EFFECT OF INSULIN ON LEVELS AND TURNOVER OF INTERMEDIATES OF BRAIN CARBOHYDRATE METABOLISM IN VIVO

Abstract— –The rates of incorporation of ¹⁴C from [U-^l4C]glucose into intermediary metabolites have been measured in rat brain in vivo. The time course of labelling of glycogen was similar to that of glutamate and of glucose, which were all maximally labelled between 20 and 40min, but different from lactate, which lost radioactivity rapidly after 20min. The extent of labelling of glycogen (d.p.m./ μ mol of glucose) was of the same order as that of glutamate at 20 and 40 min after injection of [¹⁴C]glucose. However, calculations of turnover rates showed that glutamate turns over some 8-10 times faster than glycogen. Insulin, intracisternally applied, produced after 4-5 h a 60 per cent increase in glucose-6-P and a 50 per cent increase in glycogen. There was no change in the levels of glucose, glutamate or lactate, nor in the activity or properties of the particulate and soluble hexokinase of the brain. The injection of insulin affected neither the glycogen nor glucose contents of skeletal muscle from the same animals. The effects of insulin on the incorporation of ^l4C into the metabolites contrasted with its effects on their levels. The specific activities of glycogen and glucose were unchanged and there was a slight but non-significant increase in the specific activity of glutamate. The time course of incorporation into lactate was unaffected up to 20 min, but a significant delay in the loss of ¹⁴C after 20 min occurred as a result of the insulin injection. At 40 min, the specific activity of cerebral lactate was 60 per cent higher in insulin-treated animals than in control animals. The results are interpreted in terms of an effect of insulin on glucose uptake to the brain, with possibly an additional effect on a subsequent stage in metabolism, which involves lactate.     

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.     

EFFECTS OF GAMMA-HYDROXYBUTYRIC ACID AND OTHER HYPNOTICS ON GLUCOSE UPTAKE IN VIVO AND IN VITRO

Abstract— (1) The effects of gamma-hydroxybutyrate, imidazole-4-acetic acid and pento-barbitone on mouse brain glucose, glycogen and lactate levels have been studied. All the drugs significantly increased the brain glucose content, but did not significantly alter brain glycogen levels. The increase in brain glucose following imidazole-4-acetic acid or hypnotic doses of pentobarbitone was matched by corresponding decreases in the lactate level; this was not the case with gamma-hydroxybutyrate where the total glucose equivalents in the brain, expressed as the tissue level of (glucose) + (lactate/2), were significantly increased.
(2) All drugs except imidazole-4-acetic acid significantly decreased the rate of appearance of [¹⁴C]glucose into the bloodstream in vivo but had no effect on the uptake of glucose into rat diaphragm in vitro when present at 2·5 mM concentration.
(3) Only imidazole-4-acetic acid significantly inhibited glucose uptake into the brain in vivo but at 2·5 mM had no significant effect on glucose uptake into rat cerebral cortical slices in vitro.
(4) It is concluded that the very large increase in brain glucose level observed following the injection of hypnotic doses of gamma-hydroxybutyrate cannot be explained in terms of an increased net uptake of glucose into the brain.     

BRAIN ENERGETICS IN OXYGEN-INDUCED CONVULSIONS

Mice were exposed to 6 ATA of 100% oxygen. The effect of high oxygen pressure (OHP), disulphiram and both disulphiram and oxygen as a function of the length of oxygen exposure on cerebral cortical ATP, phosphocreatine, lactate, pyruvate and glucose was determined. Neither OHP nor disulphiram altered ATP prior to the onset of convulsions. The combination of OHP and disulphiram appeared to elevate cerebral ATP, particularly during the early exposure period. OHP had no effect on phosphocreatine, however, disulphiram, both alone and in combination with OHP increased cerebral cortical phosphocreatine. ATP and phosphocreatine were unchanged in mice sacrificed either at the onset or 9 s after the onset of oxygen convulsions. Lactate and pyruvate increased as the length of time the mice were exposed to OHP increased although neither lactate nor pyruvate levels differed significantly from control levels at either the onset or 9 s after the onset of convulsions. Disulphiram by itself lowered cerebral lactate, and prevented the increase in lactate and pyruvate in mice exposed to OHP. OHP and disulphiram increased cerebral glucose with the combination of both OHP and disulphiram appearing to have an additive effect. Glucose also remained elevated at the onset or 9 s after the onset of oxygen convulsions.     

Metabolic recovery in herring larvae following strenuous activity

Larvae of spring spawning Clyde herring Clupea harengus L. were reared at 5 and 12° C. Metabolism following burst swimming was studied in 7-day-old larvae at their respective rearing temperatures. Escape responses were repeatedly elicited using tactile stimulation for a period of 3 min. Larval herring were hard to fatigue and still responded to tactile stimuli after 3 min. Whole larvae were freeze-quenched in liquid nitrogen, either immediately after exercise, or after periods of recovery of up to 24 h. Samples were freeze-dried and analysed for whole body creatine (Cr), phosphocreatine (PCr), ATP, ADP, AMP, lactate, glucose, and glycogen using high performance liquid chromatography and enzymatic methods. The exercise regime resulted in a marked decrease in PCr, ATP and glycogen concentrations and an increase in creatine, glucose and lactate concentrations whereas there was no significant change in either AMP or ADP concentrations. The extent of phosphagen hydrolysis (approx. 110 to 15μmol PCr g ⁻¹ dry body mass) and lactate accumulation (approx. 7 to 40 μmol lactate g⁻¹ dry body mass) over the exercise period was similar at the two temperatures, consistent with a relatively constant degree of effort. The rates of recovery of PCr and ATP were essentially the same at 5 and 12° C; returning to resting levels after approximately 30 min. Lactate and glycogen concentrations were restored 60 min after exercise at both temperatures. Maximum lactate clearance rates (1.2 μmol min ⁻¹ g ⁻¹ wet muscle mass) were an order of magnitude faster than reported for adult fish in the literature.     

CEREBRAL METABOLIC CHANGES IN PROFOUND, INSULIN-INDUCED HYPOGLYCEMIA, AND IN THE RECOVERY PERIOD FOLLOWING GLUCOSE ADMINISTRATION

Severe hypoglycemia was induced by insulin in lightly anaesthetized (70°_o N₂O) and artificially ventilated rats. Brain tissue was frozen in situ after spontaneous EEG potentials had disappeared for 5. 10. 15 or 30 min and cerebral cortex concentrations of labile organic phosphates, glycolytic metabolites, ammonia and amino acids were determined. In other experiments, recovery was induced by glucose injection at the end of the period of EEG silence. All animals with an isoelectric EEG showed extensive deterioration of the cerebral energy state. and gross perturbation of amino acid concentrations. The latter included a 4-fold rise in aspartate concentration and reductions in glutamate and glutamine concentrations to 20 and 5^o_o of control levels respectively. There was an associated rise in ammonia concentration to about 3μmol-g^-1. Administration of glucose brought about extensive recovery of cerebral energy metabolism. For example, after an isoelectric period of 30 min tissue concentrations of phosphocreatine returned to or above normal, the accumulation of ADP and AMP was reversed, there was extensive resynthesis of glycogen and glutamine and full normalisation of tissue concentrations of pyruvate. α-ketoglutarate. GABA and ammonia. However, even after 3 h of recovery there was a reduction in the ATP concentration and thereby in adenine nucleotide pool, moderate elevations of lactate content and the lactate pyruvate ratio, and less than complete restoration of the amino acid pool. It is concluded that some cells may have been irreversibly damaged by the hypoglycemia.     

Cerebral carbohydrate metabolism during acute hypoxia and recovery

Abstract— The levels of ATP, ADP, AMP and phosphocreatine, of four amino acids, and of 11 intermediates of carbohydrate metabolism in mouse brain were determined after: (1) various degrees of hypoxia; (2) hypoxia combined with anaesthesia; and (3) recovery from severe hypoxia. Glycogen decreased and lactate rose markedly in hypoxia, but levels of ATP and phosphocreatine were normal or near normal even when convulsions and respiratory collapse appeared imminent. During 30 s of complete ischaemia (decapitation) the decline in cerebral ATP and phosphocreatine and the increase in AMP was less in mice previously rendered hypoxic than in control mice. From the changes we calculated that the metabolic rate had decreased by 15 per cent or more during 30 min of hypoxia. Hypoxia was also associated with decreases of cerebral 6-phosphogluconate and aspartate, and increases in alanine, γ-aminobutyrate, α-ketoglutarate, malate, pyruvate, and the lactate :pyruvate ratio. Following recovery in air (10 min), increases were observed in glucose (200 per cent), glucose-6-phosphate, phosphocreatine and citrate, and there was a fall in fructose-1, 6-diphosphale. Similar measurements were made in samples from cerebral cortex, cerebellum, midbrain and medulla. Severe hypoxia produced significant increases in lactate and decreases in glycogen in all areas; γ-aminobutyrate levels increased in cerebral cortex and brain stem, but not in cerebellum. No significant changes occurred in ATP and only in cerebral cortex was there a significant fall in phosphocreatine. Phosphocreatine, ATP and glycogen were determined by quantitative histochemical methods in four areas of medulla oblongata, including the physiological respiratory centre of the ventromedial portion. After hypoxia, ATP was unchanged throughout and the changes (decreases) in phosphocreatine and glycogen were principally confined to dorsal medulla, notably the lateral zone. Thus there is no evidence that respiratory failure is caused by a ‘power’ failure in the respiratory centre. It is suggested that in extremis a protective mechanism may cause neurons to cease firing before high-energy phosphate stores have been exhausted.     

POST-ISCHEMIC CHANGES IN CERTAIN METABOLITES FOLLOWING PROLONGED ISCHEMIA IN THE GERBIL CEREBRAL CORTEX

-Eight metabolites were measured in the post-ischemic period following either 1 or 3 h of unilateral ischemia in the gerbil cerebral cortex. The levels of ATP, P-creatine, glucose, glycogen and GABA were essentially restored by 1 h after ischemia. In the 3 h ischemic animals. glycogen continued to increase to greater than control values aftcr 5 and 20 h of recirculation. The Icvels of glutamate were unchanged during the ischemic episode, but decreased to 60% of control at Smin and 1 h after either period of ischemia. The concentrations of cyclic AMP, which were 4-to 5-fold elevated during ischemia. increased an additional 6-fold 5 min after recirculation in both groups. Arter 1 h of recovery. the levels were not different from control values. After the 1 h ischemic period, lactate levels recovered between 5 and 20 h of recirculation. In the 3 h ischemic animals. lactate concentrations were still elevated even after 20 h of recirculation. These data suggest that with the exception of lactate. recovery of metabolites is not sevcrely compromiscd by either 1 or 3 h of ischemia. Furthermore, the changes in glycogen. glutamate and cyclic AMP after recirculation suggest that the recovery process is not just a rcversal of the changes observed during ischemia.     

The effect of electroshock on regional CNS energy reserves in mice

ATP, phosphocreatine, glycogen, glucose and lactate levels were measured in the cerebral cortex, thalamus, cerebellum, brain stem and spinal cord of mice following supramaximal electroshock. During the initial 17 s after the onset of a 2 s electrical stimulus high energy phosphate expenditure exceeded formation in all regions but was slower in spinal cord than in the other regions. In cerebral cortex high energy phosphate utilization continued to exceed formation for 32 s which was twice as long as in any other region studied. Altered levels of metabolites recovered most rapidly in spinal cord and least rapidly in cerebral cortex. Pretreatment with a non-anaesthetic dose of phenobarbitone influenced the effect of electroshock. Most of the clinical seizure was prevented, and increased high energy phosphate utilization was sustained for a much shorter period. Only in cerebral cortex did high energy phosphate expenditure exceed formation for as long as 15 s after the electrical stimulus; but even in this region the excess of expenditure over formation was much less than in untreated animals.     

Relationships between energy reserves and function in rat superior cervical ganglion

—Major components of the energy reserves of the isolated superior cervical ganglion (ATP, phosphocreatine, glucose, glycogen and lactate) were measured under aerobic and anaerobic conditions. Complete anaerobiosis was maintained by incubation in mineral oil through which N₂ had been bubbled. From the initial rate of change in the energy reserves, a metabolic rate was calculated which would be equivalent to the consumption of 93 m-moles of O₂ per kg per hour. Under aerobic conditions (oxygenated moist chamber) a similar metabolic rate was calculated. In contrast to the anaerobic state, initial energy expenditure was almost exclusively at the expense of glucose. Continuous supramaximal stimulation in O₂ increased energy expenditure by a factor of three; both glucose and glycogen were utilized from the outset, and lactate accumulated in the initial periods. Ganglionic transmission failed in both resting and stimulated states in spite of the continued presence of very substantial levels of ATP and phosphocreatine. Failure seemed to be associated not with ATP depletion but rather with the complete disappearance of glucose and glycogen.     

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.

     

Levels of metabolites and production of glucose in the lamprey brain

—The levels of ATP, phosphocreatine, glucose, glucose-6-P., lactate, and glycogen have been measured in brains and spinal cords of lamprey larvae. Levels of glycogen in the brains were high, sometimes exceeding 100 m-moles of glycosyl units/kg wet weight. Isolated brains incubated in oxygenated saline exhibited the same levels of ATP and phosphocreatine as brains of intact animals. The level of glucose in tissue water of isolated brains approximated that found in the medium, except at low concentrations where the brains maintained levels of glucose many times that of the medium. It is demonstrated that under metabolic stress lamprey brains produce glucose by the breakdown of glycogen and the probable action of a glucose-6-phosphatase. The hypothesis is proposed that cells of the brain exchange glucose by this mechanism.     

Lactate and glycogen metabolism during and after exercise in the lizardSceloporus occidentalis

Summary Lactate removal and glycogen replenishment were studied in the lizardSceloporus occidentalis following exhaustion at 35°C. Whole body lactate concentrations and oxygen consumption were measured inSceloporus at rest, after 2 min vigorous exercise and at intervals during a 150 min recovery period. Lactate concentrations peaked at 2.2 mg/g (24 mM) after exercise and returned to resting levels after 90 min. Oxygen consumption returned to resting rates after 66 min. In a second set of experiments, glycogen and lactate concentrations of liver, hindlimb and trunk musculature were measured over the same time periods of exercise and recovery. The decrease in muscle glycogen following exercise was identical (mg/g) to the increase in muscle lactate, and the stoichiometric and temporal relationships between lactate removal and glycogen replenishment during the recovery period were also similar. Glycogen replenishment was rapid (within 150 min) and complete in fastedSceloporus. Dietary supplement of carbohydrate during 48 h of recovery led to supercompensation of glycogen stores in the muscle (+66%) and liver (+800%). The changes were similar to the seasonal differences measured inSceloporus from the field.     

Partial Restoration of Cerebral Myelination of the Congenitally Hypothyroid Mouse by Parenteral or Breast Milk Administration of Thyroxine

The objective of the present study was to assess metabolic changes in the neocortex and hippocampus of well-oxygenated or moderately hypoxic rats in which fluorothyl-induced seizures were sustained for 5 or 20 min, or which were allowed recovery periods of 5, 15, or 45 min following cessation of 20-min seizure activity by withdrawal of the convulsant gas. Sustained fluorothyl-induced seizures were found to cause metabolic alterations qualitatively and quantitatively similar to those previously observed with other commonly used convulsants. Thus, although the phosphorylation state of the adenine nucleotide pool remained only moderately perturbed, if at all, there were decreases in tissue concentrations of phosphocreatine and glycogen, and increases in those of cyclic AMP, lactate, and pyruvate, with a calculated fall in intracellular pH of about 0.15 units and a rise in the cytoplasmic NADH/NAD+ ratio. The enhanced metabolic rate was reflected in a marked reduction in the tissue-to-plasma glucose concentration ratio. Induced moderate hypoxia (arterial PO2 40-50 mm Hg) had no metabolic effect after 5 min of seizures but moderately increased lactate concentrations after 20 min (from about 10 to about 15 mumol X g-1). On cessation of seizure discharge cyclic AMP and phosphocreatine concentrations normalized already within 5 min, whereas glycogen and lactate concentrations normalized more slowly. In the neocortex (but not the hippocampus) postepileptic tissue-to-plasma glucose concentration ratios rose above control, probably reflecting metabolic depression. The results suggest that intracellular pH promptly returned to control, and that postepileptic alkalosis developed. They also suggest that some elevation of the NADH/NAD+ ratio persisted even after 45 min of recovery.     

Metabolic support of moderate activity differs from patterns seen after extreme behavior in the desert iguana Dipsosaurus dorsalis

This study examined glucose and lactate metabolism in an iguanid lizard, Dipsosaurus dorsalis, during rest and after activity patterned on field behavior (15 s of running at 1 m/s). Metabolite oxidation and incorporation into glycogen by the whole animal, the liver, and oxidative and glycolytic muscle fibers were measured using (14)C- and (13)C-labeled compounds. Results showed that lactate metabolism is more responsive to changes that occurred between rest and recovery, whereas glucose appears to play a more steady state role. After activity, lactate oxidation produced 57 times as much ATP during 1 h of recovery than did glucose oxidation. However, lactate oxidation rates were elevated for only 30 min after activity, while glucose oxidation remained elevated beyond 1 h. Lactate was the primary source for glycogen synthesis during recovery, and glucose was the main glycogenic substrate during rest. This study supports previous research showing that brief activity in D. dorsalis is primarily supported by glycolysis and phosphocreatine breakdown, but it also suggests that there may be less of a reliance on glycolysis and a greater reliance on phosphocreatine than previously shown. The findings presented here indicate that the metabolic consequences of the behaviorally relevant activity studied are less severe than has been suggested by studies using more extreme activity patterns.     

Functional, Metabolic, and Circulatory Changes Associated with Seizure Activity in the Postischemic Brain

Abstract: The present study was undertaken to explore how transient ischemia in rats alters cerebral metabolic capacity and how postischemic metabolism and blood flow are coupled during intense activation. After 6 h of recovery following transient forebrain ischemia 15 min in duration, bicuculline seizures were induced, and brains were frozen in situ after 0.5 or 5 min of seizure discharge. At these times, levels of labile tissue metabolites were measured, whereas the cerebral metabolic rate for oxygen (CMRO₂) and cerebral blood flow (CBF) were measured after 5 min of seizure activity. After 6 h of recovery, and before seizures, animals had a 40–50% reduction in CMRO₂, and CBF. However, because CMRO₂ rose threefold and CBF fivefold during seizures, CMRO₂ and CBF during seizures were similar in control and postischemic rats. Changes in labile metabolites due to the preceding ischemia encompassed an increased phosphocreatine/ creatine ratio, as well as raised glucose and glycogen concentrations. Seizures gave rise to minimal metabolic perturbation, essentially comprising reduced glucose and glycogen contents and raised lactate concentrations. It is concluded that although transient ischemia leads to metabolic depression and a fall in CBF, the metabolic capacity of the tissue is retained, and drug-induced seizures lead to a coupled rise in metabolic rate and blood flow.     

Dynamics of cerebral metabolism during moderate hypercapnia.

The effects of hypercapnia on the kinetics of cerebral energy metabolism were evaluated in adult rats by the closed system method of LOWRY et al. (1964). Moderate hypercapnia with a Pa_co2 of 61 torr sustained for 20 min resulted in intracellular brain acidosis (7.07-6.97). During hypercapnia the tissue content of glucose increased whereas phosphocreatine, ADP, pyruvate and lactate contents, and the lactate/pyruvate ratio decreased. The ATP/ADP ratio increased from 7.7 to 9.0; the cytoplasmic NADH/NAD + ratio decreased from 2.06 × 10^-3 to 1.49 × 10^-3. There was no change in Energy Charge. Turnover rate of phosphocreatine increased from 3.84 to 4.62 mmol/kg/min, but the turnover rates of ATP, glucose and glycogen were reduced (from 1.98 to 1.86, 6.24 to 4.80, and 3.96 to 2.94 mmol/kg/min, respectively). The utilization rate of total high energy phosphate decreased from 30.6 to 25.4 mmol/kg/min while the post-decapitation EEG during hypercapnia persisted longer than during normocapnia. These results indicate that moderate hypercapnia reduces the overall kinetic activity of cerebral energy metabolism. The steady Energy Charge suggests that the reduction in the rate of high energy phosphate use is proportionally balanced by a lowered production rate of ATP.     

Cerebral Metabolic State During the Ethanol Withdrawal Reaction in the Rat

Abstract: A severe ethanol withdrawal reaction was induced in rats by means of repeated intragastric intubation during a 4-day period. At the peak of the withdrawal reaction cerebral cortical tissue was frozen in situ for analysis of glycogen, glucose, phosphocreatine, creatine, ATP, ADP, AMP, lactate, pyruvate, GAB A, β-hydroxybutyrate, acetoacetate, cAMP and cGMP. Blood glucose concentration was also measured. The level of brain glycogen was decreased during ethanol withdrawal. Brain glucose concentration was increased, probably secondary to the increase in blood glucose concentration. The calculated NADH/NAD⁺ ratio was slightly increased during the withdrawal and brain ATP concentration and adenine nucleotide pool size were decreased. The adenylate energy charge remained unchanged. The overall changes in the metabolites were in agreement with the previously shown metabolic activation during ethanol withdrawal. The brain concentrations of ketone bodies (β-hydroxybutyrate and acetoacetate) during withdrawal did not deviate from controls, indicating that no abnormal ketone metabolism had developed as a consequence of the long-lasting ethanol intoxication. No changes were observed in the concentrations of GABA, cAMP, or cGMP in the rat cerebral cortex during ethanol withdrawal.     

CARBOHYDRATE AND ENERGY METABOLISM IN PERINATAL RAT BRAIN: RELATION TO SURVIVAL IN ANOXIA

The ability of rats of different ages to survive exposure to anoxia was correlated with rates of high energy phosphate consumption (metabolic rates) of the fore-brain. Fetal rats at term, delivered by hysterotomy following maternal decapitation, survived in nitrogen at 37°C twice as long as 1-day-old neo-nates, 5 times longer than 7-day-old rats, and 45 times longer than adults. During ischemia induced by decapitation, the cerebral concentrations of the labile energy reserves (ATP, ADP, P-creatine, glucose and glycogen) and of lactate were determined in fetuses, 1- and 7-day post-natal animals. From the changes, the cerebral energy use rates were calculated to be 1·57 mmol/kg/min in fetuses, 1·33 mmol/kg/min in 1-day-olds and 2·58 mmol/kg/min in 7-day-olds. Maximal rates of lactate accumulation during ischemia, as a measure of glycolytic capacity, were comparable in fetuses and neonates, but were about twice as great in 7-day-old rats. It is concluded that in post-natal animals survival in anoxia and cerebral energy consumption are inversely, and nearly quantitatively, related. However, the reduced cerebral energy requirement cannot entirely account for the greater anoxic resistance of fetuses.     

CEREBRAL METABOLIC CHANGES DURING PROLONGED EPILEPTIC SEIZURES IN RATS

Abstract— Sustained epileptic seizures were induced in paralysed, artificially ventilated and anaesthetized (70% N₂O) rats by means of intravenous bicuculline (1.2mgkg^?1), and cerebral cortical tissue was frozen in situ after periods varying between 10 s and 2 h for analyses of labile phosphates, glycolytic metabolites, citric acid cycle intermediates, and associated amino acids and ammonia, using enzymic fluorometric techniques. Body temperature was kept at 37°C, and arterial hypotension, arterial hypoxaemia and hypoglycaemia were prevented. Cortical glycogen concentrations fell progressively (to 23% of control levels) between 1 and 20 min after seizure onset but returned to control concentrations after 120 min of seizure activity. Cortical glucose concentration fell to 30% of control after 1 min of seizure activity, remained close to 50% of control for 1 h, and fell again to 30% after 2 h of seizure activity. Cortical lactate concentration was doubled in brains frozen 10 s after bicuculline injection. It rose over the following 20 min, reaching a steady concentration of about 10μmolg^?1 wet wt. The changes in lactate and glucose concentration indicated a 34-fold increase in the rate of glycolysis during the first minute of seizure. Phosphocreatine concentration was reduced by nearly 50% after 10 and 30 s of seizure activity, and subsequently stabilized at a concentration 2/3 of normal. ATP concentration was maximally reduced (by 7%) after 30 s and remained close to normal thereafter. Larger, initial reductions occurred in ATP/ADP and ATP/ AMP ratios, as well as in the adenylate energy charge. All these parameters remained significantly reduced for the rest of the 2 h seizure period. However, the changes were moderate since the energy charge was maintained within 2% of control. Changes in citric acid cycle intermediates included initial reductions in α-ketoglutarate and oxaloacetate (calculated) and progressive increases in fumarate, malate and citrate. After long periods of seizures all citric acid cycle intermediates except oxaloacetate were increased in concentration. Ammonia increased during the first min to reach steady state values of 200% of control. Alanine increased progressively during the first 20 min, to stabilize at 200% of control thereafter. GABA increased at 5 min and subsequently rose to almost twice the control value (120 min). At 20 min and onwards there were progressive decreases in glutamate and aspartate, and a progressive increase in glutamine. The sum of amino acids measured increased significantly and the sum of ammonia equivalents rose substantially. Intracellular pH calculated from the creatine kinase equilibrium decreased by 0.25 units during the first minute. However, since the pH calculated from P_co, and cellular buffer base changes remained close to normal during this period, it is concluded that the components of the creatine kinase reaction were not in equilibrium, and the pH values calculated from this equilibrium were incorrect. Tentative calculations of NADH/NAD⁺ ratios indicated that redox changes of opposite direction occurred in cytoplasm (reduction) and mitochondria (oxidation).