Aberrations in the diurnal rhythms of plasma glucose, plasma insulin, liver glycogen, and hepatic glycogen synthase and phosphorylase activities in genetically diabetic (db/db) mice

The diurnal rhythms of plasma glucose, insulin, liver glycogen, and hepatic glycogen synthase and phosphorylase activities were determined in control and genetically diabetic (db/db) mice 8 weeks of age. The diabetic mice showed wide fluctuations in their plasma glucose levels, although being similar to controls near the end of the light period. Little variation was observed in their elevated plasma insulin levels. Liver glycogen levels in diabetic mice were not depleted to the low levels seen in controls during the last part of the light period but were maintained at significantly higher levels. However, maximum attained glycogen levels were similar in the two groups of mice. Alterations were also observed for the diurnal rhythms of glycogen synthase and phosphorylase activities, although again the daily maximums were similar in control and diabetic mice. These findings suggest that the reported changes of several of these metabolic parameters in the db/db mouse may be due to alterations in the diurnal pattern rather than to absolute changes.     

Diurnal variations in the specific activities of some hepatic enzymes in the immature domestic fowl (Gallus domesticus)

• 1.1. Any diurnal rhythms in the hepatic specific activities of some enzymes of carbohydrate, lipid and amino acid metabolism were studied over a 36 hr period in immature domestic fowl subjected to 12 hr light: 12 hr dark.
• 2.2. Only ATP-citrate lyase exhibited a diurnal fluctuation in metabolism having a higher specific activity in the light period than in the dark period.
• 3.3. The results are discussed in relation to the diurnal rhythm in energy metabolism observed in domestic fowl.

     

Diurnal variations in the activities of the glycogen metabolizing enzymes in mouse liver

The glycogen level in mouse liver was maximal during the night and decreased to the lowest level during the light period. The peak activity of phosphorylase alpha was observed during the light hours and thus paralleled the decline of hepatic glycogen concentrations. The period of rapid glycogen synthesis (1800-2200 hr) was immediately preceded by maximum glycogen synthase alpha activity. Significant diurnal rhythms for phosphorylase kinase and phosphorylase phosphatase activities were also observed and appear to play a role in regulating the diurnal rhythm of phosphorylase alpha activity.     

Daily rhythms of glycogen synthetase and phosphorylase activities in rat liver: Influences of food and light

In rats fed during the night time (from 8 p.m. to 8 a.m.) the activities of liver glycogen synthetase $\text{I}$ and phosphorylase $\text{a}$ varied rhythmically during a 24 hour period. There was an inverse relationship between their levels; the level of synthetase $\text{I}$ rose to a maximum at around 6 a.m. and that of phosphorylase $\text{a}$ attained the peak value at around 6 p.m. Eye enucleation of rats did not affect significantly the daily rhythms of the enzymes. However, when food was offered only during the day time, the phases of both enzyme rhythms were shifted by about 12 hours. On starvation for 24 hours, the glycogen level was reduced almost to nil, but the daily rhythms of the enzymes were retained. It is thus very likely that the daily variations of the enzyme activities are not merely a passive effect of food intake, and that food can be a synchronizer or zeitgeber which sets up the characteristic rhythms of glycogen metabolism in the liver.     

Diurnal changes in the glucose and glycogen levels of the blood and liver of rats in chronic consumption of alcohol and after its withdrawal

Chronic ethanol ingestion by rats exerts almost no effect on the diurnal rhythms of the blood and hepatic glucose concentrations. The rhythm of liver glycogen alters substantially in ethanol-fed animals, the phase of the rhythm being shifted and the daily mean level of glycogen being reduced by a factor of 2. Much more drastic disturbances in carbohydrate metabolism occur after ethanol withdrawal than with ethanol consumption. The diurnal rhythm of liver glycogen becomes inverted in phasing, and the rhythmic amplitude reduced greatly as compared with controls. Both the blood and hepatic glucose concentrations are maintained at nearly constant levels for 18-21 h after ethanol withdrawal, but then the level of blood glucose sharply falls, while that of hepatic glucose somewhat increases. The liver cytosolic water/blood plasma water gradient of glucose 24 h after ethanol withdrawal achieves a value of 4 and remains low 24 h later. The liver glycogen level remains relatively high over the 24 h period after ethanol withdrawal despite the severe hypoglycemia, that can be a result of a limitation of the liver cell membrane permeability for glucose.     

Diurnal rhythms of hepatic carbohydrate metabolism during development of the rat

The ;8+16' feeding schedule (8h feeding and 16h without food in each 24h cycle) was applied to nursing mother rats to study enzyme development in neonatal rats in the absence of solid food. A ;16+8' suckling schedule (16h with the mother and 8h while the mother is fed in a separate cage) was used to show that the increases in pyruvate kinase, glucokinase and aldolase B activities that occur in the late suckling period of liver development do not require the intake of solid food at this time. Their activities may, however, be modulated by the composition of the diet at the time of weaning. Adaptation to the composition of the diet can occur within one feeding period, and to the periodicity of food provision in one or two feeding periods. In the early neonatal period, diurnal rhythms of tyrosine aminotransferase, liver glycogen and glucokinase are either greatly suppressed or absent, but develop rapidly after weaning. Food-dependent rhythms of glycogen and tyrosine aminotransferase were included in the late suckling period (day 14).     

Effect of hypoxia on carbohydrate metabolism in scorpion tissues

The levels of glycogen, glucose, lactate, as well the activities of ten enzymes of carbohydrates metabolism in brain, liver and white muscle of sea scorpion have been investigated. Metabolite concentrations didn't change in brain and the levels of glycogen and lactate were constant in the rest tissues investigated. Glucose concentration decreased in the liver and increased in muscle. In brain hypoxia decreased the activity of hexokinase and increased one of pyruvate kinase, phosphoglucoisomerase and fructoso-1,6-bisphosphatase. In liver most of the enzymes showed the tendency to decrease of their activities. In muscle the activities of phosphofructokinase and phosphoglucoisomerase decreased. Mechanisms of carbohydrates metabolism regulation under hypoxia are discussed.     

A graph-theoretical analysis of metabolic regulation

A graph theoretical method is proposed for modeling metabolic networks including enzymic cascades and synergistic binding of ligands to enzymes. Formal operations on the graph of a given network leads to the identification of feedback metabolites and the enzymes which regulate the feedback. These systemic properties are thus isolated from the purely local regulation of individual enzymes. The method was applied to a model of glycogen metabolism. At low cyclic AMP and insulin levels feedback control of the system is predicted to be largely with the glycogen branching and debranching enzymes, which set the amount of glycogen in the metabolically available outer branches.     

Control of carbohydrate metabolism in an anoxia-tolerant nervous system

Anoxia-tolerant animal models are crucial to understand protective mechanisms during low oxygen excursions. As glycogen is the main fermentable fuel supporting energy production during oxygen tension reduction, understanding glycogen metabolism can provide important insights about processes involved in anoxia survival. In this report we studied carbohydrate metabolism regulation in the central nervous system (CNS) of an anoxia-tolerant land snail during experimental anoxia exposure and subsequent reoxygenation. Glucose uptake, glycogen synthesis from glucose, and the key enzymes of glycogen metabolism, glycogen synthase (GS) and glycogen phosphorylase (GP), were analyzed. When exposed to anoxia, the nervous ganglia of the snail achieved a sustained glucose uptake and glycogen synthesis levels, which seems important to maintain neural homeostasis. However, the activities of GS and GP were reduced, indicating a possible metabolic depression in the CNS. During the aerobic recovery period, the enzyme activities returned to basal values. The possible strategies used by Megalobulimus abbreviatus CNS to survive anoxia are discussed.     

Effect of tangeretin,a polymethoxylated flavone on glucose metabolism in streptozotocin-induced diabetic rats

The present study was designed to evaluate the antihyperglycemic potential of tangeretin on the activities of key enzymes of carbohydrate and glycogen metabolism in control and streptozotocin induced diabetic rats. The daily oral administration of tangeretin (100 mg/kg body weight) to diabetic rats for 30 days resulted in a significant reduction in the levels of plasma glucose, glycosylated hemoglobin (HbA1c) and increase in the levels of insulin and hemoglobin. The altered activities of the key enzymes of carbohydrate metabolism such as hexokinase, pyruvate kinase, lactate dehydrogenase, glucose-6-phosphatase, fructose-1,6-bisphosphatase, glucose-6-phosphate dehydrogenase, glycogen synthase and glycogen phosphorylase in liver of diabetic rats were significantly reverted to near normal levels by the administration of tangeretin. Further, tangeretin administration to diabetic rats improved hepatic glycogen content suggesting the antihyperglycemic potential of tangeretin in diabetic rats. The effect produced by tangeretin on various parameters was comparable to that of glibenclamide – a standard oral hypoglycemic drug. Thus, these results show that tangeretin modulates the activities of hepatic enzymes via enhanced secretion of insulin and decreases the blood glucose in streptozotocin induced diabetic rats by its antioxidant potential.     

Effects of acute starvation on carbohydrate metabolism in rat salivary glands.

1. Effects of acute starvation on enzymes of carbohydrate metabolism were determined in rat submandibular and parotid glands. 2. Activities of glycolytic enzymes were high in submandibular gland, but those of pentose phosphate pathway and glycogen metabolism were high in parotid gland. 3. Enzyme activities were lowered by acute starvation. Refeeding the rats with solid diet restored the enzyme activities, but with liquid diet, only partial recoveries were found in submandibular gland.     

Effects of denervation on the glycogen content and on the activities of enzymes of glucose and glycogen metabolism in rat diaphragm muscle

1. Changes in the content and concentration of glycogen and in the activity of a number of enzymes involved in glucose and glycogen metabolism were studied in the rat hemidiaphragm after unilateral denervation. 2. After nerve section the tissue hypertrophies; this hypertrophy is said to be confined to the smaller red fibres and not to the white. 3. The total hexokinase activity increases, whereas that of total glycogen phosphorylase decreases. The specific activity of phosphorylase a, determined after Halothane anaesthesia, remains fairly constant. 4. In fed animals the denervated tissue stores less glycogen, but in the early stages its glycogen content does not fall on starvation. 5. The effect of denervation on the specific activities of several other characteristically white-fibre enzymes are not consistent with the response of glycogen phosphorylase; the increase in content of glyceraldehyde 3-phosphate dehydrogenase and lactate dehydrogenase is thought to be related to proliferation of the sarcoplasmic reticulum. 6. The ratio of lactate dehydrogenase M/H subunits increases at the height of the hypertrophy, but then declines as the mass of the tissue falls. 7. The chronology of these changes in enzyme activities suggests a multiplicity of distinct responses after nerve section not consistent with any one model, either specific fibre development or reversion to de-differentiated, foetal-type metabolism.     

Effect of changes in feeding schedule on the diurnal rhythms and daily activity levels of intestinal brush border enzymes and transport systems.

The activities of rat intestinal enzymes, sucrase, lactase, maltase, trehalase, gamma-glutamyltransferase, leucylnaphthylamide-hydrolyzing activity, and the transport system for glucose follow diurnal rhythms on ad libitum and restricted feeding regimes. In response to 6 days of restricted feeding, food available between 1400 and 1800 Eastern Standard Time, all rhythms shifted in time and the daily levels of activities were changed. Alkaline phosphatase activity followed a diurnal rhythm only in restricted fed animals. In restricted fed rats several activity patterns were observed, some with short periods of maximum activity, 3 h or less, and some with plateaus of maximum activity, 5-9 h long. In respect to the time of day of the synchronizer, sucrase peaked before feeding, glucose transport peaked during feeding, alkaline phosphatase peaked after feeding, and the other enzymes had higher levels of activity before, during and after feeding. The effect of restricted feeding on the daily activity levels were: a decrease in leucylnaphthylamide-hydrolyzing activity, no change in alkaline phosphatase, and increases in the others. These enzyme and transport systems exhibit a large amount of individual regulation or control as reflected by the lack of a uniform activity pattern and response to the synchronizer, and the variation in direction and magnitude of the adaptations to restricted feeding.     

Effect of changes in feeding schedule on the diurnal rhythms and daily activity levels of intestinal brush border enzymes and transport systems

The activities of rat intestinal enzymes, sucrase, lactase, maltase, trehalase, γ-glutamyltransferase, leucylnaphthylamide-hydrolyzing activity, and the transport system for glucose follow diurnal rhythms on ad libitum and restricted feeding regimes. In response to 6 days of restricted feeding, food available between 1400 and 1800 Eastern Standard Time, all rhythms shifted in time and the daily levels of activities were changed. Alkaline phosphatase activity followed a diurnal rhythm only in restricted fed animals.In restricted fed rats several activity patterns were observed, some with short periods of maximum activity, 3 h or less, and some with plateaus of maximum activity, 5–9 h long. In respect to the time of day of the synchronizer, sucrase peaked before feeding, glucose transport peaked during feeding, alkaline phosphatase peaked after feeding, and the other enzymes had higher levels of activity before, during and after feeding. The effect of restricted feeding on the daily activity levels were: a decrease in leucylnaphthylamide-hydrolyzing activity, no change in alkaline phosphatase, and increases in the others.These enzyme and transport systems exhibit a large amount of individual regulation or control as reflected by the lack of a uniform activity pattern and response to the synchronizer, and the variation in direction and magnitude of the adaptations to restricted feeding.     

On the activities of glycogen phosphorylase and glycogen synthase in the liver of the rat

A procedure was developed for determination of glycogen synthase and phosphorylase activities in liver after various in vivo physiological treatments. Liver samples were obtained from anaesthetised rats by freeze-clamping in situ. Other procedures were shown to stimulate the activity of phosphorylase and depress the activity of glycogen in the liver. The direction of glycogen metabolism appears to be regulated by the relative proportions of the two enzymes, as shown by a strong positive correlation between total activities and active forms of phosphorylase and synthase. The enzyme activities responded as expected to stimuli such as insulin and glucose, which depressed phosphorylase and increased synthase activity, and glucagon, which increased phosphorylase and decreased synthase activity. In fasted animals approximately 50% of each enzyme was in the active form, which suggests the existence of a potential futile cycle for glycogen metabolism. The role for such a cycle in the regulation of glycogen synthesis and degradation is discussed.     

On the activities of glycogen phosphorylase and glycogen synthase in the liver of the rat.

A procedure was developed for determination of glycogen synthase and phosphorylase activities in liver after various in vivo physiological treatments. Liver samples were obtained from anaesthetised rats by freeze-clamping in situ. Other procedures were shown to stimulate the activity of phosphorylase and depress the activity of glycogen in the liver. The direction of glycogen metabolism appears to be regulated by the relative proportions of the two enzymes, as shown by a strong positive correlation between total activities and active forms of phosphorylase and synthase. The enzyme activities responded as expected to stimuli such as insulin and glucose, which depressed phosphorylase and increased synthase activity, and glucagon, which increased phosphorylase and decreased synthase activity. In fasted animals approximately 50% of each enzyme was in the active form, which suggests the existence of a potential futile cycle for glycogen metabolism. The role for such a cycle in the regulation of glycogen synthesis and degradation is discussed.     

Effects of altered thyroid status on beta-adrenergic actions on skeletal muscle glycogen metabolism

The effects of hypothyroidism on glycogen metabolism in rat skeletal muscle were studied using the perfused rat hindlimb preparation. Three weeks after propylthiouracil treatment, serum thyroxine was undetectable and muscle glycogen and Glc-6-P were decreased. Basal and epinephrine-stimulated phosphorylase a and phosphorylase b kinase activities were also significantly reduced, as were epinephrine-stimulated cAMP accumulation and cAMP-dependent protein kinase activity. Conversely, basal and epinephrine-stimulated glycogen synthase I activities were significantly higher while the Ka of the enzyme for Glc-6-P was lower in hypothyroid animals. Propylthiouracil-treated rats also had increased phosphoprotein phosphatase activities towards phosphorylase and glycogen synthase and decreased activity of phosphatase inhibitor 1. beta-Adrenergic receptor binding and basal and epinephrine-stimulated adenylate cyclase activities were reduced in muscle particulate fractions from hypothyroid rats. Administration of triiodothyronine to rats for 3 days after 3 weeks of propylthiouracil treatment restored the altered metabolic parameters to normal. It is proposed that the decreased beta-adrenergic responsiveness of the enzymes of glycogen metabolism in hypothyroid rat skeletal muscle is due to increased activity of phosphoprotein phosphatases and to reduced beta-adrenergic receptors and adenylate cyclase activity.     

Altered carbohydrate, lipid, and xenobiotic metabolism by liver from rats flown on Cosmos 1887

To determine the possible biochemical effects of prolonged weightlessness on liver function, samples of liver from rats that had flown aboard Cosmos 1887 were analyzed for protein, glycogen, and lipids as well as the activities of a number of key enzymes involved in metabolism of these compounds and xenobiotics. Among the parameters measured, the major differences were elevations in the glycogen content and hydroxymethylglutaryl-CoA (HMG-CoA) reductase activities for the rats flown on Cosmos 1887 and decreases in the amount of microsomal cytochrome P-450 and the activities of aniline hydroxylase and ethylmorphine N-demethylase, cytochrome P-450-dependent enzymes. These results support the earlier finding of differences in these parameters and suggest that altered hepatic function could be important during spaceflight and/or the postflight recovery period.