Brain metabolism during fasting

Nitrogen loss in normal and obese subjects during total fast

Healthy volunteers of ideal weight (12 men and 12 women) were fasted for 6 days, and obese but otherwise healthy subjects (20 men, 28 women) for 6--28 days. In all groups studied a significant increase in urinary nitrogen loss from day 1 to day 3 of fasting was followed by a steady decrease. The early rise in urinary nitrogen excretion coincided with a rise in plasma glucagon levels, suggesting a relation of the latter to increased gluconeogenesis from amino acids. At equal weight greater nitrogen losses were found in men than in women, in both normal and obese subjects. In spite of much higher weight and larger energy expenditure and nitrogen loss in obese subjects however was not higher than in normal ones. Mean daily nitrogen losses varied from 14.5 g (normal and obese men early in starvation) to 3.0 g (obese women after a 4-weeks fast). Calculating the amount of calories derived from body protien (urinary nitrogen X 6.25 X 4.1)and taking total energy expenditure from tabular metabolic values, the contribution of protein to total calorie output was found to vary from 15% (normal men 6 day fast) to 5(obese women, 4th week of fasting). The clinical significance of nitrogen loss during therapeutic fasting is discussed.

Effect of ketone infusions on amino acid and nitrogen metabolism in man

To evaluate the role of hyperketonemia in the hypoalaninemia and decreased protein catabolism of prolonged starvation, Na dl-beta-hydroxybutyrate was administered as a primed continuous 3-6-h infusion in nonobese subjects and in obese subjects in the postabsorptive state and after 3 days and 3-5 1/2 wk of starvation. An additional obese group received 12-h ketone infusions on 2 consecutive days after 5-10 wk of fasting. The ketone infusion in nonobese and obese subjects studied in the postabsorptive state resulted in total blood ketone acid levels of 1.1-1.2 mM, a 5-15 mg/100 ml decrease in plasma glucose, and unchanged levels of insulin, glucagon, lactate, and pyruvate. Plasma alanine fell by 21% (P smaller than 0.001) in 3 h. In contrast, other amino acids were stable or varied by less than 10%. Infusions lasting 6 h reduced plasma alanine by 37%, reaching levels comparable to those observed in prolonged starvation. Equimolar infusions of NaC1 and/or administration of NaHCO3 failed to alter plasma alanine levels. During prolonged fasting, plasma alanine, which had fallen by 40% below prefast levels, fell an additional 30% in response to the ketone infusion. In association with repeated prolonged (12 h) infusions in subjects fasted 5-10 wk, urinary nitrogen excretion fell by 30%, returning to base line after cessation of theinfusions and paralleling the changes in plasma alanine. Ketone infusins resulted in two- to fourfold greater increments in blood ketone acids in fasted as compared to postabsorptive subjects. It is concluded that increased blood ketone acid levels induced by infusions of Na DL-beta-hydroxybutyrate result in hypoalaninemia and in nitrogen conservation in starvation. These data suggest that hyperketonemia may be a contributory factor in the decreased availability or circulating alanine and reduction in protein catabolism characteristic of prolonged fastings9

Neurologic manifestations of diabetic comas: correlation with biochemical alterations in the brain

Coma and other neurologic abnormalities are present in patients with either diabetic ketoacidosis (DKA) or nonketotic coma (NKC), and the cause of such phenomena are not known. Patients with NKC also manifest seizures and focal neurologic changes. Treatment of diabetic coma with insulin may induce cerebral edema by as yet undefined mechanism(s). In patients with DKA, cerebral oxygen utilization is impaired, and there is hyperviscosity of the blood. A substantial part of the brain's energy source is derived from ketones, which in themselves can depress sensorium. Extracellular hyperosomolality is present, which may also contribute to the genesis of coma. In addition, most ketoacidotic patients have associated medical conditions, which may further impair consciousness. Biochemical changes in the brains of animals with DKA include impairment of both phosphofructokinase activity and pyruvate oxidation, and accumulation of citrate. The net effect upon sensorium in ketoacidotic patients probably represents the interaction of most of the above factors and differs markedly among individuals. Patients with NKC manifest not only depression of sensorium, but also focal motor seizures, hemiparesis, and other neurologic changes, such as aphasia, hypereflexia, sensory defects, autonomic changes, and brainstem dysfunction. Most of the aforementioned changes revert to normal after correction of hyperosomolality. Gamma amino butyric acid, which has been shown to elevate the seizure threshold, is normal in brains of ketoacidotic animals, but may be low in nonketotic coma. Also, hyperosomolality per se may produce seizures. Cerebral edema may complicate the treatment of either DKA or NKC. The available experimental evidence suggests that many of the commonly held theories for the production of such brain swelling probably do not occur. There is no breakdown of the sodium pump, sorbitol or fructose do not accumulate in brain, and brain glucose is only about 25 percent of that in plasma; Cerebral edema is probably produced largely by a direct action of insulin on brain at a time when plasma glucose is approaching normal values. Cerebral edema can thus theoretically be avoided by stopping insulin when plasma glucose has been lowered to values approaching normal.

What's Wrong With Carbohydrates?

A review of carbohydrates in metabolism is presented. The recent rash of books on diet and weight reduction implicate carbohydrates as the root cause of obesity and abnormal metabolism, but they actually play a positive role in normal metabolism. They are involved in energy metabolism, water balance and a host of other functions in the body. Used with intelligence and not with indulgence, they will continue to be good sources of calories, contributing to total nutritional health and well-being.

A disorder of muscle lipid metabolism and myoglobinuria. Absence of carnitine palmityl transferase

Two brothers, 29 and 33 years of age, had recurrent myoglobinuria, renal failure and azotemia, but were otherwise normal, without apparent muscle weakness or exercise intolerance. Ischemic exercise resulted in normal lactate production. Muscle glycogen content and activities of phosphorylase and phosphofructokinase were normal. Plasma triglycerides were elevated (500 mg per deciliter) on a regular diet and rose during fasting. During a 72-hour fast, serum creatine phosphokinase rose more than 10 times, and myoglobin was detected in urine. Plasma ketone production was minimal during fasting, but prompt ketonemia ( a normal response) occurred after ingestion of medium-chain triglycerides. Carnitine palmityl transferase activity was virtually absent in crude muscle extracts and mitochondrial fractions. Lack of this enzyme impairs long-chain fatty acid utilization, reflected in increased content of plasma free fatty acids and plasma triglycerides. Depletion of ATP because of this metabolic block in muscle may account for the attacks of myoglobinuria.

Effect of noradrenaline on triglyceride and glycogen concentrations in liver and muscle from man

Fifteen male subjects aged 21-29 were given noradrenaline infusions for 4 hr. The noradrenaline infusions started at a rate of 0.1 mug/kg/min and were increased stepwise at hourly intervals. Blood was sampled before and at hourly intervals during and after the noradrenaline infusions. Needle biopsies were taken from the femoral muscle in 10 subjects and frim the liver in 8 subjects before and after the noradrenaline infusion. The concentrations of FFA in lasma and of beta-hydroxybutyric acid in blood were markedly elevated during noradrenaline infusion. The concentrations of plasma TG and blood glucose were slightly elevated. In liver the TG concentration increased from 23.1 to 32.7 mmole/kh (p smaller than 0.001). It was estimated that the de novo synthesis of TG in the liver could have required about 30% of the plasma FFA entering the slanchnic region. In muscle the TG concentration was 3.6 mmole/kh lower after the noradrenaline infusion (p smaller than 0.001). No change was found in the concentrations of cholesterol and phospholipids in muscle or of cholesterol in liver. The glycogen concentration in muscle did not change during noradrenaline infusion. The decrease in liver glycogen concentration was of the same order of magnitude as previously observed in fasting man. The results suggest that noradrenaline was without major net effects on the metabolism of glycogen in liver and muscle.

Initial effect of injury on ketone bodies and other blood metabolites

The arterial and venous concentrations of ketone bodies and other metabolites were measured in twelve adults, from 2 to 24 hours after injury. Two groups could be distinguished, with or without hyperketonaemia (defined as more than 0.2 mmol per litre) in the 2-hour blood-sample. In the hyperdetonaemic group the concentrations of alanine, pyruvate, and lactate and the urinary nitrogen excretion were lower throughout the first 24 hours than in the non-hyperdetonaemic group. These preliminary results indicate that hyperketonaemia after trauma is associated with decreased protein breakdown.

Enzymatic organization of the subcommissural organ

In the subcommissural organ (SCO) of the guinea pig, rat, golden hamster, and mouse the activity and distribution of enzymes related to the energy-supplying metabolism and of some marker enzymes of different cell organelles have been investigated by means of mostly modified histochemical methods. The results were compared with findings in the ciliated ependyma of the ventricular wall and with those in the ependyma of the choroid plexus of the third ventricle. In the ependymal part of the SCO only a moderate activity of hexokinase is observed in its specialized columnar cells whereas a high activity is present both in the ciliated ependyma and the choroid plexus. - The staining pattern of glucose-6-phosphatase is similar to that of hexokinase but this enzyme is found is the SCO only. - Likewise hexokinase, glycogen granules and enzymes related to glycogen metabolism (phosphoglucomutase, uridine-diphosphoglucose pyrophosphorylase, glycogen synthetase and phosphorylase) are regularly found most numerous and active in the nuclear and supra-nuclear area of the ependymal part. These enzymes are less active in both the other ependymal regions. - Uridine-diphosphoglucose dehydrogenase could not be demonstrated in the SCO. The NADP-linked enzymes of the pentose phosphate shunt, glucose-6-phosphate and 6-phosphogluconate dehydrogenase, show a moderate activity which decreases also from the nuclear towards the apical area of the ependymal cells of the SCO. Enzymes of the glycolytic pathway, such as glucosephosphate isomerase, fructose-6-phosphate kinase, fructose-I,6-diphosphate aldolase, glyceraldehyde-3-phosphate and lactate dehydrogenase, are highly active in the SCO and are located mainly in the supranuclear area, too. Fructose-1,6-diphosphatase could not be demonstrated thus indicating that in the SCO the pathway is most probably only glycolytic but not gluconeogenetic. Compared to the ependyma of the ventricular wall and of the choroid plexus, in the SCO the M type subunits of lactate dehydrogenase predominate. Glycolytic enzymes are also very active in the choroid plexus but less in the ciliated ependyma. Compared to the ciliated ependyma and especially to the ependyma of the choroid plexus, the activities of enzymes which are only present in mitochondria (NAD-linked isocitrate dehydrogenase, succinate dehydrogenase, NAD-linked malate dehydrogenase after preextraction, cytochrome oxidase, 3-hydroxybutyrate and glycerolphosphate and glutamate dehydrogenase) are relatively low. Mitochondria are accumulated near the superior pole of the nuclei as well as in the most apical part of the ependymal cells. - The staining pattern of NADP-linked isocitrate and malate dehydrogenase as well as of NADH dehydrogenase suggests that these enzymes are localized both in and out of mitochondria. The extramitochondrial activity of the first two enzymes might be localized in the cytosol. The extramitochondrial activity of NADH dehydrogenase might be localized in the endoplasmic reticulum...

Renal conservation of ketone bodies during starvation

Renal handling of acetoacetate and beta-hydroxybutyrate was studied in 12 obese subjects undergoing total starvation. Simultaneously, the acetoacetate, beta-hydroxybutyrate, and inulin clearance rates were measured, and acetoacetate and beta-hydroxybutyrate reabsorption rates were calculated. Renal clearance of blood acetoacetate and beta-hydroxybutyrate remained constant. In contrast, acetoacetate reabsorption rate increased significantly from 47 plus or minus 10 mumoles/min on day 3 to 106 plus or minus 15, 89 plus or minus 10, and 96 plus or minus 10 mumoles/min on days 10, 17, and 24, respectively. Similarly, beta-hydroxybutyrate reabsorption rate increased significantly from 154 plus or minus 27 mumoles/min on day 3 to 419 plus or minus 53, 399 plus or minus 25, and 436 plus or minus 53 mumoles/min on days 10, 17, and 24, respectively. Both acetoacetate and beta-hydroxybutyrate reabsorption rates increased linearly when plotted against their filtered loads. Thus, no tubular maximal transport rate exists for acetoacetate or beta-hydroxybutyrate during physiologic ketonemia. Conservation 450-500 mmoles of ketone bodies/day prevents large urinary losses of cations during prolonged starvation. Since ammonium becomes the major cation excreted during prolonged fasting, the increased renal reabsorption of ketone bodies minimizes body protein loss and aids in maintaining high circulating acetoacetate and beta-hydroxybutyrate concentrations.

Oxidation of glucose and D-B-OH-butyrate by the early human fetal brain

The isolated brains of 12 previable human fetuses obtained at 12 to 21 weeks' gestation, were perfused through the interval carotid artery with glucose (3 mM) and/or DL-B-OH-butyrate (DL-BOHB), 4.5 MM, plus tracer quantities of either glucose-6-14C (G6-14C) or beta-OH-butyrate-3-14C (BOHB3-14C). Oxidative metabolism was demonstrated by serial collection of gaseous 14CO2 from the closed perfusion system, and from the recirculating medium. Glucose and BOHB were utilized at physiological rates as indicated (mean plus or minus SEM): G6-14C at 0.10 plus or minus 0.01 mumoles/min g brain (n equal 7) or 17.5 plus or minus 1.9 mumoles/min kg fetus; and BOHB3-14C at 0.16 plus or minus 0.05 mumoles/min g (n equal to 5) or 27.3 plus or minus 7.4 mumoles/min kg. Based on fetal weight, glucose metabolism by brain apparently accounted for about 1/3 of basal glucose utilization in the fetus. On a molar basis BOHB3-14C was taken up at 1.47 times the rate of G6-14C. Both BOHB3-14C and G6 14C were converted to 14CO2. The rate of BOHB3-14C conversion to 14CO2 was equal to its rate of consumption, and exceeded the conversion of glucose to CO2 because 45% of the G6-14C was incorporated into lactate-14C. Accordingly, both substrates support oxidative metabolism by brain; and BOHB is a major potential alternate fuel which can replace glucose early in human development.

Permeability changes in the blood-brain barrier: causes and consequences

1. Generalized changes in blood-brain barrier (BBB) permeability are accompanied by extravasation of plasma proteins; thus, they are readily studied with protein markers or protein-dye complexes. Selective changes in permeability involve alterations in BBB transport systems; they are best studied with techniques which detect the qualitative hallmarks of carrier-mediated transport, namely saturation, competition, and stereospecificity. 2. Quantitative assessments of the selective permeability of the BBB can be made from the saturation data expressed in terms of Michaelis-Menten kinetics. The advantages of the latter are twofold: (a) alterations elicited by modified barrier affinity (Km) can be distinguished from alterations in carrier capacity (Vmax); (b) the relative rates of flux of a metabolite across the BBB can be placed in the perspective of cerebral metabolism. Kinetic data on transport processes in the BBB are obtained by either constant infusion or single injection techniques. Results obtained with both methodologies have been comparable. 3. Independent transport systems for glucose, neutral amino acids, basic amino acids, and monocarboxylic acids have been identified in the BBB. The description of these transport systems in kinetic terms provides a background of information on intact mechanisms to which altered transport can be compared. 4. Experimental evidence indicates that the availability of key metabolic substrates, such as glucose or essential amino acids, may be rate-limiting in cerebral metabolism. A working hypothesis was developed that the consequences of a selective change in barrier permeability to one or more of these essential substrates are directly related to altered rates of reaction in substrate-limited pathways, e.g., cerebral protein or neuro-transmitter biosynthesis. 5. Toxicological causes of generalized changes in BBB permeability include hypertonic solutions, organic solvents, surface-active agents, enzymes, and heavy metals. Some agents, e.g., mercury or hypertonic urea, induce selective changes in BBB transport at doses much lower than those required for nonspecific barrier break-down. Subtle changes in transport of metabolic substrates may remain unrecognized unless specifically investigated, yet may have profound consequences on brain metabolism. 6. Pathological processes can also induce selective changes in BBB permeability. Such changes often temporally precede the more generalized alterations in permeability that can occur during pathogenesis. For example, in brain edema due to an ischemic infarct, glucose transport increases during the early cytotoxic phase, whereas generalized changes are not detected until the later vasogenic phase.

Hepatic ketogenesis and gluconeogenesis in humans

Splanchnic arterio-hepatic venous differences for a variety of substrates associated with carbohydrate and lipid metabolism were determined simultaneously with hepatic blood flow in five patients after 3 days of starvation. Despite the relative predominance of circulating beta-hydroxybutyrate, the splanchnic productions of both beta-hydroxybutyrate and acetoacetate were approximately equal, totaling 115 g/24 h. This rate of hepatic ketogenesis was as great as that noted previously after 5-6 wk of starvation. Since the degree of hyperketonemia was about threefold greater after 5-6 wk of starvation, it seems likely that the rate of ketone-body removal by peripheral tissues is as important in the development of the increased ketone-body concentrations observed after prolonged starvation as increased hepatic ketone-body production rate. Splanchnic glucose release in this study was 123 g/24 h, which was less than that noted previously after an overnight fast, but was considerably more than that noted during prolonged starvation. Hepatic gluconeogenesis was estimated to be 99 g/24 h, calculated as the sum of lactate, pyruvate, glycerol, and amino acid uptake. This was greater than that observed either after an overnight fast or after prolonged starvation. In addition, a direct relationship between the processes of hepatic ketogenesis and gluconeogenesis was observed.

Substrate turnover during prolonged exercise in man. Splanchnic and leg metabolism of glucose, free fatty acids, and amino acids

Arterial concentrations and substrate exchange across the leg and splanchnic vascular beds were determined for glucose, lactate, pyruvate, glycerol, individual acidic and neutral amino acids, and free fatty acids (FFA) in six subjects at rest and during 4 h of exercise at approximately 30% of maximal oxygen uptake. FFA turnover and regional exchange were evaluated using (14)C-labeled oleic acid. The arterial glucose concentration was constant for the first 40 min of exercise, but fell progressively thereafter to levels 30% below basal. The arterial insulin level decreased continuously, while the arterial glucagon concentration had risen fivefold after 4 h of exercise. Uptake of glucose and FFA by the legs was markedly augmented during exercise, the increase in FFA uptake being a consequence of augmented arterial levels rather than increased fractional extraction. As exercise was continued beyond 40 min, the relative contribution of FFA to total oxygen metabolism rose progressively to 62%. In contrast, the contribution from glucose fell from 40% to 30% between 90 and 240 min. Leg output of alanine increased as exercise progressed. Splanchnic glucose production, which rose 100% above basal levels and remained so throughout exercise, exceeded glucose uptake by the legs for the first 40 min but thereafter failed to keep pace with peripheral glucose utilization. Total estimated splanchnic glucose output was 75 g in 4 h, sufficient to deplete approximately 75% of liver glycogen stores. Splanchnic uptake of gluconeogenic precursors (lactate, pyruvate, glycerol, alanine) had increased 2- to 10-fold after 4 h of exercise, and was sufficient to account for 45% of glucose release at 4 h as compared to 20-25% at rest and at 40 min of exercise. In the case of alanine and lactate, the increase in precursor uptake was a consequence of a rise in splanchnic fractional extraction. It is concluded that during prolonged exercise at a low work intensity (a) blood glucose levels fall because hepatic glucose output fails to keep up with augmented glucose utilization by the exercising legs; (b) a large portion of hepatic glycogen stores is mobilized and an increasing fraction of the splanchnic glucose output is derived from gluconeogenesis; (c) blood-borne substrates in the form of glucose and FFA account for a major part of leg muscle metabolism, the relative contribution from FFA increasing progressively; and (d) augmented secretion of glucagon may play an important role in the metabolic adaptation to prolonged exercise by its stimulatory influence on hepatic glycogenolysis and gluconeogenesis.

Ketone-body production and oxidation in fasting obese humans

Rates of plasma acetoacetate and total ketone-body production and oxidation to CO2 were determined by an isotope tracer technique in eight obese subjects undergoing progressive starvation. After a brief fast and under conditions of mild ketonemia and minimal ketonuria, rates of acetoacetate and total ketone-body production and oxidation were directly related to the increasing plasma concentration. After a longer fast and with severer ketonemia, acetoacetate and total ketone-body production and oxidation rates were higher but became constant and unrelated to the plasma concentrations. The maximum rates of total ketone-body production and oxidation were about 150 g/24 h and 129 g/24 h, respectively. Although an increased ketone-body production was the primary factor responsible for the hyperketonemia, an imbalance between production and removal of the ketone bodies cannot be excluded. Such an imbalance could account, at least in part, for the developing hyperketonemia and for the lack of relationship between production rates and plasma concentrations.

Regulation of glucose and ketone-body metabolism in brain of anaesthetized rats

1. The effects of starvation and diabetes on brain fuel metabolism were examined by measuring arteriovenous differences for glucose, lactate, acetoacetate and 3-hydroxybutyrate across the brains of anaesthetized fed, starved and diabetic rats. 2. In fed animals glucose represented the sole oxidative fuel of the brain. 3. After 48h of starvation, ketone-body concentrations were about 2mm and ketone-body uptake accounted for 25% of the calculated O(2) consumption: the arteriovenous difference for glucose was not diminished, but lactate release was increased, suggesting inhibition of pyruvate oxidation. 4. In severe diabetic ketosis, induced by either streptozotocin or phlorrhizin (total blood ketone bodies >7mm), the uptake of ketone bodies was further increased and accounted for 45% of the brain's oxidative metabolism, and the arteriovenous difference for glucose was decreased by one-third. The arteriovenous difference for lactate was increased significantly in the phlorrhizin-treated rats. 5. Infusion of 3-hydroxybutyrate into starved rats caused marked increases in the arteriovenous differences for lactate and both ketone bodies. 6. To study the mechanisms of these changes, steady-state concentrations of intermediates and co-factors of the glycolytic pathway were determined in freeze-blown brain. 7. Starved rats had increased concentrations of acetyl-CoA. 8. Rats with diabetic ketosis had increased concentrations of fructose 6-phosphate and decreased concentrations of fructose 1,6-diphosphate, indicating an inhibition of phosphofructokinase. 9. The concentrations of acetyl-CoA, glycogen and citrate, a potent inhibitor of phosphofructokinase, were increased in the streptozotocin-treated rats. 10. The data suggest that cerebral glucose uptake is decreased in diabetic ketoacidosis owing to inhibition of phosphofructokinase as a result of the increase in brain citrate. 11. The inhibition of brain pyruvate oxidation in starvation and diabetes can be related to the accelerated rate of ketone-body metabolism; however, we found no correlation between the decrease in glucose uptake in the diabetic state and the arteriovenous difference for ketone bodies. 12. The data also suggest that the rates of acetoacetate and 3-hydroxybutyrate utilization by brain are governed by their concentrations in plasma. 13. The finding of very low concentrations of acetoacetate and 3-hydroxybutyrate in brain compared with plasma suggests that diffusion across the blood-brain barrier may be the rate-limiting step in their metabolism.

The glycogen storage diseases

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Glucose turnover and disposal in maturity-onset diabetes

The glucose turnover rate in maturity-onset diabetes in man has been variously reported as increased, normal, and decreased. The present experiments suggest that these discrepancies may have been due to methodology, and to nonrecognition of a circadian cycle in the glucose turnover rate that is present in health, and marked in diabetes. During the early morning hours the glucose turnover rate in maturity-onset diabetes is increased in proportion to the fasting blood glucose level. It may reach three to four times the rate found in health. During the evening hours the increments are about one-half as great. The glucose outflow rate constant, k, lower in diabetes than in health, is also lower in both groups in the evening than in the morning. An analysis of the relative contributions of glucose overproduction and underutilization to the development of hyperglycemia in maturity-onset diabetes indicates that overproduction is the greater factor. The relative role of underutilization appears to increase as the fasting blood glucose level increases. The circulating glucose oxidation rate in maturity-onset diabetes is only slightly lower than in health, but the fraction oxidized is markedly lower, and only a small fraction is excreted. The principal conclusion is that in maturity-onset diabetes there is a hypertrophied flux of endogenous glucose, most of which is neither oxidized nor excreted. The precursors and the qualitative and quantitative metabolic fates of this excess glucose are unknown.

Rapid intravenous sodium acetoacetate infusion in man. Metabolic and kinetic responses

The metabolic and kinetic responses to rapidly intravenously administered sodium acetoacetate (1.0 mmol/kg body wt) was studied after an overnight fast in 12 male and female adults weighing between 88 and 215% of average body weight. Blood was obtained before, during, and after the infusion for determination of circulating concentrations of immunoreactive insulin, glucose, acetoacetate, beta-hydroxybutyrate and free fatty acids. In three obese subjects the studies were repeated after 3 and 24 days of total starvation. After the overnight fast acetoacetate rose rapidly reaching a peak concentration at the end of the infusion; beta-hydroxybutyrate concentrations also increased rapidly and exceeded those of acetoacetate 10 min postinfusion. Total ketone body concentration at the end of the infusion period was comparable to that found after prolonged starvation. After the initial mixing period, acetoacetate, beta-hydroxybutyrate and total ketone bodies rapidly declined in a parallel manner. There were no obvious differences between the subjects with regard to their blood concentrations of ketone bodies. The mean plasma free fatty acid concentration decreased significantly during the 20th to 90th min postinfusion period; for example the control concentration of 0.61 mmol/liter fell to 0.43 mmol/liter at 60 min. In the three obese subjects studied repeatedly, fasting plasma free fatty acids decreased with acetoacetate infusion from 0.92 to 0.46 mmol/liter after the 3 day fast and from 1.49 to 0.71 mmol/liter after the 24 day fast. Acetoacetate infusion caused no changes in blood glucose concentration after an overnight fast. However, in the three obese subjects restudied after 3- and 24-day fasts blood glucose decreased, respectively, from 3.49 to 3.22 mmol/liter and from 4.07 to 3.49 mmol/liter. The mean serum insulin concentration in all subjects significantly increased from 21 to 46 muU/ml at the completion of the infusion and rapidly declined. In the three obese subjects restudied after 3- and 24-day fasts an approximate two-fold increase of serum insulin was observed after each acetoacetate infusion. The mean fractional utilization rate of exogenously derived ketone bodies for all 12 subjects after an overnight fast was 2.9% min(-1). In the three obese subjects studied after an overnight, 3 and 24 day fast the mean fractional utilization rates were 2.1%, 1.5%, and 0.6% min(-1), respectively. Ketone body volumes of distribution in the overnight fasted subjected varied from about 18% to 31% of body wt, suggesting that ketone bodies are not homogenously distributed in the body water. In the three obese subjects restudied after 3- and 24-day fasts volumes of distribution remained approximately constant. When total ketone body concentrations in the blood were below 2.0 mmol/liter, there was a linear relationship between ketone body utilization rates and ketone body concentrations; no correlation was found when blood concentrations were higher.

Cerebral glucose transport and oxygen consumption in sheep and rabbits

1. Mechanisms underlying the ability of ruminants to tolerate severe hypoglycaemia have been investigated. Anaesthetized sheep and rabbits were compared with respect to cerebral glucose transport and oxygen consumption as a function of glucose concentration in cerebral extracellular fluids.2. Glucose in plasma was decreased by insulin or increased by I.V. infusion. Measurements were made of cerebral blood flow, arteriovenous concentration differences of glucose and oxygen and the concentration of glucose in c.s.f.3. Equations for carrier-mediated transport accurately described steady-state glucose flux across the blood-brain barrier as plasma concentration was varied from 0.2 to 30 mM. In sheep, the affinity constant (K(m)) was 6 mM and the maximum transport capacity (T(m)) was 260 mumole min(-1). 100 g(1) brain. In rabbits, K(m) = 5.5 mM and T(m) = 280 mumole min(-1). 100 g(1). Transport of glucose across the blood-brain barrier of rabbits is at least as efficient as that in sheep and in both species T(m) is 10-15 times greater than normal rates of glucose utilization.4. During hypoglycaemia the concentration of glucose in c.s.f. is less in sheep than in rabbits (Fig. 5). Steady-state utilization of glucose by sheep brain decreased to 50% of normal when steady-state concentration of glucose in c.s.f. (interstitial fluid) falls to 0.1 mumole ml.(-1); in rabbits the corresponding concentration is 0.7 mumole ml.(-1) (Fig. 6). We suggest that transport capacity of membranes separating cerebral interstitial fluid from the site of glucose phosphorylation is greater in sheep than in rabbits; this may be the principal adaptation which enables ruminants to withstand severe hypoglycaemia (Discussion II).5. Approximately 30 min were required to reach a steady state of glucose transport following a sudden increment of glucose concentration in plasma (Fig. 1). 80-100 min were required to reach a new steady-state concentration of glucose in c.s.f.6. The molar ratio of steady-state cerebral glucose utilization to oxygen consumption (6G:O(2)) is normally 0.93 (S.E. +/- 0.05) but is decreased to the range 0.1-0.5 during sustained hypoglycaemia in both sheep and rabbits (Figs. 2, 3). Continued low glucose: oxygen ratios could be explained by (a) utilization of non-carbohydrate substrates derived from blood or (b) utilization of stored lipid in brain. Only 0.1 g lipid/100 g brain would suffice to account for the observed rate of non-glucose oxidative metabolism during 3 hr of severe hypoglycaemia (Discussion IV).

Influence of meal-feeding on some of the effects of dietary carbohydrate deficiency in rats

1. Experiments were conducted to investigate the effects of feeding rats on a diet based on a mixture of maize oil fatty acids as the only source of non-protein energy (fatty acid diet) and the influence on these effects of giving such a diet in a single daily meal lasting 2 h.

2. In comparison with a triglyceride diet in which the non-protein energy was in the form of maize oil, feedingad lib.with the fatty acid diet produced no significant changes in body-weight gain, plasma glucose and plasma ketones concentrations, liver glycogen concentration and protein efficiency ratio.

3. In comparison with the triglyceride diet, meal-feeding with the fatty acid diet produced significantly lower body-weight gain and protein efficiency ratio; moreover, it significantly lowered plasma glucose and liver glycogen concentrations.

4. Rats meal-fed on the fatty acid diet synthesized glucose from protein, as evidenced by the significantly higher liver glycogen concentration detected 6 h after the meal had been eaten, but the increase was significantly lower than in the animals fed on the triglyceride diet. Also, 6 h after the meal had been eaten, the amount of meal remaining in the stomach of rats meal-fed on the fatty acid diet was significantly higher than in those fed on the triglyceride diet.

Activities of hexokinase, phosphofructokinase, 3-oxo acid coenzyme A-transferase and acetoacetyl-coenzyme A thiolase in nervous tissue from vertebrates and invertebrates

1. The maximum activities of hexokinase and phosphofructokinase in nervous tissue from 18 different animals from different phyla range from 5.1 to 17.6 and from 24.0mumol/min per g fresh wt. respectively. In any one tissue the activities of these two enzymes are, in general, very similar. The rate of glucose utilization by the brain in vivo is much lower than the activities of hexokinase or phosphofructokinase. It is suggested that the high activities of these enzymes indicate a capacity for glycolysis which may be used by the brain during hypoxia or during conditions of extreme neuronal activity. 2. The activities of 3-oxo acid CoA-transferase and acetoacetyl-CoA thiolase in the nervous tissues range from 1.1 to 15.3 and from 0.7 to 4.5mumol/min per g fresh wt. respectively. Unfortunately the activities of these enzymes cannot be used to estimate maximal flux through the ketone-body-utilization pathway, since they may catalyse reactions that are close to equilibrium. Nonetheless, the presence of these enzymes in nervous tissue from a large variety of animals suggests that the importance of ketone bodies as a fuel for nervous tissue may be widespread in the animal kingdom.