Brain metabolism during fasting

Effects of insulin on peripheral and splanchnic glucose metabolism in noninsulin-dependent (type II) diabetes mellitus

The mechanism(s) and site(s) of the insulin resistance were examined in nine normal-weight noninsulin-dependent diabetic (NIDD) subjects. The euglycemic insulin clamp technique (insulin concentration approximately 100 microU/ml) was employed in combination with hepatic and femoral venous catheterization and measurement of endogenous glucose production using infusion of tritiated glucose. Total body glucose metabolism in the NIDD subjects (4.37 +/- 0.45 mg/kg per min) was 38% (P less than 0.01) lower than in controls (7.04 +/- 0.63 mg/kg per min). Quantitatively, the most important site of the insulin resistance was found to be in peripheral tissues. Leg glucose uptake in the diabetic group was reduced by 45% as compared with that in controls (6.0 +/- 0.2 vs. 11.0 +/- 0.1 mg/kg leg wt per min; P less than 0.01). A strong positive correlation was observed between leg and total body glucose uptake (r = 0.70, P less than 0.001). Assuming that muscle is the primary leg tissue responsible for glucose uptake, it could be estimated that 90 and 87% of the infused glucose was disposed of by peripheral tissues in the control and NIDD subjects, respectively. Net splanchnic glucose balance during insulin stimulation was slightly more positive in the control than in the diabetic subjects (0.31 +/- 0.10 vs. 0.05 +/- 0.19 mg/kg per min; P less than 0.07). The difference (0.26 mg/kg per min) in net splanchnic glucose balance in NIDD represented only 10% of the reduction (2.67 mg/kg per min) in total body glucose uptake in the NIDD group and thus contributed very little to the insulin resistance. The results emphasize the importance of the peripheral tissues in the disposal of infused glucose and indicate that muscle is the most important site of the insulin resistance in NIDD.

Effect of insulin on the distribution and disposition of glucose in man

Understanding the influence of insulin on glucose turnover is the key to interpreting a great number of metabolic situations. Little is known, however, about insulin's effect on the distribution and exchange of glucose in body pools. We developed a physiological compartmental model to describe the kinetics of plasma glucose in normal man in the basal state and under steady-state conditions of euglycemic hyperinsulinemia. A bolus of [3-3H]glucose was rapidly injected into a peripheral vein in six healthy volunteers, and the time-course of plasma radioactivity was monitored at very short time intervals for 150 min. A 1-mU/min kg insulin clamp was then started, thereby raising plasma insulin levels to a high physiological plateau (approximately 100 microU/ml). After 90 min of stable euglycemic hyperinsulinemia, a second bolus of [3-3H]glucose was given, and plasma radioactivity was again sampled frequently for 90 min more while the clamp was continued. Three exponential components were clearly identified in the plasma disappearance curves of tracer glucose of each subject studied, both before and after insulin. Based on stringent statistical criteria, the data in the basal state were fitted to a three-compartment model. The compartment of initial distribution was identical to the plasma pool (40 +/- 3 mg/kg); the other two compartments had similar size (91 +/- 12 and 96 +/- 9 mg/kg), but the former was in rapid exchange with plasma (at an average rate of 1.09 +/- 0.15 min-1), whereas the latter exchanged 10 times more slowly (0.12 +/- 0.01 min-1). The basal rate of glucose turnover averaged 2.15 +/- 0.12 mg/min kg, and the total distribution volume of glucose in the postabsorptive state was 26 +/- 1% of body weight. In view of current physiological information, it was assumed that the more rapidly exchanging pool represented the insulin-independent tissues of the body, while the slowly exchanging pool was assimilated to the insulin-dependent tissues. Insulin-independent glucose uptake was estimated (from published data) at 75% of basal glucose uptake, and was constrained not to change with euglycemic hyperinsulinemia. When the kinetic data obtained during insulin administration were fitted to this model, neither the size nor the exchange rates of the plasma or the rapid pool were appreciably changed. In contrast, the slow pool was markedly expanded (from 96 +/- 9 to 190 +/- 30 mg/kg, P less than 0.02) at the same time as total glucose disposal rose fourfold above basal (to 7.96 +/- 0.85 mg/min kg, P less than 0.001). Furthermore, a significant direct correlation was found to exist between the change in size of the slow pool and the insulin-stimulated rate of total glucose turnover (r=0.92, P<0.01). We conclude that hyperinsulinemia, independent of hyperglycemia, markedly increases the exchangeable mass of glucose in the body, presumably reflecting the accumulation of free, intracellular glucose in insulin-dependent tissues.

Two-day starvation does not alter the kinetics of blood--brain barrier transport and phosphorylation of glucose in rat brain

The blood-brain barrier (BBB) transport and brain phosphorylation of glucose were assessed in conscious rats subjected to 2 days of starvation. Although plasma glucose decreased, no significant changes in brain blood flow, BBB glucose transport, or 2-deoxy-D-glucose phosphorylation were observed. The data suggest that adaptive changes of brain glucose metabolism previously observed in starvation are located beyond the initial steps of brain entry and phosphorylation.

Carnitine acyltransferase activities in rat brain mitochondria. Bimodal distribution, kinetic constants, regulation by malonyl-CoA and developmental pattern

Carnitine palmitoyltransferase and carnitine octanoyltransferase activities in brain mitochondrial fractions were approx. 3-4-fold lower than activities in liver. Estimated Km values of CPT1 and CPT2 (the overt and latent forms respectively of carnitine palmitoyltransferase) for L-carnitine were 80 microM and 326 microM, respectively, and K0.5 values for palmitoyl-CoA were 18.5 microM and 12 microM respectively. CPT1 activity was strongly inhibited by malonyl-CoA, with I50 values (concn. giving 50% of maximum inhibition) of approx. 1.5 microM. In the absence of other ligands, [2-14C]malonyl-CoA bound to intact brain mitochondria in a manner consistent with the presence of two independent classes of binding sites. Estimated values for KD(1), KD(2), N1 and N2 were 18 nM, 27 microM, 1.3 pmol/mg of protein and 168 pmol/mg of protein respectively. Neither CPT1 activity, nor its sensitivity towards malonyl-CoA, was affected by 72 h starvation. Rates of oxidation of palmitoyl-CoA (in the presence of L-carnitine) or of palmitoylcarnitine by non-synaptic mitochondria were extremely low, indicating that neither CPT1 nor CPT2 was likely to be rate-limiting for beta-oxidation in brain. CPT1 activity relative to mitochondrial protein increased slightly from birth to weaning (20 days) and thereafter decreased by approx. 50%.

Activities of the enzymes of ketone body metabolism in the developing chick

The concentration of ketone bodies in the blood of the developing chick prior to and just after hatching were higher than those found in the adult. The activities of 3-oxo acid-CoA transferase and acetoacetyl-CoA thiolase in the heart, leg and pectoral muscle before and after hatching were higher than those of the adult. The activity of 3-hydroxybutyrate dehydrogenase increased constantly during incubation and after hatching in all three muscle tissues. In the liver the activities of the enzymes of ketone body synthesis increased during incubation and after hatching. It is suggested that the liver could provide fuel to the extrahepatic tissues of the developing chick and ketone bodies could contribute as fuel for oxidation in the skeletal muscle of the newly hatched bird.

Alzheimer's disease and malnutrition: a new etiological hypothesis

The author proposes an etiological hypothesis for Alzheimer's disease (malnutrition with or without malabsorption); this may explain clinical and biological phenomena which occur in this disease. A search through the literature revealed that all people suffering from Down's syndrome (and who all develop Alzheimer's disease from forty years of age) also suffer from malnutrition. This hypothesis may explain the anatomical and neurobiochemical perturbations and the inefficacy of choline. Consequent nutritional treatment of the disease is proposed.

Metabolic effects of carbohydrate in low-calorie diets

Fasting plasma glucose turnover, urinary 3-methylhistidine excretion, and fasting plasma protein profiles were compared in a 4-week randomized clinical trial of two very low-calorie weight-reduction diets. Diet A (360 kcal) provided 1.5 g egg protein per kg ideal body weight (IBW) but no carbohydrate. Diet B (340 kcal) provided 0.8 g egg protein per kg IBW plus 0.7 g carbohydrate per kg IBW. Eleven moderately obese healthy young women were studied. After 3 weeks of dieting, fasting plasma glucose appearance and oxidation decreased by equal amounts (20% and 30%, respectively) for both diets. 3-methylhistidine excretion remained at control rates for the first week on the diets, then fell by equal amounts (25% to 30%) with both diets. Similar declines were observed for both diets in serum prealbumin and retinol-binding protein concentrations. Mean serum transferrin declined with both diets, but the changes were not statistically significant. Serum albumin was unchanged by either diet. Thus, there were no significant differences between the two diets with regard to any of the measured parameters.

Effect of aging on cerebral cortex energy metabolism in hypoglycemia and posthypoglycemic recovery

Severe hypoglycemia, causing the cessation of spontaneous EEG, induced in cerebral cortex of rats of different ages, causes gross energy failure and extensive derangement of both carbohydrate and amino acid contents. During posthypoglycemic recovery of adult rats, there was moderate restitution of energy metabolism and both ATP concentration and adenine nucleotide pool remained still reduced, even if the creatine phosphate and ADP contents were close to normal. During recovery of adult rats there was a rise in glutamate and glutamine concentrations and the perturbated aspartate and gamma-aminobutyrate cerebral contents normalized. Ammonia content decreased to normal, while alanine content was markedly elevated. Aging does not affect the cerebral metabolic derangements occurring in severe hypoglycemia, but rather the metabolic changes that the brain tend to reverse during the posthypoglycemic restitution. In fact, there was lower restitution of the contents of cerebral cortical metabolites of "mature" and "senescent" rats in comparison with "adult" ones. Particularly, in older brains the contents of many amino acids and adenylate nucleotides remained largely abnormal.

Ketone body synthesis from leucine by adipose tissue from different sites in the rat

Leucine is catabolized to ketone bodies in adipose tissue, but the contribution of this output to overall ketone metabolism is not known. The intent of the present study was to determine the capacity of different adipose tissues to synthesize ketone bodies from leucine. The amino acid was readily converted into acetoacetate in epididymal, perirenal, and omental fat tissues. In rats fed ad libitum, the rate of acetoacetate synthesis in omental fat (about 2 mumol g tissue-1h-1) was at least 8 times higher than in epididymal or perirenal fat. In omental fat, the rates of acetoacetate formation from alpha-ketoisocaproic acid were 47-55% lower than from leucine at all concentrations examined. There was no significant synthesis of beta-hydroxybutyrate from leucine or alpha-ketoisocaproic acid. After oxidative decarboxylation, a greater proportion (about three-fourths) of leucine in omental fat was metabolized to acetoacetate than to CO2 production through the Krebs cycle. Although addition of glucose, pyruvate, or carnitine did not affect the production of acetoacetate, fasting for 24 h stimulated acetoacetate synthesis from leucine and alpha-ketoisocaproic acid in omental fat. The high rate of leucine conversion to acetoacetate in omental fat was related to high activities of leucine aminotransferase and branched-chain alpha-keto acid dehydrogenase. Moreover, protein content and cytochrome c oxidase activity of omental mitochondria were, respectively, 13 and 12 times higher than in epididymal mitochondria. In contrast, fat content of epididymal adipose tissue was 21 times that of omental adipose tissue. Epididymal depot consisted of 2.0% protein and 75.8% fat, whereas omental depot contains 17.2% protein and 3.6% fat, resembling that of liver and muscle. The results suggest that the high ketogenic capacity of omental fat stems in part from an augmented mitochondrial mass and high activity of branched-chain alpha-keto acid dehydrogenase.

The metabolism of D- and L-3-hydroxybutyrate in developing rat brain

The incorporation of L- and D-3-hydroxybutyrate into rat brain protein, lipid, and amino acids during development was studied. L-3-Hydroxybutyrate was found to label brain protein and amino acids in addition to sterols and fatty acids throughout the first 32 postnatal days. Age related changes in L- and D-3-hydroxybutyrate labeling of protein and amino acids were similar. Whereas L-3-hydroxybutyrate incorporation into brain lipids rose sharply between 6-15 days of age, D-3-HOB incorporation into the lipid fraction gradually increased from birth through the age of 15 days. Incorporation by both isomers into lipid was greatest during the third week of suckling and then declined when the animals were weaned. At 15 days of age, the distribution of L-3-hydroxybutyrate into glutamate, glutamine + aspartate, and gamma-aminobutyrate was similar to that obtained with D-3-hydroxybutyrate. L-3-Hydroxybutyrate was poorly oxidized to CO2 by brain slices and mitochondria. Oxidation capacity was maximal from 15-21 days of age for both isomers. The activity of L-3-hydroxybutyrl-CoA ligase increased between 6-28 days of age, and its increase is well correlated with the developmental pattern of L-3-hydroxybutyrate incorporation and mitochondrial oxidation. L-3-Hydroxybutyrate was not detected in the blood of palmitate-injected pups or fasted adult animals. These results suggest that although L-3-hydroxybutyrate can be utilized for the synthesis of brain components during development, its negligible blood concentration precludes a significant contribution to either tissue synthesis or energy balance during the suckling period.

Butanediol induced cerebral protection from ischemic-hypoxia in the instrumented Levine rat

To determine if 1,3-Butanediol (BD), which protects mice from hypoxia, would extend the tolerance of rats to ischemic-hypoxia, the Levine rat (unilateral carotid ligation and conscious hypoxic exposure) was modified to record mean arterial pressure (BP), heart rate (HR), central venous pressure (CVP), spontaneous respiration and EEG. Age and weight matched, male, Sprague-Dawley rats were anesthetized under halothane (1-2%), ligated, instrumented, and recovered 2 hrs before hypoxia (4.5% oxygen). Thirty minutes prior to hypoxia, groups of rats received, BD (47 mmoles/kg i.v.; n = 7), equal volumes of saline (S) (n = 6) or no-infusion (NI) (n = 7). Since no significant difference was observed between S and NI they were combined into a single control group (C). In a parallel group administered BD, resultant beta- hydroxybutarate ( BHB ) levels increased from 0.13 +/- 0.02 to 0.84 +/- 0.03 mM and temperature declined only 1.5 degrees C. The EEG of all ischemic-hypoxic rats invariably became isoelectric before cessation of spontaneous respiration and eventual loss of BP. BD significantly (p less than 0.01, Student's t) increased ischemic-hypoxic tolerance (time to isoelectric EEG) from 875 +/- 56 for the control group to 1338 +/- 67 seconds for the BD group, without changing the interval from isoelectric EEG to loss of BP. Further, EEG activity persisted at a lower mean BP (p less than 0.01) in the BD group (44 +/- 5 mm Hg) than in the control group (66 +/- 4 mm Hg). In summary, isoelectric EEG invariably precedes ventilatory failure and cardiovascular collapse. BD increases ischemic-hypoxic tolerance in the conscious rat by extending, at a lower mean BP, the time to isoelectric EEG.

Effects of lactate on glucose metabolism of developing rat brain

The effects of hyperlactatemia on cerebral glucose metabolism of normoglycemic 20-day-old rats were studied in animals breathing air or 20% CO2:21% O2:59% N2. Sodium lactate or sodium bicarbonate were given intraperitoneally, together with a mixture of [3H]deoxyglucose and [2-14C]glucose. Animals were sacrificed in a freeze-blowing apparatus at intervals of 2-15 min after injection. Blood lactate levels in the lactate-injected rats were 4-6 mM. Hyperlactatemia caused a gradual decline in the brain rate of glucose utilization in air-breathing animals to 50-70% of control rates. Results with both tracers were similar. Concentrations of Krebs cycle intermediates and glutamate did not decrease. These findings indicate that lactate can partially replace glucose as an oxidative fuel for developing rat brain. Hypercapnia depressed the rate of glucose utilization by developing brain and rates were 30-40% lower still in lactate-injected hypercapnic rats. Decreases in levels of Krebs cycle intermediates and glutamate were similar in both groups. Thus, lactate and CO2 are additive in their depressant effects on brain glucose utilization. The observation that lactate did not prevent the decreases in Krebs cycle intermediates and glutamate caused by hypercapnic acidosis suggests an inhibition of flux through pyruvate dehydrogenase during hypercapnia. The data from this study, coupled with data on lactate transport across the blood-brain barrier, indicate that the direction of movement of lactate and its rate of utilization by developing brain are functions of its concentration on blood relative to brain. Physiological and pathological conditions which elevate blood lactate levels above those in brain will, then, have a sparing effect upon brain glucose utilization.

Glucagon stimulates ketone utilization by rat brain slices

Glucagon has been shown previously to increase further the enhanced tolerance for hypoxia observed in mice with elevated blood ketones. Glucagon is also known to increase blood glucose and to alter directly the metabolism of some (liver) cells. Both the increase in blood glucose and altered cellular metabolism could contribute to the increase in tolerance for hypoxia observed in mice given glucagon in combination with the ketone, beta-hydroxybutyrate. To evaluate the systemic component of this hypothesis, blood glucose, beta-hydroxybutyrate, and glucagon were elevated alone or simultaneously and hypoxic tolerance of mice was measured. To identify possible cellular effects of glucagon on glucose or ketone metabolism, we measured the incorporation of radiolabeled glucose or beta-hydroxybutyrate into CO2 or total lipid in isolated rat brain slices. Both glucagon and glucose increased hypoxic tolerance of ketotic mice but our data do not support the hypothesis that glucagon's action was only through an elevation of blood glucose. In brain slices glucagon stimulated the incorporation of beta-hydroxybutyrate into CO2 both in the presence or absence of additional glucose. These results demonstrate that glucagon has a direct effect on brain metabolism which may contribute to the increased tolerance for hypoxia. They, however, do not exclude the possibility that glucagon is working in addition to increase hypoxic survival in ketotic mice by increasing the availability of glucose to the brain.

Insulin and trauma: Some thoughts

Trauma and/or sepsis produce a net negative nitrogen balance due mainly to muscle protein breakdown. Lymphokines (interleukins) appear to be the hormonal signal which negates insulin's anabolic effect on muscle resulting in net catabolism. In contrast, their effects on immune and reparative cells appear additive to insulin's anabolic effect augmenting tissue repair and increasing immunologic defense, but at the expense of muscle nitrogen. This logical evolutionary adaptation did not anticipate that parenteral alimentation could provide both the needed glucose for tissue repair, thus obviating muscle protein catabolism to provide precursor amino acids for hepatic gluconeogenesis and the amino acids for tissue reconstruction.

Hypoxia induced preferential ketone utilization by rat brain slices

When exposed to hypoxia, intact mice, with elevated blood ketones, live longer than mice with normal blood ketones. To evaluate a possible mechanism responsible for this phenomenon a rat brain slice preparation was used to determine if brain tissue would utilize glucose or ketones preferentially during exposure to reduced oxygen. Reducing available oxygen in the incubation medium from 95%, in steps, to 5% produced the expected gradual reduction in the carbon dioxide formation from glucose. In contrast, reducing the oxygen level to 40 and 20% resulted in a statistically significant stimulation of the production of carbon dioxide from the ketone beta-hydroxybutyrate. At very low oxygen levels carbon dioxide production from either substrate was reduced. These results are consistent with the hypothesis that ketones can be used in addition to glucose as a substrate for brain energy production even during reduced oxygen availability. If the increase in carbon dioxide production from ketones can be equated with an increase in energy production from this supplemental substrate then ketones may be therapeutically useful in avoiding the collapse of brain function during moderate hypoxia.

The regulation of substrate utilization

The factors which regulate the utilization of blood-borne substrates by mammalian tissues are discussed. These include the availability of the substrates, the blood flow to tissues, the permeability of cells to the substrate, the activities of initiating enzymes and the regulation of the metabolic pathway. These various factors are illustrated by specific examples. In addition, the methodology involved in measurement of substrate utilization by tissues is briefly outlined.

Regional enzyme development in rat brain. Enzymes associated with glucose utilization

The development of key enzyme activities concerned with glucose metabolism was studied in six regions of the rat brain in animals from just before birth (-2 days) through the neonatal and suckling period until adulthood (60 days old). The brain regions studied were the cerebellum, medulla oblongata and pons, hypothalamus, striatum, mid-brain and cortex. The enzymes whose developmental patterns were investigated were hexokinase (EC 2.7.1.1), aldolase (EC 4.1.2.13), lactate dehydrogenase (EC 1.1.1.27) and glucose-6-phosphate dehydrogenase (EC 1.1.1.49). Hexokinase, aldolase and lactate dehydrogenase activities develop as a single cluster in all the regions studied, although the timing of this development varies from region to region. Glucose-6-phosphate dehydrogenase activity, however, declines relative to glycolytic enzyme activity as the brain matures. When the different brain regions are compared, it is clear that the medulla develops its glycolytic potential, as indicated by its potential enzyme activity, considerably earlier than the other regions (hypothalamus, striatum and mid-brain), with the cortex and cerebellar activities developing even later. This enzyme developmental sequence correlates well with the neurophylogenetic development of the brain and adds support to the hypothesis that the development of the potential for glycolysis in the brain is a necessary prerequisite for the development of neurological competence.

Nutritional support of children in the intensive care unit

Nutritional support is an integral and essential part of the management of 5-10 percent of hospitalized children. Children in the intensive care unit are particularly likely to develop malnutrition because of the nature and duration of their illness, and their inability to eat by mouth. This article reviews the physiology of starvation and the development of malnutrition in children. A method of estimating the nutritional requirements of children is presented. The techniques of nutritional support, including enteral, peripheral, and central parenteral nutrition are discussed in detail. Appropriate formulas are given for different age groups. Electrolyte, vitamin, and mineral supplements are discussed. Guidelines are provided for choosing between peripheral and central total parenteral nutrition. A monitoring protocol is suggested and complications of nutritional therapy are reviewed. Safe and effective nutritional support requires considerable investment of time and effort by members of the nutrition team.

Role of tissue lactate and substrate availability in 1,3-butanediol-enhanced hypoxic survival in the mouse

Previously we found that 1,3-butanediol-treated mice live longer during hypoxia. We hypothesized that 1,3-butanediol could reduce the brain's accumulation of potentially cytotoxic lactate and/or elevate brain substrate availability (ketones or glucose) and thus maintain the brain's energy producing capability even during reduced oxygen availability. To test these hypotheses, whole brain metabolites from normoxic and hypoxic mice, pretreated with 1,3-butanediol or insulin, were compared to saline controls. During hypoxia both pretreated groups had lower brain lactate than controls. If lactate accumulation was the sole factor responsible for hypoxic tolerance, insulin should have increased brain lactate since insulin has been shown previously to reduce hypoxic tolerance. In normoxic mice the ratio of lactate to pyruvate and the level of malate and fumarate were not changed by 1,3-butanediol as is found with other agents known to protect the hypoxic animal. When substrate availability was directly elevated by beta-hydroxybutyrate and glucose administration hypoxic survival time increased thus implicating substrate availability as an important factor in hypoxic tolerance. We conclude that reduced brain lactate and augmented substrate availability both contribute to 1,3-butanediol-enhanced hypoxic tolerance in this animal model.

Effects of ketosis on glucose flux in children and adults

Sequential glucose flux studies were carried out in five normal and six epileptic children and ten adult volunteers using [6,6-2H2]glucose to determine the effect of ketosis on carbohydrate homeostasis in children and adults. All subjects were studied after 14 and 30-38 h of fasting while consuming a normal diet and the epileptic children under 14 h of fasting while consuming an isocaloric ketogenic diet (75% fat wt/wt). Glucose flux, when expressed per kilogram body weight, was inversely correlated with the degree of ketosis in children (P less than 0.001) and in adults (P less than 0.01), but not when both children and adults were considered together (r = 0.078). When glucose flux was corrected for estimated brain weight, the relationship between glucose flux and ketonemia was linearly related in children (P less than 0.001), in adults (P less than 0.02), and when all subjects were considered together (P less than 0.001). The inverse relationship between ketonemia and glucose flux corrected for estimated brain mass is consistent with a partial replacement of glucose by ketone bodies for cerebral metabolism and may provide a more rational means of expressing glucose flux data to take into account the higher brain-to-body ratio in children.

Effect of somatostatin-induced insulinopenia on glucose oxidation in man

In the basal state the body utilizes glucose at a rate of 2.2 - 2.3 mg.kg-1.min-1; of this approximately 1.2 - 1.3 mg.kg-1.min-1 is oxidized, while the remaining 1.0 mg.kg-1.min-1 must be utilized by non-oxidative pathways. Little information is, however, available concerning the insulin dependency of these processes. To examine the role of basal insulin levels on glucose oxidation, glucose storage and total body glucose uptake, somatostatin (10 microgram/min) was infused for 2 h in nine volunteers while maintaining plasma glucose concentration constant at basal levels by an exogenous glucose infusion. Basal plasma insulin fell by about 50% (13 +/- 2 to 7 +/- 1 mU/l, p less than 0.01). Total body glucose metabolism (3H-3-glucose) declined from 2.3 +/- 0.1 to 1.9 +/- 0.1 mg.kg-1.min-1 (p less than 0.01). This decrease was entirely accounted for by a fall in basal glucose oxidation (measured by indirect calorimetry) from 1.3 +/- 0.1 to 0.7 +/- 0.1 mg.kg-1.min-1 (p less than 0.001). To assess the specific role of insulin deficiency in the decline in glucose oxidation, subjects were restudied with somatostatin plus basal insulin replacement (0.07 mg.kg-1.min-1). Fasting insulin concentration (14 +/- 1 mU/l) remained constant during somatostatin plus insulin infusion (13 +/- 1 mU/l) and basal rates of glucose oxidation (1.2 +/- 0.1 mg.kg-1.min-1) and total body glucose uptake did not change significantly. After 2 h, the basal insulin infusion was stopped and somatostatin was continued. Over the subsequent hour, glucose oxidation declined by 0.4 +/- 0.1 mg.kg-1.min-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Palmitate incorporation into different brain regions in the awake rat

A quantitative method is presented to examine palmitate flux into a stable metabolic compartment in individual brain regions of awake rats. Following the i.v. injection of [14C]palmitate, brain radioactivity rose and then fell until, at 4 h, a stable concentration was reached that was maintained for up to 24 h. The flux of plasma palmitate into this 4 h compartment was calculated by dividing regional brain radioactivity at 4 h, as determined by quantitative autoradiography, by the integral of the plasma palmitate specific activity. Palmitate flux varied from 2.0 x 10(-5) mumol/g.s into the internal capsule to 9.3 x 10(-5) mumol/g.s into the arcuate nucleus, and generally was proportional to the regional cerebral metabolic rate for glucose, as measured with 2-deoxy-D-[1-14C]glucose. The results demonstrate that it is possible to determine unidirectional palmitate flux into a stable metabolic compartment in individual brain regions of awake rats, that flux into gray matter regions generally exceeds flux into white matter, and that palmitate flux is proportional to published values for regional brain oxidative metabolism.

Local cerebral glucose metabolism in rats with chronic portacaval shunts

Local cerebral glucose utilization was measured with the autoradiographic [14C]deoxyglucose method in rats at 2 days and 1, 4, 8, and 12 weeks after the construction of a portacaval shunt, and in weight-matched controls. Local glucose utilization in brain was altered in shunted animals, but the magnitude and direction of the changes differed among 36 neuroanatomical structures, depending upon the length of time that the animals had been shunted. In rats shunted for 4 weeks or less, glucose utilization did not differ from control (30 of 36 structures) or was decreased (6 structures). The largest decreases of glucose utilization, noted at 1 week, occurred in the parietal (-25%) and frontal cortices (-28%) and subcortical white matter (-50%. In rats shunted for 8 weeks, however, glucose utilization was higher than control in many brain structures (13 of 36), and after 12 weeks it was higher than control in most structures (25 of 36). Only the parietal cortex did not follow this trend; it exhibited a decreased rate of glucose utilization in rats shunted for 8 weeks (-21%) that normalized in animals shunted for 12 weeks. Portal-systemic shunting of blood increased arterial blood ammonia concentrations to twice the control value of 85 +/- 3 microM in animals shunted for 1 week, and to approximately 2.5 times control in animals shunted for 4-12 weeks. Kety-Schmidt measurements of cerebral blood flow and arterial-venous (torcular) differences for ammonia across the brains of control rats and rats with portacaval shunts for 8 weeks revealed an increased cerebral uptake of ammonia in the shunted animals. The late-developing morphological changes known to occur in astrocytes and the delayed increases in local glucose consumption in rats with portacaval shunts may be related, and represent an adaptive response to sustained hyperammonemia.

Studies of cerebral beta-hydroxybutyrate transport by carotid injection; effects of age, diet and injectant composition

Transport of beta-hydroxybutyrate at the blood--brain barrier was studied by the carotid-injection technique of Oldendorf. beta-hydroxybutyrate permeability declined sharply with age (80-300 g rats) and, in adult rats, increased 5-fold during one week on a high fat diet. Acetoacetate and lactate permeabilities showed age and diet dependences which were similar in direction whereas DMO, urea and mannitol did not show age and diet dependent permeabilities. There was no apparent increase of beta-hydroxybutyrate Km with age, so the decline of permeability was attributed to a decline of Vmax. beta-Hydroxybutyrate permeability was inversely related to pH of the injectant in the alkaline range but not in the acid range, suggesting that the pH dependence reflected titration of a carrier rather than titration of the permeant. Permeability was independent of [Na+], [K+], [Cl-] and [SO2-(4)]. Replacing a portion of the Na+ with ammonium enhanced beta-hydroxybutyrate uptake. This effect appeared to be due to trans alkalization, and was as expected of an A-/H+-symport or A-/OH- -antiport mechanism. Pyruvate, 4-hydroxy-alpha-cyanocinnamate and tetracaine inhibited, but SITS, DIDS, phloretin and methyl-isobutylxanthine did not. The data are consistent with transport by an A-/H+ -symporter or A-/OH- -antiporter with properties similar to those found in erythrocytes and other cells. Induction of its activity during ketosis would spare carbohydrate both by favoring ketoacid uptake and by favoring lactate output.

Ketotic hypoglycaemia

Ketotic hypoglycaemia is the most common form of childhood hypoglycaemia. This disorder classically manifests itself between the ages of 18 months and 5 years, and generally remits spontaneously before 8 or 9 years of age. A presumptive diagnosis is made by documenting a low blood sugar in association with ketonuria, ketonaemia and typical symptoms of hypoglycaemia. The definitive diagnosis is established by demonstrating an inability to tolerate a provocative ketogenic diet, or a fast. Susceptible or affected children develop severe hypoglycaemia and ketosis on this diet within 24 hours. Plasma alanine concentrations on either a normal or ketogenic diet were significantly lower in ketotic hypoglycaemic children compared with normal children. In contrast to adults, even normal children develop hypoglycaemia and ketonaemia when calorically deprived for relatively short periods of time (32 to 36 hrs).

Insulin action during acute starvation: evidence for selective insulin resistance in normal man

The effects of insulin on glucose utilization, lipolysis, and potassium and phosphate metabolism were studied during short-term fasting in six lean subjects using a sequential euglycemic glucose clamp technique (two additional subjects were used in 70 mU/m2/min clamp studies). The subjects were infused with insulin for four hours at four rates ranging from 6 to 442 mU/m2/min before and after a 48-hour fast. Insulin was infused for one hour at each rate in all experiments. Fasting markedly reduced glucose utilization at all insulin infusion rates. On the other hand, the decline in levels of free fatty acids that occurred at insulin concentrations of 30 microU/ml was virtually identical before and after fasting. After insulin was infused for four hours, serum phosphate had decreased in all subjects (P less than 0.001) and strongly correlated with glucose disposal rates (r = 0.76, P less than 0.005). The plasma potassium level also declined in all subjects but did not relate to fasting or glucose disposal. These studies demonstrate that starvation produces selective insulin resistance. The biologic effect of insulin on glucose utilization and plasma phosphate shifts is clearly diminished. Free fatty acid and potassium metabolism are unaffected by starvation.

Lactic acidosis and hypoglycemia: experimental evidence to show lactate prevents insulin induced seizures in rats

A 52-year-old man with lactic acidosis and severe hypoglycemia was fully conscious and alert with a blood sugar of 8 mgs%. We believe normal level of consciousness was maintained due to the presence of hyperlactatemia. We show experimental evidence to suggest that lactate prevents the development of insulin-induced seizures in rats.

Cerebral metabolism of amino acids and glucose in fed and fasted sheep

Net cerebral uptake from or release into whole blood of oxygen, carbon dioxide, glucose, amino acids, lactate, pyruvate, ketone bodies, and acetate was estimated in fed, 3-day-fasted, and 6-day-fasted sheep. The respiratory quotient was similar in all three groups of sheep (approximately 0.95). Glucose uptake (35 mumol X min-1 X 100 g-1) was maintained during fasting, and about 94% of the cerebral oxygen consumption could have been accounted for by glucose oxidation in all sheep. A cerebral uptake of the branched-chain amino acids (leucine, isoleucine, and valine) and proline also was observed with a concomitant production of glutamine and asparagine. The brains of fed and 3-day-fasted sheep were in nitrogen balance, but a small net release of nitrogen occurred in 6-day-fasted sheep (2 mumol N. min-1 X 100 g-1). A small amount of pyruvate was always released (1.4 mumol X min-1 X 100 g-1) into the blood, whereas lactate was released (6 mumol X min-1 X 100 g-1) only in 6-day-fasted sheep. Ketone body and acetate utilization always was negligible when compared with that for glucose. The total cerebral nonglucose carbon release found for 6-day-fasted sheep was equivalent to 23% of the glucose carbon taken up, although only 8% could have been derived directly from glucose. Thus, metabolism by the ovine brain seems resistant to prolonged periods of hypoglycemia with only small adaptations occurring after a 6-day fast.

Loss of acetoacetate coenzyme A transferase activity in tumours of peripheral tissues

The presence of succinyl-coenzyme A: acetoacetate CoA--transferase (3-oxo acid-CoA transferase), an initiator of ketone body utilization in non-hepatic tissue was examined in a number of animal and human tumours of peripheral tissues. While enzyme levels in heart, kidney, lymphocytes and bladder were high, the tumours contained low or non-detectable levels of transferase activity, comparable with that of normal liver. The activities of acetoacetyl-CoA thiolase paralleled that of the transferase, except for the high activity in liver, and in all cases the tumour content of the enzyme was lower than that of the brain. The activity of 3-hydroxybutyrate dehydrogenase was similar in both normal and tumour tissue. The results indicate that tumours of non-hepatic tissues may be unable to metabolize ketone-bodies and suggest a therapeutic strategy for selective starvation of the tumour by dietary modification.

An hypothesis of function for the avian glycogen body: a novel role for glycogen in the central nervous system

Our own and other recent data have led us to hypothesize that the glycogen body, heretofore generally considered to be metabolically inert, may be functionally geared to support the process of myelin formation in the avian central nervous system (CNS). We envision that the abundant glycogen stores in this tissue, unlike those in the liver or in skeletal muscle, can serve as a recyclable substrate for the ultimate production of reducing equivalents that would be available for the synthesis of myelin lipid cholesterol. In addition, it is suggested that such glycogen also may serve as a source of organic acids which might provide alternate substrates to the CNS under conditions of metabolic stress.

Effect of weight reduction on circulating lipids: an integration of possible mechanisms

The effect of weight reduction on circulating lipids was studied in 17 normotensive, normolipemic obese women consuming diets with differing carbohydrate contents. Ten subjects consumed an 800-calorie, 10-gram carbohydrate diet, and 7 subjects followed an 800-calorie, 70-gram carbohydrate diet. At the end of 12 weeks of dieting, the mean weight loss was 12.3 kg for both groups (P less than 0.001) but not significantly different between the two groups. Serum insulin decreased by 31% (P less than 0.05) and a reduction of 18% (P less than 0.025) was observed in serum triiodothyronine. Serum potassium and magnesium remained unchanged; however, serum magnesium was marginally low for the duration of the study. Only four nonspecific changes were observed in a total of 68 electrocardiograms (5.8%); there were no arrhythmias. The response of serum cholesterol was biphasic, decreasing initially and returning to the baseline by the twelfth week. The high-density lipid (HDL) cholesterol followed a pattern similar to that of total cholesterol. A possible mechanism explaining the changes in circulating lipids is proposed. The results indicate that, in spite of substantial changes in weight, the cholesterol-to-HDL cholesterol ratio remained unchanged. Although weight reduction did not alter the risk factor in a group of normotensive, normolipemic obese women, hypocaloric diets should be prescribed under medical supervision, especially when cardiovascular or other diseases coexist.

Modulation of substrate transport to the brain

Variations of substrate transport across the cerebral capillary endothelium were examined in response to variations of the substrate demand of the brain tissue, and to variations of substrate concentration in the blood. The substrates examined included glucose and ketone bodies. The transport changes were measured in rats, using an indicator fractionation method modified by the reviewer. Four mechanisms appeared to contribute to the adjustment of substrate transport to variations in substrate demand. The first and least important mechanism was the change of concentration gradient across the endothelium that occurred when the substrate consumption rate changed. The second mechanism was the flow-dependency of the average capillary substrate concentration: the higher the perfusion rate, the higher the average capillary concentration. This mechanism failed to account for the changes of substrate transport observed during marked increases of the metabolic rate. The third and most important mechanism was a change of the capillary diffusion capacity, probably associated with a change of the number of perfused capillaries. The fourth mechanism, not previously described, was an adaptation of transport to permanent changes of substrate concentration in the blood. This mechanism appeared to reflect changes of the concentration (and affinity?) of transport proteins in the plasma membranes of endothelial cells, possibly in association with changes of cellular protein synthesis and gene expression.

Unique effects of infectious or inflammatory stress on fat metabolism in rats

Infectious or inflammatory stress in the rat causes very typical functional and metabolic alterations. Among the most typical are elevation in body temperature, insulin, and glucagon and depression in the concentrations of plasma ketones and free fatty acids. These changes occur only with infectious or inflammatory stress and not with noninflammatory stresses such as femoral fracture, screen restraint, or exercise. It appears that the depression in plasma ketone bodies during infection or inflammation is closely related to the rise in plasma insulin. During infection imposed on experimentally induced diabetes, inhibition of plasma ketones is not apparent. In a similar fashion, infection in hypophysectomized rats causes no elevation in plasma insulin and no depression in plasma ketones. Discussion concerning the implications of these observations in the rat and primate is included.

Fasting: the history, pathophysiology and complications

An appreciation of the physiology of fasting is essential to the understanding of therapeutic dietary interventions and the effect of food deprivation in various diseases. The practice of prolonged fasting for political or religious purposes is increasing, and a physician is likely to encounter such circumstances. Early in fasting weight loss is rapid, averaging 0.9 kg per day during the first week and slowing to 0.3 kg per day by the third week; early rapid weight loss is primarily due to negative sodium balance. Metabolically, early fasting is characterized by a high rate of gluconeogenesis with amino acids as the primary substrates. As fasting continues, progressive ketosis develops due to the mobilization and oxidation of fatty acids. As ketone levels rise they replace glucose as the primary energy source in the central nervous system, thereby decreasing the need for gluconeogenesis and sparing protein catabolism. Several hormonal changes occur during fasting, including a fall in insulin and T(3) levels and a rise in glucagon and reverse T(3) levels. Most studies of fasting have used obese persons and results may not always apply to lean persons. Medical complications seen in fasting include gout and urate nephrolithiasis, postural hypotension and cardiac arrhythmias.

[Regulation of ketone body levels before and following elective surgical operations during different intravenous feedings]

44 patients who had to undergo gastric resection and 28 patients who had to undergo cholecystectomy were divided into 4 groups each. Each group received parenterally a different energy source and calorie-nitrogen ratio. We intended to investigate the influence of different intravenous regimens on pre- and postoperative acetoacetate and beta-hydroxybutyrate levels. Patients undergoing gastric resection who received 0.36 g glucose/kg BW x h together with 1.14 g/kg BW x day 1-crystalline amino acids had the lowest postoperative ketone body concentration. A comparable group who received 0.36 g/kg BW x day of a carbohydrate-mixture solution consisting of glucose-fructose and xylitol in a proportion of 1:1:1 had significantly higher ketone bodies. The comparison of glucose with xylitol in a hypocaloric dosage of 0.11 g/kg BW x h led to a physiologic ketosis only in the group with xylitol as energy source from postoperative day 2 on. In patients undergoing cholecystectomy, the sole infusion of amino acids in a dosage of 1.14 g/kg BW x h led to the highest ketone bodies from the operation day on. The intravenous infusion of a polyol-mixture solution containing xylitol and sorbitol in a relation of 1:1 in a dosage of 4.2 g/kg BW x day led to the lowest ketone body production. The infusion of a polyol-mixture solution in a dosage of 2 g/kg BW x day enabled the development of a physiologic ketosis. In this study we could demonstrate that the infusion of xylitol or a polyol-mixture solution in a dosage of 2-3 g/kg BW x day after elective surgery enables the development of physiologic ketosis.