THE INCORPORATION OF ISOTOPIC CARBON 14C INTO THE CEREBRAL GLYCOGEN OF NORMAL AND X-IRRADIATED RATS

Maximal incorporation of ¹⁴C from labelled glucose into cerebral glycogen of the rat occurred at 3-6 h following intravenous injection of the precursor. A reduction of the rate of glycogen breakdown is the most likely cause of the glycogen accumulation observed in rat brain following exposure to 10 krad of X-radiation.     

Effects of insulin-induced hypoglycaemia on the fate of glucose carbon atoms in the mouse

1. [U-(14)C]Glucose was injected into mice and the distribution of (14)C in various chemical fractions of the whole body was determined at times from 15min. to 8hr. after injection. 2. At 1hr. after injection 31.8% of the recovered (14)C was found in the expired air and 26.7% was found in the isolated glycogen, lipids, proteins, nucleic acids and in other acid-insoluble carbon compounds (;residual (14)C'). The rest (41.5%) was combined in acid-soluble substances. 3. When insulin was injected 5min. or 1hr. before injection of [U-(14)C]glucose, and the mouse was killed 1hr. later, the (14)C content of expired air, glycogen, protein and ;residual (14)C' was not significantly affected; but the incorporation of (14)C into lipids was increased two- to three-fold. 4. Chromatography of the lipids on silicic acid columns and by thin-layer chromatography showed that the main effect of insulin injection was to increase the incorporation of (14)C into fatty acids. 5. A significant increase of (14)C after insulin injection was also found in a glyceride in which the (14)C was combined in glycerol.     

Delayed labelling of brain glutamate after an intra-arterial [13C]glucose bolus: evidence for aerobic metabolism of guinea pig brain glycogen store.

Glycogen in glial cells is the largest store of glucose equivalents in the brain. Here we describe evidence that brain glycogen contributes to aerobic energy metabolism of the guinea pig brain in vivo. Five min after an intra-arterial bolus injection of d-[U-14C]glucose, 28+/-11% of the radioactivity in brain tissue was associated with the glycogen fraction, indicating that a significant proportion of labelled glucose taken up by the brain is converted to glycogen shortly after bolus infusion. Incorporation of 13C-label into lactate generated by brains made ischaemic after d-[1-13C]glucose injection confirms that these glucose equivalents can be mobilised for anaerobic glucose metabolism. Aerobic metabolism was monitored by following the time course of 13C-incorporation into glutamate in guinea pig cortex and cerebellum in vivo. After an intra-arterial bolus injection of d-[1-13C]glucose, glutamate labelling reached a maximum 40-60 min after injection, suggesting that a slowly metabolised pool of labelled glucose equivalents was present. As the concentration of 13C-labelled glucose in blood was shown to decrease below detectable levels within 5 min of bolus injection, this late phase of glutamate labelling must occur with mobilisation of a brain storage pool of labelled glucose equivalents. We interpret this as evidence that glucose equivalents in glycogen may contribute to energy metabolism in the aerobic guinea pig brain.     

Effects of insulin on the pattern of glucose metabolism in the perfused working and Lagendorff heart of normal and insulin-deficient rats

1. The metabolic pattern of [U-(14)C]glucose in the isolated rat heart has been studied, with both retrograde aortic (Langendorff) and atrially (working) perfused preparations in the presence and absence of insulin, in normal animals, animals rendered insulin-deficient (by injection of anti-insulin serum 1hr. before excision of the heart) and animals rendered diabetic by streptozotocin injection 7 days before use. 2. Radioautochromatograms of heart extracts show that the pattern of glucose metabolism in heart muscle is more complex than in diaphragm muscle. In addition to (14)CO(2), glycogen, oligosaccharides, phosphorylated sugars and lactate (the main metabolites formed from [(14)C]glucose in diaphragm muscle), (14)C label from [(14)C]glucose appears in heart muscle in glutamate, glutamine, aspartate and alanine, and in tricarboxylic acid-cycle intermediates. 3. By a quantitative scanning technique of two-dimensional chromatograms it was found that a mechanical work load stimulates glucose metabolism, increasing by a factor of 2-3 incorporation of (14)C into all the metabolites mentioned above except lactate and phosphorylated sugars, into which (14)C incorporation is in fact diminished; (14)CO(2) production is equally stimulated. 4. Addition of insulin to the perfusion fluid of the working heart causes increases in (14)C incorporation, by a factor of about 1.5 into (14)CO(2), by a factor of about 3-5 into glycogen, lactate and phosphorylated sugars, by a factor of about 2-3 into glutamate and tricarboxylic acid-cycle intermediates and by a factor of about 0.5 into aspartate, whereas incorporation into alanine and glutamine is not affected. The effect of a work load on the pattern of glucose metabolism is thus different from that of insulin. 5. Increasing the concentration of glucose in the perfusion fluid from 1 to 20mm leads to changes of the pattern of glucose metabolism different from that brought about by insulin. (14)CO(2) production steadily increases whereas [(14)C]lactate and glycogen production levels off at 10mm-glucose, at values well below those reached in the presence of insulin. 6. In Langendorff hearts of animals rendered insulin-deficient by anti-insulin serum or streptozotocin, glucose uptake, formation of (14)CO(2) and [(14)C]lactate, and (14)C incorporation into glycogen and oligosaccharides are decreased. In insulin-deficient working hearts, however, glucose uptake and (14)CO(2) production are normal, whereas incorporation of (14)C into glycogen and [(14)C]lactate production are greatly decreased. 7. Insulin added to the perfusion fluid restores (14)C incorporation from glucose into (14)CO(2), glycogen and lactate in the Langendorff heart from animals rendered insulin-deficient by anti-insulin serum; in hearts from streptozotocin-diabetic animals addition of insulin restores (14)C incorporation into glycogen and lactate, but (14)CO(2) production remains about 50% below normal. 8. The bearing of these results on the problem of the mode of action of insulin is discussed.     

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

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

Rapid activation and inactivation of fatty acid synthesis from glucose in vivo

The flux of glucose carbon to total body fatty acids was measured in unanesthetized mice either after fasting or 50-80 min after they nibbled a small test meal containing 120 mg of glucose (fasted-refed). Flux was calculated from plasma [(14)C]glucose specific activity curves and from total body (14)C-labeled fatty acid 30 min after intravenous injection of tracer [(14)C]glucose. Mobilization of liver glycogen, changes in the body glucose pool size, and total flux of carbon through the glucose pool during periods of fasting and refeeding were defined. Liver glycogen was almost completely depleted 8 hr after food removal. Body glucose pool size fell during fasting and increased after refeeding the test meal. Irreversible disposal rate of glucose C varied directly with body glucose pool size; but flux of glucose C into fatty acids increased exponentially as body glucose concentration increased. Within an hour after nibbling a small test meal, the flux of glucose C into total body fatty acids increased 700% in mice previously starved for 24 hr. However, flux of glucose C into fatty acids in postabsorptive mice (food removed for 2 hr; livers rich in glycogen) was only about 2% of the value calculated from published studies in which the incorporation of an intubated [(14)C]glucose load into total body fatty acid was measured in mice. A possible explanation for this phenomenon is presented.     

Restoration of glycogenesis in hepatocytes from starved rats.

Hepatocytes isolated from the liver of rats starved for two days synthesized glycogen only when incubated in the presence of both glucose and glucogenic precursors (combinations of alanine, glycerol, pyruvate, lactate or fructose). Unlabeled glucogenic precursors facilitated the incorporation of [U-14C]glucose into glycogen. Unlabeled glucose likewise greatly enhanced glycogen synthesis from isotopically labeled lactate and other glucogenic precursors.In those systems which contained no added endocrines glucose dampened glycogen phosphorylase activity in a cAMP-independent fashion. Fructose is unable to mimic the effects of glucose on glycogen deposition and on glycogen phosphorylase activity.     

The mechanism of thyrotrophin action in relation to lipid metabolism in thyroid tissue

1. The effects of thyrotrophin in vitro on the incorporation of [(14)C]-glucose, -glycerol, -palmitate and -oleate into the lipids of thyroid tissue were examined. 2. Thyrotrophin increased the incorporation of these (14)C-labelled precursors into phosphatidylinositol specifically. 3. Thyrotrophin also increased the proportion of (14)C radioactivity from labelled glucose, glycerol, palmitate and oleate incorporated into the 1,2-diglycerides. 4. The addition of thyrotrophin to thyroid slices for 10min., after 2hr. of prelabelling with [(14)C]glycerol, also increased the proportion of (14)C radioactivity incorporated into the 1,2-diglyceride fraction. 5. After incubation of thyroid tissue with [1-(14)C]palmitate, thyrotrophin caused a two- to three-fold increase in the specific radioactivity of palmitate isolated from phosphatidylinositol and 1,2-diglycerides. In contrast, the specific radioactivity of palmitate isolated from the choline and ethanolamine phosphoglycerides, 1,3-diglycerides and triglycerides was not increased by thyrotrophin.     

Regulation of glycogen synthesis from glucose and gluconeogenic precursors by insulin in periportal and perivenous rat hepatocytes.

Glycogen synthesis in hepatocyte cultures is dependent on: (1) the nutritional state of the donor rat, (2) the acinar origin of the hepatocytes, (3) the concentrations of glucose and gluconeogenic precursors, and (4) insulin. High concentrations of glucose (15-25 mM) and gluconeogenic precursors (10 mM-lactate and 1 mM-pyruvate) had a synergistic effect on glycogen deposition in both periportal and perivenous hepatocytes. When hepatocytes were challenged with glucose, lactate and pyruvate in the absence of insulin, glycogen was deposited at a linear rate for 2 h and then reached a plateau. However, in the presence of insulin, the initial rate of glycogen deposition was increased (20-40%) and glycogen deposition continued for more than 4 h. Consequently, insulin had a more marked effect on the glycogen accumulated in the cell after 4 h (100-200% increase) than on the initial rate of glycogen deposition. Glycogen accumulation in hepatocyte cultures prepared from rats that were fasted for 24 h and then re-fed for 3 h before liver perfusion was 2-fold higher than in hepatocytes from rats fed ad libitum and 4-fold higher than in hepatocytes from fasted rats. The incorporation of [14C]lactate into glycogen was 2-4-fold higher in periportal than in perivenous hepatocytes in both the absence and the presence of insulin, whereas the incorporation of [14C]glucose into glycogen was similar in periportal and perivenous hepatocytes in the absence of insulin, but higher in perivenous hepatocytes in the presence of insulin. Rates of glycogen deposition in the combined presence of glucose and gluconeogenic precursors were similar in periportal and perivenous hepatocytes, whereas in the presence of glucose alone, rates of glycogen deposition paralleled the incorporation of [14C]glucose into glycogen and were higher in perivenous hepatocytes in the presence of insulin. It is concluded that periportal and perivenous hepatocytes utilize different substrates for glycogen synthesis, but differences between the two cell populations in the relative utilization of glucose and gluconeogenic precursors are dependent on the presence of insulin and on the nutritional state of the rat.     

Sub-compartmentation of the 'cytosolic' glucose 6-phosphate pool in cultured rat hepatocytes

[14C]Glucose release either from endogenous 14C-prelabelled glycogen or from added 14C-labelled glucose 6-phosphate was measured in filipin-treated, permeabilized hepatocytes in 48 h culture. [14C]Glucose output from prelabelled glycogen was not altered by the addition of 5 mM glucose 6-phosphate to the incubation medium. Conversely, [14C]glucose release from 5 mM labelled glucose 6-phosphate was not influenced by different glycogen concentrations in the cells. Moreover, in the permeabilized cells the anion transport inhibitor DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid) inhibited only the liberation of [14C]glucose from labelled glucose 6-phosphate but not from glycogen. It is therefore concluded that there exist at least 2 separate, mutually non-accessible glucose 6-phosphate pools in cultured rat hepatocytes, one linked to glycogenolysis and the other to gluconeogenesis.     

Selective inhibition of glucose oxidation by triethyltin in rat brain in vivo

Results are reported of a comparative study in vivo of the metabolism of [2-(14)C]-glucose and [1-(14)C]acetate in brains of rats intoxicated with triethyltin sulphate. The incorporation of (14)C from glucose into glutamate, glutamine, gamma-aminobutyrate and aspartate was greatly decreased. The incorporation of (14)C from acetate into these amino acids was unaffected. The experimental data indicated that the main action of triethyltin was to decrease the rate at which pyruvate formed from glucose is oxidized. Glycolysis was not inhibited. Changes in glucose metabolism in the brain are shown not to be directly due to hypothermia. Some of the advantages of measuring the labelling of intermediates at very short time intervals after the injection of the labelled glucose are demonstrated.     

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

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

Studies in lipogenesis in vivo: Fatty acid and cholesterol synthesis during starvation and re-feeding

1. Lipogenesis in vivo has been studied in mice given a 250mg. meal of [U-¹⁴C]glucose (2·5μc) or given an intraperitoneal injection of 25μg. of [U-¹⁴C]glucose (2·0μc). 2. The ability to convert a [U-¹⁴C]glucose meal into fatty acid was not significantly depressed by 6–7hr. of starvation. In contrast, incorporation of ¹⁴C into fatty acid in the liver after the intraperitoneal dose of [¹⁴C]glucose was depressed by 80% and by more than 90% by 1 and 2hr. of starvation respectively. Carcass fatty acid synthesis from the [U-¹⁴C]glucose meal was not depressed by 12hr. of starvation, whereas from the tracer dose of [U-¹⁴C]glucose the depression in incorporation was 80% after 6hr. of starvation. 3. Re-feeding for 3 days, after 3 days' starvation, raised fatty acid synthesis and cholesterol synthesis in the liver fivefold and tenfold respectively above the levels in non-starved control mice. These increases were associated with an increased amount of both fatty acid and cholesterol in the liver. 4. After 18hr. of starvation incorporation of a [U-¹⁴C]glucose meal into carcass and liver glycogen were both increased threefold.     

THE BIOSYNTHESIS OF CHOLESTEROL AND OTHER STEROLS BY BRAIN TISSUE: DISTRIBUTION IN SUBCELLULAR FRACTIONS AS A FUNCTION OF TIME AFTER INJECTION OF [2-14C]MEVALONIC ACID, SODIUM [2-14C]ACETATE AND [U-14C]GLUCOSE INTO 15-DAY-OLD RATS

The distribution of [¹⁴C]-labelled material into subcellular fractions of 15-day-old rat brain was studied at 2 and 24 h following intraperitoneal and intracerebral injection of [2-¹⁴C]sodium acetate, [U-¹⁴C]glucose and [2-¹⁴C]mevalonic acid respectively. The total quantity of labelled isoprenoids in the brain was, except for glucose, greater when the precursor was administered intracerebrally. The intraperitoneal route was more advantageous in the case of [U-¹⁴C]glucose. The subcellular distribution of both labelled total isoprenoid material and sterol was distinct for each labelled precursor. Intracerebrally injected [U-¹⁴C]glucose at both time periods studied suggested no dominance of labelling in any fraction. After intraperitoneal injection of [U-¹⁴C]glucose the microsomes were more prominently labelled. Both methods of administration of sodium [2-¹⁴C]acetate resulted in heavy labelling of the myelin fraction after 24 h. The total labelled isoprenoids resided mainly in the microsomes 24 h after injection of [2-¹⁴C]mevalonic acid. Labelled sterol was found to be localized more in the myelin and microsomal fractions for all three precursors than was the labelled total isoprenoids. Depending on the type of experiment to be conducted, each of these precursors can give different results, which must be interpreted accordingly.     

Gluconeogenesis from lactate in the developing rat. Studies in vivo

1. The specific radioactivity of plasma l-lactate and the incorporation of (14)C into plasma d-glucose, liver glycogen and skeletal-muscle glycogen were measured as a function of time after the intraperitoneal injection of l-[U-(14)C]lactate into 2-, 10- and 30-day-old rats. 2. Between 15 and 60min after the injection of the l-[U-(14)C]lactate, the specific radioactivity of plasma lactate decreased with a half-life of 20-33min in animals at all three ages. 3. At all times after injection examined, the specific radioactivity of plasma glucose of the 2- and 10-day-old rats was at least fourfold greater than that of the 30-day-old rats. 4. Although (14)C was incorporated into liver glycogen the amount incorporated was always less than 5% of that present in plasma glucose. 5. The results are discussed with reference to the factors that may influence the rate of incorporation of (14)C into plasma glucose, and it is concluded that the rate of gluconeogenesis in the 2- and 10-day-old suckling rat is at least twice that of the weaned 30-day-old animal.     

Brain cholesterol XVIII: EFFECt of methylphenidate (Ritalin) on [U-14C] glucose and [2-3H] acetate incorporation.

The effect of a single injection of methylphenidate (Ritalin, 4 mg/kg) on precursor ([2-3H]acetate and [U-14C]glucose) incorporation into brain cholesterol was studied. The drug caused a steady decrease in the concentration of brain cholesterol during the 24-hr period examined. Incorporation studies during this time with [U-14C]glucose indicated higher than normal incorporation for all time periods studied. The most significant incorporation increases took place 2 and 4 hr after drug injection. Experiments using [2-3H]acetate as the sterol precursor gave incorporation values which tended (not significantly) to be lower than control values at 2 and 4 h. The values after 12 hr were less than normal, while the 24-hr group indicated an increase to or slightly higher than normal values. These data suggest that the pharmacological effect of methylphenidate may be due to lowering of brain cholesterol levels directly or on some more basic metabolic process leading to a decreased level of membrane sterols.     

Increased gluconeogenesis in rats exposed to hyper-G stress

The role of gluconeogenesis on the increase in plasma glucose and liver glycogen of rats exposed to hyper-G (radial acceleration) stress was determined. Overnight-fasted, male Sprague-Dawley rats (250-300 g) were injected i.p. with uniformly labeled 1 4C lactate, alanine, or glycerol (5 microCi/rat) and immediately exposed to 3.1G for 0.25, 0.50, and 1.0 hr. 1 4C incorporation of the labeled substrates into plasma glucose and liver glycogen was measured and compared to uncentrifuged control rats injected in a similar manner. Significant increases in 1 4C incorporation of all three labeled substrates into plasma glucose were observed in centrifuged rats at all exposure periods; 1 4C incorporation into liver glycogen was significantly increased only at 0.50 and 1.0 hr. The i.p. administration (5 mg/100-g body wt) of 5-methoxyindole-2-carboxylic acid, a potent gluconeogenesis inhibitor, prior to centrifugation blocked the increase in plasma glucose and liver glycogen during the first hour of centrifugation. The increase in plasma glucose and liver glycogen was also abolished in adreno-demedullated rats exposed to centrifugation for 1.0 hr. Propranolol, a beta-adrenergic blocker, suppressed the increase in plasma glucose of rats exposed to centrifugation for 0.25 hr. From the results of this study, it is concluded that the initial, rapid rise in plasma glucose as well as the increase in liver glycogen of rats exposed to hyper-G stress can be attributed to an increased rate of gluconeogenesis, and that epinephrine plays a dominant role during the early stages of exposure to centrifugation.     

Lactate metabolism in the perfused rat hindlimb.

A preparation of isolated rat hindleg was perfused with a medium consisting of bicarbonate buffer containing Ficoll and fluorocarbon, containing glucose and/or lactate. The leg was electrically prestimulated to deplete partially muscle glycogen. The glucose was labelled uniformly with 14C and with 3H in positions 2, 5 or 6, and lactate uniformly with 14C and with 3H in positions 2 or 3. Glucose carbon was predominantly recovered in glycogen, and to a lesser extent in lactate. The 3H/14C ration in glycogen from [5-3H,U-14C]- and [6-3H,U-14C]-glucose was the same as in glucose. Nearly all the utilized 3H from [2-3H]glucose was recovered as water. Insulin increased glucose uptake and glycogen synthesis 3-fold. When the muscle was perfused with a medium containing 10 mM-glucose and 2 mM-lactate, there was little change in lactate concentration. 14C from lactate was incorporated into glycogen. There was a marked exponential decrease in lactate specific radioactivity, much greater with [3H]- than with [14C]-lactate. The 'apparent turnover' of [U-14C]lactate was 0.28 mumol/min per g of muscle, and those of [2-3H]- and [3-3H]-lactate were both about 0.7 mumol/min per g. With 10 mM-lactate as sole substrate, there was a net uptake of lactate, at a rate of about 0.15 mumol/min per g, and the apparent turnover of [U-14C]lactate was 0.3 mumol/min per g. The apparent turnover of [3H]lactate was 3-5 times greater. When glycogen synthesis was low (no prestimulation, no insulin), the incorporation of lactate carbon into glycogen exceeded that from glucose, but at high rates of glycogen deposition the incorporation of lactate carbon was much less than that of glucose. Lactate incorporation into glycogen was similar in fast-twitch white and fast-twitch red muscle, but was very low in slow-twitch red fibres. We find that (a) pyruvate in muscle is incorporated into glycogen without randomization of carbon, and synthesis is not inhibited by mercaptopicolinate or cycloserine; (b) there is extensive lactate turnover in the absence of net lactate uptake, and there is a large dilution of 14C-labelled lactate from endogenous supply; (c) there is extensive detritiation of [2-3H]- and [3-3H]-lactate in excess of 14C utilization.     

Effect of growth hormone on glycogenesis in cerebral cortical slices

The uptake of glucose by cerebral cortical slices of rats was found to be enhanced by insulin by Rafaelsen (1961) and Genes and Charnaya (1966). This was confirmed by Prasannan and Subrah-manyam (1965) and more recently by Nelson , Schultz , Pasoneau and Wry (1968). Eisenberg and Seltzer (1962) and Gotistein , Held , Sebenng and Walpurger (1965) obtained evidence for a direct effect of insulin on the entry of glucose into brain and on its metabolism in this tissue. A marked resynthesis of glycogen was demonstrated with glucose as substrate by Lebaron (1955) and Mcilwain and Tresize (1956) in cerebral cortical slices of the guinea pig. Prasannan and Subrahmanyam (1965) obtained evidence for a similar resynthesis of glycogen in cerebral cortical slices of the rat. Addition of 0.2 unit of insulin per 3.5 ml of incubating medium gave rise to an increase of 60 per cent in the resynthesis of glycogen in these slices. The incorporation of ¹⁴C from labelled glucose into glycogen and CO₂ by cerebral cortical slices of normal and alloxan diabetic rats and the stimulation of the incorporation into glycogen by insulin in vitro was reported by Visweswaran , Prasannan and Subrahmanyam (1969). An insulin-like action of growth hormone on the carbohydrate metabolism was reported by Ketterer , Randle and Young (1967) and Manchester and Young (1961). It was believed to be due to the formation of a polypeptide breakdown product of growth hormone which has biological insulin-like properties. Park , Brown , Cornbluth , Daughaday and Krahl . (1952) reported an increased uptake of glucose by isolated rat diaphragm due to the action of growth hormone which is similar to that of insulin. Hence, it was considered appropriate to study the incorporation of ¹⁴C from labelled glucose into glycogen and CO₂ by cerebral slices of growth hormone treated rats and the effect of growth hormone treatment on the activities of the enzymes concerned with glycogenesis in rat cerebral cortex.     

Studies on lipogenesis in vivo. Effect of dietary fat or starvation on conversion of [14C]glucose into fat and turnover of newly synthesized fat

1. Lipogenesis was studied in vivo by giving mice 250mg. meals of [U-(14)C]glucose and measuring the disposition and incorporation of label. About 48% of the (14)C dose was eliminated as (14)CO(2) in the first 2hr. At 60min. after administration, 1.0, 1.9 and 11.9% of the dose was recovered as liver glycogen, liver fatty acid and carcass fatty acid respectively. Of the [(14)C]glucose converted into fat in the epididymal pads about 90% was present as glyceride fatty acid and 10% as glyceride glycerol. 2. Hepatic synthesis of fatty acid was depressed by dietary fat to a much greater extent than was synthesis outside the liver. Both feeding with fat and starvation decreased the proportion of the label taken up by adipose tissue present as fat (triglyceride) and increased the proportion of triglyceride label present as glyceride glycerol. These results are consistent with the hypothesis that the primary action of both these conditions in decreasing fat synthesis is to inhibit synthesis of fatty acids. 3. Turnover of body fat labelled in vivo from [U-(14)C]glucose was estimated from the decline in radioactivity measured over the first 24hr. of the experiment. The half-life of liver and extrahepatic fatty acids (excluding epididymal fat) was 16hr. and 3 days respectively. In contrast, no measurable decrease in radioactivity of the fatty acids of epididymal fat was observed for 7 days after administration of the [U-(14)C]glucose.