Brain metabolism during fasting

Metabolic profiles in patients with insulinoma

Twelve-hour metabolic profiles have been measured in six patients with insulinoma and results compared with normal subjects of similar age and weight. Fasting blood glucose was lower (mean +/- SEM 2.9 +/- 0.3 mmol/l vs 5.0 +/- 0.2 mmol/l) and plasma insulin higher (20.0 +/- 3.9 mU/l vs 7.2 +/- 1.6 mU/l) in insulinoma patients. Over the 12-h period blood glucose, pyruvate and glycerol were significantly lower, and plasma insulin, blood lactate, alanine and plasma non-esterified fatty acids (NEFA) significantly higher in insulinoma patients. Overall the concentration of blood total ketone bodies was significantly higher in insulinoma patients. Values were higher in the early part of the day but lower later in the day and did not show the marked pre-meal rise observed in the normal subjects. The raised NEFA and ketone bodies are of particular interest as they may be a source of fuel supply in the presence of relative glucose deficiency.

Changes of amino acid by the deprivation of energy sources in the cerebral cortex

After cerebral cortex slices from adult or infant rats had been incubated in medium containing glucose or beta-hydroxybutyrate as an energy source, the concentrations of ATP and amino acids in the tissue and incubation medium were examined. In the adult cerebral cortex, on the substitution of beta-hydroxybutyrate for glucose, the levels of ATP and amino acids were not maintained. The concentrations of glutamate, glutamine and gamma-aminobutyrate decreased, and that of aspartate increased together with the decrease in ATP in the tissue. Similar changes were observed when iodoacetate (10 microM) was added to the incubation medium. Moreover, the depletion of an energy source led to more drastic changes. On the other hand, in the infant cerebral cortex, the substitution of beta-hydroxybutyrate did not affect the levels of ATP and glutamate, glutamine and gamma-aminobutyrate. There are good correlations between the concentrations of ATP and glutamate and related amino acids in the cerebral cortex.

Protective effect of fasting upon cerebral hypoxic-ischemic injury

This study was designed to determine the effect of fasting upon cerebral hypoxic-ischemic injury. In the first part of the study the effect of fasting was determined for survival, brain tissue water and kation contents, and blood-brain barrier integrity. In the second part of the study the administration of the substrates beta-hydroxybutyrate (BHB) and glucose has been evaluated regarding their influence upon the effect of fasting. The study used the Levine-Klein model of unilateral carotid occlusion and hypoxia because it mimics clinical situations of ischemia with hypoxia. The data show that fasting did protect rats from developing brain infarction following hypoxia-ischemia. Hypoglycemia seems to be involved in the mitigation of ischemic blood-brain barrier disruption. The plasma glucose level seems to be not the only factor involved in the genesis of the tissue kation changes. Starvation-induced ketosis probably does not play a role in the protection mechanism.

Neural dysfunction during hypoglycaemia

There is controversy over the definition of hypoglycaemia in neonates and children and over its significance when 'asymptomatic'. We measured sensory evoked potentials in relation to blood glucose concentration in 17 children: 13 were fasted or given insulin to investigate endocrine or metabolic abnormalities and four had spontaneous episodes of hypoglycaemia. Abnormal evoked potentials were recorded in 10 of the 11 children whose blood glucose concentration fell below 2.6 mmol/l; five of these 10 children were 'asymptomatic'. No change in evoked potentials was recorded in the six children whose blood glucose concentration remained above 2.6 mmol/l. Our findings suggest that the blood glucose concentration should be maintained above 2.6 mmol/l to ensure normal neural function in children irrespective of the presence or absence of abnormal clinical signs.

The metabolic and nutritional effects of injury and sepsis

The existence of a co-ordinated response to stress of a variety of causes has clearly been established. Basically, this consists of an elevation in energy expenditure and an increased breakdown of skeletal muscle protein. In addition, glucose level in the plasma increases as a result of increased synthesis and decreased uptake of glucose into cells. Release of fatty acid into the plasma is also increased, and an elevation in the proportion of energy derived from oxidation of fatty acids is observed. This response is qualitatively very different from that seen in simple starvation, where a progressive reduction in energy expenditure and a reduction in the synthesis of glucose allows fat to become the major energy-producing substrate and also allows sparing of body protein stores. The mechanisms responsible for this altered pattern of metabolism are probably primarily hormonal in nature, with adrenaline, cortisol and glucagon being the major catabolic stimulants. Some evidence exists, however, for alteration in intracellular pathway metabolism. Within the past decade a new class of mediators of the stress response, the cytokines, has been recognized. These substances are protein products of circulating monocytes and the way in which they integrate into the control of the stress response has not been completely elucidated. At present there is evidence that they can stimulate production of catabolic hormones, and also they may well have direct effects in enhancing protein catabolism in muscle. At present the main method for modification of the stress response remains the provision of energy and amino acid, either intravenously or enterally. In the present state of our knowledge, 30-40 kcal kg-1 day-1 would appear to be adequate for most patients, with half provided as fat. Amino acids 3 g kg-1 day-1 will provide adequate nitrogen. It must be said, however, that the most effective method of modifying the stress response is removal of the source of stress by surgery, antibiotics or other primary therapy.

Hormone and metabolite plasma levels after oral glucose in bulimia and healthy controls

Bulimia patients claim to crave sweets and since as clinical evidence suggests that the food consumed during eating binges often contains large amounts of carbohydrates, hormones involved in carbohydrate metabolism might be affected in bulimia. We therefore performed a 4-hr glucose tolerance test (GTT), using 100 g oral glucose and inquired about attitudes toward sweets. Thirteen female patients, with a mean age of 23.3 years, who had had bulimia from 3 to 7 years but whose binge-eating/vomiting behavior was largely controlled at the time of testing, were compared to 14 age-matched healthy female controls with a mean age of 24.4 years. All bulimic patients and most controls had liked sweets as children and still liked sweets. Significantly more bulimic patients than controls stated they overate on sweets and avoided sweets. Glucose utilization and the insulin, glucagon, growth hormone (GH), and pancreatic polypeptide (PP) response curves in the bulimic patients were within the normal range. Fasting plasma levels of glucose, insulin, glucagon, GH, cortisol, free fatty acids (FFA), and PP were not different from controls. There was a trend in bulimic patients to have lower plasma FFA levels and higher plasma cortisol levels during the GTT than controls. The findings suggest that, given body weight maintenance and adequate nutrition, patients with bulimia nervosa have normal glucose tolerance and normal hormonal responses following an oral glucose load.

Eating attitudes and glucose tolerance in anorexia nervosa patients at 8-year followup compared to control subjects

This study analyzed eating attitudes and plasma glucose, insulin, unesterified fatty acid (FFA), human growth hormone (GH), and cortisol responses to an oral (100 g) glucose load in 26 female anorexia nervosa patients at an 8-year outcome evaluation in comparison to 14 age-matched female control subjects. Recovered patients who were of normal body weight and had cyclical menstruation (n = 19) showed glucose tolerance curves and insulin, cortisol, and GH responses that were indistinguishable from those of normal subjects, although patients tended to be more diet-conscious than controls and showed elevated fasting FFA levels. Two of 19 recovered patients met criteria for impaired glucose tolerance. Nonrecovered patients (n = 7) showed abnormal eating attitudes at an average underweight of 20% with persistent amenorrhea or oligomenorrhea. They had high fasting FFA plasma levels, significantly greater than normal rises in plasma glucose, a significant delay in serum insulin secretion, higher mean glucose levels before and after controlling for amount of exercise, and paradoxical release of GH. One of seven patients met criteria for diabetes mellitus and two of seven had impaired glucose tolerance. The findings suggest that fasting plasma FFA levels may reflect patients' eating and exercise habits more accurately than their verbal or written reports.

Enzymes of fatty acid beta-oxidation in developing brain

Developmental profiles were determined for the activities of eight enzymes involved in fatty acid beta-oxidation in rat brain. The enzymes studied were the palmitoyl-CoA, octanoyl-CoA, butyryl-CoA, glutaryl-CoA, and 3-hydroxyacyl-CoA dehydrogenases, the enoyl-CoA hydratase (crotonase), and the C4- and C10-thiolases. With the exception of the thiolases, all of the activities (expressed on the basis of brain weight) increased during the postnatal period of brain maturation. The activity of octanoyl-CoA dehydrogenase was elevated markedly compared to that of palmitoyl-CoA dehydrogenase at all developmental stages and in all brain regions in the rat. A similar relationship between these enzymes was observed in various regions of adult human brain. Comparisons of the activities of the beta-oxidation enzymes in human brain versus human skeletal muscle and in cultured neural cell lines (neuroblastoma and glioma) versus cultured skin fibroblasts revealed that the elevated activity of octanoyl-CoA dehydrogenase relative to palmitoyl-CoA dehydrogenase was specific to the neural tissues. This relationship was particularly evident when the enzyme activities were normalized to the activity of crotonase. The data support previous findings with radiochemical tracers, indicating that the brain is capable of utilizing fatty acids as substrates for oxidative energy metabolism. The relatively high activity of the medium-chain fatty acyl-CoA dehydrogenase in neural tissue may represent an adaptive mechanism to protect the brain from the known encephalopathic effects of octanoate and other medium-chain fatty acids that readily cross the blood-brain barrier.

Transport of 3-hydroxy[3-14C]butyrate by dissociated cells from rat brain

Recent studies suggest that the utilization of oxidizable substrates by the brain may be regulated in part by transport across the plasma membrane. Dissociated brain cells obtained by mechanical disruption of rat brain were used to measure the uptake of 3-hydroxy[3-14C]butyrate. Total uptake revealed two mechanisms (diffusion and a carrier-mediated system). A Lineweaver-Burk plot of the latter component yielded an apparent Km of 1.47 mM and a maximal velocity (Vmax) of 5 nmol.min-1.mg protein-1. The rates of uptake were temperature dependent and were significantly higher at pH 6.2 than at pH 7.4 or 8.2. Preloading the cells and increasing the intracellular concentration of 3-hydroxybutyrate using 12.5 and 25 mM increased the rate of uptake 143 and 206%, respectively, indicative of an accelerative exchange mechanism. Uptake was inhibited approximately 50% by (in mM) 10 phenylpyruvate, 10 alpha-ketoisocaproate, 10 KCN, and 1.5 NaAsO2. Uptake was also decreased by (in mM) 5 lactate, 5 methyl malonic acid, 1 alpha-cyano-4-hydroxycinnamate, and 1 mersalyl. Dissociated brain cells from 14- to 16-day-old rats accumulated 3-hydroxybutyrate at a rate more than two-fold greater than cells from either younger (2-day-old) or older (28-day-old and adult) animals. These data are consistent with the proposal that 3-hydroxybutyrate is taken up by the brain by both diffusion and a carrier-mediated transport system, and they support the hypothesis that transport at the cellular level contributes to the regulation of substrate utilization by the brain.

Effect of beta-hydroxybutyrate on whole-body leucine kinetics and fractional mixed skeletal muscle protein synthesis in humans

Because intravenous infusion of beta-hydroxybutyrate (beta-OHB) has been reported to decrease urinary nitrogen excretion, we investigated in vivo metabolism of leucine, an essential amino acid, using L-[1-13C]leucine as a tracer during beta-OHB infusion. Leucine flux during beta-OHB infusion did not differ from leucine flux during normal saline infusion in nine normal subjects, whereas leucine oxidation decreased 18-41% (mean = 30%) from 18.1 +/- 1.1 mumol.kg-1.h-1 (P less than 0.01), and incorporation of leucine into skeletal muscle protein increased 5-17% (mean = 10%) from 0.048 + 0.003%/h (P less than 0.02). Since blood pH during beta-OHB infusion was higher than the pH during saline infusion, we performed separate experiments to study the effect of increased blood pH on leucine kinetics by infusing sodium bicarbonate intravenously. Blood pH during sodium bicarbonate infusion was similar to that observed during the beta-OHB infusion, but bicarbonate infusion had no effect on leucine flux or leucine oxidation. We conclude that beta-OHB decreases leucine oxidation and promotes protein synthesis in human beings.

Ketone body utilization for lipid synthesis in the murine sciatic nerve: alterations in the dysmyelinating trembler mutant

This work demonstrates that in vitro sciatic nerves of normal and trembler adult mice can use ketone bodies (beta-hydroxybutyrate and acetoacetate) and butyrate for lipid synthesis. In normal sciatic nerves, beta-hydroxybutyrate is incorporated in total lipids to a larger extent than acetoacetate (141% and 33%, respectively, of acetate incorporation), whereas for trembler sciatic nerves, these percentages are only 69% and 27%. Incorporation of ketone bodies is greater into sterols than into other lipids. Lipid metabolism of ketone bodies in trembler nerves is altered and could reflect a process similar to Wallerian degeneration: a dramatic decrease of sterol and free fatty acid synthesis and an increased synthesis of triglycerides. Moreover, differences seen in precursor incorporation into lipids between normal and trembler sciatic nerves suggest that their lipid metabolism is not the same.

Brain energy metabolism in streptozotocin-diabetes

Regional brain glucose use was measured in rats with streptozotocin-induced diabetes (65 mg/kg intravenously) of 1 or 4 weeks duration, by using [6-14C]glucose and quantitative autoradiography. The concentrations of several metabolites were measured in plasma and brain. Results were compared with those from normal untreated rats. Glucose concentrations were increased in both plasma and brain, to similar degrees in both diabetic groups. Plasma ketone-body concentrations were 0.25, 1.0, and 3.15 mumol/ml in the control, 1-week and 4-week groups respectively (sum of acetoacetate and 3-hydroxybutyrate). Glucose use was increased throughout the brain (differences were statistically significant in 55 of 59 brain areas) after 1 week of diabetes, with an increase of 25% for the brain as a whole. In contrast, normal rates were found throughout the brain after 4 weeks of diabetes. None of the brain areas was affected significantly differently from the others, in either diabetic group. There was no significant loss of 14C as lactate or pyruvate during the experimental period, nor was there any indication of net production of lactate in any of the groups. Other methodological considerations that could have affected the results obtained in the diabetic rats were likewise ruled out. Because the ketone bodies are expected to supplement glucose as a metabolic fuel for the brain, our results indicate that brain energy consumption is increased during streptozotocin-diabetes.

Cloning of rat brain succinyl-CoA:3-oxoacid CoA-transferase cDNA. Regulation of the mRNA in different rat tissues and during brain development

3-Oxoacid CoA-transferase, which catalyses the first committed step in the oxidation of ketone bodies, is uniquely regulated in developing rat brain. Changes in 3-oxoacid CoA-transferase activity in rat brain during the postnatal period are due to changes in the relative rate of synthesis of the enzyme. To study the regulation of this enzyme, we identified, with a specific polyclonal rabbit anti-(rat 3-oxoacid CoA-transferase), two positive cDNA clones (approx. 800 bp) in a lambda gtll expression library, constructed from poly(A)+ RNA from brains of 12-day-old rats. One of these clones (lambda CoA3) was subcloned into M13mp18 and subjected to further characterization. Labelled single-stranded probes prepared by primer extension of the M13mp18 recombinant hybridized to a 3.6 kb mRNA. Rat brain mRNA enriched by polysome immunoadsorption for a single protein of size 60 kDa which corresponds to the precursor form of 3-oxoacid CoA-transferase was also found to be similarly enriched for the hybridizable 3.6 kb mRNA complementary to lambda CoA3. Affinity-selected antibody to the lambda CoA3 fusion protein inhibited 3-oxoacid CoA-transferase activity present in rat brain mitochondrial extracts. The 3.6 kb mRNA for 3-oxoacid CoA-transferase was present in relative abundance in rat kidney and heart, to a lesser extent in suckling brain and mammary gland and negligible in the liver. The specific mRNA was also found to be 3-fold more abundant in the brain from 12-day-old rats as compared with 18-day-old foetuses and adult rats, corresponding to the enzyme activity and relative rate of synthesis profile during development. These data suggest that 3-oxoacid CoA-transferase enzyme activity is regulated at a pretranslational level.

Ketone body utilization for energy production and lipid synthesis in isolated rat brain capillaries

Isolated brain capillaries from 2-month-old rats were incubated for 2 h in the presence of [3-14C]acetoacetate, D-3-hydroxy[3-14C]butyrate, [U-14C]glucose, [1-14C]acetate or [1-14C]butyrate. Labelled CO2 was collected as an index of oxidative metabolism and incorporation of label precursors into lipids was determined. The rate of CO2 production from glucose was slightly higher than from the other substrates. Interestingly, acetoacetate was oxidized at nearly the same rate as glucose. This shows that ketone bodies could be used as a source of energy by brain capillaries. Radiolabelled substrates were also used for the synthesis of lipids, which was suppressed by the addition of albumin. The incorporation of [U-14C]glucose in total lipids was 10-times higher than that from other precursors. However, glucose labelled almost exclusively the glycerol backbone of phospholipids, especially of phosphatidylcholine. Ketone bodies as well as glucose were incorporated mainly into phospholipids, whereas acetate and butyrate were mainly incorporated into neutral lipids. The contribution to fatty acid synthesis of various substrates was in the following order: butyrate greater than or equal to acetate greater than ketone bodies greater than or equal to glucose. All precursors except glucose were used for sterol synthesis. Glucose produced almost exclusively the glycerol backbone of phospholipids.

In vivo glucose metabolism in the awake rat: tracer and insulin clamp studies

The goals of this study were twofold: (1) to determine the in vivo dose-response relationship in the conscious, unstressed rat between the plasma insulin concentration and total body glucose uptake, and between plasma insulin and suppression of endogenous glucose production; and (2) to develop a physiologic compartmental model to describe the kinetics of plasma glucose in the rat in the basal state. In order to perform repeat insulin clamp studies in the same rat, chronic catheters were implanted in the aortic arch (via the carotid artery) and in the cardiac atrium (via the jugular vein), exteriorized, and fixed to the back of the skull with a dental cement cap. Insulin was infused at rates of 1.2, 2.4, 4.8, 12, and 24 mU/min.kg, and the plasma glucose was held constant at the basal level by a variable glucose infusion (euglycemic insulin clamp). The resulting steady-state plasma insulin concentrations ranged from 40 to 1,300 microU/mL. The dose-response curve for glucose uptake was sigmoidal in shape: in the basal state, total glucose utilization averaged 6.8 mg/min.kg at an insulin concentration of 9 microU/mL, half-maximal glucose uptake (18.3 mg/kg.min) occurred at a plasma insulin concentration between 70 and 80 microU/mL, and maximal uptake (36.6 mg/kg.min) was seen at an insulin level in excess of 100 microU/mL. Residual endogenous glucose production was evaluated by a prime-continuous infusion of (3-3H)-glucose. The dose-response curve for suppression of endogenous glucose output also was sigmoidal.(ABSTRACT TRUNCATED AT 250 WORDS)

Regional brain glucose use in unstressed rats after two days of starvation

Regional brain glucose use was measured in conscious, unrestrained, fed rats and after 2 days of starvation, using quantitative autoradiography and [6-14C]glucose. Plasma glucose, lactate, and ketone body concentrations and brain glucose and lactate content were measured in separate groups of rats. Glucose concentrations were lower in starved rats in both plasma and brain; plasma ketone body concentrations were elevated. Glucose use was found to be lower throughout the brain by about 12%. While some areas seemed to be affected more than others, statistical analysis showed that none were exceptionally different. The results could not be explained by increased loss of 14C as lactate or pyruvate during the experimental period, because the arteriovenous differences of these species were insignificant. The calculated contribution by ketone bodies to the total energy consumption was between 3 and 9% for the brain as a whole in the starved rats and could, therefore, partially account for the depression seen in glucose use. It was concluded that glucose oxidation is slightly depressed throughout the brain after 2 days of starvation.

Protective action of 1,3-butanediol in cerebral ischemia. A neurologic, histologic, and metabolic study

1,3-Butanediol (BD) is converted in the body to beta-hydroxybutyrate, and previous studies have shown that hyperketonemia had beneficial effects in experimental models of generalized hypoxia. The aim of this study was to determine if BD would reduce brain damage following cerebral ischemia. A transient forebrain ischemia of 30-min duration was induced by the four-vessel occlusion technique in control and BD-treated rats (25 mmol/kg, i.p.; 30 min prior to ischemia). BD treatment led to significant improvement of neurologic deficit during the 72-h recovery period and reduced neuronal damage in the striatum and cortex but not in the CA1 sector of the hippocampus. Evaluation of cerebral energy metabolism before and at the end of the ischemic period showed that the treatment did not change the preischemic glycolytic and energy metabolite levels but attenuated the ischemia-induced metabolic alterations. It increased energy charge, phosphocreatine, and glucose levels, and reduced lactate accumulation. The decrease in brain lactate concentration might account for the beneficial effects of BD by minimizing the neuropathological consequences of lactic acidosis.

Inhibition of muscular amino acid release by lipid infusion in man

Amino acid balances of healthy postabsorptive volunteers were investigated with the forearm technique under the influence of an intravenous infusion of triglyceride emulsions (LCT; MCT/LCT) [corrected]. A decrease of the basal muscular release of most of the amino acids, respectively an increase of pre-existent uptake rates [corrected]. In parallel, arterial concentrations of these amino acids declined. With constant insulin levels and substantially unchanged blood glucose levels, this inhibition of muscular proteolysis and/or stimulation of proteosynthesis is most probably due to the increased level of free fatty acids.

Reduction of weight loss and tumour size in a cachexia model by a high fat diet

An attempt has been made to reverse cachexia and to selectively deprive the tumour of metabolic substrates for energy production by feeding a ketogenic regime, since ketone bodies are considered important in maintaining homeostasis during starvation. As a model we have used a transplantable mouse adenocarcinoma of the colon (MAC 16) which produces extensive weight loss without a reduction in food intake. When mice bearing the MAC16 tumour were fed on diets in which up to 80% of the energy was supplied as medium chain triglycerides (MCT) with or without arginine 3-hydroxybutyrate host weight loss was reduced in proportion to the fat content of the diet, and there was also a reduction in the percentage contribution of the tumour to the final body weight. The increase in carcass weight in tumour-bearing mice fed high levels of MCT was attributable to an increase in both the fat and the non-fat carcass mass. Blood levels of free fatty acids (FFA) were significantly reduced by MCT addition. The levels of both acetoacetate and 3-hydroxybutyrate were elevated in mice fed the high fat diets, and tumour-bearing mice fed the normal diet did not show increased plasma levels of ketone bodies over the non-tumour-bearing group despite the loss of carcass lipids. Both blood glucose and plasma insulin levels were reduced in mice bearing the MAC16 tumour and this was not significantly altered by feeding the high fat diets. The elevation in ketone bodies may account for the retention of both the fat and the non-fat carcass mass. This is the first example of an attempt to reverse cachexia by a diet based on metabolic differences between tumour and host tissues, which aims to selectively feed the host at the expense of the tumour.

Regional cerebral glucose utilization during hyperglycemia

Previous data indicate that regional cerebral blood flow (rCBF) decreases during acute and chronic hyperglycemia. To test the hypothesis that the decrease in rCBF is secondary to a decrease in cerebral metabolic rate, the rate of regional cerebral glucose utilization (rCMRgl) was measured in awake-restrained rats during acute and chronic hyperglycemia. Acute hyperglycemia was produced by intraperitoneal injection of glucose, and chronic hyperglycemia was produced by treatment with streptozotocin. The rCMRgl was measured over a 10-min period using [6-14C]glucose. Glucose utilization was normal during acute hyperglycemia but decreased by 13% following 3 weeks of chronic hyperglycemia. The absence of a decrease in rCMRgl measured during acute hyperglycemia indicates that decreased rCBF cannot be explained by a change in the metabolic rate of the brain. The decrease in rCMRgl measured during chronic hyperglycemia does not necessarily indicate the presence of a drop in the metabolic rate of the brain because ketone bodies are available as an alternate fuel for oxidative metabolism. Therefore, it is unlikely that the decrease in rCMRgl measured during chronic hyperglycemia accounts for decreased rCBF.

Enzyme activities in regions of the hypothalamus

The activities of certain key enzymes have been measured in the ventral medial and ventral lateral areas of the hypothalamus, which are implicated in feeding behaviour, and compared with enzyme activities in the cortex and brainstem. The enzymes measured are concerned with glucose metabolism [hexokinase (EC 2.7.1.1) and glucose-6-phosphate dehydrogenase (EC 1.1.1.49)], ketone body metabolism [3-hydroxybutyrate dehydrogenase (EC 1.1.1.30)], fatty acid utilisation [carnitine palmitoyl transferase (EC 2.3.1.7)], citric acid cycle activity [pyruvate dehydrogenase (EC 1.2.4.2) and citrate synthase (EC 4.1.3.7)] and neurotransmitter synthesis [glutamate dehydrogenase (EC 1.4.1.3)].

Quantitative alterations in the metabolism of carbonyl compounds due to diet-induced lipid peroxidation in rats

Following the dietary induction of lipid peroxidation in rats (verified by the levels of malonaldehyde and glutathione peroxidase), the urinary carbonyl compounds were followed chromatographically. Through a headspace gas chromatographic procedure, increases of several aldehydes and furan derivatives were noticed. Liquid chromatography of the dinitrophenylhydrazone derivatives of urinary carbonyls provided a more definitive experiment, in which the increased-peroxidation animals could be compared to those maintained on a control diet. Several carbonyl metabolites, identified by mass spectrometry, were elevated following the lipid peroxidation induction.

Chronic hypoglycemia increases brain glucose transport

Glucose transport into the brain is depressed in chronically hyperglycemic (diabetic) rats. To determine whether hypoglycemia has the opposite effect, brain transport of hexoses and other substrates was examined in chronically and acutely hypoglycemic rats. We produced chronic hypoglycemia by implanting insulin-secreting tumors or insulin-releasing osmotic mini-pumps or by repeated injection of protamine zinc insulin (PZI) and acute hypoglycemia by intravascular injection of regular insulin. Blood-brain barrier (BBB) transport was measured using the brain uptake index (BUI) method. In the three models of chronic hypoglycemia, brain glucose extraction was increased compared with controls. The extraction of deoxyglucose and several other hexoses was also increased by chronic hypoglycemia. Acute hypoglycemia had no effect on brain transport. The transport of other substrates was either not affected or depressed, suggesting increased brain hexose transport is specific. Studies of freeze-blown brain in insulinoma-engrafted rats showed that brain glucose levels were depressed while creatine phosphate, ATP, and glucose 6-phosphate were maintained. Tumor removal led to a reversion of brain glucose transport to control rates but only after 5-25 days. These findings support the view that glucose transport across the BBB is modulated by chronic alterations in the ambient glucose concentration. They also may explain why some patients with chronic hypoglycemia tolerate low blood glucose concentrations.

Fat metabolism and cancer

Progressive weight loss and anorexia are frequent phenomena in cancer patients. Although cachexia is an expected occurrence in the terminal stages of nearly all malignancies, it may be a presenting sign when the tumor burden is quite small. Lipid depletion occurs out of proportion to the protein loss and accounts for most of the weight loss in cancer. Lipids, more specifically fatty acids, are the major source of fuel in mammals and may also be used in the synthesis of new cell products. Lipolysis and lipogenesis are under the influence of several important enzymes and peptide hormones that may be modulated by a variety of exogenous factors. There is evidence that cancer patients have lost the normal homeostatic responses to decreased energy intake or starvation that allow a decrease in oxygen consumption and protein sparing. An increase in Cori cycle activity or futile recycling of metabolic products occurs with a net energy expenditure rather than energy production. Clinical studies have shown that the body lipid depletion accompanying tumor progression is not solely secondary to decreased food intake and may be reproduced by the transplantation of certain noninvasive tumors to normal hosts. Elevated basal lipolysis has occasionally been seen early in tumor growth. Such findings suggest the presence of a tumor-associated factor responsible for this increase in lipid mobilization. Some of the potential mechanisms for the altered lipid metabolism seen in cancer have been discussed. Metabolic substrates may be remodeled and directed away from fuel-efficient into energy-requiring pathways. An increased energy expenditure may occur as a result of the energy costs of tumor synthesis, an uncoupling of oxidative phosphorylation, or energy-requiring futile cycling. An overall depletion of lipid may be the final outcome of the inhibition of lipid deposition. TNF/cachectin has recently been found to suppress the activity and synthesis of several key lipogenic enzymes, including lipoprotein lipase. Abnormalities in insulin secretion or sensitivity may be involved in the decrease of fat storage in malignancy. Insulin also exerts a significant antilipolytic effect by its antagonism of hormone-sensitive lipase. Mediators of lipolysis and abnormal lipid metabolism may occur in a number of clinical conditions and include ectopic hormone production, growth factors, and tumor-associated lipolytic factors (lipid mobilizing factor, toxohormone).

The role of glucose, insulin and glucagon in the regulation of food intake and body weight

Glucose and related pancreatic hormones play a major role in the metabolism of monogastric mammals yet their influence on hunger and/or satiety is, as yet, poorly understood. Glucose, insulin and glucagon rise during a meal and gradually decline to baseline levels shortly after a meal. A sudden drop in plasma glucose as well as insulin have been reported just prior to the onset of a meal but the functional significance of this is not yet clear. Systemic injections of glucose have no acute satiety effects but intraduodenal and intrahepatic infusions reduce food intake and free-feeding and deprived animals respectively. Treatments which decrease cellular glucose utilization directly (2-DG) or indirectly (insulin) increase food intake while exogenous glucagon (which produces hyperglycemia) decreases it. There is considerable evidence that some or all of these effects may be due to a direct central action of glucose, 2-DG, insulin, and glucagon on brain mechanisms concerned with the regulation of hunger and satiety although influences on peripheral "glucoreceptors" have been demonstrated as well. The functional significance of glucoprivic feeding is, however, questioned. The feeding response to 2-DG and related compounds is capricious, and its temporal course does not parallel the hyperglycemic reaction which presumably reflects cellular glucopenia. Moreover, numerous brain lesions which increase, decrease, or have no effect on ad lib intake and often have no effect on the response to deprivation have been shown to severely impair or abolish feeding responses to systemic injections of 2-DG that produce severe central as well as peripheral glucopenia. Feeding responses to insulin are intact after most of these lesions, suggesting that this hormone may influence food intake in a fundamentally different fashion. The mechanism of insulin action is not understood--the classic feeding response is obtained only with doses that are pharmacological when compared to normal plasma levels and there is increasing evidence that lower doses may have opposite, inhibitory effects on food intake and body weight. Relatively small doses of glucagon decrease food intake (although opposite facilitatory effects have been reported after even smaller doses) but the effect does not appear to be due to hepatic mobilization of glucose as initially assumed. Decreases in food intake after intracranial injections of very small doses suggest a direct central action.

Glucose carbon recycling and oxidation in human newborns

Total carbohydrate oxidation, plasma glucose oxidation, and glucose carbon recycling were measured in 11 fasting newborns using a constant infusion of D-[U-13C]glucose combined with respiratory calorimetry. The "true" rate of glucose appearance (Ra) was quantified from the enrichment of the nonrecycling tracer species (m + 6), while the "apparent" rate of glucose appearance was quantified from the enrichment of glucose C - 1. The plasma glucose concentration remained constant at approximately 50 mg/dl (2.8 mM) throughout the study. The true rate of glucose production was 5.02 +/- 0.41 mg X kg-1 X min-1, (means +/- SD). Glucose was oxidized at a rate of 2.67 +/- 0.34 mg X kg-1 X min-1 and represented 53% of the glucose turnover. Recycling of glucose carbon represented 36% of the glucose production rate, or 1.87 +/- 0.74 mg X kg-1 X min-1. The oxidation of plasma glucose provided 15.8 +/- 2.0 kcal X kg-1 X day-1, whereas total carbohydrate oxidation (measured by respiratory calorimetry) provided 19.9 +/- 6.6 kcal X kg X day. The data indicate that 1) recycling of glucose carbon accounts for about one-third of glucose production, demonstrating active gluconeogenesis in the fasting newborn; 2) the oxidation of plasma glucose represents only 80% of total carbohydrate oxidation, the remaining 20% possibly representing the local oxidation of tissue glycogen stores; and 3) as the measured rate of glucose oxidation will be insufficient to supply the entire calculated cerebral metabolic requirements, these data suggest that fuels in addition to glucose may be important for cerebral metabolism in the fasting human newborn.

Regional kinetic constants for blood-brain barrier pyruvic acid transport in conscious rats by the monocarboxylic acid carrier

The present investigation using labeled pyruvate describes the regional distribution and kinetics of the monocarboxylic acid carrier at the blood-brain barrier of conscious rats. The experimental procedure involved the arterial injection of a single bolus of 200 microliter containing [1-14C]pyruvate, [3H]water, and varying concentrations of unlabeled pyruvate into the common carotid via an indwelling externalized catheter. The hemisphere ipsi-lateral to the injection and rostral to the midbrain was removed and dissected into five regions. A kinetic analysis revealed no significant regional differences in Km values with an overall average of 1.37 mM. However, there was regional variation in the density of the monocarboxylic acid carrier as indicated by varied levels of the kinetic constant Vmax. The cortex showed the highest Vmax value of 0.42 +/- 0.08 mumol/min/g whereas values for the caudate/putamen, thalamus/hypothalamus, and remaining portion of hemisphere ranged significantly lower at 0.22-0.27 mumol/min/g. The Vmax for the hippocampus was intermediate at 0.37 +/- 0.12 mumol/min/g. The nonsaturable carrier described kinetically by KD had an overall average of 0.034 ml/min/g. The present study confirms quantitatively previous results suggesting a variable regional distribution of the monocarboxylic acid carrier.

Beta-hydroxybutyrate reverses insulin-induced hypoglycemic coma in suckling-weanling mice despite low blood and brain glucose levels

In normal suckling-weanling mice, DL-beta-hydroxybutyrate (30 mmol/kg ip) stimulated insulin secretion and reduced plasma glucose levels. In the brains of these animals, glucose levels were tripled due to a reduced rate of glucose utilization (determined by deoxyglucose phosphorylation). Other metabolite changes were compatible with inhibition of hexokinase, phosphofructokinase, glyceraldehyde-P-dehydrogenase, and pyruvate dehydrogenase activities. In contrast to the decrease in cerebral glycolysis, metabolite changes were compatible with an increase in the Krebs citric acid metabolic flux. The brain energy charge was also elevated. While it is generally believed that ketone bodies cannot sustain normal brain metabolism and function in the absence of glucose, DL-beta-hydroxybutyrate (20 or 30 mmol/kg ip) reversed insulin (100 U/kg sc)-induced hypoglycemia despite the persistence of a critically reduced plasma glucose concentration and near-zero brain glucose levels. Metabolic correlates of possible significance in the behavioral recovery from coma were reductions of the elevated levels of brain aspartate to below normal and ammonia levels to normal. Levels of acetyl CoA were unchanged both before and after treatment with beta-hydroxybutyrate.

Regional glucose and beta-hydroxybutyrate use by developing rat brain

Rates of glucose and D-beta-hydroxybutyrate use were determined in five brain regions of 20-day-old rats. The regions studied were cerebral cortex, thalamus, striatum, cerebellum, and brain stem. The tracers for determining rates of substrate use were [3H]fluorodeoxyglucose and [3-14C]-D-beta-hydroxybutyrate. Two or five minutes after isotope administration the animals were sacrificed in a 6-kW, 2450-MHz focused microwave device. Ten minutes prior to isotope administration the animals were injected intraperitoneally with normal saline or DL-beta-hydroxybutyrate (10 mmol/kg). Blood D-beta-hydroxybutyrate levels averaged 0.21 mumol/ml in saline-injected and 3.13 mumol/ml in hyperketonemic rats. Rates of glucose utilization were significantly heterogeneous between regions in both groups: thalamus greater than cerebral cortex greater than or equal to striatum greater than brain stem greater than cerebellum. These rates were 20-35% lower in hyperketonemic rats. Rates of D-beta-hydroxybutyrate use varied significantly between regions only in the saline group, with the brain stem rate being significantly lower than that in cortex or cerebellum. Regional rates of D-beta-hydroxybutyrate use did not correlate significantly with regional rates of glucose use in either the saline or the hyperketonemic groups. Regional rates of glucose use were strongly and positively correlated between conditions, as were regional rates of D-beta-hydroxybutyrate use. Thus, in 20-day-old rats, the regional heterogeneity of brain glucose use is similar to that in adult rats. D-beta-Hydroxybutyrate use is much less regionally heterogeneous.(ABSTRACT TRUNCATED AT 250 WORDS)

3-Hydroxy-3-methylglutaryl-coenzyme a lyase deficiency: a review

Children with 3-hydroxy-3-methylglutaryl-CoA lyase deficiency (HMG-CoA-LD; McKusick 24645), have inherited two areas of metabolic weakness. Firstly, they are unable to metabolize fully the carbon skeleton of leucine, and secondly, they cannot make ketone bodies in response to prolonged fasting. In the first year of life infants with HMG-CoA-LD run a high risk of developing severe hypoglycaemia which can lead to death if prompt intervention does not occur. The metabolic crisis develops when the infant is first introduced to dietary protein soon after birth, or later, when a reduced intake of glucose, often during a viral infection, results in a drain on the infant's circulating glucose levels. However, where diets are adequately adjusted to limit protein and fat intake, the metabolic handicaps of individuals with HMG-CoA-LD are not exposed and they are virtually symptomless. As children with HMG-CoA-LD grow older the incidence of hypoglycaemic attacks diminishes and they usually develop normally. This article reviews literature on cases of HMG-CoA-LD and interprets data on altered metabolism in these children.

Ketone body transport in the human neonate and infant

Using a continuous intravenous infusion of D-(-)-3-hydroxy[4,4,4-2H3]butyrate tracer, we measured total ketone body transport in 12 infants: six newborns, four 1-6-mo-olds, one diabetic, and one hyperinsulinemic infant. Ketone body inflow-outflow transport (flux) averaged 17.3 +/- 1.4 mumol kg-1 min-1 in the neonates, a value not different from that of 20.6 +/- 0.9 mumol kg-1 min-1 measured in the older infants. This rate was accelerated to 32.2 mumol kg-1 min-1 in the diabetic and slowed to 5.0 mumol kg-1 min-1 in the hyperinsulinemic child. As in the adult, ketone turnover was directly proportional to free fatty acid and ketone body concentrations, while ketone clearance declined as the circulatory content of ketone bodies increased. Compared with the adult, however, ketone body turnover rates of 12.8-21.9 mumol kg-1 min-1 in newborns fasted for less than 8 h, and rates of 17.9-26.0 mumol kg-1 min-1 in older infants fasted for less than 10 h, were in a range found in adults only after several days of total fasting. If the bulk of transported ketone body fuels are oxidized in the infant as they are in the adult, ketone bodies could account for as much as 25% of the neonate's basal energy requirements in the first several days of life. These studies demonstrate active ketogenesis and quantitatively important ketone body fuel transport in the human infant. Furthermore, the qualitatively similar relationships between the newborn and the adult relative to free fatty acid concentration and ketone inflow, and with regard to ketone concentration and clearance rate, suggest that intrahepatic and extrahepatic regulatory systems controlling ketone body metabolism are well established by early postnatal life in humans.

Rates of noninsulin-mediated glucose uptake are elevated in type II diabetic subjects

Although insulin is extremely potent in regulating glucose transport in insulin-sensitive tissues, all tissues are capable of taking up glucose by facilitated diffusion by means of a noninsulin-mediated glucose uptake (NIMGU) system. Several reports have estimated that in the postabsorptive state the majority of glucose disposal occurs via a NIMGU mechanism. However, these estimates have been either derived or extrapolated in normal humans. In the present study we have directly measured NIMGU rates in 11 normal (C) and 7 Type II noninsulin-dependent diabetic subjects (NIDDM; mean +/- SE fasting serum glucose, 249 +/- 24 mg/dl). To accomplish this, the serum glucose was clamped at a desired level during a period of insulin deficiency induced by a somatostatin infusion (SRIF, 550 micrograms/h). With a concomitant [3-3H]glucose infusion, we could isotopically quantitate glucose disposal rates (Rd) during basal (basal insulin present) and insulin-deficient (SRIF) conditions. With this approach we found that (a) basal Rd was greater in NIDDM than in C, 274 +/- 31 vs. 150 +/- 7 mg/min, due to elevated hepatic glucose output, (b) NIMGU composes 75 +/- 5% of basal Rd in C and 71 +/- 4% in NIDDM, (c) NIDDMS have absolute basal NIMGU rates that are twice that of C (195 +/- 23 vs. 113 +/- 8 mg/min, P less than 0.05), (d) when C were studied under conditions of insulin deficiency (SRIF infusion) and at a serum glucose level comparable to that of the NIDDM group (250 mg/dl), their rates of NIMGU were the same as that of the NIDDM group (186 +/- 19 vs. 195 +/- 23 mg/min; NS). We conclude that (a) in the postabsorptive state, NIMGU is the major pathway for glucose disposal for both C and NIDDM; (b) for a given glucose level the efficiency of NIMGU (NIMGU divided by serum glucose level) is equal in C and NIDDM, but since basal Rd is elevated in NIDDMs their absolute basal rates of NIMGU are higher; and (c) elevated basal rates of NIMGU in NIDDM may play a role in the pathogenesis of the late complications of diabetes.

Reduction of neurologic deficit by 1,3-butanediol induced ketosis in levine rats

The objective of this study was to determine if 1,3-butanediol would reduce a neurologic deficit in rats exposed to ischemic-hypoxia (Levine rats). Age and weight matched male Sprague-Dawley rats were anesthetized with 2% halothane. The right common carotid and external jugular vein were ligated and cannulated and EEG screws were implanted followed by a 2 hour recovery period. Thirty minutes prior to exposure the rats received either 1,3-butanediol (47 mmole/kg i.v.; n = 11) or an equal volume of saline (n = 10). The rats were then exposed to 4.5% O2 until mean arterial blood pressure fell to 70 mm Hg. The oxygen level was then increased to 8% for 30 minutes, after which the rats were returned to room air. Posture, hemiparesis, circling, shuffling, activity, and ability to hang on to a vertical screen were scored 1 (normal) to 5 (severe deficit) at 2 and 20 hours after insult. The time to 70 mm Hg was extended from 7.9 +/- 0.9 min for saline treated rats to 19.0 +/- 2.3 min for the 1,3-butanediol treated rats (p less than 0.001). All eleven 1,3-butanediol treated rats survived the hypoxic insult; 90% (9/10) saline treated rats died. In an attempt to reduce the insult, six additional saline treated rats were switched to 8% O2 at 75 mm Hg and still 4/6 died. The mean score at 20 hours for three surviving saline treated rats was 3.4. A significantly better (p less than 0.002) mean 20 hour score for the surviving 8/11 1,3-butanediol treated rats was 1.2. 1,3-butanediol increases survival and decreases the neurologic deficits associated with this ischemic-hypoxic insult.

Hepatic and renal metabolism before and after portasystemic shunts in patients with cirrhosis

Hepatic cirrhosis with portal hypertension and gastroesophageal hemorrhage is a disease complex that continues to be treated by surgical portasystemic shunts. Whether or not a reduction or diversion of portal blood flow to the liver adversely affects the ability of the liver to maintain fuel homeostasis via gluconeogenesis, glycogenolysis, and ketogenesis is unknown. 11 patients with biopsy-proven severe hepatic cirrhosis were studied before and after distal splenorenal or mesocaval shunts. Hepatic, portal, and renal blood flow rates and glucose, lactate, pyruvate, glycerol, amino acids, ketone bodies, free fatty acids, and triglyceride arteriovenous concentration differences were determined to calculate net precursor-product exchange rates across the liver, gut, and kidney. The study showed that hepatic contribution of glucose and ketone bodies and the caloric equivalents of these fuels delivered to the blood was not adversely affected by either a distal splenorenal or mesocaval shunt. In addition to these general observations, isolated findings emerged. Mesocaval shunts reversed portal venous blood and functionally converted this venous avenue into hepatic venous blood. The ability of the kidney to make a substantial net contribution of ketone bodies to the blood was also observed.

Glucose counterregulation, hypoglycemia, and intensive insulin therapy in diabetes mellitus

The prevention or correction of hypoglycemia is the result of both dissipation of insulin and activation of counterregulatory systems. In the models studied to date, glucagon and epinephrine have been shown to be the key counterregulatory factors; the potential roles of other hormones, neural factors, or substrate mechanisms in other models and during more gradual recovery from hypoglycemia remain to be defined. Deficient glucagon responses to decrements in plasma glucose, which are common in patients with IDDM and occur in some patients with NIDDM, result in altered counterregulation. But counterregulation is generally adequate, because epinephrine compensates for it. Defective glucose counterregulation due to combined deficiencies of glucagon and epinephrine secretory responses occurs in many patients, typically those with longstanding diabetes, and must be added to the list of factors known to increase the risk of hypoglycemia, at least during intensive therapy. From the material reviewed, it should be apparent that much has been learned about glucose counterregulation. It should be equally clear that much remains to be learned. Among the many possibilities, we consider four worthy of emphasis. First of all, we need to examine the physiology and pathophysiology of glucose counterregulation in additional models (e.g., during exercise) and over longer periods. Secondly, we need to determine whether central nervous system adaptation to antecedent glycemia occurs and, if so, identify its mechanisms. Thirdly, we need to develop better methods of insulin delivery or learn to correct or compensate for defective counterregulatory systems, if we are to achieve euglycemia safely in diabetic patients with defective glucose counterregulation. Finally, we need to know whether effective control of diabetes mellitus prevents development of defective glucose counterregulation.

Regulation of succinyl-CoA:3-oxoacid CoA-transferase in developing rat brain: responsiveness associated with prenatal but not postnatal hyperketonemia

Activities of ketone body-metabolizing enzymes in rat brain rise 3- to 5-fold during the suckling period, then fall more than 50% after weaning. Our purpose was to determine the mechanism of the developmental changes in activity of 3-oxoacid CoA-transferase in rat brain and to study its regulation by dietary modification. Purified rat brain 3-oxoacid CoA-transferase was used to generate specific antibody. Immunotitrations of the enzyme from brains of 4-, 24-, and 90-day-old rats indicated that changes in 3-oxoacid CoA-transferase activity during development are due to changes in content of the enzyme protein. Pulse-labeling studies showed that changes in enzyme specific activity reflected changes in its relative rate of synthesis, which increased 2.5-fold between the nineteenth day of gestation and the third postnatal day, remained at this high level until the twelfth postnatal day, and declined thereafter, returning by Day 38 to the level observed in utero. The enzyme is apparently degraded very slowly during early postnatal life. Fetal hyperketonemia induced by feeding pregnant rats a high-fat diet was associated with an increase in the relative rate of synthesis of 3-oxoacid CoA-transferase in brains of 19-day-old fetuses and newborn rats and with an increase in the specific activity of the enzyme at birth. To examine the role of postnatal hyperketonemia in the development of the enzyme in brains of suckling rats, neonates received intragastric cannulas and were fed, for up to 13 days, a modified milk formula low in fat. Postnatal hyperketonemia was abolished but cerebral 3-oxoacid CoA-transferase specific activity on Days 10 and 17 was not significantly affected. Thus, the physiological hyperketonemia caused by the high fat content of rat milk is not required for the normal development of 3-oxoacid CoA-transferase in rat brain.