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Salman TM, Iyanda MA, Alli-Oluwafuyi AM, Sulaiman SO, Alagbonsi AI. Telfairia occidentalis stimulates hepatic glycolysis and pyruvate production via insulin-dependent and insulin-independent mechanisms. Metabol Open 2021; 10:100092. [PMID: 33997754 PMCID: PMC8095178 DOI: 10.1016/j.metop.2021.100092] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/24/2021] [Revised: 04/05/2021] [Accepted: 04/06/2021] [Indexed: 11/02/2022] Open
Abstract
Background Telfairia occidentalis (TO), a plant consumed for its nutritional and medicinal values, exhibits hypoglycaemic effect. However, the metabolic fate of the glucose following TO-induced insulin secretion and consequent hypoglycaemia is not clear. Objective This study determined the effect of ethyl acetate and n-hexane fractions of TO leaf extracts on some biochemical parameters in the glucose metabolic pathway to explain the possible fate of blood glucose following TO-induced hypoglycaemia. Methods Eighteen male Wistar rats (180-200 g) divided into control, n-hexane TO fraction- and ethyl acetate TO fraction-treated groups (n = 6/group) were used. The control animals received normal saline while the treated groups received TO at 100 mg/kg for seven days. After 24 h following the last dose, the animals were anaesthetised using ketamine; blood samples were collected and livers harvested to determine some biochemical parameters. Results Ethyl acetate TO fraction significantly increased plasma insulin, liver glucokinase activity and plasma pyruvate concentration, but significantly decreased plasma glucose and liver glycogen, without significant changes in plasma lactate, glucose-6-phosphate, liver glucose-6-phosphatase and lactate dehydrogenase activities when compared with control. N-hexane TO fraction significantly reduced liver glucose-6-phosphatase activity and glycogen but significantly increased plasma pyruvate, without significant changes in plasma glucose, insulin, glucose-6-phosphate and lactate concentrations; and liver glucokinase and lactate dehydrogenase activities. Conclusion The present study showed that insulin-mediated TO-induced hypoglycaemia resulted in the stimulation of glycolysis and pyruvate production via insulin-dependent and insulin-independent mechanisms.
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Key Words
- ANOVA, Analysis of Variance
- ATP, Adenosine triphosphate
- EATO, Ethyl acetate TO fraction
- ELISA, Enzyme-linked immunosorbent assay
- G6P, Glucose-6-phosphate
- G6PD, Glucose-6-phosphate dehydrogenase
- G6Pase, Glucose-6-phosphatase
- GCK, Glucokinase
- GLUT, Glucose transporter
- GSIS, glucose-stimulated insulin secretion
- Glucoregulatory enzymes
- Glucose metabolites
- Glycogen
- HClO4, Perchloric acid
- HRP, Horseradish Peroxidase
- IMGU, Insulin-mediated glucose uptake
- Insulin
- KOH, Potassium hydroxide
- LDH, Lactate dehydrogenase
- MCT, Monocarboxylate transporters
- NAD, Nicotinamide adenine dinucleotide
- NHTO, N-hexane TO fraction
- Plasma glucose
- SEM, Standard error of mean
- TCA, Tricarboxylic acid cycle
- TO, Telfairia occidentalis
- Telfairia occidentalis
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Affiliation(s)
- Toyin Mohammed Salman
- Department of Physiology, College of Health Sciences, University of Ilorin, Ilorin, Nigeria
| | - Mayowa Adewale Iyanda
- Department of Physiology, College of Health Sciences, University of Ilorin, Ilorin, Nigeria
| | | | - Sheu Oluwadare Sulaiman
- Physiology Department, Kampala International University - Western Campus, Ishaka-Bushenyi, Uganda.,Department of Morphology (Cell Biology), Instituto de Ciências Biológicas, Universidade Federal de Minas Gerais, Belo Horizonte, Brazil
| | - Abdullateef Isiaka Alagbonsi
- Department of Clinical Biology (Physiology), School of Medicine and Pharmacy, College of Medicine and Health Sciences, University of Rwanda, Huye Campus, Rwanda
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Wan M, Leavens KF, Hunter RW, Koren S, von Wilamowitz-Moellendorff A, Lu M, Satapati S, Chu Q, Sakamoto K, Burgess SC, Birnbaum MJ. A noncanonical, GSK3-independent pathway controls postprandial hepatic glycogen deposition. Cell Metab 2013; 18:99-105. [PMID: 23823480 PMCID: PMC3725134 DOI: 10.1016/j.cmet.2013.06.001] [Citation(s) in RCA: 53] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 08/21/2012] [Revised: 02/02/2013] [Accepted: 05/21/2013] [Indexed: 01/19/2023]
Abstract
Insulin rapidly suppresses hepatic glucose production and slowly decreases expression of genes encoding gluconeogenic proteins. In this study, we show that an immediate effect of insulin is to redirect newly synthesized glucose-6-phosphate to glycogen without changing the rate of gluconeogenesis. This process requires hepatic Akt2, as revealed by blunted insulin-mediated suppression of glycogenolysis in the perfused mouse liver, elevated hepatic glucose production during a euglycemic-hyperinsulinemic clamp, or diminished glycogen accumulation during clamp or refeeding in mice without hepatic Akt2. Surprisingly, the absence of Akt2 disrupted glycogen metabolism independent of GSK3α and GSK3β phosphorylation, which is thought to be an essential step in the pathway by which insulin regulates glycogen synthesis through Akt. These data show that (1) the immediate action of insulin to suppress hepatic glucose production functions via an Akt2-dependent redirection of glucose-6-phosphate to glycogen, and (2) insulin increases glucose phosphorylation and conversion to glycogen independent of GSK3.
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Affiliation(s)
- Min Wan
- The Institute for Diabetes, Obesity, and Metabolism, University of Pennsylvania, Philadelphia, PA 19104, USA
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3
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Martineau LC. Large enhancement of skeletal muscle cell glucose uptake and suppression of hepatocyte glucose-6-phosphatase activity by weak uncouplers of oxidative phosphorylation. Biochim Biophys Acta Gen Subj 2012; 1820:133-50. [DOI: 10.1016/j.bbagen.2011.11.012] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/12/2011] [Revised: 11/16/2011] [Accepted: 11/19/2011] [Indexed: 01/08/2023]
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Abstract
Type 2 diabetes is a complex disorder with diminished insulin secretion and insulin action contributing to the hyperglycemia and wide range of metabolic defects that underlie the disease. The contribution of glucose metabolic pathways per se in the pathogenesis of the disease remains unclear. The cellular fate of glucose begins with glucose transport and phosphorylation. Subsequent pathways of glucose utilization include aerobic and anaerobic glycolysis, glycogen formation, and conversion to other intermediates in the hexose phosphate or hexosamine biosynthesis pathways. Abnormalities in each pathway may occur in diabetic subjects; however, it is unclear whether perturbations in these may lead to diabetes or are a consequence of the multiple metabolic abnormalities found in the disease. This review is focused on the cellular fate of glucose and relevance to human type 2 diabetes.
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Affiliation(s)
- Clara Bouché
- Harvard Medical School, Boston, Massachusetts 02115, USA
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5
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Haaxma CA, Kim PK, Andrejko KM, Raj NR, Deutschman CS. Transcription factors C/EBP-alpha and HNF-1alpha are associated with decreased expression of liver-specific genes in sepsis. Shock 2003; 19:45-9. [PMID: 12558143 DOI: 10.1097/00024382-200301000-00009] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
Previous studies have demonstrated sepsis-specific changes in the transcription of key hepatic genes. However, the role of hepatic transcription factors in sepsis-associated organ dysfunction has not been well established. We hypothesize that the binding activities of C/EBPalpha and beta, HNF-1alpha, and HNF-3 transiently decrease during mild sepsis but persistently decrease after fulminant sepsis, and that the decrease in this binding activity correlates in time and severity with previously described decreases in the transcription of key hepatic genes. Male C57/BL6 mice had nonlethal sepsis induced by cecal ligation and single puncture (CLP) and fulminant sepsis via cecal ligation and double puncture (2CLP). Sham-operated and unoperated animals served as controls. Transcription factor binding activity was assessed with electrophoretic mobility shift assays. C/EBP-a and HNF-1alpha binding activity decreased transiently after CLP and persistently after 2CLP. Binding activity of both C/EBP-beta and HNF-3 were unchanged. The decrease in C/EBP-a and HNF-1alpha binding activities correlated in time and magnitude with the decreased hepatic gene transcription previously observed in sepsis. Furthermore, the loss of activity after 2CLP correlated in time with outcome. Sepsis decreases DNA binding activities of C/EBPalpha and HNF-1alpha, two key hepatocyte transcription factors, in a time course consistent with down-regulation of their target hepatic genes. Therefore, alterations in transcription factor binding are likely important in the transcriptional modulation that is characteristic of hepatic dysfunction in sepsis.
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Affiliation(s)
- Charlotte A Haaxma
- Department of Anesthesia, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104, USA
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6
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Yoshiuchi I, Shingu R, Nakajima H, Hamaguchi T, Horikawa Y, Yamasaki T, Oue T, Ono A, Miyagawa JI, Namba M, Hanafusa T, Matsuzawa Y. Mutation/polymorphism scanning of glucose-6-phosphatase gene promoter in noninsulin-dependent diabetes mellitus patients. J Clin Endocrinol Metab 1998; 83:1016-9. [PMID: 9506766 DOI: 10.1210/jcem.83.3.4659] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
Abstract
Glucose-6-phosphatase (G6Pase) catalyzes the rate-limiting step of gluconeogenesis, and hepatic G6Pase activity is increased in diabetes. We have cloned and analyzed the human G6Pase gene promoter region and identified putative regulatory sequences for insulin, cAMP, glucocorticoid, and hepatocyte nuclear factors. The promoter region of the G6Pase gene was analyzed in 154 noninsulin-dependent diabetes mellitus patients and 90 control subjects by PCR-single strand conformation polymorphism and direct sequencing methods. Polymorphisms were not found in any subjects. The results suggested that in noninsulin-dependent diabetic patients, the major cause of the hepatic glucose overproduction was not attributed to dysregulation of the G6Pase gene due to mutation/polymorphism of its promoter region.
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Affiliation(s)
- I Yoshiuchi
- Second Department of Internal Medicine, Osaka University Medical School, Suita, Japan
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7
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Mithieux G, Daniele N, Payrastre B, Zitoun C. Liver microsomal glucose-6-phosphatase is competitively inhibited by the lipid products of phosphatidylinositol 3-kinase. J Biol Chem 1998; 273:17-9. [PMID: 9417039 DOI: 10.1074/jbc.273.1.17] [Citation(s) in RCA: 41] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
We have studied the effect of various phospholipids on the activity of glucose-6-phosphatase (Glc6Pase) in untreated and detergent-treated rat liver microsomes. Glc6Pase is inhibited in the presence of phosphoinositides in a dose-dependent manner within a range of concentration 0.5-10 microM. The order of efficiency in untreated microsomes is: phosphatidylinositol (PI) 3,4,5P3 > PI3,4P2 = PI4,5P2 > PI3P = PI4P > PI. In contrast, Glc6Pase is not inhibited in the presence of phosphatidylserine, phosphatidylcholine, and phosphatidylethanolamine, diacylglycerol, and inositol 1,4, 5-trisphosphate at concentrations up to 100 microM. The mechanism of Glc6Pase inhibition by PI4,5P2, PI3,4P2, and PI3,4,5P3 is competitive in both untreated and detergent-treated microsomes. In untreated microsomes, the Ki for PI3,4,5P3 (1.7 +/- 0.3 microM, mean +/- S.D. n = 3) is significantly lower (p < 0.01) than that for PI3, 4P2 (5.0 +/- 0.8 microM) and for PI4,5P2 (4.7 +/- 0.7 microM). In detergent-treated microsomes, Glc6Pase is less sensitive to the inhibition and there is no difference anymore among the Ki values for the three compounds: 8.3 +/- 0.8, 11.1 +/- 0.5 and 8.9 +/- 0.4 microM for PI3,4,5P3, PI3,4P2, and PI4,5P2, respectively. This inhibition phenomenon might be of special importance with regards to the insulin's inhibition of hepatic glucose production.
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Affiliation(s)
- G Mithieux
- Institut National de la Santé et de la Recherche Médicale, Unit 449, Faculty of Medicine R. T. H. Laënnec, 69372 Lyon Cédex 08, France.
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8
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Christ B, Nath A. Impairment by interleukin 1 beta and tumour necrosis factor alpha of the glucagon-induced increase in phosphoenolpyruvate carboxykinase gene expression and gluconeogenesis in cultured rat hepatocytes. Biochem J 1996; 320 ( Pt 1):161-6. [PMID: 8947481 PMCID: PMC1217911 DOI: 10.1042/bj3200161] [Citation(s) in RCA: 22] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Abstract
The influence of the inflammatory mediators interleukin 1 beta (IL1 beta) and tumour necrosis factor alpha (TNF alpha) on the glucagon-induced expression of phosphoenolpyruvate carboxykinase (PCK) and on glucose formation via gluconeogenesis was investigated in cultured rat hepatocytes. Gene expression was monitored by determination of mRNA levels and of enzyme activity. Glucose formation was estimated with newly synthesized radioactive glucose derived from a radiolabelled lactate precursor. Glucagon (0.1 or 1 nM) induced PCK mRNA transiently to a maximum 2 h after its application. In the presence of recombinant human (rh) IL1 beta or rhTNF alpha the increase in PCK mRNA levels was totally inhibited at 0.1 nM glucagon, whereas at 1 nM glucagon the maximal increase was inhibited by only 25%. Glucagon (0.1 or 1 nM) induced PCK activity to a maximum after 4 h (4-fold and 6-fold over prestimulatory activity respectively). In the presence of rhIL1 beta or rhTNF alpha the maximal increase was inhibited by approx. 50%. Addition of rhIL1 beta or rhTNF alpha 2 h after glucagon, at the maximal glucagon-induced PCK mRNA levels, accelerated the decay of PCK mRNA. Glucagon (1 or 10 nM) [corrected] increased glucose formation from lactate by 1.3-fold and 1.7-fold respectively over unstimulated rates. In the presence of rhIL1 beta or rhTNF alpha this increase in glucose formation was inhibited by 60-90%. At 0.1 nM, glucagon doubled the intracellular cAMP concentration. This increase was prevented by rhIL1 beta or rhTNF alpha. At 1 nM, glucagon increased cAMP concentrations by 10-fold. In the presence of rhIL1 beta or rhTNF alpha this increase was inhibited by 70%. From the results it is suggested that rhIL1 beta and rhTNF alpha prevented glucagon-stimulated PCK gene expression and gluconeogenesis at least in part by inhibition of the glucagon-stimulated increase in cAMP concentrations.
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Affiliation(s)
- B Christ
- Institut für Biochemie und Molekulare Zellbiologie, Georg-August-Universität, Göttingen, Federal Republic of Germany
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9
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Abstract
There are several hypothalamic theories of aging, none of which has been validated. An approach to validation is to search for consequences of anatomic ablations of hypothalamic regions that are functional hallmarks of aging, or consequences of ablation that postpone the appearance of hallmarks of aging or extend longevity. Ablation of the hypothalamic ventromedial nucleus (VMN) in the weanling rat is associated with subsequent increased body fat, glucose intolerance, hyperlipidemia, and decreased renal function. Each of these consequences is characteristic of aging in humans and in several animal models of aging. Ablation of the hypothalamic dorsomedial nucleus (DMN) in the weanling rat leads to a symmetrically smaller animal with normal glucose and lipid metabolism, decreased body fat for size, and reduced risk of decreased renal function and circulating IGF-I levels. These are findings consistent with calorie restriction models in rodents that significantly extend life span. This review compares outcomes of lesions in the VMN, DMN, and lateral hypothalamic area (LHA) for relevance to aging. To establish a relationship between these anatomic areas of the hypothalamus and aging, it is concluded that the VMN, DMN, and LHA lesions should be examined for impact on longevity and compared with data obtained from simultaneously studied intact ad-lib-fed and 40% calorie-restricted animals. Lesioned animals also should be rigorously studied for neurotransmitters (e.g., neuropeptide Y, beta-endorphin, serotonin, corticotropin-releasing factor, and galanin), and for behavioral changes consistent with aging, for accumulation of specific tissue lipofuscin and amyloid that are associated with normal aging and for other age-dependent findings, such as incidence of tumors and cataract.
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Affiliation(s)
- L L Bernardis
- Neurovisceral-Neuroendocrine Laboratory, Veterans Affairs Medical Center, Buffalo, NY 14215, USA
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10
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Mithieux G, Zitoun C. Mechanisms by which fatty-acyl-CoA esters inhibit or activate glucose-6-phosphatase in intact and detergent-treated rat liver microsomes. EUROPEAN JOURNAL OF BIOCHEMISTRY 1996; 235:799-803. [PMID: 8654431 DOI: 10.1111/j.1432-1033.1996.00799.x] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
We have studied the effects of fatty-acyl-CoA esters on the activity of glucose-6-phosphatase (Glc6Pase) in untreated and detergent-treated liver microsomes. Fatty-acyl-CoA esters with chain lengths less than or equal to nine carbons do not inhibit Glc6Pase. Medium-chain fatty-acyl-CoA esters (10-14 carbons) inhibit Glc6Pase of untreated microsomes in a dose-dependent manner in the range 1-20 microM. The inhibitory effect is also dependent on the acyl-chain length. The higher the chain length, the stronger the inhibitory effect. It is also dependent on the microsomal protein concentration. The higher the protein concentration, the lower the inhibitory effect. Fatty-acyl-CoA esters with longer chain length (equal to or higher than 16 carbons) inhibit Glc6Pase of untreated microsomes within the range 1-2 microM. However, the inhibitory effect is either partially or totally cancelled, or even changed into an activation effect at higher concentrations. This is due to the release of mannose-6-phosphatase latency. The inhibition is fully reversible in the presence of bovine serum albumin. The mechanism of the Glc6Pase inhibition in untreated microsomes is uncompetitive (Ki for myristoyl-CoA = 1.2 +/- 0.3 microM, mean +/- SD, n = 3). Glc6Pase of detergent-treated microsomes is also inhibited by fatty-acyl-CoA esters, albeit less efficiently. In this case, the mechanism is non-competitive (Ki for myristoyl-CoA = 29 +/- 3 microM).
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Affiliation(s)
- G Mithieux
- Institut National de la Santé et de la Recherche Médicale, Unité 449, Faculté de Médecine A. Carrel, Lyon, France
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11
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Haber BA, Chin S, Chuang E, Buikhuisen W, Naji A, Taub R. High levels of glucose-6-phosphatase gene and protein expression reflect an adaptive response in proliferating liver and diabetes. J Clin Invest 1995; 95:832-41. [PMID: 7860767 PMCID: PMC295564 DOI: 10.1172/jci117733] [Citation(s) in RCA: 112] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Abstract
The regenerating liver after partial hepatectomy is one of the few physiologic models of cellular proliferation in the adult animal. During hepatic regeneration, the animal is able to maintain metabolic homeostasis despite the acute loss of two thirds of hepatic tissue. In examining the molecular mechanisms regulating hepatic regeneration, we isolated novel immediate-early genes that are rapidly induced as the remnant liver undergoes the transition from its normal quiescent state into the G1 phase of the cell cycle. One of the most rapidly and highly induced genes which we initially termed RL-1, encodes rat glucose-6-phosphatase (rG6Pase). G6Pase mRNA peaks at 30 min and 36-48 h after hepatectomy correlating with the first and second rounds of cell division. This finding is compatible with studies that showed that G6Pase enzyme activity increases during liver regeneration. However, the increase in G6Pase mRNA is much more dramatic, indicating that it is a more sensitive indicator of this regulation. G6Pase gene expression peaks in the perinatal time period in the liver and remains elevated during the first month of life. The expression of the G6Pase gene is also dramatically elevated in BB diabetic rats, again higher than the enzyme elevation, and its relative induction after partial hepatectomy is blunted in these animals. Insulin treatment of partially hepatectomized diabetic animals downregulates the expression of G6Pase mRNA. Using specific antibodies against G6Pase, we detect a 36-kD G6Pase protein, and its level is elevated in regenerating and diabetic livers. The pattern of G6Pase mRNA expression appears to reflect similar changes in insulin and glucagon levels which accompany diabetes and hepatic proliferation. The elevation of G6Pase expression in these conditions is indicative of its importance as a regulator of glucose homeostasis in normal and abnormal physiologic states.
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Affiliation(s)
- B A Haber
- Department of Genetics, Children's Hospital of Philadelphia, Pennsylvania
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12
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Printz R, Koch S, Potter L, O'Doherty R, Tiesinga J, Moritz S, Granner D. Hexokinase II mRNA and gene structure, regulation by insulin, and evolution. J Biol Chem 1993. [DOI: 10.1016/s0021-9258(18)53521-3] [Citation(s) in RCA: 134] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/16/2023] Open
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13
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Marin JJ, Bravo P, Perez Barriocanal F, el-Mir MY, Villanueva GR. Hyperglycemia-induced cholestasis in the isolated perfused rat liver. Hepatology 1991; 14:184-91. [PMID: 2066067 DOI: 10.1002/hep.1840140130] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
In a previous report we showed that cholestasis in diabetic rats is due in part to hyperglycemia. To gain information about the mechanism responsible for this phenomenon, bile flow was studied in isolated perfused rat livers. The perfusion media were modified erythrocyte-free Krebs-Henseleit solutions. Under these experimental conditions, no cholestasis was observed in isolated rat livers obtained from rats treated with streptozotocin (6 mg/100 gm body wt) 6 days before the experiments. We then proceeded to use normal animals. The composition of the perfusion media was modified to maintain the osmolality even after increasing D-glucose concentrations from 0 to 35 mmol/L. Bile flow was not affected with doses up to 15 mmol/L D-glucose. Beyond a threshold value for plasma D-glucose concentrations between 15 and 20 mmol/L, cholestasis was observed. Using D-glucose analogs such as L-glucose and 3-O-methyl-D-glucose, bile flow was also reduced (by 0.54 and 0.53 microliters/min/gm liver, respectively). Isosmotic sucrose-containing perfusion media were also observed to impair bile flow (by 0.66 microliters/min/gm liver). However, i-erythritol and D-mannitol failed to inhibit bile formation. The study of bile/plasma concentration ratios determined using tracer amounts of radioactive sugars indicated that this value was much lower for cholestatic sucrose (0.11) and L-glucose (0.31) than for noncholestatic i-erythritol (0.99) and D-mannitol (0.98). Cholestasis was partly reversed after induction by 35 mmol/L D-glucose if perfusion media were replaced by sugar-free ones, but also by media containing 25 mmol/L D-glucose. Insulin given during the perfusion with sugar-free media was observed to have no effect on bile flow.(ABSTRACT TRUNCATED AT 250 WORDS)
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Affiliation(s)
- J J Marin
- Department of Physiology and Pharmacology, Faculty of Pharmacy, University of Salamanca, Spain
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14
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Bànhegyi G, Fulceri R, Bellomo G, Romani A, Pompella A, Benedetti A. Role of a nonmitochondrial Ca2+ pool in the synergistic stimulation by cyclic AMP and vasopressin of Ca2+ uptake in isolated rat hepatocytes. Arch Biochem Biophys 1991; 287:320-8. [PMID: 1654813 DOI: 10.1016/0003-9861(91)90485-2] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Abstract
The subcellular distribution of 45Ca2+ accumulated by isolated rat hepatocytes exposed to dibutyryl cyclic AMP (dbcAMP) followed by vasopressin (Vp) was studied by means of a nondisruptive technique. When treated with dbcAMP followed by vasopressin, hepatocytes obtained from fed rats accumulated an amount of Ca2+ approximately fivefold higher than that attained under control conditions. Ca2+ released from the mitochondrial compartment by the uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) accounted for only a minor portion of the accumulated Ca2+. The largest portion was released by the Ca2+ ionophore A23187 and was attributable to a nonmitochondrial compartment. DbcAMP + Vp-treatment also caused a maximal stimulation of glucose production and a twofold increase in cellular glucose 6-phosphate levels. In hepatocytes obtained from fasted rats, dbcAMP + Vp-stimulated Ca2+ accumulation was lower, although with the same subcellular distribution, and was associated with a minimal glucose production. In the presence of gluconeogenetic substrates (lactate plus pyruvate) hepatocytes from fasted rats were comparable to cells isolated from fed animals. However, Ca2+ accumulation and glucose 6-phosphate production could be dissociated in the absence of dbcAMP, in the presence of lactate/pyruvate alone. Under this condition in fact Vp induced only a minimal accumulation of Ca2+ in hepatocytes isolated from fasted rats, although glucose production was markedly increased. Moreover, treatment of fed rat hepatocytes with 1 mM ATP caused a maximal activation of glycogenolysis, but only a moderate stimulation of cellular Ca2+ accumulation. In this case, sequestration of Ca2+ occurred mainly in the mitochondrial compartment. By contrast, the addition of ATP to dbcAMP-pretreated hepatocytes induced a large accumulation of Ca2+ in a nonmitochondrial pool. Additional experiments using the fluorescent Ca2+ indicator Fura-2 showed that dbcAMP pretreatment can enlarge and prolong the elevation of cytosolic free Ca2+ caused by Vp. A nonmitochondrial Ca2+ pool thus appears mainly responsible for the Ca2+ accumulation stimulated by dbcAMP and Vp in isolated hepatocytes, and cyclic AMP seems able to activate Ca2+ uptake in such a nonmitochondrial pool.
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Affiliation(s)
- G Bànhegyi
- Istituto di Patologia Generale, Università di Siena, Italy
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15
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Fulceri R, Bellomo G, Gamberucci A, Benedetti A. Liver glucose-6-phosphatase activity is not modulated by physiological intracellular Ca2+ concentrations. Biochem J 1991; 275 ( Pt 3):805-7. [PMID: 1645522 PMCID: PMC1150127 DOI: 10.1042/bj2750805] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Abstract
1. In the presence of MgATP and increasing amounts of added Ca2+, isolated liver microsomal vesicles accumulate approx. 10 nmol of Ca2+/mg of protein and buffer ambient free Ca2+ at increasing concentrations (0.22-10.9 microM). Under these experimental conditions, microsomal glucose-6-phosphatase activity is unaffected by the concentration of extravesicular free Ca2+. 2. Different levels of intravesicular Ca2+ were obtained by treating microsomes with the Ca2+ ionophore A23187 and by stimulating active microsomal Ca2+ accumulation with Pi (3 mM). In both instances, microsomal glucose-6-phosphatase activity is unaffected by the level of intravesicular Ca2+.
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Affiliation(s)
- R Fulceri
- Istituto di Patologia Generale, University of Siena, Italy
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16
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Ljungqvist O, Boija PO, Esahili H, Larsson M, Ware J. Food deprivation alters liver glycogen metabolism and endocrine responses to hemorrhage. THE AMERICAN JOURNAL OF PHYSIOLOGY 1990; 259:E692-8. [PMID: 2240208 DOI: 10.1152/ajpendo.1990.259.5.e692] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
Liver glycogen content, blood glucose, insulin, glucagon, and epinephrine were determined during 1 h hemorrhagic hypotension at 60 mmHg and 23 h thereafter in fed and two groups of 24-h food-deprived rats receiving either no infusion or 30% glucose intravenously during hemorrhage. Liver glycogen content was reduced by greater than 90% after 24-h food deprivation. Fed and food-deprived rats given glucose developed similar and substantial elevations of blood glucose during hemorrhage, whereas changes in blood glucose were modest in food-deprived rats given no infusion. In fed rats, liver glycogen was reduced by 60% during the 1-h bleed, but within 2 h after hemorrhage repletion of liver glycogen content commenced. By 6 h, approximately 75% of the glycogen lost during hemorrhage had been restored, and 23 h after hemorrhage liver glycogen content was six times greater compared with nonbled controls. Although glycogen levels increased after hemorrhage in food-deprived animals, the increase was negligible compared with that found in fed rats. Infusion of glucose during hemorrhage or adrenergic blockade after hemorrhage did not alter glycogen repletion in food-deprived rats. Posthemorrhage fed animals had high levels of insulin, glucagon, and epinephrine during hemorrhage, whereas insulin levels remained low in food-deprived rats despite exogenously induced hyperglycemia. It is concluded that rapid and substantial glycogen repletion can occur even immediately poststress. The conditions seem to be related to the nutritional state at the time of the insult.
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Affiliation(s)
- O Ljungqvist
- Department of Surgery, Karolinska Hospital and Institute, Stockholm, Sweden
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17
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Probst I, Quentmeier A, Schweickhardt C, Unthan-Fechner K. Stimulation by insulin of glycolysis in cultured hepatocytes is attenuated by extracellular ATP and puromycin through purine-dependent inhibition of phosphofructokinase 2 activation. EUROPEAN JOURNAL OF BIOCHEMISTRY 1989; 182:387-93. [PMID: 2525468 DOI: 10.1111/j.1432-1033.1989.tb14843.x] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
Activation of glycolysis by insulin in cultured rat hepatocytes is preceded by an activation of phosphofructokinase 2 (PFK 2) and subsequent rise of the fructose 2,6-bisphosphate [Fru(2,6)P2] level. Extracellular addition of ATP or puromycin prevented the hormonal effect on glycolysis. The mechanism through which the purines abolished glycolytic stimulation was investigated. 1. 50 microM ATP completely prevented the 3-5-fold insulin-dependent increase of glycolysis, irrespective of whether the cells initially possessed a low or a high Fru(2,6)P2 content. 50 microM puromycin prevented the stimulation of glycolysis by insulin only in cells whose initial Fru(2,6)P2 levels were low and had to be increased by insulin prior to the increase in glycolysis. It did not antagonize the action of insulin cells with initial high Fru(2,6)P2 content. 2. ATP exerted effects on its own; it decreased initially high Fru(2,6)P2 levels by 95% within 10 min and decreased the basal glycolytic rate by 60%. Half-maximal effects on the Fru(2,6)P2 level were obtained with about 25 microM ATP or 15 microM adenosine 5'[beta, gamma-methylene]triphosphate. ADP and adenosine-5-[gamma-thio]triphosphate were as effective as ATP, whereas 100 microM adenosine 5'[alpha, beta-methylene]triphosphate elicited no effect. Puromycin neither decreased high Fru(2,6)P2 levels nor inhibited basal glycolysis. 3. Extracellular ATP (100 microM) led to inhibition of the active form of PFK 2. Intracellular levels of Glc6P, citrate, ATP, ADP and AMP were increased by extracellular ATP, the phosphoenolpyruvate content was decreased, Fru6P and glycerol 3-phosphate levels stayed constant. Puromycin did not inhibit PFK 2. 4. Both puromycin and ATP prevented the insulin-dependent rise of the Fru(2,6)P2 level, they abolished the activation of PFK 2 by the hormone. Puromycin did not block the accumulation of Fru(2,6)P2 provoked by glucose addition; ATP also antagonized the glucose-dependent increase. 5. 100 microM ATP elevated the cAMP-dependent protein kinase activity ratio from 0.1 to 0.38 and increased the level of inositol trisphosphate by 16-fold within 5 min, whereas puromycin was without effect on either level. It is concluded that the two purines block the insulin effect on glycolysis by preventing the hormone increasing the Fru(2,6)P2 level. The mode of action, however, seems to be different: ATP antagonizes insulin action in that it leads to increased inhibition of PFK 2 whereas puromycin prevents the activation of PFK 2 by insulin.
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Affiliation(s)
- I Probst
- Institut für Biochemie, Fachbereich Medizin, Georg-August-Universität Göttingen, Federal Republic of Germany
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18
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van de Werve G. Liver Glucose-6-phosphatase Activity Is Modulated by Physiological Intracellular Ca2+ Concentrations. J Biol Chem 1989. [DOI: 10.1016/s0021-9258(18)83308-7] [Citation(s) in RCA: 19] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022] Open
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19
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Romani A, Fulceri R, Pompella A, Benedetti A. MgATP-dependent, glucose 6-phosphate-stimulated liver microsomal Ca2+ accumulation: difference between rough and smooth microsomes. Arch Biochem Biophys 1988; 266:1-9. [PMID: 2972255 DOI: 10.1016/0003-9861(88)90231-7] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/03/2023]
Abstract
Some features of the MgATP-dependent Ca2+-accumulating capacity of rough as compared to smooth liver microsomal fraction were studied. Smooth microsomes accumulate somewhat higher amounts of Ca2+ than rough ones in the presence of MgATP. In the presence of glucose 6-phosphate, which markedly stimulates MgATP-dependent Ca2+ accumulation in both fractions, smooth microsomes exhibit a much higher Ca2+-accumulating capacity than rough ones. Possible reasons of the differences observed between the two fractions were investigated. Smooth microsomes exhibit a higher Ca2+-dependent ATPase activity, suggesting a higher Ca2+ inward transport into smooth vesicles. Also, following the inhibition of active Ca2+ transport by means of vanadate, smooth microsomes appear to release the Ca2+ previously accumulated--both in the absence (i.e., with MgATP only) and in the presence of glucose 6-phosphate--at a lower rate than rough ones. This indicates a lower passive backflux of Ca2+ accumulated in smooth vesicles. On the basis of these data, differences can be envisaged with respect to cellular Ca2+ handling by different domains of endoplasmic reticulum in the liver.
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Affiliation(s)
- A Romani
- Istituto di Patologia Generale, Università di Siena, Italy
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20
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Müller A, Unthan-Fechner K, Probst I. Activation of phosphofructokinase 2 by insulin in cultured hepatocytes without accompanying changes of effector levels or cAMP-stimulated protein kinase activity ratios. EUROPEAN JOURNAL OF BIOCHEMISTRY 1988; 176:415-20. [PMID: 2843374 DOI: 10.1111/j.1432-1033.1988.tb14298.x] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
Abstract
Activation of glycolysis by insulin in cultured adult rat hepatocytes is accompanied by an activation of phosphofructokinase 2 (PFK 2). PFK 2 activation might be caused by insulin-dependent changes of (a) metabolite levels, (b) basal and (c) Br8cAMP-stimulated cAMP-dependent protein kinase activity; this problem was investigated. 1. Cells cultured with 0.1 nM insulin for 48 h exhibited a low glycolytic rate and low fructose 2,6-bisphosphate [Fru(2,6)P2] levels. Addition of insulin increased Fru(2,6)P2 and Fru(1,6)P2 levels sequentially which points to PFK 2 as first target enzyme of insulin action. 2. Concentrations of Glc6P, Fru6P, phosphoenolpyruvate, glycerol 3-phosphate and citrate, which modulate PFK 2/fructose 2,6-bisphosphatase 2 activity, were not altered by insulin. 3. Activation of PFK 2 by insulin occurred without changes in the levels of total and protein-bound cAMP. Bound cAMP amounted to about 14% of total cAMP. 4. Insulin neither decreased the basal dissociation state of the cAMP-dependent protein kinase nor lowered the sensitivity of the kinase towards cAMP in cell extracts. 5. Addition of the phosphodiesterase-resistant Br8cAMP to the cultures increased cAMP levels 3-4-fold, elevated the protein kinase activity ratio from 0.14 to 0.6 and decreased the Fru(2,6)P2 level and the rate of glycolysis. When Br8cAMP and insulin were given together, insulin was capable of counteracting Br8cAMP in that it activated glycolysis and PFK 2 and elevated the Fru(2,6)P2 level; however, it did not decrease the elevated protein kinase activity ratio. It is concluded that insulin presumably does not activate PFK 2 through changes in cAMP and effector levels or through inhibition of cAMP-dependent protein kinase dissociation. The data support the hypothesis that insulin may act via activation of PFK 2 phosphatase.
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Affiliation(s)
- A Müller
- Institut für Biochemie, Universität Göttingen, Federal Republic of Germany
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21
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MgATP-dependent glucose 6-phosphate-stimulated Ca2+ accumulation in liver microsomal fractions. Effects of inositol 1,4,5-trisphosphate and GTP. J Biol Chem 1988. [DOI: 10.1016/s0021-9258(18)69094-5] [Citation(s) in RCA: 54] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
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22
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Bartels H, Vogt B, Jungermann K. Glycogen synthesis via the indirect gluconeogenic pathway in the periportal and via the direct glucose utilizing pathway in the perivenous zone of perfused rat liver. HISTOCHEMISTRY 1988; 89:253-60. [PMID: 3403300 DOI: 10.1007/bf00493149] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/05/2023]
Abstract
The isolated liver from 24 h fasted rats was perfused in a non-recirculating manner in the ortho- and retrograde direction with erythrocyte-containing (20% v/v) media to provide adequate oxygenation of the liver. Glucose and/or gluconeogenic precursors were added as substrates. Glycogen formation was determined biochemically and demonstrated histochemically. With glucose as the sole exogenous substrate glycogen was deposited in the perivenous area, with gluconeogenic precursors it was formed in the periportal zone during ortho- and retrograde flow. When glucose and gluconeogenic compounds were offered together, glycogen was deposited in both zones. The results corroborate the model of metabolic zonation predicting that periportal glycogen is synthesized indirectly from gluconeogenic precursors while perivenous glycogen is formed directly from glucose.
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Affiliation(s)
- H Bartels
- Institut für Biochemie, Georg-August-Universität, Göttingen, Federal Republic of Germany
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23
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Ballé C, Beuers U, Engelhardt R, Jungermann K. Intracellular mechanism of action of sympathetic hepatic nerves on glucose and lactate balance in perfused rat liver. EUROPEAN JOURNAL OF BIOCHEMISTRY 1987; 170:193-9. [PMID: 2826151 DOI: 10.1111/j.1432-1033.1987.tb13686.x] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
Abstract
In rat liver perfused in situ stimulation of the nerve plexus around the hepatic artery and the portal vein caused an increase in glucose output and a shift from lactate uptake to output. The effects of nerve stimulation on some key enzymes, metabolites and effectors of carbohydrate metabolism were determined and compared to the actions of glucagon, which led to an increase not only of glucose output but also of lactate uptake. 1. Nerve stimulation caused an enhancement of the activity of glycogen phosphorylase a to 300% and a decrease of the activity of glycogen synthase I to 40%, while it left the activity of pyruvate kinase unaltered. Glucagon, similarly to nerve action, led to a strong increase of glycogen phosphorylase and to a decrease of glycogen synthase; yet in contrast to the nerve effect it lowered pyruvate kinase activity clearly. 2. Nerve stimulation increased the levels of glucose 6-phosphate and of fructose 6-phosphate to 200% and 170%, respectively; glucagon enhanced the levels to about 400% and 230%, respectively. The levels of ATP and ADP were not altered, those of AMP were increased slightly by nerve stimulation. 3. Nerve stimulation enhanced the levels of the effectors fructose 2,6-bisphosphate and cyclic AMP only slightly to 140% and 125%, respectively; glucagon lowered the level of fructose 2,6-bisphosphate to 15% and increased the level of cyclic AMP to 300%. 4. In calcium-free perfusions the metabolic responses to nerve stimulation showed normal kinetics, if calcium was re-added 3 min before, but delayed kinetics, if it was re-added 2 min after the onset of the stimulus. The delay may be due to the time required to refill intracellular calcium stores. The hemodynamic alterations dependent on extracellular calcium were normal in both cases. The activation of glycogen phosphorylase, the inhibition of glycogen synthase and the increase of glucose 6-phosphate can well explain the enhancement of glucose output following nerve stimulation. The unaltered activity of pyruvate kinase and the marginal increase of fructose 2,6-bisphosphate cannot be the cause of the nerve-stimulation-dependent shift from lactate uptake to output. The very slight increase of the level of cyclic AMP after nerve stimulation cannot elicit the observed activation of glycogen phosphorylase.(ABSTRACT TRUNCATED AT 400 WORDS)
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Affiliation(s)
- C Ballé
- Institut für Biochemie, Georg-August-Universität Göttingen, Federal Republic of Germany
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24
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Bartels H, Vogt B, Jungermann K. Glycogen synthesis from pyruvate in the periportal and from glucose in the perivenous zone in perfused livers from fasted rats. FEBS Lett 1987; 221:277-83. [PMID: 3622767 DOI: 10.1016/0014-5793(87)80940-7] [Citation(s) in RCA: 29] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Abstract
The isolated liver of 24 h fasted rats was perfused in a non-recirculating manner in the orthograde or retrograde direction with media containing glucose and/or gluconeogenic precursors. Glycogen formation was determined biochemically and demonstrated histochemically. With glucose as the only exogenous substrate glycogen was formed exclusively in the perivenous area during both orthograde and retrograde perfusion. With gluconeogenic precursors as the exogenous substrates glycogen was deposited in the periportal zone during orthograde perfusion and in the intermediate zone during retrograde perfusion. Supply of glucose and gluconeogenic substrates initiated glycogen synthesis only in the upstream region, i.e. in the periportal zone during orthograde and in the perivenous zone during retrograde perfusion. This localization of glycogen synthesis was probably due to an unavoidable, insufficient oxygen supply of the respective downstream area. In general, the results confirm the hypothesis that periportal and perivenous glycogen was synthesized from different substrates.
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25
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Christ B, Jungermann K. Sub-compartmentation of the 'cytosolic' glucose 6-phosphate pool in cultured rat hepatocytes. FEBS Lett 1987; 221:375-80. [PMID: 3622776 DOI: 10.1016/0014-5793(87)80959-6] [Citation(s) in RCA: 32] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Abstract
[14C]Glucose release either from endogenous 14C-prelabelled glycogen or from added 14C-labelled glucose 6-phosphate was measured in filipin-treated, permeabilized hepatocytes in 48 h culture. [14C]Glucose output from prelabelled glycogen was not altered by the addition of 5 mM glucose 6-phosphate to the incubation medium. Conversely, [14C]glucose release from 5 mM labelled glucose 6-phosphate was not influenced by different glycogen concentrations in the cells. Moreover, in the permeabilized cells the anion transport inhibitor DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid) inhibited only the liberation of [14C]glucose from labelled glucose 6-phosphate but not from glycogen. It is therefore concluded that there exist at least 2 separate, mutually non-accessible glucose 6-phosphate pools in cultured rat hepatocytes, one linked to glycogenolysis and the other to gluconeogenesis.
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26
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Benedetti A, Fulceri R, Romani A, Comporti M. Stimulatory effect of glucose 6-phosphate on the non-mitochondrial Ca2+ uptake in permeabilized hepatocytes and Ca2+ release by inositol trisphosphate. BIOCHIMICA ET BIOPHYSICA ACTA 1987; 928:282-6. [PMID: 3032281 DOI: 10.1016/0167-4889(87)90187-x] [Citation(s) in RCA: 21] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/03/2023]
Abstract
The relationships between Ca2+ transport and glucose-6-phosphatase activity, previously studied in isolated liver microsomes, were investigated in permeabilized hepatocytes in the presence of mitochondrial inhibitors. It was found that the addition of glucose 6-phosphate to the cells markedly stimulates the MgATP-dependent Ca2+ uptake. A progressive increase in the stimulation of Ca2+ uptake was seen with increasing amounts of glucose 6-phosphate up to 5 mM concentrations. Vanadate, when added in adequate concentrations (20-40 microM) to the hepatocytes inhibits both the glucose-6-phosphatase activity and the stimulation of Ca2+ uptake by glucose 6-phosphate, while not affecting the MgATP-dependent Ca2+ uptake. The addition of inositol 1,4,5-trisphosphate to permeabilized hepatocytes in which Ca2+ had been accumulated in the presence of MgATP and glucose 6-phosphate, results in a rapid release of Ca2+.
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27
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Davidson AL, Arion WJ. Factors underlying significant underestimations of glucokinase activity in crude liver extracts: physiological implications of higher cellular activity. Arch Biochem Biophys 1987; 253:156-67. [PMID: 3813560 DOI: 10.1016/0003-9861(87)90648-5] [Citation(s) in RCA: 160] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Abstract
M. Kuwajima, C. B. Newgard, D. W. Foster, and J. D. McGarry (1986, J. Biol. Chem. 261, 8849-8853) have concluded that the reason postprandial hepatic glycogenesis occurs primarily from gluconeogenic precursors rather than glucose is because glucokinase activity is insufficient to support the observed rates of glycogen synthesis. F. L. Alvares and R. C. Nordlie (1977, J. Biol. Chem. 252, 8404-8414) have concluded that the combined activities of glucokinase and hexokinase are less than the apparent rates of hepatic glucose uptake. We have identified several factors in the assays used in these studies which lead to substantial underestimations of glucokinase activity. Glucokinase was assayed either by allowing glucose 6-phosphate to accumulate over 10 min (discontinuous assay) or by coupling the formation of glucose 6-phosphate with its oxidation by Leuconostoc mesenteroides glucose 6-phosphate dehydrogenase and NAD (continuous assay). Accurate determinations of glucokinase at 37 degrees C with subsaturating glucose require both 100 mM KCl and 2.5 mM dithioerythritol in the assay medium; 2-mercaptoethanol will not substitute for dithioerythritol. When both KCl and dithioerythritol are absent (Kuwajima et al.) glucokinase activity is underestimated by 3- to 5-fold. The discontinuous assay as used previously (Alvares and Nordlie) underestimates glucokinase activity in crude extracts by 2- to 2.5-fold, due in part to the hydrolysis of glucose 6-phosphate and its transformation to other hexose monophosphates. Under optimized conditions at 37 degrees C both assays yield similar results in extracts from fed rats, i.e., 2-3 and 4-5 units/g liver at 10 and 100 mM glucose, respectively. Some implications of the finding that total hepatic glucose phosphorylating capacity at physiological concentrations significantly exceeds the observed rates of postprandial glycogen synthesis are discussed.
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