Rat liver responsiveness to gluconeogenic substrates during insulin-induced hypoglycemia

Hepatic responsiveness to gluconeogenic substrates during insulin-induced hypoglycemia was investigated. For this purpose, livers were perfused with a saturating concentration of 2 mM glycerol, 5 mM L-alanine or 5 mM L-glutamine as gluconeogenic substrates. All experiments were performed 1 h after an ip injection of saline (CN group) or 1 IU/kg of insulin (IN group). The IN group showed higher (P<0.05) hepatic glucose production from glycerol, L-alanine and L-glutamine and higher (P<0.05) production of L-lactate, pyruvate and urea from L-alanine and L-glutamine. In addition, ip injection of 100 mg/kg glycerol, L-alanine and L-glutamine promoted glucose recovery. The results indicate that the hepatic capacity to produce glucose from gluconeogenic precursors was increased during insulin-induced hypoglycemia.

Rat liver responsiveness to gluconeogenic substrates during insulin-induced hypoglycemia.

Hepatic responsiveness to gluconeogenic substrates during insulin-induced hypoglycemia was investigated. For this purpose, livers were perfused with a saturating concentration of 2 mM glycerol, 5 mM L-alanine or 5 mM L-glutamine as gluconeogenic substrates. All experiments were performed 1 h after an ip injection of saline (CN group) or 1 IU/kg of insulin (IN group). The IN group showed higher (P<0.05) hepatic glucose production from glycerol, L-alanine and L-glutamine and higher (P<0.05) production of L-lactate, pyruvate and urea from L-alanine and L-glutamine. In addition, ip injection of 100 mg/kg glycerol, L-alanine and L-glutamine promoted glucose recovery. The results indicate that the hepatic capacity to produce glucose from gluconeogenic precursors was increased during insulin-induced hypoglycemia.

Combined administration of glucose precursors is more efficient than that of glucose itself in recovery from hypoglycemia.

The purpose of the present study was to investigate the effect of the combined administration of hepatic gluconeogenic substrates (glycerol + L-lactate + L-alanine + L-glutamine) on glucose recovery during insulin induced hypoglycemia (IIH), in rats. IIH was obtained by an ip injection of regular insulin (1 U/kg). Thus, 150 min after insulin administration the rats received an ip injection of glycerol + L-lactate + L-alanine + L-glutamine (each 100 mg/kg). In these experiments control groups, which received saline, glucose or isolated precursors (100 mg/kg), were employed. Glycemia was measured 30 min later, i.e., 180 min after insulin injection. The results showed that the combined administration of gluconeogenic precursors is more efficient than that of glucose itself to promote glycemia recovery. Since, the blood levels of hepatic glucose precursors were decreased (glycerol, L-lactate and L-alanine) or maintained (L-glutamine) during IIH, the ability of the liver to produce glucose from these gluconeogenic substrates was investigated. The results showed that the maximal capacity of the liver to produce glucose from glycerol (2 mM), L-lactate (2 mM), L-alanine (5 mM) and L-glutamine (5 mM) was increased. To L-alanine and L-glutamine, not only the glucose production was increased (P < 0.05) but also the production of L-lactate, pyruvate and urea. Therefore, the results suggest that the decreased availability of glucose precursors, promoted by insulin administration, limits the participation of hepatic gluconeogenesis to glycemia recovery. However, the administration of gluconeogenic precursors could overcome this limitation and promote better glycemia recovery than glucose itself.

Acute effects of leptin on glucose metabolism of in situ rat perfused livers and isolated hepatocytes.

OBJECTIVE: To investigate whether leptin interferes directly with glycogenolysis and gluconeogenesis in isolated rat hepatocytes and also in in situ rat perfused livers. ANIMALS: Male albino rats (200-250 g) were used in all experiments. MEASUREMENTS: D-glucose, L-lactate and pyruvate production. RESULTS: In the present study, no differences were found for the rates of glycolysis, as expressed by the areas under the curves, among control (24.2+5.0 mmol¿g), leptin (32.0+4.5 mmol¿g), glucagon (24.7+3.0 mmol¿g), and the leptin + glucagon (23.8+3.4 mmol¿g) groups. No difference was found for the rates of glycogenolysis between the control and the leptin perfused livers (15.2+3.9 and 15.0+3.2 mmol¿g, respectively). In the presence of glucagon, the areas under the curves for the rate of glycogenolysis rose to 108.6+3.8 mmol¿g. When leptin was combined with glucagon, the area under the curve for glycogenolysis was 43. 7+4.3 mmol¿g. In fact, leptin caused a reduction of almost 60% (P<0. 001) in the rate of glucagon-stimulated glycogenolysis. Under basal conditions, the addition of leptin (100 ng¿ml) to the incubation medium did not elicit any alteration in glucose production by isolated hepatocytes. However, in the presence of leptin, the production of glucose from glycerol (2 mM), L-lactate (2 mM). L-alanine (5 mM) and L-glutamine (5 mM) by the isolated hepatocytes was significantly reduced (30%, 30%, 23% and 25%, respectively). The rate of glucose production (glycogenolysis) by isolated hepatocytes was not different between the control and the leptin incubated groups (445.0+/-91.0 and 428.0+/-72.0 nmol¿106 cells¿h, respectively). CONCLUSION: We conclude that leptin per se does not directly affect either liver glycolysis or its glucose production, but a physiological leptin concentration is capable of acutely inducing a direct marked reduction on the rate of glucagon-stimulated glucose production in in situ rat perfused liver. Leptin is also capable of reducing glucose production from different gluconeogenic precursors in isolated hepatocytes.

Hypoglycemia induced by insulin increases hepatic capacity to produce glucose from gluconeogenic amino acids.

AIM: To investigate the hepatic capacity to produce glucose during hypoglycemia induced by insulin (HII). METHODS: Livers from 24-h fasted rats which received i.p. insulin (HII rats) or saline (control rats) were perfused in situ. The gluconeogenic substrates L-alanine (5 mmol/L), L-glutamine (5 mmol/L), L-lactate (2 mmol/L), and glycerol (2 mmol/L) were employed. The gluconeogenic activity was measured as the difference between rates of glucose released during and before the substrate infusion. In part of the experiments the production of urea was measured. Before the liver perfusion blood was collected for determination of glycemia and insulinemia. RESULTS: HII rats showed: (a) hypoglycemia and hyperinsulinemia; (b) increased hepatic capacity to produce glucose from L-alanine and L-glutamine; (c) increased hepatic ureogenesis from L-alanine and L-glutamine; and (d) increased hepatic glucose production from glycerol. However, hepatic glucose production from L-lactate was not affected by hypoglycemia. CONCLUSION: In spite of hyperinsulinemia the hepatic capacity to produce glucose from L-glutamine and L-alanine increased during HII. These results can be attributed to the higher hepatic catabolism of both amino acids, since the ability of the liver to produce glucose was not affected by hypoglycemia.

Changes in glycemia induced by exercise in rats: contribution of hepatic glycogenolysis and gluconeogenesis.

The participation of hepatic glycogenolysis and gluconeogenesis to the glycemic changes promoted by exercise was investigated. For this purpose, we employed swimming rats (2.5% body weight extra load attached to the tail, at 24 degrees C) using a favorable condition to measure hepatic glycogenolysis (fed rats) and a favorable condition to measure hepatic gluconeogenesis (fasted rats). This experimental approach permits us to compare the contribution of hepatic glycogenolysis and gluconeogenesis to glucose changes for a specific schedule of exercise. The animals were investigated at rest, after 5 minutes of swimming and after swimming to exhaustion. Our results show that hepatic glycogen has a crucial role to determine hyperglycemia during exercise. In contrast, hypoglycemia developed during exercise when glycogen was depleted. However, the ability of the liver to produce glucose from L-lactate, glycerol and L-glutamine was increased during exercise. Taken together, these findings suggest that the hepatic capacity to produce glucose from gluconeogenic substrates (except for L-alanine) was increased when hepatic glycogen stores were depleted. Thus, the increased capacity to produce glucose shown by livers from exercising rats must to be an important metabolic adaptation to protect against severe hypoglycemia.