scholarly journals The rôle of mitochondrial pyruvate transport in the stimulation by glucagon and phenylephrine of gluconeogenesis from l-lactate in isolated rat hepatocytes

1981 ◽  
Vol 198 (3) ◽  
pp. 551-560 ◽  
Author(s):  
A P Thomas ◽  
A P Halestrap
Keyword(s):  
Pyruvate Kinase ◽  
Phosphoenolpyruvate Carboxykinase ◽  
Glucose Production ◽  
Rat Hepatocytes ◽  
Pyruvate Kinase Activity ◽  
Isolated Rat Hepatocytes ◽  
Isolated Liver Cells ◽  
Additional Role ◽  
Isolated Liver

The sensitivity of glucose production from L-lactate by isolated liver cells from starved rats to inhibition by alpha-cyano-4-hydroxycinnamate was studied. A small percentage of the maximal rate of gluconeogenesis was insensitive to inhibition by alpha-cyano-4-hydroxycinnamate, and evidence is presented to show that this is due to pyruvate entry into the mitochondria as alanine. After subtraction of this rate, Dixon plots of the reciprocal of the rate of gluconeogenesis against inhibitor concentration were linear both in the absence and presence of glucagon, phenylephrine or valinomycin, each of which stimulated gluconeogenesis by 30-50%. Pyruvate kinase activity was decreased by glucagon, but not by phenylephrine or valinomycin. Inhibition of gluconeogenesis by quinolinate (inhibitor of phosphoenolpyruvate carboxykinase) or monochloroacetate (probably inhibiting pyruvate carboxylation) caused a significant deviation from linearity of the Dixon plot obtained with alpha-cyano-4-hydroxycinnamate. Amytal, however, inhibited gluconeogenesis without affecting the linearity of this plot. These data, coupled with a computer simulation study, suggest that pyruvate transport may control gluconeogenesis from L-lactate and that hormones may stimulate this process through an effect on the respiratory chain. An additional role for pyruvate kinase and pyruvate carboxylase is quite compatible with the data presented.

scholarly journals Role of fructose 2,6-bisphosphate in the control by glucagon of gluconeogenesis from various precursors in isolated rat hepatocytes

1984 ◽  
Vol 218 (1) ◽  
pp. 165-170 ◽  
Author(s):  
L Hue ◽  
R Bartrons
Keyword(s):  
Pyruvate Kinase ◽  
Glucose Production ◽  
Rat Hepatocytes ◽  
Isolated Rat Hepatocytes ◽  
Lactate And Pyruvate ◽  
Isolated Rat

Hepatocytes from overnight-starved rats were incubated with 1-20 mM-fructose, -dihydroxyacetone, -glycerol, -alanine or -lactate and -pyruvate with or without 0.1 microM-glucagon. The production of glucose and lactate was measured, as was the content of fructose 2,6-bisphosphate. The concentrations of fructose (below 5 mM) and dihydroxyacetone (above 1 mM) that gave rise to an increase in fructose 2,6-bisphosphate were those at which a glucagon effect on the production of glucose and lactate could be observed. Glycerol had no effect on fructose 2,6-bisphosphate content or on production of lactate, and glucagon did not stimulate the production of glucose from this precursor. With alanine or lactate/pyruvate as substrates, glucagon stimulated glucose production whether the concentration of fructose 2,6-bisphosphate was increased or not. The extent of inactivation of pyruvate kinase by glucagon was not affected by the presence of the various gluconeogenic precursors. The role of fructose 2,6-bisphosphate in the effect of glucagon on gluconeogenesis from precursors entering the pathway at the level of triose phosphates or pyruvate is discussed.

scholarly journals Physiological concentrations of 2-oxoglutarate regulate the activity of phosphoenolpyruvate carboxykinase in liver

1992 ◽  
Vol 285 (3) ◽  
pp. 767-771 ◽  
Author(s):  
M A Titheradge ◽  
R A Picking ◽  
R C Haynes
Keyword(s):  
Pyruvate Kinase ◽  
Rat Liver ◽  
Phosphoenolpyruvate Carboxykinase ◽  
Rat Hepatocytes ◽  
Fasted State ◽  
Stimulation Of

2-Oxoglutarate was found to inhibit purified rat liver phosphoenolpyruvate carboxykinase when the assay was performed in the direction of either phosphoenolpyruvate or oxaloacetate synthesis. The inhibition was competitive with respect to oxaloacetate or phosphoenolpyruvate, the Ki values being 0.32 +/- 0.04 mM 0.63 +/- 0.19 mM respectively. 2-Oxoglutarate inhibited non-competitively when tested against GTP or Mn2+. The reported cytosolic concentrations of 2-oxoglutarate in rat hepatocytes are such that the enzyme is likely to be significantly inhibited under basal conditions. The cytosolic concentration of 2-oxoglutarate is known to fall precipitously under the influence of glucagon and other hormones that stimulate gluconeogenesis, and it is suggested that the hormone-induced decrease in 2-oxoglutarate content would alleviate the inhibition of phosphoenolpyruvate carboxykinase and stimulate flux from oxaloacetate to phosphoenolpyruvate. The implications of this finding to the rationalization of the role of pyruvate kinase in the stimulation of gluconeogenesis in the fasted state are discussed.

scholarly journals Differential effects of tryptophan on glucose synthesis in rats and guinea pigs

1978 ◽  
Vol 176 (3) ◽  
pp. 817-825 ◽  
Author(s):  
S A Smith ◽  
K R F Elliott ◽  
C I Pogson
Keyword(s):  
Rat Liver ◽  
Phosphoenolpyruvate Carboxykinase ◽  
Glucose Production ◽  
Rat Hepatocytes ◽  
Liver Cells ◽  
Isolated Rat Hepatocytes ◽  
Rat Liver Cells ◽  
Glucose Output ◽  
Isolated Rat

1. Tryptophan inhibition of gluconeogenesis in isolated rat liver cells is characterized by a 20 min lag period before linear rates of glucose output are attained. 2. Half-maximal inhibition of gluconeogenesis in isolated rat hepatocytes is produced by approx. 0.1 mM-tryptophan. 3. Tryptophan inhibits gluconeogenesis from all substrates giving rise to oxaloacetate, but stimulates glycerol-fuelled glucose production. 4. Gluconeogenesis in guinea-pig hepatocytes is insensitive to tryptophan. 5. Changes in metabolite concentrations in rat liver cells are consistent with a locus of inhibition at the step catalysed by phosphoenolpyruvate carboxykinase. 6. Inhibition of gluconeogenesis persists in cells from rats pretreated with tryptophan in vivo. 7. Tryptophan has no effect on urea production from alanine, but decreases [1-14C]palmitate oxidation to 14CO2 and is associated with an increased [hydroxybutyrate]/[acetoacetate] ratio. 8. These results are discussed with reference to the control of gluconeogenesis in various species.

scholarly journals Modulation of glucagon-induced glucose production by dexfenfluramine in rat hepatocytes

1995 ◽  
Vol 310 (1) ◽  
pp. 61-66 ◽  
Author(s):  
B Comte ◽  
A Romanelli ◽  
S Tchu ◽  
G van de Werve
Keyword(s):  
Cyclic Amp ◽  
Pyruvate Kinase ◽  
Glucose Production ◽  
Rat Hepatocytes ◽  
Significant Drop ◽  
Isolated Rat Hepatocytes ◽  
Dependent Inhibition ◽  
Dose Dependent Inhibition ◽  
Inhibition Of Glycolysis ◽  
State Concentration

The mechanism of the antihyperglycaemic action of dexfenfluramine (DEXF) was investigated in isolated rat hepatocytes exposed to glucagon. Preincubation of hepatocytes with DEXF caused a dose-dependent inhibition of cyclic AMP formation by 100 nM glucagon (Ki = 0.29 mM) that was almost complete at 1 mM DEXF. Surprisingly, glucagon-induced phosphorylase activation was not affected by DEXF despite the significant drop in cyclic AMP levels. Glucose production stimulated by glucagon was inhibited by up to 48% by 1 mM DEXF, and the rate of glucose production correlated positively with the steady-state concentration of glucose 6-phosphate. DEXF also partially restored lactate + pyruvate production which was abolished by an optimal concentration of glucagon. Although DEXF was not able to prevent the inactivation of pyruvate kinase by glucagon, the lack of further accumulation of phosphoenolpyruvate in DEXF-treated cells supports the conclusion that the flux through pyruvate kinase is stimulated, probably via the increase in fructose 2,6-bisphosphate, thereby increasing glycolysis. Our results thus indicate that DEXF counteracts the inhibition of glycolysis by glucagon and that this property might contribute to the antihyperglycaemic effect of this drug. Furthermore, this study shows that, in the presence of the drug, glucagon caused phosphorylase activation and pyruvate kinase inactivation without a significant increase in cyclic AMP levels.

scholarly journals Bovine serum albumin decreases 4-methyl-2-oxovalerate utilization by isolated rat hepatocytes

1983 ◽  
Vol 212 (3) ◽  
pp. 655-658 ◽  
Author(s):  
G Livesey
Keyword(s):  
Bovine Serum Albumin ◽  
Serum Albumin ◽  
Acid Concentration ◽  
Bovine Serum ◽  
Rat Hepatocytes ◽  
Isolated Rat Hepatocytes ◽  
Isolated Liver Cells ◽  
Rate Limiting ◽  
Isolated Liver

Binding of 4-methyl-2-oxo[1-14C]valerate to defatted bovine serum albumin inhibited the utilization of this 2-oxo acid by fed-rat hepatocytes in vitro. With 0-50g of albumin/l in the presence of 0.05mM 2-oxo acid or on increasing the 2-oxo acid concentration from 0 to 2mM in the presence of 26g of albumin/l, the extent of inhibition was essentially dependent on the change in the free 2-oxo acid concentration. Intrahepatocyte 4-methyl-2-oxo[1-14C]valerate concentrations were similar to extracellular free 2-oxo acid concentrations, suggesting equilibration so that the plasma membrane appears not to be rate-limiting for the utilization of this substrate by the isolated liver cells.

scholarly journals Inhibition of gluconeogenesis in isolated rat hepatocytes after chronic treatment with phenobarbital

1991 ◽  
Vol 280 (3) ◽  
pp. 663-669 ◽  
Author(s):  
D Argaud ◽  
S Halimi ◽  
F Catelloni ◽  
X M Leverve
Keyword(s):  
Phosphoenolpyruvate Carboxykinase ◽  
Glucose Production ◽  
Rat Hepatocytes ◽  
Inhibitory Effect ◽  
Glycerol Metabolism ◽  
Isolated Rat Hepatocytes ◽  
Flux Control Coefficient ◽  
Lactate And Pyruvate ◽  
Pyruvate Ratio

Gluconeogenesis was studied in hepatocytes isolated from phenobarbital-pretreated rats fasted for 24 h. In closed vial incubations, glucose production from lactate (20 mmol/l) and pyruvate (2 mmol/l), alanine (20 mmol/l) or glutamine (20 mmol/l) was suppressed by about 30-45%, although glycerol metabolism was not affected. In hepatocytes perifused with lactate and pyruvate (ratio 10:1), glucose production was inhibited by 50%, even at low gluconeogenic flux. From the determination of gluconeogenic intermediates at several steady states of gluconeogenic flux, we have found a single relationship between phosphoenolpyruvate and the rate of glucose production (Jglucose), and two different curves between cytosolic oxaloacetate and Jglucose in controls and in phenobarbital-pretreated hepatocytes. By using 3-mercaptopicolinate to determine the flux control coefficient of phosphoenolpyruvate carboxykinase we found that phenobarbital pretreatment led to an increase in this coefficient from 0.3 (controls) to 0.8 (phenobarbital group). These observations were confirmed by the finding that the activity of phosphoenolpyruvate carboxykinase was decreased by 50% after phenobarbital treatment. Hence we conclude that the inhibitory effect of phenobarbital on gluconeogenesis is due, at least partly, to a decrease in the flux through phosphoenolpyruvate carboxykinase.

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