Glucagon stimulation of hepatic gluconeogenesis in rats fed a high-protein, carbohydrate-free diet

Abstract

We previously observed increased gluconeogenesis in isolated liver of rats fed a high protein, carbohydrate-free (HP) diet and postulated that this was due to enhanced secretion of glucagon. To test this hypothesis, the effects of glucagon, dibutyryl cyclic AMP (DB-CAMP), and insulin on hepatic gluconeogenesis were examined. In perfused liver of control rats, alanine conversion to carbohydrate amounted to 20.39 ± 3.15 μmole/100 g body wt/90 min. Gluconeogenesis was increased to 35.14 ± 2.66 by glucagon; to 31.40 ± 1.0 by DB-CAMP, and suppressed to 14.56 + 1.15 by insulin. When both glucagon and insulin were added to the medium, only the stimulatory action of glucagon was evident. In perfused liver of HP-fed rats, alanine incorporation into carbohydrate totalled 51.79 ± 4.36. Gluconeogenesis was elevated by glucagon to 65.23 + 1.78, by DB-CAMP to 62.27 + 2.41, and was unaffected by insulin, 49.73 + 3.11. In the presence of both glucagon and insulin, only the glucagon effect was apparent. Levels of intermediary metabolites involved in gluconeogenesis were determined in control and HP livers. Concentrations of alanine, lactate, and pyruvate were reduced in HP liver, while phosphoenol-pyruvate (PEP) was increased, findings that suggest enhanced conversion of pyruvate to PEP. Fructose diphosphate concentration was normal; however, fructose-6-phosphate was greatly elevated, suggesting that fructose diphosphatase was stimulated by HP feeding. Similar changes in hepatic metabolites have been found following exposure of normal liver to glucagon. Plasma insulin values were not altered by HP feeding (C = 36.7 ± 3.7 μU/ml; HP = 32.3 ± 2.8). Plasma-glucagon levels were higher in HP rats than in controls (C = 80 ± 7.5 pg/ml; HP = 137.8 ± 16.7; p < 0.01). When the molar ratio of insulin to glucagon in plasma was calculated, the value in HP-fed rats was reduced, thus demonstrating a relative as well as an absolute rise of glucagon concentration (C = 11.1 ± 1.1; HP = 5.76 ± 0.6; p < 0.001). Liver cyclic AMP was greater in HP-fed rats than in controls (C = 0.83 ± 0.04 nmole/g; HP = 1.17 ± 0.07; p < 0.001), a finding that indicates that the liver was exposed to an increased amount of glucagon.