The hepatic response to a fructose challenge for control rats, and rats subjected to an acute sublethal dose of carbon tetrachloride (CCl4) or bromobenzene (BB), was compared using dynamic in vivo 31P MRS. Fructose loading conditions were used in which control rats showed only a modest increase in hepatic phosphomonoester (PME), and a small decrease in ATP, Pi, and intracellular pH after fructose administration. Both CCl4 and BB-treated rats showed a much greater fructose-induced accumulation of PME than did controls. Trolox C, a free radical scavenger, prevented most of this PME increase. BB-treated rats, given sufficient time to recover from the hepatotoxic insult, responded to the fructose load similarly to controls. Liver aldolase activities of control, toxicant-treated rats, and toxicant plus Trolox C-treated rats correlated inversely with PME accumulation after fructose loading (correlation coefficient: -0.834, P < 0.05). Perchloric acid extracts of rat livers studied by in vitro 31P MRS confirmed that the PME accumulation after fructose loading is mainly due to an increase in fructose 1-phosphate. These studies are consistent with the aldolase-catalyzed cleavage of fructose 1-phosphate being rate-limiting in hepatic fructose metabolism, and that the CCl4 and BB treatment modify and inactivate the aldolase enzyme.
We used dynamic in vivo 31P magnetic resonance spectroscopy to noninvasively study the metabolism of glycerol by the liver in living rats, as a means of detecting subtle metabolic changes induced by chronic ethanol consumption. Rats subjected to chronic ethanol consumption and their pair-fed controls were given a metabolic load of glycerol (0.75 or 1.3 mL glycerol x kg body mass(-1), i.p. or i.v) under normoxic or hyperoxic (98% O2) conditions. Changes in the level of glycerol 3-phosphate were followed in situ by monitoring the hepatic 31P phosphomonoester resonance every 7 or 13 min for up to 330 min. When challenged with a large dose of glycerol, chronic ethanol-treated rats exhibited less accumulation of glycerol 3-phosphate than controls, independent of the route of administration of the glycerol or whether the two groups were fasted or fed. For example, 1.3 mL glycerol x kg(-1) i.v. under normoxic conditions resulted in a two-fold increase in phosphomonoester in ethanol-treated rats compared with a five-fold increase in controls. The ethanol-treated rats also showed a slower rate of phosphorylation of glycerol and slower oxidation of glycerol 3-phosphate than controls, indicating decreased activities of the glycerol kinase and glycerol 3-phosphate dehydrogenase steps, and hence slower glycerol utilization. The rate of glycerol utilization was dose and oxygen concentration dependent. Kinetic analysis indicated that the chronic ethanol-induced decrease in the glycerol 3-phosphate dehydrogenase reaction was due to a decreased rate of NADH reoxidation in the liver, likely owing to a decrease in oxygen supply or utilization in the ethanol-treated rats. These observations support the hypothesis of pre-existing hypoxia in rat liver after chronic ethanol administration. This study demonstrates the utility of dynamic in vivo 31P magnetic resonance spectroscopy in following the metabolism of a glycerol load as a sensitive, nonperturbing, and potentially clinically applicable test of liver function.
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