Portal Insulin and Glucagon in Rats Fed Proteins as a Meal: Immediate Variations and Circadian Modulations

Abstract

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.

Chemical Specificity of the Effects of Branched Chain Amino Acids on Glucagon and Insulin Release in the Suckling Rat

“…In the adult mammals, the branched chain AAs are considered as glucagonotropic in vi tro [ 15,32], and as weak inhibitors of periph eral glucagonemia [33,34] or stimulators of portal glucagonemia [6,35,36] in vivo. Our results emphasize the stimulatory Ile-LeuVal effect on glucagon secretion in vitro in suckling rats.…”

“…So we manipulated the suckling rat in vitro. An experimental sched-ule is proposed in this paper: previous work [6,7] has shown that some AAs exhibited the same pattern in the blood after a high protein meal. At the same time, Ala-Gly-Ser-Thr dropped when Ile-Leu-Val was increased as well as other AAs, namely Arg-Asn-PhePro-Thr.…”

Glucagon kinetics in growing rats fed different levels of protein and/or energy

“…These metabolic adaptable responses could be correlated with an increment of glucagon effects on liver since this hormone enhances amino-acid transport ang gluconeogenesis in liver (Le Cam and Freychet, 1976 ;Fehlmann, Le Cam and Freychet, 1979 ;Fehlmann et al, 1981 ;Morin, Fehlmann and Freychet 1982 ;Ayuso-Parilla, 1975, 1976). High levels of plasma glucagon have generally been associated with high protein diets (Tiedgen and Seitz, 1980 ;Eisenstein and Strack, 1978 ;Eisenstein et al, 1979 ;Peret et al, 19811. Changes in plasma hormone concentration may result from a stimulation of pancreatic a cells by amino acids (Dencker et al, 1975 ;Jarrousse et al, 1980) (Genuth, 1972 ;Sherwin et al, 1974 (fig. 3) ; the mean regression lines were highly significant (P < 0.0011.…”

Glucagon binding to purified liver plasma membranes from growing rats undergoing energy restriction

“…It probably parallels an increase in circulating blood glucagon since hyperglucagonemia (following exogenous administration of glucagon and observed in spontaneous obesity) induces a drop in glucagon binding to hepatocytes and liver plasma membranes (Bhatena et al, 1978 ;Srikant et aL, 1977). Furthermore, fasting, energy restriction or high-protein diets have been known to increase blood glucagon in different species (Unger, 1972 ;Eisenstein and Strack, 1978 ;Eisenstein et al, 1979 ;Jarrousse et al, .1980 ;Peret et al, 1981 ;Aguilar-Parada et al, 1969 ;Gerich, 1976). The age increment from the controls to the experimental animals cannot explain this since glucagon binding to its liver receptors is unaltered or increased from neonatal to adult rats (Lockwood and East, 1978 ;Basquez et al, 1976 ;Pingoud et al, 19821. Decreased glucagon binding after feeding the experimental diet could not explain the increase in amino acid breakdown observed in such animals (Grizard et al, 1975) (Broer et aL, 1977 ;Fourchereau-Peron et aL, 1976).…”