeability pore in the inner mitochondrial membrane.…”

Keeping cool in acute liver failure: Rationale for the use of mild hypothermia

“…The prevention of brain edema by hypothermia in hyperammonemia or ALF, however, is not accompanied by reduction of brain glutamine [78] and [92], similar to findings with indomethacin [95]. These observations challenge the notion of glutamine as the major determinant of brain edema in ALF.…”

“…Since the brain cannot synthesize urea, detoxification of ammonia relies almost entirely on glutamine synthetase localized in astrocytes [88]. An increase of brain glutamine is a major feature in hyperammonemia or ALF [87], [89], [90], [91], [92], [93] and [94], and inhibition of glutamine synthesis attenuates ammonia-induced brain edema [87] and [89]. The osmotic effects of glutamine may partly explain the selective astrocytic swelling in ALF.…”