CHANGES IN GLYCOGEN PHOSPHORYLASE ACTIVITY AND GLYCOGEN LEVELS OF MOUSE CEREBRAL CORTEX DURING CONVULSIONS INDUCED BY HOMOCYSTEINE

Glycogen phosphorylase activity and glycogen levels were investigated in the cerebral cortex of mice of two different strains under the influence of homocysteine. Control levels of glycogen and total phosphorylase activity (i. e. activity in the presence of 1 mM-AMP) were higher in the inbred strain A, whereas a higher proportion of phosphorylase in its active form (activity without 5′-AMP) was obtained in the ICR strain (probably due to slower fixation of brain in this strain). Changes occurring after the administration of homocysteine were similar in both strains. With the onset of first clonic seizures a marked increase of phosphorylase a occurred (increase 99 per cent in strain A and 46.5 per cent in ICR, respectively). During the latter phase of tonic seizures active phosphorylase a did not significantly differ from control values. Five minutes after the end of a tonic seizure, i. e. when partial recovery could already be observed, a marked decrease of active phosphorylase a in comparison with control values, was evident (decrease against control values of 45.5 per cent in strain A and 30.5 per cent in ICR, respectively). The total phosphorylase activity was not affected in strain A, whereas a slight increase during clonic seizures was seen in the ICR strain. In accordance with the enhanced activation of phosphorylase at the onset of clonic seizures, a marked decrease in glycogen levels (35-50 per cent) was observed in both strains of mice. This decrease persisted even during the 5 min recovery period. When seizures were prevented by Na phenobarbital or glycine, the activation of phosphorylase was either completely prevented (by a non-anaesthetic dose of phenobarbital) or reduced (by glycine). The present results have demonstrated that changes in glycogen metabolism occurring during homocysteine seizures differ distinctly from those previously found during seizures induced by methionine sulphoximine, a substance structurally related to homocysteine.     

Glycogen, ammonia and related metabolities in the brain during seizures evoked by methionine sulphoximine

Abstract— The levels of ATP, P-creatine, glucose, glycogen, lactate, glutamate and ammonia were measured in mouse brain after administration of the convulsive agent methionine sulphoximine (MSO). No changes were observed in ATP and P-creatine levels either before or during the seizures. Lactate levels were unchanged until the onset of seizures (4–5 hr) at which time the levels increased an average of 65 per cent. Glucose and glycogen levels increased progressively. Just before the onset of seizures the levels had increased 95 and 62 per cent, respectively. During the seizures both substances had increased a total of 130 per cent. Comparable changes were found in cerebral cortex, cerebellum and subcortical forebrain. Through the use of quantitative histochemical methods it was found that the greatest increases in glycogen occurred in layers I and III (layers II and IV were not analysed). Progressively smaller changes were found in layers V and VI and no increase at all was found in the subjacent white matter. Glucose, in contrast to glycogen, increased to about the same degree in all cerebral layers and in subjacent white matter. The increase in glycogen after MSO administration may be related to the fact that MSO also causes an increase in the ratio of brain to serum glucose levels. This would indicate that an increase in intracellular glucose had occurred. Ammonia levels were increased 300–400 per cent in both cerebrum and cerebellum. A time study in cerebellum showed that the increase begins early and reaches maximal levels long before the onset of seizures. Glutamate levels were reduced by small but statistically significant amounts in both cerebrum and cerebellum. Administration of methionine sulphoximine completely prevented seizures and the increase in lactate, but did not prevent the increases in glycogen and glucose. The rise in ammonia was reduced but not prevented. During 20 sec of complete ischaemia (decapitation) ATP, P-creatine and glucose fell somewhat more rapidly than normal in brain of animals undergoing MSO seizures. From the changes it was calculated that the metabolic rate had been increased about 20 per cent by the seizure. A new sensitive and specific enzymic method for determination of tissue ammonia is presented together with evised enzymic procedures for lactate and glutamate.     

FACTORS AFFECTING THE TURNOVER OF CEREBRAL GLYCOGEN AND LIMIT DEXTRIN IN VIVO

The turnover of cerebral glycogen in mice has been investigated by using [U-¹⁴C]glucose as a precursor. The time required for turnover of total glycogen and limit dextrin has been determined in normal animals and animals given phenobarbital or hydrocortisone. In all 3 groups, the turnover time for limit dextrin was twice that of total glycogen. Phenobarbital increased the time for turnover of total glycogen and limit dextrin approximately 2-fold, whereas hydrocortisone diminished the turnover time of both fractions to one-half. The accumulation of glycogen during phenobarbital anesthesia (2·5-fold) is attributed to the decrease in rate of phosphorolysis rather than elevated glycogenesis. The ratio of phosphorylase a to total phosphorylase was significantly decreased in the brains of phenobarbital-treated mice, while the ratio of glycogen synthetase I to total synthetase activity was not affected. The administration of hydrocortisone had no effect on either the phosphorylase or synthetase of mouse brain. A mathematical model was devised to determine the rate constants for incorporation of labelled glucose into brain glycogen and the subsequent loss of radioactivity. Metabolite levels and enzyme activities have been correlated with the observed changes in glycogen turnover.     

THE EFFECT OF METHIONINE SULPHOXIMINE ON THE INCORPORATION OF LABELLED GLUCOSE, ACETATE, PHENYLALANINE AND PROLINE INTO GLUTAMATE AND RELATED AMINO ACIDS IN THE BRAINS OF MICE

—The incorporation of radioactivity from labelled glucose, acetate, phenylalanine and proline into glutamate, aspartate and glutamine was measured in mice treated with methionine sulphoximine and in the control animals. The labelled precursors were injected and their incorporation determined before the onset of convulsions. The incorporation of radioactivity from labelled glucose into the dicarboxylic amino acids was reduced, in particular the incorporation into glutamine. The incorporation of radioactivity from labelled acetate and phenylalanine into glutamate and aspartate was increased by methionine sulphoximine, while the incorporation into glutamine was not changed very much. The labelling of glutamine, relative to glutamate, was reduced with all precursors, indicating that glutamine synthetase was inhibited in vivo by methionine sulphoximine. It is very likely that methionine sulphoximine affects many aspects of energy metabolism in brain; in particular the metabolism of glucose seems to be inhibited, while the rate of conversion of substrates other than glucose seems to be increased.     

THE INCORPORATION OF GLUCOSE INTO BRAIN GLYCOGEN AND THE ACTIVITIES OF CEREBRAL GLYCOGEN PHOSPHORYLASE AND SYNTHETASE: SOME EFFECTS OF AMPHETAMINE

Abstract— The effects of amphetamine sulphate (5 mg/kg intraperitoneally) on the incorporation of radioactive carbon from [U-¹⁴C]glucose into the glycogen of mouse cerebral cortex, midbrain and hind-brain have been investigated. In all brain regions studied amphetamine induced a rapid decrease in glycogen followed by a slower return to control values. No significant alterations were observed in the steady state concentration of cerebral glucose. The initial fall in glycogen was associated with a fall in its specific radioactivity relative to that of cerebral glucose, whereas the resynthesis of the polysaccharide was associated with a marked increase in the relative specific radioactivity of glycogen. Other experiments demonstrated that amphetamine initially stimulates the breakdown of prelabelled glycogen and that the resulting molecule has fewer 1,4 linked glucose side chains.
Studies of the relative forms of the enzymes glycogen phosphorylase and glycogen synthetase suggested that rapid post mortem changes were less likely to occur if cerebral tissue was fixed by means of a freeze-blowing technique. Amphetamine administration resulted in a rapid though transient elevation of phosphorylase a activity in mouse forebrain. The level of glycogen synthetase I activity was unchanged initially but was markedly elevated during the period when there was a large increase in the rate of incorporation of glucose into glycogen. It is suggested that cerebral glycogen metabolism is controlled, at least in part, by the interconversion of the 'active' and 'inactive' forms of glycogen phosphorylase and synthetase.     

IN VIVO INHIBITION OF γ-GLUTAMYLCYSTEINE SYNTHETASE BY l-METHIONINE-RS-SULFOXIMINE; INFLUENCE ON INTERMEDIATES OF THE γ-GLUTAMYL CYCLE

—The inhibition of γ-glutamylcysteine synthetase and its influence on the concentration of intermediates associated with the metabolism of glutathione was studied in mice receiving methionine sulfoximine, a convulsant agent. The activity of the enzyme decreased significantly in the liver and kidney 1-4 h after administration of methionine sulfoximine; the activity of the enzyme in the brain was unchanged after 1 and 2 h but decreased significantly after 4 h. There was a rapid and sharp decrease in the concentration of glutathione in the kidney and a slower decrease in the liver. Brain glutathione concentrations were unaffected. Methionine sulfoximine in vivo, inhibited the synthesis of l -γ-glutamyl-l -α-aminobutyrate after administration of l -α-aminobutyrate, a reaction catalyzed by γ-glutamylcysteine synthetase. The inhibitor also lowered the concentration of pyrrolidone carboxylate in mouse tissues and prevented the accumulation of this intermediate after administration of l -α-aminobutyrate. The results show that methionine sulfoximine in vivo affects the metabolism of glutathione and that this action may contribute to its convulsive properties.     

THE INCORPORATION OF ISOTOPIC CARBON 14C INTO THE CEREBRAL GLYCOGEN OF NORMAL AND X-IRRADIATED RATS

Maximal incorporation of ¹⁴C from labelled glucose into cerebral glycogen of the rat occurred at 3-6 h following intravenous injection of the precursor. A reduction of the rate of glycogen breakdown is the most likely cause of the glycogen accumulation observed in rat brain following exposure to 10 krad of X-radiation.     

The effect of methionine on the regional and intracellular disposition of [3H]-methionine sulphoximine in rat brain

—The intracellular disposition of the convulsant agent, methionine sulphoximine (MSO), administered as methyl-labelled [³H]MSO, was examined in rat brain. Intraperitoneal (i.p.) and intrathecal (i.th.) routes were compared. The effect of simultaneous administration of methionine on the uptake, the regional distribution and the intracellular disposition of [³H]MSO was also assessed: (1) The peak uptake of i.p. [³H]MSO was at 2 h and amounted to about 1 per cent of the dose; the peak uptake of i.th. [³H]MSO was at 30 min post-injection and amounted to 40 per cent of the administered dose. The uptake was effectively reduced when methionine was simultaneously administered. (2) The regional distribution of [³H]MSO as a function of time after injection revealed a rather uniform penetration of the entire brain by the drug. A maximum of 43 per cent of the tissue radioactivity was found in the cerebellum 2 h after i.p. injection, while 49 per cent accumulated in the extracortical portion of the brain 3·5 h after i.th. administration. Methionine did not affect the regional distribution of [³H]MSO. (3) Differential centrifugation of samples of cortex and cerebellum revealed an association of [³H]MSO with intracellular particulate fractions. Since closely similar proportions of MSO occurred in the crude mitochondrial and the microsomal fractions, these fractions were analysed further: (a) [³H]MSO was bound to nerve endings sedimenting at the 1·0 m–1·2 m-sucrose interface; this binding was not abolished by prior increase of the endogenous cerebral methionine pool; and (b) [³H]MSO was released by subjecting the nerve endings to osmotic shock. However, the striking finding was that [³H]MSO could not be released from the nerve endings of the cerebellum from animals pre-treated with methionine. (4) An association of [³H]MSO was observed with the membranes of the endoplasmic reticulum and specifically with its agranular component. (5)The results implicate the cerebellum as the primary target for MSO, in confirmation of the original observations of Lodin (1958).     

CYCLIC NUCLEOTIDES IN MURINE BRAIN: EFFECT OF HYPOTHERMIA ON ADENOSINE 3'',5'' MONOPHOSPHATE, GLYCOGEN PHOSPHORYLASE, GLYCOGEN SYNTHASE AND METABOLITES FOLLOWING MAXIMAL ELECTROSHOCK OR DECAPITATION

Abstract —The accumulation of adenosine-3',5'-cyclic monophosphate (cyclic AMP) has been investigated in murine brain following electroconvulsive shock and decapitation. Animals were made hypothermic (20°C) to minimize the freezing time of the brain and to delay metabolic events. Cyclic AMP concentrations were decreased in the cerebral cortex of hypothermic rats and mice. Furthermore, the changes in cyclic AMP elicited by electroconvulsive shock and decapitation were delayed. In hypothermic animals, the metabolic rate as determined by high energy phosphate use was decreased to 65% of control values. The interconversions of the active and inactive forms of glycogen phosphorylase and glycogen synthase were sufficiently retarded in hypothermic animals to correlate with changes in cyclic AMP concentrations. The conversion of phosphorylase b to a and synthase a to b occurred when cyclic AMP concentrations had increased from 2 to 5 μmol/kg, following either electroconvulsive shock or decapitation. The results indicate that cyclic AMP plays a role in regulation of glycogen metabolism in cerebral cortex.     

Role of glucose and insulin in regulating glycogen synthase and phosphorylase activities in rainbow trout hepatocytes

This study, using ¹³C nuclear magnetic resonance spectroscopy showed enrichment of glycogen carbon (C1) from ¹³C-labelled (C1) glucose indicating a direct pathway for glycogen synthesis from glucose in rainbow trout (Oncorhynchus mykiss) hepatocytes. There was a direct relationship between hepatocyte glycogen content and total glycogen synthase, total glycogen phosphorylase and glycogen phosphorylase a activities, whereas the relationship was inverse between glycogen content and % glycogen synthase a and glycogen synthase a/glycogen phosphorylase a ratio. Incubation of hepatocytes with glucose (3 or 10 mmol·1^-1) did not modify either glycogen synthase or glycogen phosphorylase activities. Insulin (porcine, 10^-8 mol·1^-1) in the medium significantly decreased total glycogen phosphorylase and glycogen phosphorylase a activities, but had no significant effect on glycogen synthase activities when compared to the controls (absence of insulin). In the presence of 10 mmol·1^-1 glucose, insulin increased % glycogen synthase a and decreased % glycogen phosphorylase a activities in trout hepatocytes. Also, the effect of insulin on the activities of % glycogen synthase a and glycogen synthase a/glycogen phosphorylase a ratio were more pronounced at low than at high hepatocyte glycogen content. The results indicate that in trout hepatocytes both the glycogen synthetic and breakdown pathways are active concurrently in vitro and any subtle alterations in the phosphorylase to synthase ratio may determine the hepatic glycogen content. Insulin plays an important role in the regulation of glycogen metabolism in rainbow trout hepatocytes. The effect of insulin on hepatocyte glycogen content may be under the control of several factors, including plasma glucose concentration and hepatocyte glycogen content.     

Depletion of glycogen synthetase and increase of glucose 6-phosphate dehydrogenase in livers of ethionine-treated mice

1. Ethionine-treated mice showed a marked depletion in liver glycogen, a decrease of glycogen-synthetase activity, an increase in activity of glucose 6-phosphate dehydrogenase and the solubilization of phosphorylase. 2. The administration of cortisol or glucose did not alleviate these changes but the effect of ethionine was completely prevented in animals given methionine as well as ethionine. 3. The activities of the following enzymes were unchanged: hexokinase, glucokinase, glucose 6-phosphatase, phosphoglucomutase, 6-phosphogluconate dehydrogenase, UDP-glucose pyrophosphorylase, UDP-glucose dehydrogenase and pyruvate kinase.     

Disruption of the allosteric phosphorylase a regulation of the hepatic glycogen-targeted protein phosphatase 1 improves glucose tolerance in vivo

Type 2 diabetes is characterised by elevated blood glucose concentrations, which potentially could be normalised by stimulation of hepatic glycogen synthesis. Under glycogenolytic conditions, the interaction of hepatic glycogen-associated protein phosphatase-1 (PP1–G_L) with glycogen phosphorylase a is believed to inhibit the dephosphorylation and activation of glycogen synthase (GS) by the PP1–G_L complex, suppressing glycogen synthesis. Consequently, the interaction of G_L with phosphorylase a has emerged as an attractive anti-diabetic target, pharmacological disruption of which could provide a novel mechanism to lower blood glucose levels by increasing hepatic glycogen synthesis. Here we report for the first time the in vivo consequences of disrupting the G_L–phosphorylase a interaction, using a mouse model containing a Tyr284Phe substitution in the phosphorylase a-binding region of the G_L protein. The resulting G_L^Y284F/Y284F mice display hepatic PP1–G_L activity that is no longer sensitive to allosteric inhibition by phosphorylase a, resulting in increased GS activity under glycogenolytic conditions, demonstrating that regulation of G_L by phosphorylase a operates in vivo. G_L^Y284F/Y284F and G_L^Y284F/+ mice display improved glucose tolerance compared with G_L^+/+ littermates, without significant accumulation of hepatic glycogen. The data provide the first in vivo evidence in support of targeting the G_L–phosphorylase a interaction for treatment of hyperglycaemia. During prolonged fasting the G_L^Y284F/Y284F mice lose more body weight and display decreased blood glucose levels in comparison with their G_L^+/+ littermates. These results suggest that, during periods of food deprivation, the phosphorylase a regulation of G_L may prevent futile glucose–glycogen cycling, preserving energy and thus providing a selective biological advantage that may explain the observed conservation of the allosteric regulation of PP1–G_L by phosphorylase a in mammals.     

CEREBRAL ENERGY METABOLISM DURING TRANSIENT ISCHEMIA AND RECOVERY IN THE GERBIL1

Energy metabolism was studied in the cerebral cortex of gerbils during and following ischemia induced by 1 h of unilateral carotid artery occlusion. An aneurysm clip was applied to the right common carotid artery of 50-70 g gerbils under brief halothane anesthesia, and the clip was removed 1 h later. Clinical state (gait, responsiveness, seizures) was evaluated during carotid occlusion, and 40% of the animals showed clinical evidence of stroke. Cortical energy stores (2 ATP + ADP + P-creatine) were more than half depleted in the ipsilateral cortex of clinically-affected gerbils, and glucose fell by 75%; lactate rose over 7-fold in the same specimens. After release of the carotid clip, clinical state improved, and biochemical abnormalities partially resolved. However, even after 24 h, the concentration of ATP and the total pool of adenine nucleotides remained subnormal. Metabolic activity in the ischemic cortex, assessed as the utilization of high-energy phosphates following decapitation, was normal after 1 h of recovery and decreased (-50%) after 24 h but was increased by more than 50% after 4 h. Cerebral glucose utilization, evaluated from autoradiographs prepared after intravenous administration of 2-[1-¹⁴C]deoxyglucose, was also increased in the cortex, hippocampus, and thalamus after 4 h of recovery. This post-ischemic hypermetabolism in tissue damaged by ischemia may identify a critical period for cell repair, when therapy could be decisive.     

FLOW IN VIVO OF GLUCOSE CARBON TO BRAIN PROTEIN IN RATS: EFFECT OF STARVATION

—The conversion of plasma glucose into brain proteins in vivo was measured in rats after various periods of food deprivation. Rates of flow of glucose carbon into both soluble and insoluble brain proteins were calculated from the curve representing the decrease of plasma [¹⁴C]-glucose specific activity with time, and from the specific activity of brain protein 180 min after intravenous injection of a tracer dose of d -[¹⁴C]-glucose. Compared to the post-absorptive rats, food deprivation for 72 h caused a 30 per cent reduction in the rate of flow of glucose carbon into soluble brain proteins but did not affect the flow into insoluble proteins. Results of experiments in which the soluble brain proteins were separated by isoelectric focusing suggest that prolonged fasting in adult rats causes substantial differences in the conversion of glucose to different proteins.     

THE PRESENCE OF BIOLOGICALLY LABILE COMPOUNDS DURING ISCHEMIA AND THEIR RELATIONSHIP TO THE EEG IN RAT CEREBRAL CORTEX AND HYPOTHALAMUS

—Two surgical methods are described in the present paper, allowing for the approximate determination of in vivo levels of ATP, lactate, glucose, pyruvate and glycogen in anatomically uninjured cortex and hypothalamus from unanaesthetized rats. It was not possible to obtain such levels for P-creatine in the 2 mm thick samples used in the present investigation. No fundamental difference was observed between the cortical and the hypo-thalamic levels of these substrates nor in their fluxes. The substrate fluxes during ischemia were correlated with electrical activity in the rat cortex and hypothalamus, recorded by means of telemetrically transmitted electroencephalograms. The electrical activity declined precipitously at 9.6 s after decapitation in the cortex, and after 12.1 s in the hypothalamus. High levels of glycogen, glucose and ATP were present at this moment, while P-creatine had declined sharply.     

Carbohydrate metabolism in starved fifth instar larvae of Manduca sexta

The influence of starvation on carbohydrate metabolism in fifth instar larvae of Manduca sexta was studied. The percentage of active fat body glycogen phosphorylase increased from 10% to approximately 50% within 3 h of starvation; afterward the enzyme was slowly inactivated. The increase of phosphorylase activity might have been caused by a peptide(s) from the CC. The amount of fat body glycogen in starved animals decreased over 24 h by approximately 20 mg. The released glucose molecules seem to be converted mainly to trehalose because the hemolymph trehalose concentration in starved animals was always slightly higher than in the fed controls, and the glucose concentration decreased even when phosphorylase was activated. The chitosan content in starved larvae increased during the first 9 h of treatment to the same extent as in fed controls. It is suggested that fat body glycogen phosphorylase was activated during starvation to provide substrates for chitin synthesis and energy metabolism.     

RATES OF CEREBRAL GLUCOSE UTILIZATION IN RATS ANAESTHETIZED WITH PHENOBARBITONE

Abstract—

• 1 Intraperitoneal injection of phenobarbitone (250 mg/kg body wt.) into rats caused increased brain concentrations of glucose (100 per cent), glucose 6-phosphate (16 per cent) and ATP (12 per cent) and decreased concentrations of lactate (33 per cent) and ADP (15 per cent). A 31 per cent decrease in glutamate content was not statistically significant. No significant change occurred in the cerebral contents of glycogen or creatine phosphate.
• 1 The rates of increase in the brain of specific activities, in the first few minutes after systemic injection of [U-¹⁴C]glucose, of glucose, lactate, glutamate and glycogen were all halved by phenobarbitone. Calculated flux rates of ¹⁴C from glucose into metabolic intermediates and from lactate to glutamate were also decreased by 27–47 per cent; the effects on rate constants showed inconsistencies. The rate constants for conversion of glucose to lactate and to glutamate were decreased by 60–70 per cent, but that from lactate to glutamate was virtually unchanged. The rate constant for the flux from glucose to glycogen was reduced by 39 per cent, but the accumulation of glucose meant that the actual flux into glycogen increased by 20 per cent.
• 1 The results are interpreted in terms of an effect of the barbiturate not only on glucose transport, but also at an enzymic stage in glycolysis, possibly hexokinase or phosphofructokinase.

     

IN VIVO CHANGES OF CEREBRAL CYCLIC ADENOSINE 3'',5''-MONOPHOSPHATE INDUCED BY BIOGENIC AMINES: ASSOCIATION WITH PHOSPHORYLASE ACTIVATION

—The intravenous injection of adrenaline, isoprenaline and histamine to 4-6-day-old chicks resulted in a rapid increase in the cyclic AMP content of cerebral hemispheres that had been removed and frozen within 0·5 s using a freeze-blowing technique. Noradrenaline, dopamine, adenosine, 5-HT and acetylcholine did not significantly alter the nucleotide concentration in vivo. Addition of adrenaline, isoprenaline and histamine to incubated chick cerebral cortex slices also increased the cyclic AMP content of the tissue. Noradrenaline was considerably less potent than these amines and adenosine was ineffective. Low phosphorylase a levels (16 per cent of total activity) were observed in instantaneously frozen cerebral hemispheres of untreated chicks. The injection of adrenaline, isoprenaline and histamine resulted in a rapid conversion of phosphorylase b to a and a significant fall in tissue glycogen. Administration of noradrenaline was without effect on the relative forms of phosphorylase and also failed to influence cerebral glycogen. Phosphorylase activation was not observed in chick cerebral slices under conditions producing large increases in cyclic AMP. It is suggested that in vivo phosphorylase activation and subsequent glycogenolysis may occur, at least in part, in glia and that these cells may be damaged during preparation of cerebral slices. The results provide evidence of a metabolic role for cyclic AMP in cerebral tissue.     

The effect of streptozotocin-induced diabetes on glycogen metabolism in rat kidney and its relationship to the liver system.

The effects in kidney of streptozotocin-induced diabetes and of insulin supplementation to diabetic animals on glycogen-metabolizing enzymes were determined. Kidney glycogen levels were approximately 30-fold higher in diabetic animals than in control or insulintreated diabetic animals. The activities of glycogenolytic enzymes i.e., phosphorylase (both a and b), phosphorylase kinase, and protein kinase were not significantly altered in the diabetic animals. Glycogen synthase (I form) activity decreased in the diabetic animals whereas total glycogen synthase (I + D) activity significantly increased in these animals. The activities were restored to control values after insulin therapy. Diabetic animals also showed a 3-fold increase in glucose 6-phosphate levels. These data suggest that higher accumulation of glycogen in kidneys of diabetic animals is due to increased amounts of total glycogen synthase and its activator glucose 6-phosphate.     

AXONAL TRANSPORT OF [3H]GLUCOSE RADIOACTIVITY IN THE OPTIC SYSTEM OF SCARDINIUS ERYTHROPHTHALMUS1

Abstract— The optic system of Scardinius erythrophthalmus has been used to study the axonal translocation of radioactivity from [³H]glucose. Intraocularly injected precursors were transported intra-axonally along the optic nerve towards the contralateral optic tectum. In comparison with the well known properties of axonal protein transport there were remarkable differences in the proximo-distal translocation of [³H]glucose. These were: (1) a delay in the labelling of the structures investigated, after tracer application; (2) only a rapid phase of transport; and (3) no accumulation of radioactivity in the region of nerve terminals in the optic tectum connected with the injected eye. The transported material was almost exclusively in the form of TCA-soluble compounds and was mainly glucose itself or its low molecular derivatives, but not glycogen. The rate of transport was decreased by lowered temperatures and was not immediately dependent on retinal protein synthesis. Colchicine blocked the axonal transport of glucose by up to 60–70 per cent.