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Uptake of amino acids and nucleic acid precursors by regenerating rat liver.

ABSTRACT: 1. At 0.5-1.0h after partial hepatectomy the intracellular acid-soluble fraction of rat liver took up twice as much radioactivity from [(3)H]orotic acid, [(3)H]uridine and [(3)H]thymidine as did similar fractions from sham-operated animals. This increase in penetration was not prevented by adrenalectomy or actinomycin, both of which decreased precursor uptake into nuclear RNA at this time. The increase in entry was still shown by thymidine at 22h after operation, at the height of the S period. Adenine penetration was not increased 1h after partial hepatectomy. 2. Plasma concentrations of ornithine and possibly methionine, tyrosine and lysine were raised 1.5h after partial hepatectomy. [(3)H]Lysine entry into regenerating liver at this time was increased by 60%; [(3)H]valine uptake was unaffected. Intracellular amounts of tyrosine, phenylalanine and ornithine in the liver were also increased. 3. The relation of these events to the start of liver regeneration is discussed.

SUBMITTER: Ord MG

PROVIDER: S-EPMC1174053 | biostudies-other | 1972 Aug

REPOSITORIES: biostudies-other

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Amino acid requirement for the growth-hormone stimulation of incorporation of precursors into protein and nucleic acids of liver slices.

Project description:1. Incorporation of [(14)C]leucine into protein in rat liver slices, incubated in vitro, increased as the concentration of unlabelled amino acids in the incubation medium was raised. A plateau of incorporation was reached when the amino acid concentration was 6 times that present in rat plasma. Labelling of RNA by [(3)H]orotic acid was not stimulated by increased amino acid concentration in the incubation medium. 2. When amino acids were absent from the medium, or present at the normal plasma concentrations, no effect of added growth hormone on labelling of protein or RNA by precursor was observed. 3. When amino acids were present in the medium at 6 times the normal plasma concentrations addition of growth hormone stimulated incorporation of the appropriate labelled precursor into protein of liver slices from normal rats by 31%, and into RNA by 22%. A significant effect was seen at a hormone concentration as low as 10ng/ml. 4. Under the same conditions addition of growth hormone also stimulated protein labelling in liver slices from hypophysectomized rats. Tissue from hypophysectomized rats previously treated with growth hormone did not respond to growth hormone in vitro. 5. No effect of the hormone on the rate or extent of uptake of radioactive precursors into acid-soluble pools was found. 6. Cycloheximide completely abolished the hormone-induced increment in labelling of both RNA and protein. 7. It was concluded that, in the presence of an abundant amino acid supply, growth hormone can stimulate the synthesis of protein in rat liver slices by a mechanism that is more sensitive to cycloheximide than is the basal protein synthesis. The stimulation of RNA labelling observed in the presence of growth hormone may be a secondary consequence of the hormonal effect on protein synthesis. 8. The mechanism of action of growth hormone on liver protein synthesis in vitro was concluded to be similar to its mechanism of action in vivo.

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The control of nucleic acid and nicotinamide nucleotide synthesis in regenerating rat liver.

Project description:1. The effect of injecting nicotinamide on the incorporation of [(14)C]orotate into the hepatic nucleic acids of rats after partial hepatectomy was investigated. 2. At 3h after partial hepatectomy the rapid incorporation of [(14)C]orotate into RNA, and at 20h after partial hepatectomy the incorporation of [(14)C]orotate into both RNA and DNA, were inhibited in a dose-dependent fashion by the previous injection of nicotinamide. 3. The injection of nicotinamide at various times before the injection of [(14)C]orotate at 20h after partial hepatectomy revealed an inhibition of the incorporation of orotate into RNA and DNA which was non-linear with respect to the duration of nicotinamide pretreatment. 4. The induction of a hepatic ATP depletion by ethionine demonstrated that the synthesis of hepatic NAD and NADP in partially hepatectomized rats was more susceptible to an ATP deficiency than in control rats. 5. The total hepatic activity of ribose phosphate pyrophosphokinase (EC 2.7.6.1) was assayed at various times after partial hepatectomy and found to be only marginally greater than the maximum rate of hepatic NAD synthesis induced in vivo by nicotinamide injection between 12 and 24h after partial hepatectomy. 6. It is suggested that a competition exists between NAD synthesis and purine and pyrimidine nucleotide synthesis for available ATP and particularly 5-phosphoribosyl 1-pyrophosphate. In regenerating liver the competition is normally in favour of the synthesis of nucleic acid precursors, at the expense of NAD synthesis. This situation may be reversed by the injection of nicotinamide with a subsequent inhibition of nucleic acid synthesis.

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