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

Abstract

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.

Amino Acid Incorporation into Liver Proteins during Extrahepatic Cholestasis in the Rat

“…The effect of cholestasis on leucine incorporation seems thus to be quite independent of amino acid content in the media, in contrast to, for example, the absolute dependence on amino acids for growth hormone stimulation of protein synthesis in liver slices [9]. The free soluble ra dioactivity was similar in control and cholestatic material.…”

“…Part of this difference may be due to a continuous decrease of the spe cific activity of the precursor pool because of protein degradation. A contributory factor may also be that for 3H-leucine, a larger proportion of the free radioactivity is located in water and metabolites of amino ac ids [5], The higher rate of leucine incorporation in medium 2 compared to medium 1 seems to be due mainly to the higher concentration of leu cine in medium 2 [9]. There is little indication that the amino acid mix ture modifies the increase in leucine incorporation between 0.05 itim (medium 1) and 0.5 mM (medium 2) leucine concentration in the medi um.…”

Influence of amino acid supply on protein synthesis by rat liver in vitro

“…The present results are relevant to the observed amino acid requirement for an enhancement of protein synthesis by growth hormone in the incubated liver-slice system (Clemens & Korner, 1970). The hormone may stimulate translation by increasing the availability ofamino acids to the protein-synthesizing complex, or alternatively a high concentration of amino acids may be necessary in order that ribosomal activity is not already rate-limiting.…”

“…until death or starved overnight (16h). The tissue was incubated for 1 h in a medium containing either normal plasma concentrations of 20 amino acids or six times these concentrations (Clemens & Korner, 1970). Post-mitochondrial supernatants, ribosomes and cell sap were prepared from the slices and from unincubated liver, and cell-free amino acid incorporation was assayed as described by Clemens & Korner (1971).…”

Regulation of protein synthesis in rat liver cell-free systems by amino acids

“…The relationship of amino acids to the mechanism of action of growth hormone Jefferson & Korner (1967) and Clemens & Korner (1970) have shown that growth hormone stimulates directly the labelling of protein and RNA in perfused liver or liver slices from normal rats, but only when amino acids are present in the medium at three or six times plasma levels. These results suggest that the protein synthetic ability of the liver may be limited unless amino acids are in plentiful supply.…”