Portal insulin and glucagon in rats fed proteins as a meal: immediate variations and circadian modulations.

In adult rats, proteins fed as a meal apart from the remainder of the diet induce alterations of protein metabolism characterized by the simultaneous stimulation of protein synthesis and breakdown. These alterations occur in parallel with an acceleration of glycogenolysis. The purpose of this work was to investigate whether these metabolic changes are related to variations in portal insulin and glucagon levels or to insulin-glucagon balance. Portal hormone concentrations, aortic glycemia and aminoacidemia, liver glycogen contents were followed over a day-night cycle in rats adapted either to mixed feeding (10% protein) or to separate feeding (protein meal given 2 hours after the onset of the light phase). Insulin and glucagon were assayed by radioimmunoassay, glucagon with antibody K 964 specific for 3500 MW glucagon. During the 3 hours following the protein meal, the portal ratio of insulin to glucagon decreased; liver glycogenolysis and glucogenic amino acid catabolism were enhanced. This glucagonotropic and glucogenic response to a protein meal administered during daytime is consistent with the increase in protein turnover previously observed. Separate feeding did not alter the overall circadian pattern of portal insulinemia which rose at night but it did alter the overall circadian pattern of portal insulinemia which rose at night but it did alter that of portal glucagonemia by maintaining it at a low level during the nightly prandial period. No correlation could be evidenced between portal insulin concentrations and the aortic levels of any amino acid in either mixed-fed or separately-fed animals. Portal glucagonemia appeared to be weakly correlated with the aortic level of arginine in both experimental groups. In the separately fed group, highly significant correlation could be evidenced between portal insulin concentrations and the aortic levels of any amino acid in either mixed-fed or separately-fed animals. Portal glucagonemia appeared to be weakly correlated with the aortic level of arginine in both experimental groups. In the separately fed group, highly significant correlations were found between portal glucagonemia and aortic concentrations of the three branched and the two aromatic amino acids.