Paradoxical increase in liver ketogenesis during long-term insulin-induced hypoglycemia in diabetic rats.

It is well established that insulin inhibits liver ketogenesis. However, during insulin-induced hypoglycemia (IIH) the release of counterregulatory hormones could overcome the insulin effect on ketogenesis. To clarify this question the ketogenic activity in livers from alloxan-diabetic rats submitted to long-term IIH was investigated. Moreover, liver glycogenolysis, gluconeogensis, ureagenesis and the production of L-lactate were measured, and its correlation with blood levels of ketone bodies (KB), L-lactate, glucose, urea and ammonia was investigated. For this purpose, overnight fasted alloxan-diabetic rats (DBT group) were compared with control non-diabetic rats (NDBT group). Long-term IIH was obtained with an intraperitoneal injection of Detemir insulin (1 U/kg), and KB, glucose, L-lactate, ammonia and urea were evaluated at 0, 2, 4, 6, 8 or 10 h after insulin injection. Because IIH was well established two hours after insulin injection this time was used for liver perfusion experiments. The administration of Detemir insulin decreased (P < 0.05) blood KB and glucose levels, but there was an increase in the blood L-lactate levels and a rebound increase in blood KB during the glucose recovery phase of IIH. In agreement with these results, the capacity to produce KB from octanoate was increased in the livers of DBT rats. Moreover, the elevated blood L-lactate levels in DBT rats could be attributed to the higher (P < 0.05) glycogenolysis when part of glucose from glycogenolysis enters glycolysis, producing L-lactate. In contrast, except glycerol, gluconeogenesis was negligible in the livers of DBT rats. Therefore, during long-term IIH the higher liver ketogenic capacity of DBT rats increased the risk of hyperketonemia. In addition, in spite of the fact that the insulin injection decreased blood KB, there was a risk of worsening lactic acidosis.

Investigation of glycemia recovery with oral administration of glycerol, pyruvate, and L-lactate during long-term, insulin-induced hypoglycemia.

AIM: The acute effect of oral administration of isolated or combined glycerol, pyruvate, and L-lactate on glycemia recovery (GR) during long-term, insulin-induced hypoglycemia (IIH) was compared. METHODS: Glycemia of 24 h-fasted rats that received intraperitoneal injection (1.0 U/kg) of regular insulin (IIH group) or saline (COG group) and, 15, 150, or 165 min later, oral saline (control IIH), glycerol (100 mg/kg), pyruvate (100 mg/kg), L-lactate (100 mg/kg), or combined glycerol+pyruvate+L-lactate (each 33.3 or 100 mg/kg) was compared. In addition, for comparative purposes, a group that received glucose (100 mg/kg) was included. Glycemia was measured 180 min after insulin or saline injection. To investigate the participation of the hepatic availability of gluconeogenic substrates to GR, livers from IIH and COG rats that received physiological or supraphysiological concentrations of isolated or combined glycerol, pyruvate, and L-lactate were compared. Liver experiments were done 180 min after insulin or saline injection. RESULTS: Oral glycerol, pyruvate, and L-lactate (isolated or combined) or glucose promoted GR. Moreover, the best GR was obtained with combined glycerol+pyruvate+L-lactate (100 mg/kg). In agreement, livers that received supraphysiological concentrations of glycerol, pyruvate, and L-lactate (isolated or combined) showed higher glucose release than livers that received physiological concentrations of these substances (isolated or combined). CONCLUSION: The best GR obtained with combined administration of glycerol, pyruvate, and L-lactate (100 mg/kg) during long-term IIH was a consequence of the higher liver availability of these substances associated with a maintained liver ability to produce glucose from gluconeogenic substrates.

Contribution of hepatic glycogenolysis and gluconeogenesis in the defense against short-term insulin induced hypoglycemia in rats.

In this study, the contribution of liver glycogenolysis and gluconeogenesis in the defense against short-term insulin induced hypoglycemia (IIH) was investigated. For this purpose, we used an experimental model in which IIH was obtained by administering an IP injection of a pharmacological dose (1 U/kg) of regular insulin to rats that had been deprived of food for a period of six hours. This experimental model is suitable to study the simultaneous participation of glycogen breakdown and gluconeogenesis in the defense against IIH. The livers of IIH rats showed insignificant changes in the glycogen concentration, total phosphorylase, active phosphorylase, and percent of active phosphorylase. Our results also indicated that the livers of IIH rats that received the concentration of L-alanine, L-glutamine, L-lactate, or glycerol found in the blood during IIH (basal values) showed negligible glucose production. Nonetheless, glucose, urea, and pyruvate production increased (P<0.05) if the livers were perfused with a saturating concentration of gluconeogenic precursors. In agreement with these results, IIH rats that received intragastric L-alanine, L-glutamine, or L-lactate showed increased (P<0.05) glycemia 30 min after the administration of these substances. However, when using glycerol, higher glycemia (P<0.05) was observed at 2 and 5 min, but not 30 min after the administration of this hepatic gluconeogenic precursor. Thus, we can conclude that the oral availability of gluconeogenic precursors could allow for their use as important antidote in the defense against IIH.

Glycemia recovery with oral amino acid administration during experimental short-term insulin-induced hypoglycemia.

AIM: The acute effects of the oral administration of L-alanine (L-ala), L-glutamine (L-gln), L-ala+L-gln, and L-alanyl-L-glutamine (AGP) on glycemia recovery during short-term insulin-induced hypoglycemia (IIH) were compared. METHODS: For this purpose, the blood glucose levels of 24-h-fasted rats that received intraperitoneal injections of regular insulin (IIH group) or saline [control (COG) group] and, 15 min later, oral administration of L-ala (100 mg/kg), L-gln (100 mg/kg), L-ala (50 mg/kg)+L-gln (50 mg/kg), or AGP (100 mg/kg) were compared. Liver perfusion experiments and blood collection to measure blood glucose levels were performed 30 min after insulin (1.0 U/kg) or saline injection. Livers from the IIH and COG groups were perfused with saturating concentrations of L-ala, L-gln, L-ala+L-gln, or AGP, and the maximal hepatic production of glucose, urea, ammonia, L-lactate, and pyruvate was evaluated. RESULTS: In contrast with L-gln, L-ala+L-gln, or AGP, the oral administration of L-ala promoted glycemia recovery. In agreement with these results, livers from IIH rats showed maximal hepatic production of glucose and urea from L-ala with 50% of the amount used to obtain the maximal hepatic production of glucose and urea in livers from COG rats. In contrast with L-gln, L-ala+L-gln, or AGP, the maximal hepatic production of urea from L-ala occurred in the absence of ammonia accumulation. CONCLUSION: The results indicate that the best glycemia recovery promoted by the oral administration of L-ala was a consequence of the higher efficiency of the livers from IIH rats in producing glucose from L-ala.

Comparative acute effects of l-carnitine and dl-carnitine on hepatic catabolism of l-alanine and l-glutamine in rats.

AIM: To compare the acute effects of l-carnitine (LCT) and dl-carnitine (DLC) on hepatic catabolism of l-alanine and l-glutamine in rats. METHODS: Livers from 24 h fasted and fed rats were perfused in situ. The substrates l-alanine (5 mmol/L) and l-glutamine (5 mmol/L) were employed. The gluconeogenic and ureogenic activity was measured as the difference between the rates of glucose and urea released during and before the infusion of l-glutamine or l-alanine. RESULTS: LCT (60 micromol/L) but not DLC (60 micromol/L and 120 micromol/L) increased the production of glucose and urea from l-glutamine. However, neither LCT (60 micromol/L and 120 micromol/L) nor DLC (60 micromol/L and 240 micromol/L) showed any significant effect on hepatic glucose and urea production from l-alanine. CONCLUSION: The results showed a different acute effect of LCT and DLC on the activation of hepatic gluconeogenesis and ureagenesis promoted by l-glutamine, reinforcing the idea that DLC could not replace LCT.

Effect of diet supplementation with l-carnitine on hepatic catabolism of l-alanine in rats.

AIM: To investigate the effects of chronic supplementation with l-carnitine (LCT) on hepatic catabolism of l-alanine. METHODS: Two groups of male adult Wistar rats were used: 1) supplemented with LCT (1.2 mmol . kg-1 . d-1) dissolved in the drinking water (LCT group) and 2) control group (COG) without LCT supplementation. After one week of LCT supplementation livers from 24 h-fasted rats were perfused in situ and the production of glucose, urea, pyruvate, and l-lactate from l-alanine (5 mmol/L) were measured. RESULTS: LCT decreased the production of glucose and urea from l-alanine. In agreement, pyruvate and l-lactate production from l-alanine were decreased. However, the supplementation with LCT did not show any significant effect on hepatic glucose production from pyruvate (5 mmol/L) and l-lactate (2 mmol/L). CONCLUSION: LCT supplementation decreased the conversion of l-alanine