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Comparison of the effects of various amino acids on glycogen synthesis, lipogenesis and ketogenesis in isolated rat hepatocytes.

ABSTRACT: Several amino acids were found to stimulate glycogen synthesis and lipogenesis, and to inhibit ketogenesis in isolated rat hepatocytes. When hepatocytes were incubated in the presence of 20 mM-glucose, the amino acids could be classified in decreasing order of efficiency as follows: glutamine and proline, alanine, aminoisobutyric acid, asparagine and histidine for stimulation of glycogen synthesis; glutamine, proline and alanine for stimulation of lipogenesis; proline and glutamine for inhibition of ketogenesis. The study of the time course revealed that the rates were not linear and were preceded by a lag period. In all conditions studied, glutamine and proline were found to have similar quantitative effects on glycogen synthesis and lipid metabolism. However, their effects differ qualitatively. Indeed, the effects of proline on glycogen synthesis, lipogenesis and glutamate and aspartate content were faster. Moreover, proline increased the hydroxybutyrate/acetoacetate ratio, whereas glutamine did not change it. Incubation of hepatocytes with aminoisobutyric acid or under hypo-osmotic conditions, which increased cell volume and mimicked the amino acid-induced stimulation of glycogen synthesis, had little effect on lipogenesis. In hepatocytes incubated without glucose, ketogenesis was inhibited, in decreasing order of efficiency, by alanine, asparagine, glutamine and proline. Under these conditions, glutamine increased, alanine decreased and asparagine did not affect the concentration of malonyl-CoA. This indicates that the latter cannot be responsible for the inhibition of ketogenesis by alanine and asparagine.

SUBMITTER: Baquet A

PROVIDER: S-EPMC1149878 | biostudies-other | 1991 Jan

REPOSITORIES: biostudies-other

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Project description:1. Hepatocytes from starved rats or fed rats whose glycogen content was previously depleted by phlorrhizin or by glucagon injections, form glycogen at rapid rates when incubated with 10mM-glucose, gluconeogenic precursors (lactate, glycerol, fructose etc.) and glutamine. There is a net synthesis of glucose and glycogen. 14C from all three types of substrate is incorporated into glycogen, but the incorporation from glucose represents exchange of carbon atoms, rather than net incorporation. 14C incorporation does not serve to measure net glycogen synthesis from any one substrate. 2. With glucose as sole substrate net glucose uptake and glycogen deposition commences at concentrations of about 12--15mM. Glycogen synthesis increases with glucose concentrations attaining maximal values at 50--60mM, when it is similar to that obtained in the presence of 10mM glucose and lactate plus glutamine. 3. The activities of the active (a) and total (a+b) forms of glycogen synthase and phosphorylase were monitored concomitant with glycogen synthesis. Total synthase was not constant during a 1 h incubation period. Total and active synthase activity increased in parallel with glycogen synthesis. 4. Glycogen phosphorylase was assayed in two directions, by conversion of glycose 1-phosphate into glycogen and by the phosphorylation of glycogen. Total phosphorylase was assyed in the presence of AMP or after conversion into the phosphorylated form by phosphorylase kinase. Results obtained by the various methods were compared. Although the rates measured by the procedures differ, the pattern of change during incubation was much the same. Total phosphorylase was not constant. 5. The amounts of active and total phosphorylase were highest in the washed cell pellet. Incubation in an oxygenated medium, with or without substrates, caused a prompt and pronounced decline in the assayed amounts of active and total enzyme. There was no correlation between phosphorylase activity and glycogen synthesis from gluconeogenic substrates. With fructose, active and total phosphorylase activities increased during glycogen syntheses. 6. In glycogen synthesis from glucose as sole substrate there was a decline in phosphorylase activities with increased glucose concentration and increased rates of glycogen deposition. The decrease was marked in cells from fed rats. 7. To determine whether phosphorolysis and glycogen synthesis occur concurrently, glycogen was prelabelled with [2-3H,1-14C]-galactose. During subsequent glycogen deposition there was no loss of activity from glycogen in spite of high amounts of assayable active phosphorylase.

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Comparison of the effects of various amino acids on glycogen synthesis, lipogenesis and ketogenesis in isolated rat hepatocytes.

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