Brain metabolism during fasting

Failure of thermoregulation in the cold during hypoglycaemia induced by exercise and ethanol

1. After young men had exercised for approximately 2 hr at 70% maximum O(2) uptake, and taken 28 ml. ethanol by mouth, their mean blood glucose fell to 2.17 mM. It fell further to 1.77 mM during a 30 min exposure to air at 14.5 degrees C. Plasma lactate, glycerol, beta-hydroxybutyrate and free fatty acid concentrations increased.2. Rectal temperature fell to reach a mean level of 34.49 degrees C by the end of the cold exposure; oesophageal temperature fell to as low as 33.00 degrees C in one case.3. Virtually no increase in metabolic rate and no visible shivering occurred during the cold exposure.4. Administration of glucose (mean 60.4 g) prevented the falls in temperature, and restored metabolic response to the cold to the size found in control experiments without exercise or ethanol.5. Neither exercise without ethanol or ethanol without exercise significantly lowered the blood glucose or impaired the maintenance of body temperature in the cold.6. One obese subject showed almost as great a fall in blood glucose and depression of metabolic response to cold as the thinner men, but no fall in body temperature.

Ketogenesis from butyrate and acetate by the caecum and the colon of rabbits

1. When studied in vitro, tissue from the caecum and the proximal colon of rabbits converted butyrate into ketone bodies. The conversion was similar to that observed with liver slices. The ketogenic activity was associated with the mucosa rather than the muscle of the gut wall and, in the colon, diminished as the distance from the caecal-colonic junction increased. 2. Tissue from the wall of the ileum, caecum, proximal colon and distal colon was also shown to metabolize [1-(14)C]butyrate to carbon dioxide. 3. Enzyme assays showed that in both liver tissue and caecal mucosa the activity of hydroxymethylglutaryl-CoA synthase was more than ten times that of acetoacetyl-CoA deacylase. Labelling experiments in vitro gave confirmation of the hydroxymethylglutaryl-CoA pathway. 4. The significance of the conversion of butyrate into ketone bodies is discussed.

Resistance to symptomatic insulin reactions after fasting

This study was carried out to determine if, in fasting, an adaptation to utilization of ketones could prevent cerebral dysfunction during periods of acute, insulin-induced glucopenia. In the course of standard insulin tolerance tests (0.1-0.2 U/kg), nine obese subjects manifested frank hypoglycemic reactions resulting in an increase in urinary catecholamine excretion from 61 to 113 mug/24 hr (P < 0.01). After fasting 2 months, administration of weight-adjusted doses of insulin produced identical maximum insulin concentrations and disappearance curves. However, no insulin reactions nor significant rises in catecholamine excretion occurred despite equal extent and rate of glucose fall. Glucose concentrations as low as 0.5 mmoles/liter (9 mg/100 ml) failed to precipitate hypoglycemic reactions. During the postfast insulin tolerance tests, mean plasma 2-hydroxybutyrate (beta-OHB) decreased from 8.02 to 6.69 mmoles/liter (P < 0.01). In another five fasting subjects tested, the A-V difference for beta-OHB across brain increased progressively from 0.21 to 0.70 mmoles/liter whereas across the forearm no consistent uptake could be demonstrated. Simultaneously, the A-V difference across the brain for glucose decreased from 0.24 to 0.07 mmoles/liter of plasma. In addition to insulin-induced suppression of hepatic ketogenesis, the augmented cerebral ketone uptake during insulin hypoglycemia contributes to the net fall in plasma beta-OHB. Ketoacids, extracted by the fast-adapted brain, supplant glucose as a metabolic substrate preventing overt hypoglycemic reactions during acute glucopenia.

Newer techniques of cerebral blood flow measurement

A variety of methods for measuring cerebral blood flow have been developed in the past 25 years since Kety and Schmidt developed their method based on the Fick principle. None of the currently used methods approaches the clinical ideal, since none of the techniques are accurate, reproducible and noninvasive. Most accurate techniques involve catheterization of internal carotid artery and/or jugular vein with those attendant risks. There has been considerable enhancement of our understanding of the pathophysiology of cerebral circulation, particularly in the areas of brain injury and disordered systemic metabolism, but the clinical usefulness of the test at the present time is limited.

Abnormal hemispheric blood flow and metabolism in cerebrovascular disease. I. Disordered patterns of hemispheric metabolism

Hemispheric blood flow (HBF) and metabolism were studied in 46 patients with various types and stages of ischemic cerebrovascular disease and were found to be abnormal in all patients, including those with transient ischemic attacks. Bilateral depression of HBF and metabolism was observed in association with acute unilateral cerebral infarction. Hemispheric respiratory quotient (HRQ) was higher in the ischemic hemisphere than in the healthy side. The degree of elevation of HRQ on the diseased side correlated well with the acuteness of infarction, increasing age of the patients, and the severity of the neurological deficit. However, the HRQ in the healthy hemisphere was lower than unity, which suggests that substances other than glucose were being metabolized. The high hemispheric glucose:oxygen ratio (HG:O) correlated well with the severity of infarction and the age of the patient. Patients with brain stem ischemia had the lowest HBF values, suggesting that central neurogenic vasomotor control had been impaired or that diaschisis was greater in those with brain stem lesions than in patients with other types of lesions. A significant uptake of β-hydroxybutyrate was observed in both the healthy and diseased hemispheres but was highest in the healthy side. Evidence for abnormal cerebral metabolic patterns, such as anaerobic glycolysis, in patients with cerebrovascular disease, and oxidation of nonglucose substances such as β-hydroxybutyrate, are discussed.

Simple behavioural tests for defective vision in cats (film)

Images

Mechanism of hyperventilation in acute cerebrovascular accidents

Lumbar cerebrospinal fluid and arterial blood acid-base state were assessed in 19 patients within 24 hours of an acute cerebrovascular accident. Those with haemorrhage into the C.S.F. showed a lower C.S.F. pH and higher C.S.F. lactate than those without haemorrhage but the Pco(2), was similar in the two groups, suggesting that this greater C.S.F. acidity was not responsible for a greater degree of hyperventilation. In those without haemorrhage an inverse relation was found between C.S.F. pH and arterial Pco(2), suggesting that a non-chemical ventilatory drive-for example, due to central neurological damage-was responsible for the acid-base changes observed.

Human forearm metabolism during progressive starvation

Forearm muscle metabolism was studied in eight obese subjects after an overnight, 3 and 24 day fast. Arterio-deep-venous differences of oxygen, carbon dioxide, glucose, lactate, pyruvate, free fatty acids, acetoacetate, and beta-hydroxybutyrate with simultaneous forearm blood flow were measured. Rates of metabolite utilization and production were thus estimated. Oxygen consumption and lactate and pyruvate production remained relatively constant at each fasting period. Glucose, initially the major substrate consumed, showed decreased consumption after 3 and 24 days of fasting. Acetoacetate and beta-hydroxybutyrate consumption after an overnight fast was low. At 3 days of fasting with increased arterial concentrations of acetoactate and beta-hydroxybutyrate, consumption of these substrates rose dramatically. At 24 days of fasting, despite further elevation of arterial levels of acetoacetate and beta-hydroxybutyrate, the utilization of acetoacetate did not increase further and if anything decreased, while five out of eight subjects released beta-hydroxybutyrate across the forearm. Acetoacetate was preferentially extracted over beta-hydroxybutyrate. At 24 days of starvation, free fatty acids were the principal fuels extracted by forearm muscle; at this time there was a decreased glucose and also ketone-body consumption by skeletal muscle.

Incorporation of label from D- -hydroxy( 14 C)butyrate and (3- 14 C)acetoacetate into amino acids in rat brain in vivo

The metabolism of ketone bodies by rat brain was studied in vivo. Rats starved for 48h were given either d-beta-hydroxy[3-(14)C]butyrate or [3-(14)C]acetoacetate by intravenous injection and killed after 3 or 10min. Total radioactivity in the acid-soluble material of the brain and the specific radioactivities of the brain amino acids glutamate, glutamine, aspartate and gamma-aminobutyrate were determined. A group of fed animals were also given d-beta-hydroxy[3-(14)C]butyrate. In the brains of all animals (14)C was present in the acid-soluble material and the specific radioactivity of glutamate was greater than that of glutamine.

Ketone-body utilization by adult and suckling rat brain in vivo

1. Ketone-body utilization in fed and starved adult and suckling rats has been investigated by measuring arterio-venous differences across the brain. Venous blood was collected from the confluence of sinuses and arterial blood from the femoral artery in adult rats and by cardiac puncture in suckling rats. 2. During starvation the arterio-venous difference of ketone bodies increased in proportion to their concentrations in the blood and reached a value of 0.16mm at 48h. At a given concentration of the respective ketone bodies the arterio-venous differences of acetoacetate were about twice those of 3-hydroxybutyrate. 3. Fed rats in which the concentrations of ketone bodies were raised by intravenous infusion of sodium acetoacetate had the same arterio-venous differences as starved rats at corresponding ketone-body concentrations. Thus the ability of the rat brain to utilize ketone bodies is independent of the nutritional state. 4. The concentrations of glucose, acetoacetate and 3-hydroxybutyrate were much lower in the brain than in the arterial blood. The measured (blood concentration)/(brain concentration) ratio was 4.4 for glucose, 4.5 for acetoacetate and 8.1 for 3-hydroxybutyrate in 48h-starved rats. 5. The mean arterio-venous difference of glucose across the brain was 0.51mm in fed rats and 0.43mm in 96h-starved rats. 6. Conversion of glucose into lactate rose from negligible values in the fed state to 0.2mm after 48h starvation and decreased to zero after 96h starvation. 7. In 16-22-day-old suckling rats the arterio-venous differences of ketone bodies across the brain were also proportional to the ketone-body concentration, but they were about 3-4 times greater than in adult rats at the same blood ketone-body concentration. 8. Arterio-venous differences of glucose were about the same in adult and suckling rats. 9. The brain of fed suckling rats formed more lactate from glucose than fed adult rats. 10. The results indicate that ketone bodies are major metabolic fuels of the brain of the suckling rat under normal conditions.

Metabolic response to human growth hormone during prolonged starvation

The metabolic response to human growth hormone (HGH) was studied in five obese subjects in the fed state and during prolonged (5-6 wk) starvation. In the fed state (three subjects), HGH induced an elevation in basal serum insulin concentration, a minimal increase in blood and urine ketone levels, and a marked reduction in urinary nitrogen and potassium excretion resulting in positive nitrogen and potassium balance. In prolonged fasting (four subjects), HGH administration resulted in a 2- to 3-fold increase in serum insulin which preceded a 50% elevation in blood glucose. Persistence of the lipolytic effects of HGH was indicated by a rise in free fatty acids and glycerol. The response differed markedly from the fed state in that blood beta-hydroxybutyrate and acetoacetate levels rose by 20-40%, resulting in total blood ketone acid concentrations of 10-12 mmoles/liter, ketonuria of 150-320 mmoles/day, and increased urinary potassium loss. The subjects complained of nausea, vomiting, weakness, and myalgias. Despite a 50% reduction in urea excretion during HGH administration, total nitrogen loss remained unchanged as urinary ammonia excretion rose by 50% and correlated directly with the degree of ketonuria. It is concluded that in prolonged starvation (a) HGH may have a direct insulinotropic effect on the beta cell independent of alterations in blood glucose concentration, (b) persistence of the lipolytic action of HGH results in severe exaggeration of starvation ketosis and interferes with its anticatabolic action by necessitating increased urinary ammonia loss, and (c) failure of HGH to reduce net protein catabolism in starvation suggests that this hormone does not have a prime regulatory role in conserving body protein stores during prolonged fasting.

Activities of enzymes involved in acetoacetate utilization in adult mammalian tissues

1. The activities in rat tissues of 3-oxo acid CoA-transferase (the first enzyme involved in acetoacetate utilization) were found to be highest in kidney and heart. In submaxillary and adrenal glands the activities were about one-quarter of those in kidney and heart. In brain it was about one-tenth and was less in lung, spleen, skeletal muscle and epididymal fat. No activity was detectable in liver. 2. The activities of acetoacetyl-CoA thiolase were found roughly to parallel those of the transferase except for liver and adrenal glands. The high activity in the latter two tissues may be explained by additional roles of thiolase, namely, the production of acetyl-CoA from fatty acids. 3. The activities of the two enzymes in tissues of mouse, gerbil, golden hamster, guinea pig and sheep were similar to those of rat tissues. The notable exception was the low activity of the transferase and thiolase in sheep heart and brain. 4. The activities of the transferase in rat tissues did not change appreciably in starvation, alloxan-diabetes or on fat-feeding, where the rates of ketone-body utilization are increased. Thiolase activity increased in kidney and heart on fat-feeding. 5. The activity of 3-hydroxybutyrate dehydrogenase did not change in rat brain during starvation. 6. The factors controlling the rate of ketone-body utilization are discussed. It is concluded that the activities of the relevant enzymes in the adult rat do not control the variations in the rate of ketone-body utilization that occur in starvation or alloxan-diabetes. The controlling factor in these situations is the concentration of the ketone bodies in plasma and tissues.

Fatty acid metabolism in the perfused rat liver

1. The formation of acetoacetate, beta-hydroxybutyrate and glucose was measured in the isolated perfused rat liver after addition of fatty acids. 2. The rates of ketone-body formation from ten fatty acids were approximately equal and independent of chain length (90-132mumol/h per g), with the exception of pentanoate, which reacted at one-third of this rate. The [beta-hydroxybutyrate]/[acetoacetate] ratio in the perfusion medium was increased by long-chain fatty acids. 3. Glucose was formed from all odd-numbered fatty acids tested. 4. The rate of ketone-body formation in the livers of rats kept on a high-fat diet was up to 50% higher than in the livers of rats starved for 48h. In the livers of fat-fed rats almost all the O(2) consumed was accounted for by the formation of ketone bodies. 5. The ketone-body concentration in the blood of fat-fed rats rose to 4-5mm and the [beta-hydroxybutyrate]/[acetoacetate] ratio rose to 11.5. 6. When the activity of the microsomal mixed-function oxidase system, which can bring about omega-oxidation of fatty acids, was induced by treatment of the rat with phenobarbitone, there was no change in the ketone-body production from fatty acids, nor was there a production of glucose from even-numbered fatty acids. The latter would be expected if omega-oxidation occurred. Thus omega-oxidation did not play a significant role in the metabolism of fatty acids. 7. Arachidonate was almost quantitatively converted into ketone bodies and yielded no glucose, demonstrating that gluconeogenesis from poly-unsaturated fatty acids with an even number of carbon atoms does not occur. 8. The rates of ketogenesis from unsaturated fatty acids (sorbate, undecylenate, crotonate, vinylacetate) were similar to those from the corresponding saturated fatty acids. 9. Addition of oleate together with shorter-chain fatty acids gave only a slightly higher rate of ketone-body formation than oleate alone. 10. Glucose, lactate, fructose, glycerol and other known antiketogenic substances strongly inhibited endogenous ketogenesis but had no effects on the rate of ketone-body formation in the presence of 2mm-oleate. Thus the concentrations of free fatty acids and of other oxidizable substances in the liver are key factors determining the rate of ketogenesis.

Effect of insulin and acute diabetes on plasma FFA and ketone bodies in the fasting rat

The metabolism of FFA and ketone bodies was studied in fasted rats by infusing at a constant rate tracer amounts of FFA-(3)H, beta-hydroxybutyrate-(14)C or acetoacetate-(14)C for periods up to 2 hr. Blood that was removed for analyses was replaced by continuous transfusion. The rates of turnover of FFA, beta-hydroxybutyrate, and acetoacetate in rats fasted for 2 days were, respectively, 3.2, 5.6, and 2.5 mumoles/100 g body weight per min. Infusion of mannoheptulose with anti-insulin serum increased plasma glucose, FFA, and ketone body concentrations and decreased the specific activity of plasma FFA. Injection of insulin (20 mU i.v.) decreased almost simultaneously plasma glucose, FFA, and ketone body concentrations and increased the specific activity of FFA, but it did not affect the plasma concentration of FFA-(3)H. The findings indicate that insulin deprivation increased and insulin injection decreased the release of FFA from body tissues in fasting rats. The plasma FFA concentration in fasting rats was increased by infusing chylomicrons and heparin, but this had very little effect on either plasma ketone body or glucose concentrations. Insulin injection (20 mU i.v.) lowered the plasma ketone body concentration in these animals. Studies using beta-hydroxybutyrate-(14)C showed that insulin (50 mU i.v.) decreased ketogenesis in the presence of a sustained high plasma FFA concentration and had no effect on uptake of circulating ketone bodies. The results indicate that plasma FFA concentration is not the sole determinant of plasma ketone body concentration and that insulin can suppress ketone body production through some means other than lowering plasma FFA concentration.

Glycerol metabolism in the neonatal rat

1. The possible role of glycerol as a precursor in neonatal gluconeogenesis in the rat was investigated by recording the activities of glycerol kinase and l-glycerol 3-phosphate dehydrogenase in the liver, kidney and other tissues around birth and during the neonatal period. 2. Blood glycerol concentrations in the neonatal rat are high. 3. There is a marked increase after birth in the ability of both liver and kidney slices to convert glycerol into glucose plus glycogen that correlates with the increase in glycerol kinase activity. 4. High hepatic and renal l-glycerol 3-phosphate dehydrogenase activities are also found in the neonatal period. 5. The marked capacity for neonatal gluconeogenesis from glycerol thus demonstrated and the role of glycerol kinase in its control are discussed.

Changes in the concentration of specific amino acids in the serum of experimentally malnourished pigs

1. The concentrations of individual free serum amino acids have been studied during the course of chronic protein malnutrition in experimentally malnourished pigs.

2. Until growth was markedly impaired, reducing the protein content of the diet had no significant effect on serum amino acid composition.

3. During the period when growth had practically ceased but kcal intake per kg body-weight was normal the concentrations of valine, leucine, isoleucine, threonine and eventually tyrosine fell, the levels of alanine and serine were above control levels and aspartic acid, glycine, methionine, phenylalanine, proline, arginine, histidine and lysine remained unaltered.

4. When the kcal intake per kg fell below control levels, owing to anorexia, and the animals began to lose weight, the levels of alanine, glycine, serine and methionine fell and valine, leucine, isoleucine, threonine and tyrosine remained well below control levels.

5. The physiological, nutritional and metabolic significance of these findings is considered. The changes in alanine and glycine concentration are discussed in terms of the regulation of gluconeogenesis. The reduction of the branched-chain amino acids is interpreted on the basis of their availability for synthetic and catabolic needs.

Alanine: key role in gluconeogenesis

Of 20 amino acids measured, alanine is the principal amino acid released by forearm muscle of man, in accord with its being the principal amino acid extracted by liver for gluconeogenesis. This occurs in both the postabsorptive state and after 4 to 6 weeks of starvation, when total amino acid release is markedly diminished.

Acetoacetate metabolism in infant and adult rat brain in vitro

1. Acetoacetate or dl-beta-hydroxybutyrate increases the rate of oxygen consumption to a smaller extent than that brought about by glucose or pyruvate in adult rat brain-cortex slices but to the same extent as that in infant rat brain-cortex slices. 2. The rate of (14)CO(2) evolution from [1-(14)C]glucose considerably exceeds that from [6-(14)C]glucose in respiring infant rat brain-cortex slices, in contrast with adult brain-cortex slices, suggesting that the hexose monophosphate shunt operates at a greater rate in the infant rat brain than in the adult rat brain. 3. The rate of (14)CO(2) evolution from [3-(14)C]acetoacetate or dl-beta-hydroxy[3-(14)C]butyrate, in the absence of glucose, is the same in infant rat brain slices as in adult rat brain slices. It exceeds that from [2-(14)C]glucose in infant rat brain but is less than that from [2-(14)C]glucose in adult rat brain. 4. Acetoacetate is oxidized in the brain through the operation of the citric acid cycle, as shown by the accelerating effect of glucose on acetoacetate oxidation in adult brain slices, by the inhibitory effects of malonate in both infant and adult brain slices and by its conversion into glutamate and related amino acids in both tissues. 5. Acetoacetate does not affect glucose utilization in adult or infant brain slices. It inhibits the rate of (14)CO(2) formation from [2-(14)C]glucose or [U-(14)C]-glucose the effect not being wholly due to isotopic dilution. 6. Acetoacetate inhibits non-competitively the oxidation of [1-(14)C]pyruvate, the effect being attributed to competition between acetyl-CoA and CoA for the pyruvate-oxidation system. 7. Acetoacetate increases the rate of aerobic formation of lactate from glucose with both adult and infant rat brain slices. 8. The presence of 0.1mm-2,4-dinitrophenol diminishes but does not abolish the rate of (14)CO(2) formation from [3-(14)C]acetoacetate in rat brain slices. This points to the participation of ATP in the process of oxidation of acetoacetate in infant or adult rat brain. 9. The presence of 5mm-d-glutamate inhibits the rate of (14)CO(2) formation from [3-(14)C]acetoacetate, in the presence or absence of glucose. 10. Labelled amino acids are formed from [3-(14)C]acetoacetate in both adult and infant rat brain-cortex slices, but the amounts are smaller than those found with [2-(14)C]glucose in adult rat brain and greater than those found with [2-(14)C]glucose in infant rat brain. 11. Acetoacetate is not as effective as glucose as a precursor of acetylcholine in adult rat brain but is as effective as glucose in infant rat brain slices. 12. Acetoacetate or beta-hydroxybutyrate is a more potent source of acetyl-CoA than is glucose in infant rat brain slices but is less so in adult rat brain slices.