Glycogen Turnover Research Articles

Neural and pancreatic influences on net hepatic glucose uptake and glycogen synthesis.

The role of the liver nerves in the disposition of peripherally administered glucose was examined in seven hepatic innervated (HI) and nine hepatic denervated (HD) 42-h-fasted conscious dogs. After a 40-min basal period, there was a 4-h experimental period during which the hepatic glucose load was increased twofold via peripheral glucose infusion. Somatostatin was infused to suppress pancreatic endocrine secretion, and insulin and glucagon were infused intraportally to produce a fourfold increase in insulin and a gradual decrease (approximately 25%) in glucagon. The area under the curve of net hepatic glucose uptake (NHGU) during the glucose infusion period totaled 483 +/- 82 and 335 +/- 32 mg/kg in HD and HI, respectively (P < 0.05). The area under the curve of the hepatic fractional extraction of glucose was 27% greater in HD (P < 0.05). Net hepatic lactate output was similar in the two groups, and net hepatic glycogen synthesis was 3.8 +/- 0.8 vs. 2.7 +/- 0.5 mg.kg dog wt-1.min-1 in HD and HI, respectively (P = 0.13). The direct pathway of glycogen synthesis was responsible for 54-58% of net hepatic glycogen synthesis in both HI and HD (n = 6 for both). In summary 1) NHGU in response to peripheral glucose infusion was approximately 44% greater in HD than in HI, 2) net hepatic glycogen synthesis was enhanced by 41% in HD although the probability of this change was 0.13, and 3) the contribution of the direct pathway to glycogen synthesis was the same in HD and HI. These data are consistent with a role for the liver nerves in regulating the magnitude of NHGU in response to glucose administration. They also indicate that the absence of liver nerves may reduce glycogen turnover during glucose infusion.

Contribution of glycogen to aerobic myocardial glucose utilization.

We determined glycogen turnover and the contribution of glycogen as a source of glucose for aerobic myocardial glycolysis and glucose oxidation in parallel series of isolated, working rat hearts subjected to pulse-chase perfusions. Myocardial glycogen of isolated, working rat hearts was radiolabeled, after 30 minutes of substrate-free glycogen depletion, by perfusion for 60 minutes with buffer designed to stimulate resynthesis of glycogen (1.2 mmol/L palmitate, 11 mmol/L [U-14C]- or [5-3H]glucose, 0.5 mmol/L lactate, and 100 microU/mL insulin). Rates of glucose oxidation (14 CO2 production) and glycolysis (3H20 production) were then measured by perfusing the hearts for 40 minutes with buffer designed to simulate physiological conditions (0.4 mmol/L palmitate, 0.5 mmol/L lactate, 11 mmol/L [5-3H]- or [U-14C]- glucose, 100 microU/mL insulin) containing radiolabeled glucose different from that used during resynthesis. During the chase perfusion, rates of glycolysis and glucose oxidation from exogenous glucose were significantly greater than those from endogenous glycogen. Nevertheless, glycogen contributed significantly to myocardial energy production (41% of the overall ATP produced from glucose), and a significantly greater fraction of the glucose from glycogen that passed through glycolysis was oxidized (>50%) compared with the fraction oxidized from exogenous glucose (<20%, P<.05). Myocardial glycogen was simultaneously synthesized and degraded (ie, glycogen turnover) during the chase perfusion, despite net glycogenolysis. Furthermore, enrichment of labeled glucose in glycogen at the end of the chase perfusion, when corrected for newly synthesized glycogen, did not differ from that at the end of the labeling period. Thus, glycogen contributes significantly to aerobic myocardial glucose use under these experimental conditions, and the glucose derived from glycogen is oxidized preferentially compared with exogenous glucose. Additionally, substantial myocardial glycogen turnover occurs, and the manner in which glycogen is degraded does not fit the ordered hypothesis of "last glucose on, first glucose off."

Overexpression of Muscle Glycogen Phosphorylase in Cultured Human Muscle Fibers Causes Increased Glucose Consumption and Nonoxidative Disposal

The effect of increased expression of glycogen phosphorylase on glucose metabolism in human muscle was examined in primary cultured fibers transduced with recombinant adenovirus AdCMV-MGP encoding muscle glycogen phosphorylase. Increments of 20-fold in total enzyme activity and of 14-fold of the active form of the enzyme were associated with a 30% reduction in basal glycogen levels. Total glycogen synthase activity was doubled in AdCMV-MGP-transduced cells even though the activity ratio was decreased. Incubation with forskolin, which inactivated glycogen synthase and activated glycogen phosphorylase, induced greater net glycogenolysis in engineered cells. In unstimulated fibers, lactate production was three times higher in AdCMV-MGP fibers as compared with controls, despite similar rates of glycogenolysis. In transduced fibers incubated with 2-deoxyglucose, the level of 2-deoxyglucose 6-phosphate was about 8-fold elevated over the control even though hexokinase activity was unmodified in AdCMV-MGP fibers. Overexpression of glycogen phosphorylase also led to enhancement of [U-14C]glucose incorporation into glycogen, lactate, and lipid. Accordingly, determination of lipid cell content revealed that engineered cells were accumulating lipids. Furthermore, 14CO2 formation from [U-14C]glucose was 1.6-fold higher, whereas 14CO2 formation from [6-14C]glucose was unmodified, in AdCMV-MGP fibers. Our data show that in human skeletal muscle cells in culture, the increase in glycogen phosphorylase activity is able to up-regulate glycogen synthase activity indicating the enhancement of glycogen turnover. We suggest that the increase in glycogen phosphorylase and, thereby, in glycogen metabolism, is sufficient to enhance glucose uptake in the muscle cell. Glucose taken up by engineered muscle cells is essentially disposed of through nonoxidative metabolism and converted into lactate and lipid.

Effect of physical exercise on glycogen turnover and net substrate utilization according to the nutritional state

To determine the metabolic effects of a single bout of exercise performed after a meal or in the fasting state, nine healthy subjects were studied over two 8-h periods during which net substrate oxidation was monitored by indirect calorimetry. On one occasion, exercise was performed 90 min after ingestion of a meal labeled with [U-13C]glucose [protocol meal-exercise (M-E)]. On the second occasion, exercise was performed after an overnight fast and was followed 30 min later by ingestion of an identical meal [protocol exercise-meal (E-M)]. Energy balances were similar in both protocols, but carbohydrate balance was positive (42.2 +/- 5.1 g), and lipid balance was negative (-11.1 +/- 2.0) during E-M, whereas they were nearly even during M-E. Total glycogen synthesis was calculated as carbohydrate intake minus oxidation of exogenous 13C-labeled carbohydrate (calculated from 13CO2 production). Total glycogen synthesis was increased by 90% (from 47.6 +/- 3.8 to 90.7 +/- 5.4 g, P < 0.0001) during E-M vs. M-E. Endogenous glycogen breakdown was calculated as net carbohydrate oxidation minus oxidation of exogenous carbohydrate and was increased by 44% (from 35.8 +/- 5.6 to 51.7 +/- 6.6 g, P < 0.004) during E-M. It is concluded that exercise performed in the fasting state stimulates glycogen turnover and fat oxidation.

Glycogen metabolism in quail embryo muscle. The role of the glycogenin primer and the intermediate proglycogen.

Cultured quail embryo muscle has proven to be an excellent model system for studying the synthesis of macromolecular glycogen from, and its degradation to, glycogenin, the autocatalytic, self-glucosylating primer for glycogen synthesis. We recently demonstrated that proglycogen, a low-M(r) form of glycogen, is an intermediate in the synthesis. Here we show that proglycogen also functions as an intermediate in macroglycogen degradation and, in one set of circumstances, represents an arrest point in glycogen breakdown, which does not continue to glycogenin. We suggest that in the nutritionally dependent turnover of glycogen in tissues, the molecules cycle between proglycogen and macromolecular glycogen and are not normally degraded to glycogenin. Nevertheless, when this does happen, the released glycogenin is active, capable of re-initiating glycogen synthesis. Under culture conditions where the conversion of proglycogen into glycogenin does take place, the intermediates lying between form a discrete rather than a continuous series, suggestive of a cluster structure for proglycogen and indicating that breakdown is stepwise. Evidence of post-translational modification of glycogenin was obtained by the finding that, in glycogen from cultured muscle, glycogenin is phosphorylated.

Glycogenolytic response of primary chick and mouse cultures of astrocytes to noradrenaline across development

Glycogen is the brain's largest energy store and it is mainly localised in astrocytes. Glycogen turnover is extremely rapid in the brain, especially during sudden increased demand when glucose supplies are insufficient. Previous culture studies have reported on the glycogenolytic effect of noradrenaline on 3–4 week-old primary mouse astrocyte cultures. This effect is believed to be mediated by the β-adrenergic-cAMP signal transduction system. Recent evidence has shown a drop in forebrain glycogen levels at a specific time point during memory formation for a passive avoidance task in the day-old chick. This ‘memory-related’ glycogenolysis may be initiated by noradrenaline-induced rises in cAMP occurring around this time point, but it is unknown whether astrocytic glycogenolysis is stimulated by noradrenaline in day-old chicks. This question was approached in the present study and it was shown that noradrenaline is capable of stimulating both cAMP formation and glycogen breakdown in chick primary astrocyte cultures at developmental age (10–14 days in culture) comparable to the newborn chick. In contrast, noradrenaline did not have a corresponding glycogenolytic effect on 10-day-old mouse astrocyte cultures (equivalent to the 1-week-old mouse), although it induced a considerable amount of glycogen breakdown in older cultures (18 and 24–26 days).

Glycogen Turnover in the Isolated Working Rat Heart

The isolated working rat heart was adapted for simultaneous determination of glycogen synthesis and degradation using a dual isotope technique. After prelabeling of glycogen with [U-14C]glucose, glycogenolysis was determined continuously from the washout of 14CO2 plus [14C]lactate. Glycogen synthesis was determined during the same period from incorporation of [5-3H]glucose. In the absence of added hormones, hearts were predominantly glycogenolytic (1.5 mumol/min/g, dry weight), and there was simultaneous synthesis (11% of the rate of glycogenolysis). The percentage of glucose taken up by the heart that could traverse the glycogen pool as a consequence of glycogen turnover was minor (5%). Insulin (10 milliunits/ml) predictably stimulated glycogen synthesis (3.6-fold) and nearly abolished glycogenolysis. Addition of glucagon (1 microgram/ml) increased contractile performance and initially stimulated glycogenolysis (3.8-fold) until glycogen was largely depleted. Net tritium incorporation was unaffected by glucagon. Both hormones stimulated glycolytic flux from exogenous glucose (3H2O from [5-3H]glucose) as well as total glycolytic flux (3H2O plus glycogenolysis). The initial stimulation in total glycolytic flux with glucagon was largely from glycogen, explaining the lag in stimulation from exogenous glucose. The relationship between the specific radioactivity and amount of glycogen remaining after different degrees of glycogenolysis suggests that the preference of glycogenolysis for newly synthesized glycogen is only partial.

718-5 Differential Regulation of Phosphorylase Kinase in Failing Human Left Ventricular Myocardium

Congestive heart failure is characterized by a hyperadrenergic state and increased intracellular Ca++levels in failing human ventricular myocardium. Phosphorylase kinase is a key enzyme in the β-adrenergic pathway and is primarily regulated by cyclic AMP dependent protein kinase and calcium. In the case of phosphorylase kinase in failing human heart, an increase in β-ladrenergic drive and intracellular Ca++would be expected to increase the rate of glycogenolysis, leading to a depletion of glycogen. However, glycogen stores appear to increase in failing as compared to nonfailing human left ventricular myocardium (unpublished observations). Therefore, we tested the hypothesis that in heart failure, the hyperadrenergic state results in differential regulation of phosphorylase kinase in failing human ventricular myocardium. An enzymatic assay of [γ-32p] ATP was used to measure the rate of incorporation of 32p into the endogenous substrate of phosphorylase kinase, phosphorylase b. We have shown that phosphorylase kinase in crude extracts from failing (IDC, n = 10) human left ventricular myocardium has a specific activity (nmol/min·mg) that is twofold higher than in nonfailing (n = 8) left ventricular myocardium (0.09 ± 0.016 vs. 0.22 ± 0.02 nmol/min.mg, p &lt; 0.05*). Moreover, activation of the enzyme in failing heart upon binding an antibody raised to the N-terminal region of the catalytic subunit results in higher specific activities than nonfailing heart (0.40 ± 0.09 vs. 0.64 ± 0.16 nmol(min·mg, p &lt; 0.05**), suggesting altered function within the regulatory subunits. These results support the hypothesis that in failing human heart, phosphorylase kinase is differentially regulated. Moreover, these results indicate that in failing myocardium, other downstream enzymatic processes in the glycogenolytic pathway may be affected, such as phosphorylase b to a conversion, which prevent appropriate glycogen turnover under the influence of increased adrenergic drive.

Simultaneous and Separable Flux of Pathways for Glucose and Glycogen Utilization Studied by 13C-NMR

Glucose utilization and glycogen turnover was studied by 13C-NMR in segments of hog carotid artery smooth muscle. After superfusion of carotid segments for 8-16h at 37°C with C-1 labeled glucose. 13C-NMR spectra revealed substantial incorporation into glycogen, lactate and into resonances tentatively identified as glutamate at the C-2 and C-4 positions. The rate of net glycogen incorporation was approximately linear over 8h in unstimulated muscle. After washing out the initial labeled glucose, carotids were contracted by 80 mM KCl in the presence of C-2 labeled glucose. During the contraction simultaneous flux of glycogenolysis and glycolysis was observed with production of lactate labeled at the C-2 position (from glucose labeled at the second carbon) and with the magnitude of the glycogen resonance decreasing. However, no lactate labeled at the C-3 position (derived from C-3 glycogen) was observed at the end of a prolonged contraction. If complete mixing of the intermediates of the two pathways occurred, lactate derived from both glucose and glycogen would have been observed. When similar experiments are performed in the presence of 5 mM NaCN to block oxidative metabolism, then lactate derived from glucose and from glycogen was clearly observed. Therefore, when oxidative metabolism was intact, the intermediates of glycogenolysis and glycolysis did not normally appear to fully mix despite simultaneous flux of the two pathways during contraction. These separable cytosolic pathways for carbohydrate utilization are proposed to be governed by a reaction-diffusion class of mechanism. Such an organization of cytosolic enzymes may represent a general feature of cell metabolism.

Turnover of human muscle glycogen with low-intensity exercise

To determine whether glycogen turnover occurs during prolonged low-intensity exercise, five subjects performed plantar flexion of the right leg at 15% MVC for 5 h. At rest and during the initial 2.5 h of exercise gastrocnemius glycogen was monitored in both legs with natural abundance 13C NMR. At 2.5 h exercise, a step-up infusion of 99% enriched 1-13C glucose was begun and maintained over the next 1.5 h of continued exercise to monitor 1-13C glucose incorporation into the exercising muscle's glycogen pool. Exercise was continued for an hour following the infusion, and NMR scans were performed throughout the session. During the first 2 h of exercise, glycogen 1-13C signal amplitudes dropped approximately 30% and remained there at 2.5 h, indicating that glycogen concentration had leveled. Following infusion, glycogen signal amplitudes rose to 123% of resting values, remaining there during an hour of subsequent exercise. There was no change of glycogen 1-13C signal in the nonexercising leg. Venous glucose levels remained stable until the infusion was begun and then rose < 7% (5.57-5.96 mmol.l-1) during the infusion. Venous insulin and C-peptide levels did not change during the infusion. We conclude that the human gastrocnemius can degrade and synthesize glycogen simultaneously during prolonged low-intensity exercise.

Liver glycogen turnover in fed and fasted humans

Whether liver glycogen synthesis and breakdown occur simultaneously during net glycogen synthesis was assessed in fed and fasted healthy humans. The peak intensity of the carbon-1 (C1) resonance of the glycosyl units of glycogen was monitored with 13C nuclear magnetic resonance spectroscopy during [1-13C]glucose infusion followed by unlabeled glucose infusion. The C1 peak intensity increased almost linearly during the [1-13C]glucose infusion, reflecting a near linear rate of glycogen synthesis. When switched to unlabeled glucose, the C1 peak intensity reached a plateau in the fasted subjects and declined in the fed subjects, reflecting active glycogenolysis during a time of net glycogen synthesis. We conclude that liver glycogen synthesis and degradation occur simultaneously in humans under conditions of net glycogen synthesis. The relative turnover rate was significantly higher in the fed (57 +/- 3%) than in the fasted state (31 +/- 8%; P < 0.01). The results indicate that glycogen may regulate its rate of breakdown and that liver glycogen turnover may be an important factor in limiting the accumulation of liver glycogen in humans.

Glycogen turnover during refeeding in the postabsorptive dog: Implications for the estimation of glycogen formation using tracer methods

Recent 13C nuclear magnetic resonance ( 13C-NMR) studies in the anesthetized rat and perfused liver suggest that hepatic glycogen is simultaneously synthesized and degraded, even during combined hyperglycemia and hyperinsulinemia. The presence of glycogen turnover would confound efforts to study glycogen repletion with the use of tracer methods during feeding, particularly if the liver is not glycogen-depleted. To ascertain whether glycogen turnover occurs during normal feeding, we measured live uptake of glucose in 10 awake, healthy, postabsorptive dogs with long-term arterial, portal, and hepatic venous catheters before and for 3 hours after a meal of either glucose alone (1.5 g/kg) or glucose supplemented with crystalline amino acids (0.7 g/kg); the meal was labeled with d-[3- 3H]glucose and [U- 14C]alanine. Liver glycogen level was measured in biopsies obtained before and at 180 minutes after the meal. The postabsorptive liver glycogen content was 4.3 ± 0.9 g 100g , and net hepatic glucose release averaged 1.8 ± 0.3 mg/min/kg. Over the 3 hours following feeding, the liver took up glucose (0.37 ± 0.14 and 0.33 ± 0.16 g/kg body weight in dogs receiving glucose and glucose with amino acids, respectively). At 3 hours, glycogen synthesis from d-[3- 3H]glucose in the two groups averaged 0.24 ± 0.09 and 0.22 ± 0.05 g/kg, or approximately 15% of the ingested glucose load. 14C-glucose also was found in liver glycogen, demonstrating ongoing hepatic gluconeogenesis. Net hepatic glycogen synthesis averaged only 7 ± 5 and 4 ± 4 g 3 h in glucose- and glucose + amino acid-fed dogs, and overall net hepatic glycogen synthesis was inversely related to the basal glycogen content ( r = .74, P < .01). We conclude that in the awake dog following either a glucose or a mixed glucose/amino acid meal, (1) there is net uptake of both glucose and gluconeogenic amino acids by the liver, and both substrates are incorporated into newly synthesized liver glycogen; (2) net glycogen synthesis with either meal is inversely related to preexisting glycogen content; and (3) significant turnover of the liver glycogen pool occurs, and this precludes the use of tracer methods to estimate net changes of liver glycogen from the direct and indirect pathway.

Glycolipids isolated from cultured rat hepatocytes: Analysis of their role in insulin signal transduction

The precise mechanism by which insulin elicits its effects remains to be fully determined. A glycophospholipid, isolated from H35 cells, has been proposed as a possible precursor for an insulin-generated second messenger that mediates the intracellular effects of insulin. This glycolipid contains a hexosamine moiety, inositol, galactose and palmitate. We have isolated a glycolipid from cultured rat hepatocytes that exhibits chromatographic and radiolabelling characteristics similar to this proposed precursor. The glycolipid can be radiolabelled with glucosamine, galactosamine and palmitate, but not myristate or myo-inotisol. Incorporation of radiolabel into this glycolipid was insensitive to the presence of either insulin (10 −7 M) or phosphatidylinositol-specific phospholipase C (PI-PLC) in the culture medium. The cultured hepatocytes used exhibited normal insulin responses with respect to glycogen turnover and gene expression. Treatment of partially purified glycolipid with either PI-PLC or nitrous acid did not result in the generation of an aqueous soluble phosphooligosaccharide indicating that the glycolipid was not cleaved by either agent. This is in contrast to the reported cleavage of the glycolipids found in H35 hepatoma and lymphocytes. These results question the role of the putative phosphooligosaccharide mediator in the intracellular transduction system activated by insulin.

Depression of carbon flow to the glycogen pool induced by nitrogen assimilation in intact cells of Anacystis nidulans

The influence of nitrate and ammonium assimilation on glycogen metabolism has been determined in intact Anacystis nidulans cell actively fixing CO2. Assimilation of nitrate or ammonium resulted in significant decreases in both the incorporation into glycogen of newly fixed carbon and the accumulation of glycogen by the cells, the magnitude of these effects depending on the light intensity. The depression in glycogen synthesis induced by nitrogen assimilation was more marked at low light intensity, and especially when ammonium was the nitrogen source. Under these conditions, specific radioactivity of the glycogen pool was particularly high, indicating enhanced turnover of glycogen. Thus, in addition to a more general depressing effect of nitrogen assimilation on the carbon flow to glycogen, degradation of glycogen appears to be stimulated by ammonium assimilation at low (but not at high) light intensity.

Nature and localization of avian lens glycogen by electron microscopy and Raman spectroscopy

Electron microscopy confirms the presence of a high concentration of glycogen particles in the lens nuclear region of birds of flying habit such as the ring-neck dove and pigeon. This observation is consistent with Raman spectroscopy. The glycogen particles in the dove lens, which are approximately 35 nm in diameter, are classified as beta type particles. Although this type has been previously characterized by high rates of glycogen turnover in other tissues, its localization in the lens nucleus indicates that it may serve a structural function rather than as a storage depot of carbohydrate in the lens. In a comparative electron microscopy study, glycogen particles were not observed in the chicken lens.

Effect of Growth Hormone on Liver Glycogen Accumulation in Suckling Rats

The regulation of liver glycogen turnover in the neonatal period differs from that of the adult state. Little is also known about the regulation exerted by GH in the first period of life. To shed light on the regulation of glycogen production and in particular to study the role of GH on liver glycogen accumulation, we investigated the effect of GH administration in control or GH-deficient neonatal rats. A slow-release GH preparation was injected subcutaneously on days 9 and 12 of life, in normal and neonatally treated rats with thyroxine or cortisol. Seventy-two hours after the last GH administration, liver glycogen was increased without concomitant elevation of plasma insulin and corticosterone levels and in addition without sequential inactivation of glycogen synthase. These data strongly suggest that the current concepts on the regulation of the hepatic synthesis of glycogen should be revised.

Amylin or CGRP (8–37) fragments reverse amylin-induced inhibition of 14C-glycogen accumulation

The peptides amylin and calcitonin-gene related peptide (CGRP) have been shown to have similar effects on glycogen metabolism in vivo and in vitro. However, it is not clear whether they act via separate receptors. Peptide fragments based on the amino acid sequence of amylin or CGRP were evaluated for their ability to inhibit the action of the peptides in vitro. Insulin-stimulated glycogen turnover, as measured by 14C-glycogen accumulation, was inhibited about 70% by amylin (10nM) and 85% by CGRP (10nM). In the absence of exogenous peptide, peptide fragments based on the 8-37 and 10-37 amino acid sequences of rat amylin (10 uM) had no affect on 14C-glycogen accumulation. In the presence of amylin (10nM), the 8-37 and 10-37 fragments blocked amylin-induced inhibition of 14C-glycogen accumulation 100% and 11.4%, respectively. The 8-37 and 10-37 amylin fragments blocked CGRP inhibition of 14C-glycogen accumulation by 23.2% or 28.6%, respectively. The CGRP 8-37 fragment was equally effective as the amylin 8-37 reversing the effects of amylin than at reversing the effects of CGRP. These results demonstrate that amylin (8-37) completely antagonizes the effects of amylin with limited ability to block CGRP. Removing the eighth and ninth amino acids reduced the effectiveness of the inhibitor by about 90%.

Glucose metabolism and glycogen turnover in vascular smooth muscle studied by 13C-NMR

Glucose metabolism and glycogen turnover in vascular smooth muscle studied by 13C-NMR

Glucose metabolism and glycogen turnover in vascular smooth muscle studied by 13C-NMR

Glucose metabolism and glycogen turnover in vascular smooth muscle studied by 13C-NMR

Labeling of hepatic glycogen after short‐ and long‐term stimulation of glycogen synthesis in rats injected with 3H‐galactose

The effects of short- and long-term stimulation of glycogen synthesis elicited by dexamethasone were studied by light (LM) and electron (EM) microscopic radioautography (RAG) and biochemical analysis. Adrenalectomized rats were fasted overnight and pretreated for short- (3 hr) or long-term (14 hr) periods with dexamethasone prior to intravenous injection of tracer doses of 3H-galactose. Analysis of LM-RAGs from short-term rats revealed that about equal percentages (44%) of hepatocytes became heavily or lightly labeled 1 hr after labeling. The percentage of heavily labeled cells increased slightly 6 hr after labeling, and unlabeled glycogen became apparent in some hepatocytes. The percentage of heavily labeled cells had decreased somewhat 12 hr after labeling, and more unlabeled glycogen was evident. In the long-term rats 1 hr after labeling, a higher percentage of heavily labeled cells (76%) was observed compared to short-term rats, and most glycogen was labeled. In spite of the high amount of labeling seen initially, the percentage of heavily labeled hepatocytes had decreased considerably to 55% by 12 hr after injection; and sparsely labeled and unlabeled glycogen was prevalent. The EM-RAGs of both short- and long-term rats were similar. Silver grains were associated with glycogen patches 1 hr after labeling; 12 hr after labeling, the glycogen patches had enlarged; and label, where present, was dispersed over the enlarged glycogen clumps. Analysis of DPM/mg tissue corroborated the observed decrease in label 12 hr after administration in the long-term animals. The loss of label observed 12 hr after injection in the long-term pretreated rats suggests that turnover of glycogen occurred during this interval despite the net accumulation of glycogen that was visible morphologically and evident from biochemical measurement.