The metabolic zonation of glycogen synthesis in rat liver after fasting and refeeding.

The quantitative analysis of total glycogen and two fractions of the glycogen content was made by means of cytophotometry in hepatocytes with respect to the portal and central zones of the liver lobule after 48 hr starvation and 15, 30, 60, 120 min after refeeding using the Magiscan image analyzer. It was shown that glycogen content was minimal after 48 hr starvation, although a few cells of the central zone contained a noticeable glycogen quantity. Glycogen synthesis initiation was observed after 15 min refeeding. Glycogen synthesis has been characterized by an increasing glycogen content in the portal zone of the liver lobule compared to the pericentral zone, and this difference increased with time. The distinctive morphological changes were observed in the total glycogen content as well as fractions with different optical density in the process of glycogen synthesis after starvation of rats.     

Effect of partial hepatectomy on the glycogen level in hepatocytes in the portal and central lobule zones in cirrhotically-changed rat liver

Using cytophotometric method, the content of glycogen was studied in hepatocytes of the portal and central zones of a liver lobule in norm, in cirrhosis, and 1, 3, and 6 months after a partial hepatectomy of the normal and cirrhotic rat liver. As we showed earlier, glycogen content in cirrhotic liver hepatocytes rose 2-3-fold, along with obvious impairment of glycogen metabolic heterogeneity in these. In cirrhotic liver glycogen dominates in the central zone, whereas in norm more glycogen is observed in the portal one. The objective of this study was to find out to what degree a partial hepatectomy of cirrhotic liver may promote recovery of the metabolic glycogen heterogeneity in hepatocytes. Glycogen was determined in hepatocytes, using a quantitative variant of PAS-reaction on sections of the material obtained from serial supravital punctate liver biopsies. Glycogen amount in hepatocytes of different liver lobule zones was determined by an image analyzer technique that allows to bring together the cytophotometric analysis of the substance with its localization in a particular liver lobule. Results of these studies have shown that a partial hepatectomy of cirrhotic liver promotes restoration of the hepatocyte metabolic heterogeneity in the liver lobule.     

Dynamics of glycogen content in the normal and cirrhotic liver following glucose administration to starving rats

Using biochemical, cytofluorimetric and television cytophotometric methods, glycogen contents were studied in normal and cirrhotic rat liver at various intervals after glucose administration to fasting animals. The obtained data indicate that after a 48 h fasting glycogen contents in normal and cirrhotic liver are equally poor. A marked rise of glycogen content in cirrhotic liver was observed only 20-30 min after glucose administration to rats. It has been established that at all intervals after glucose administration to rats hepatocytes of the portal lobule zone, both in normal and in cirrhotic liver, accumulate more glycogen than those of the central zone. Again, the intensity of glycogen accumulation in cirrhotically altered liver is significantly lower than in normal liver, due, presumably, to a lower rate of glycogen synthesis in pathologically changed liver.     

Metabolic heterogeneity of glycogen in hepatocytes of patients with liver cirrhosis

Concentrations of the total glycogen (TG) and of its labile and stable fractions (LF and SF, respectively) were determined in hepatocytes of portal and central zones of the normal human liver and in the liver of patients with cirrhosis of viral and alcohol etiology. Using the PAS reaction, TG and its LF and SF were revealed in histological sections of the material obtained by liver punction biopsies. Concentrations of TG and its fractions were measured by television cytophotometry. In liver cirrhosis, concentrations of TG, LF, and SF in both zones of the hepatic lobule were much higher than in the normal liver. The ratio between hepatocyte TG concentration in the portal zone and that in the central zone (P/C ratio), both in norm and in viral cirrhosis, exceeds 1.0 to reach, respectively, 1.26 +/- 0.02 and 1.03 +/- 0.01. The glycogen fraction composition in cells of both liver lobule zones in viral cirrhosis does not significantly differ from that in norm. On the contrary, in the liver of patients with alcoholic cirrhosis, the P/C ratio falls to 0.82 +/- 0.02 to be accompanied by qualitative changes in glycogen composition.     

Effects of Feeding a Histidine-excess Diet and Subsequent Starvation on Liver and Muscle Glycogen,and on Serum Glucose

The effects of feeding with a histidine-excess diet and subsequent starvation on liver and muscle glycogen, and on serum glucose were investigated in young and adult rats.

Feeding with a histidine-excess diet resulted in the accumulation of liver glycogen in both young and adult rats. The hepatic glycogen continued to decrease during starvation, and the liver became almost totally depleted of glycogen after starvation for 48 hr. Glycogen in the liver of young rats starved for 24 hr after previous feeding with a histidine-excess diet was significantly higher than that of young rats starved for 24 hr after previous feeding with a basal diet.

Muscle glycogen after feeding and subsequent starvation was not affected by the types of diets fed previously, muscle glycogen during starvation showing a slight decrease in young rats and a slight increase in adult rats.

Feeding with a histidine-excess diet caused a significant decrease of serum glucose in young rats, but not in adult rats. Serum glucose in young rats was markedly reduced by starvation after previous feeding with a basal diet, but not after previous feeding with a histidine-excess diet. In adult rats, there were no changes in serum glucose between rats starved after feeding with either a basal diet or a histidine-excess diet, and serum glucose was decreased slightly by starvation after feeding with the test diets.

The overall results indicate that the maintenance of serum glucose in young rate even during starvation after previous feeding with a histidine-excess diet might be partially concerned with the export of glucose from the accumulated glycogen in the liver due to the diet.     

Heterogeneity of glycogen synthesis upon refeeding following starvation.

1. Starvation of rats for 40 hr decreased the body weight, liver weight and blood glucose concentration. The hepatic and skeletal muscle glycogen concentrations were decreased by 95% (from 410 mumol/g tissue to 16 mumol/g tissue) and 55% (from 40 mumol/g tissue to 18.5 mumol/g tissue), respectively. 2. Fine structural analysis of glycogen purified from the liver and skeletal muscle of starved rats suggested that the glycogenolysis included a lysosomal component, in addition to the conventional phosphorolytic pathway. In support of this the hepatic acid alpha-glucosidase activity increased 1.8-fold following starvation. 3. Refeeding resulted in liver glycogen synthesis at a linear rate of 40 mumol/g tissue per hr over the first 13 hr of refeeding. The hepatic glycogen store were replenished by 8 hr of refeeding, but synthesis continued and the hepatic glycogen content peaked at 24 hr (approximately 670 mumol/g tissue). 4. Refeeding resulted in skeletal muscle glycogen synthesis at an initial rate of 40 mumol/g tissue per hr. The muscle glycogen store was replenished by 30 min of refeeding, but synthesis continued and the glycogen content peaked at 13 hr (approximately 50 mumol/g tissue). 5. Both liver and skeletal muscle glycogen synthesis were inhomogeneous with respect to molecular size; high molecular weight glycogen was initially synthesised at a faster rate than low molecular weight glycogen. These observations support suggestions that there is more than a single site of glycogen synthesis.     

Pyruvate dehydrogenase and glycogen synthase activity at transition from fasted to fed state

The aim of the present study was to evaluate whether the PDC and GS activities at the transition from fasted into fed state are consistent with indirect pathway for glycogen synthesis, as suggested previously. Refeeding of glucose given to rats after 72 hr of starvation did not reactivate PDC in the liver; however, the PDC activity in the muscle was increased. In comparison to PDC, glucose refeeding leads to an opposite effect on GS in both liver and muscle as evidenced by the immediate increase in the active form of GS. The low activity of liver PDC restricts 3-carbon flux through the Krebs cycle and enables their transfer to the gluconeogenic pathway for glycogen synthesis. In contrast, an immediate activation of muscle PDC following refeeding indicates that 3-carbon flux will be oxidized in the citric acid cycle, which thereby eliminates the indirect pathway for glycogen synthesis in this tissue. Glucose infusion increased plasma lactate, insulin, and glycogen content in the liver and muscle to the same extent as observed in the fed rats. The results are in agreement with the suggestion that at the transition from fasted to fed state, liver glycogen synthesis occurs mainly from 3-carbon precursors.     

Regulatory role of glucose-6 phosphatase in the repletion of liver glycogen during refeeding in fasted rats

We analyzed the biochemical mechanisms involved in the liver glycogen repletion upon refeeding for 360 min in 48 and 96 h-fasted rats. In 48 h-fasted rats, the glycogen synthesis involved a rapid and further sustained induction of glucokinase (GK) (increased twice from 90 min) and a rapid but transient activation of glycogen synthase a (GSa) (maximal increase by 150% at 90 min). It did not involve the inhibition of glycogen phosphorylase a (GPa). In 96 h-fasted rats, the glycogen repletion did not involve the induction of GK for the first 180 min of refeeding. It involved a slow activation of GSa (maximal 150% increase at 180 min) and a rapid inhibition of GPa (significant from 90 min, maximal 50% inhibition by 180 min). In both groups of rats, there was a progressive inhibition of the glucose-6 phosphatase (Glc6Pase) activity (maximal suppression by 30% in both groups at 360 min). These results highlighted a key role for the inhibition of Glc6Pase activity in the liver glycogen repletion upon refeeding.     

The activity of glycogen synthase phosphatase limits hepatic glycogen deposition in the adrenalectomized starved rat.

Hepatocytes from adrenalectomized 48 h-starved rats responded to increasing glucose concentrations with a progressively more complete inactivation of phosphorylase. Yet no activation of glycogen synthase occurred, even in a K+-rich medium. Protein phosphatase activities in crude liver preparations were assayed with purified substrates. Adrenalectomy plus starvation decreased synthase phosphatase activity by about 90%, but hardly affected phosphorylase phosphatase activity. Synthase b present in liver extracts from adrenalectomized starved rats was rapidly and completely converted into the a form on addition of liver extract from a normal fed rat. Glycogen synthesis can be slowly re-induced by administration of either glucose or cortisol to the deficient rats. In these conditions there was a close correspondence between the initial recovery of synthase phosphatase activity and the amount of synthase a present in the liver. The latter parameter was strictly correlated with the measured rate of glycogen synthesis in vivo. The decreased activity of synthase phosphatase emerges thus as the single factor that limits hepatic glycogen deposition in the adrenalectomized starved rat.     

Effect of fasting on glycogen metabolism and activities of glycolytic and gluconeogenic enzymes in the mudskipper Boleophthalmus boddaerti

During starvation, muscle glycogen in Boleophthalmus boddaerti was utilized preferentially over liver glycogen. In the first 10 days of fasting, the ratio of the active‘a’form of glycogen phosphorylase to total phosphorylase present in the liver was small. During this period, the active‘I’form of glycogen synthetase increased in the same tissue. In the muscle, the phosphorylase‘a’activity declined during the first 7 days and increased thereafter while the total glycogen synthetase activity showed a drastic decline during the first 13 days of fasting. The glycogen level in the liver and muscle of mudskippers starved for 21 days increased after refeeding. After 6 and 12 h refeeding, liver glycogen level was 8·5 ± 2·3 and 6·9 ± 4·5 mg·g wet wt ¹, respectively, as compared to 5·8 ± l·6mg·g wet wt ¹ in unfed fish. Muscle glycogen level after 6 and 12 h refeeding was 0·96±0·76 and 0·82 ± 0·50 mg·g wet wt ¹, respectively, as opposed to 0·21 ± 0·12 mg·g wet wt ¹ in the 21-days fasted fish. At the same time, activities of glycogen phosphorylase in the muscle and liver increased while the active‘I’form of glycogen synthetase showed higher activity in the liver. Since glycogen was resynthesized upon refeeding, this eliminated the possibility that glycogen depletion during starvation was due to stress or physical exhaustion after handling by the investigator. Throughout the experimental starvation period, the body weight of the mudskipper decreased, with a maximum of 12% weight loss after 21 days. Liver lipid reserves were utilized at the onset of fasting but were thereafter resynthesized. Muscle proteins were also metabolized as the fish were visibly thinner. However, no apparent change in protein content expressed as per gram wet weight was detected as the tissue hydration state was maintained constant. The increased degradation of liver and muscle reserves was coupled to an increase in the activities of key gluconeogenic enzymes in the liver (G6Pase, FDPase, PEPCK, MDH and PC). The increase in glucose synthesis was possibly necessary to counteract hypoglycemia brought about by starvation in B. boddaerti.     

The effect of starvation and refeeding on lipogenic enzymes in mammary glands and livers of lactating rats.

Lactating rats were starved for 48 h and refed a high-carbohydrate diet for a further 48 h. Starvation stops milk secretion, which resumes shortly after refeeding. Three lipogenic enzymes, fatty acid synthase, glucose 6-phosphate dehydrogenase (EC 1.1.1.49) and 'malic' enzyme (EC 1.1.1.40) all decrease in the mammary gland during starvation and are restored to the pre-starvation levels 48 h after refeeding. The same enzymes in liver also decrease during starvation, but increase to values significantly higher than those for the normal fed rats after refeeding the high-carbohydrate diet. For the fatty acid synthase these values were four times the pre-starvation values. Serum insulin and prolactin concentrations also increased upon refeeding the high-carbohydrate diet.     

Use of deuterium labelled glucose in evaluating the pathway of hepatic glycogen synthesis

Deuterium labelled glucose has been used to study the pathway of hepatic glycogen synthesis during the fasted-refed transition in rats. Deuterium enrichment of liver glycogen was determined using nuclear magnetic resonance as well as mass spectroscopy. Sixty minutes after oral administration of deuterated glucose to fasted rats, the portal vein blood was fully enriched with deuterated glucose. Despite this, less than half of the glucose molecules incorporated into liver glycogen contained deuterium. The loss of deuterium label from glucose is consistent with hepatic glycogen synthesis by an indirect pathway requiring prior metabolism of glucose. The use of deuterium labelled glucose may prove to be a useful probe to study hepatic glycogen metabolism. Its use may also find application in the study of liver glycogen metabolism in humans by a noninvasive means.     

饥饿和再投喂对草鱼鱼种生物化学组成的影响

分析了饥饿１５天和再投喂２１天的草鱼鱼种肝脏和肌肉生物化学组成的变化，结果表明：（１）饥饿降低白肌ＲＮＡ／ＤＮＡ比值，蛋白质含量和肝脏ＲＮＡ／ＤＮＡ比值，使肝脏蛋白质含量升高，再投喂后，肝脏ＲＮＡ／ＤＮＡ比值，蛋白质含量和白蛋白质含量均恢复至正常投喂组水平，白肌ＲＮＡ／ＤＮＡ比值升高并显著高于正常投喂组水平；（２）饥饿状态下，肝脏和肌肉的脂类含量降低，水含量升高，再投喂后，肝脏和肌肉的脂类含量升高     

Direction of carbon flux in starvation and after refeeding: in vitro and in vivo effects of 3-mercaptopicolinate

3-Mercaptopicolinate (3-MPA) is a specific inhibitor of phosphoenolpyruvate carboxykinase (PEP CK). In vivo the hypoglycaemic action of 3-MPA in 24 h-starved rats was abolished on intragastric glucose refeeding. Nonetheless, 3-MPA decreased hepatic glycogen content and rate of synthesis in starved animals re-fed glucose. The inference is that on re-feeding after starvation hepatic glycogen is synthesised mainly de novo via glyconeogenesis involving PEP CK. 3-MPA increased hepatic lipogenesis in water- and glucose-fed normal and diabetic rats. This increase is presumed to result from inhibition of PEP CK and consequent diversion of pyruvate from gluconeogenesis to lipogenesis. In contrast, 3-MPA inhibited brown-fat lipogenesis in water- and glucose-fed rats.     

Change in tissue glycogen reserves in tumor bearers as a reflection of a hypoglycemic stress syndrome

Glycogen content in the brain, liver and skeletal muscles of rats bearing ascite Zajdela hepatoma (AZH) and solid 27 hepatoma (27-H) has been studied. Serum glucose levels directly correlated with liver glycogen reserves. In the terminal stage of tumor growth depletion of liver glycogen was observed, while the stores of muscle glycogen did not diminish. Within 1-4 days (AZH) and 15-30 days (27-H) after implantation the stores even exceeded those of control healthy rats. In the terminal stage, in spite of hypoglycaemia development, the content of brain glycogen was significantly elevated in both groups of animals.     

Glycogen synthesis from pyruvate in the periportal and from glucose in the perivenous zone in perfused livers from fasted rats

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.     

Glycogen synthase of Hymenolepis diminuta. II. Nutritional state, interconversion of forms, and primer glycogen molecular weight as control factors.

Glycogen synthase I (UDP glucose: glycogen alpha-4-glycosyltransferase, EC2.4.1.11) of the tapeworm Hymenolepis diminuta is the form of the enzyme which is active in vivo, while the D-form represents an inactive "storage form." Utilizing the differential effect of inorganic phosphate (Pi) on the I and D-forms, the ratio of the 2 forms in vivo has been determined under conditions of starvation of the host and refeeding of the parasite with glucose. This procedure reveals that conversion of the inactive D-form to the active I-form takes place when glycogen-depleted worms are incubated in glucose. The activity of glycogen synthase I also is affected by the molecular weight of the primer glycogen. With certain molecular weight fractions, enzymatic activity is higher than with others. This specificity of the glycogen primer could explain the relatively low concentrations of those molecular weight fractions which confer the highest synthase activity.     

Hepatic carbon flux after re-feeding. Hyperthyroidism blocks glycogen synthesis and the suppression of glucose output observed in response to carbohydrate re-feeding.

1. The work investigated hepatic glycogen synthesis and glucose output after the intragastric administration of glucose or glycerol or the provision of chow ad libitum to 48 h-starved euthyroid or hyperthyroid rats. 2. After glucose administration, glycogen synthesis via the indirect pathway [Newgard, Hirsch, Foster & McGarry (1983) J. Biol. Chem. 258, 8046-8052] occurred concomitantly with reversal of glucose flux across the liver and re-activation of pyruvate kinase in the euthyroid controls. Glycogen synthesis was decreased and net glucose output continued in the hyperthyroid rats, but normal re-activation of pyruvate kinase was observed. 3. Use of 3-mercaptopicolinate indicated that the glucose released from liver of hyperthyroid rats was synthesized from substrates metabolized via the gluconeogenic pathway. 4. Hepatic glycogen synthesis was also impaired in hyperthyroid rats after administration of glycerol or chow. Measurement of portal-minus-hepatovenous concentration differences and arterial glucose concentrations after the administration of glycerol in combination with 3-mercaptopicolinate indicated that flux from triose phosphate to glucose 6-phosphate was not decreased. 5. Inhibited glycogen synthesis after chow re-feeding was associated with accelerated re-activation of hepatic pyruvate dehydrogenase complex in the hyperthyroid rats. 6. The results indicate three distinct and independent actions of hyperthyroidism after re-feeding: (i) it inhibits the reversal of glucose flux across the liver normally observed in response to carbohydrate; (ii) it affects glycogen deposition at a site distal to glucose 6-phosphate; (iii) it allows more rapid re-activation of liver pyruvate dehydrogenase complex in response to a mixed diet.     

Indirect versus direct routes of hepatic glycogen synthesis

During refeeding after a brief period of starvation, glucose carbon is deposited into hepatic glycogen by both a direct and an indirect route. In the indirect route glucose is first metabolized to 3-carbon precursors, which then transverse the gluconeogenic pathway before being deposited into glycogen. Recent studies have yielded widely different estimates of the percentage of glucose carbon that follows the indirect route. Work summarized here demonstrates that the relative contributions of glucose carbon to hepatic glycogen formation by the indirect and direct pathways are greatly dependent on experimental design, and at least in vitro, are possibly dependent on the extent of glycogen/glucose 1-P recycling. Under physiological refeeding conditions in vivo, both pathways are used, each contributing approximately 50% of the amount of carbon appearing in glycogen. The level of glucokinase activity does not appear to be responsible for poor glucose utilization in liver. Poor glucose utilization in isolated liver preparations may result from the absence of a neurophysiological feedback loop that senses the arterial/portal glucose gradient and then regulates whole liver glucose uptake.     

补偿生长对中国林蛙肝脏生化组成的影响

在室内环境条件下研究了不同饥饿程度和再投喂后中国林蛙肝脏中蛋白质、糖原和脂肪含量的变化,并对中国林蛙在补偿生长过程中能量来源的变化进行了分析.结果 表明,饥饿状态下,中国林蛙的能量消耗主要来源于肝糖原,其次是蛋白质,即饥饿时中国林蛙首先动用糖原和蛋白质提供能量.