Evidence for the presence of a pool of glycerides with a rapid rate of turnover in brown fat from newborn rabbits

1. The specific radioactivity of [(14)C]glycerol released during the incubation of brown fat with [(14)C]glucose is much greater than that of the tissue lipid glycerol. 2. From a study of the release of [(14)C]glycerol from pre-labelled brown fat, it is concluded that the tissue contains a pool of glycerides with a higher rate of turnover than those in the main lipid store. 3. This pool contains newly synthesized glycerides, has a half-life of 25-30min and supplies about 25% of the glycerol liberated by brown fat. 4. Thus, a significant fraction of the total (14)C incorporated from glucose into brown-fat lipids is released as [(14)C]glycerol during an incubation.     

The effect of human placental lactogen upon the metabolism of rat and human adipose tissue.

The metabolic effects of human placental lactogen (HPL) on rat and human white fat were tested in vitro. When tested against rat tissue, HPL resembled insulin in stimulating uptake of glucose and incorporation of [14C] glucose into CO2, triglyceride and glycogen, but differed from insulin in stimulating glycerol release and in failing to stimulate the incorporation of [14C] The stimulation of [14C] glucose incorporation and the inhibition of glycerol release by insulin were antagonized by HPL. The effects of HPL on human white fat resembled those on rat white fat,except that glycerol release was not stimulated in human tissue. The possible role of HPL in causing the diabetogenic stress of pregnancy is discussed in the light of these findings.     

Lipogenesis in rat brown adipocytes. Effects of insulin and noradrenaline, contributions from glucose and lactate as precursors and comparisons with white adipocytes.

1. Brown adipocytes were isolated from the interscapular depot of male rats maintained at approx. 21 degrees C. In some experiments parallel studies were made with white adipocytes from the epididymal depot. 2. Insulin increased and noradrenaline decreased [U-14C]glucose incorporation into fatty acids by brown adipocytes. Brown adipocytes differed from white adipocytes in that exogenous fatty acid (palmitate) substantially decreased fatty acid synthesis from glucose. Both noradrenaline and insulin increased lactate + pyruvate formation by brown adipocytes. Brown adipocytes converted a greater proportion of metabolized glucose into lactate + pyruvate and a smaller proportion into fatty acids than did white adipocytes. 3. In brown adipocytes, when fatty acid synthesis from [U-14C]glucose was decreased by noradrenaline or palmitate, incorporation of 3H2O into fatty acids was also decreased to an extent which would not support proposals for extensive recycling into fatty acid synthesis of acetyl-CoA derived from fatty acid oxidation. 4. In the absence of glucose, [U-14C]lactate was a poor substrate for lipogenesis in brown adipocytes, but its use was facilitated by glucose. When brown adipocytes were incubated with 1 mM-lactate + 5 mM-glucose, lactate-derived carbon generally provided at least 50% of the precursor for fatty acid synthesis. 5. Both insulin and noradrenaline increased [U-14C]glucose conversion into CO2 by brown adipocytes (incubated in the presence of lactate) and, in combination, stimulation of glucose oxidation by these two agents showed synergism. Rates of 14CO2 formation from glucose by brown adipocytes were relatively small compared with maximum rates of oxygen consumption by these cells, suggesting that glucose is unlikely to be a major substrate for thermogenesis. 6. Brown adipocytes from 6-week-old rats had considerably lower maximum rates of fatty acid synthesis, relative to cell DNA content, than white adipocytes. By contrast, rates of fatty acid synthesis from 3H2O in vivo were similar in the interscapular and epididymal fat depots. Expressed relative to activities of fatty acid synthase or ATP citrate lyase, however, brown adipocytes synthesized fatty acids as effectively as did white adipocytes. It is suggested that the cells most active in fatty acid synthesis in the brown adipose tissue are not recovered fully in the adipocyte fraction during cell isolation. Differences in rates of fatty acid synthesis between brown and white adipocytes were less apparent at 10 weeks of age.     

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.     

Blood glycerol is an important precursor for intramuscular triacylglycerol synthesis.

The utilization of blood glycerol and glucose as precursors for intramuscular triglyceride synthesis was examined in rats using an intravenous infusion of [2-(14)C]glycerol and [6-(3)H]glucose or [6-(14)C]glucose. In 24-h fasted rats, more glycerol than glucose was incorporated into intramuscular triglyceride glycerol in soleus (69 +/- 23 versus 4 +/- 1 nmol/micromol triglyceride/h, respectively, p = 0.02 glycerol versus glucose) and in gastrocnemius (25 +/- 5 versus 9 +/- 2 nmol/micromol triglyceride/h, respectively, p = 0.02). Blood glucose was utilized more than blood glycerol for triglyceride glycerol synthesis in quadriceps. In fed rats, the blood glycerol incorporation rates (4 +/- 2, 8 +/- 3, and 9 +/- 3 nmol/micromol triglyceride/h) were similar (p > 0.3) to those of glucose (5 +/- 2, 8 +/- 2, and 5 +/- 2 nmol/micromol triglyceride/h for quadriceps, gastrocnemius, and soleus muscle, respectively). Glucose incorporation into intramuscular triglycerides was less with [6-(3)H]glucose than with [6-(14)C]glucose, suggesting an indirect pathway for glucose carbon entry into muscle triglyceride. The isotopic equilibrium between plasma and intramuscular free glycerol ([U-(13)C]glycerol) was complete in quadriceps and gastrocnemius, but not soleus, within 2 h after beginning the tracer infusion. We conclude that blood glycerol is a direct and important precursor for muscle triglyceride synthesis in rats, confirming the presence of functionally important amounts of glycerol kinase in skeletal muscle.     

Glucose metabolism "in vitro" in brown adipose tissue from pregnant and lactating rats

(U-14C)Glucose utilization has been studied "in vitro" in brown adipose tissue pieces from virgin, 20-day pregnant and 15-day lactating rats. Brown fat pieces from virgin rats increased their (U-14C)glucose utilization for (14C)CO2 production and for (14C)fatty acid and (14C)glycogen synthesis when insulin was present in the medium. Opposite changes were observed due to the presence of noradrenaline. Brown fat from late pregnant rats does not present any essential alteration in its capacity of metabolizing glucose and showed a pattern of responses to insulin and noradrenaline similar to that from virgins. Brown fat from mid lactating rats showed an intrinsic reduction in (U-14C)glucose utilization for oxidative pathways as well as for fatty acid synthesis, this reduction was present in all hormonal conditions. This data suggests a relationship between the lowered glucose metabolism and the known reduction in brown fat thermogenesis during mid lactation.     

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.     

Acute hyperinsulinemia inhibits intramyocellular triglyceride synthesis in high-fat-fed obese rats

Hyperinsulinemia is common in obesity, but whether it plays a role in intramyocellular triglyceride (imcTG) buildup is unknown. In this study, hyperinsulinemic-euglycemic clamp experiments were performed in overnight-fasted lean and high-fat-fed obese rats, awake, to determine the effect of insulin on imcTG synthesis (incorporation of [(14)C]glycerol, [(14)C]glucose, and [(3)H]oleate). Insulin infusion at 25 (low insulin) and 100 (high insulin) pmol/kg/min increased plasma insulin by 5- and 16-fold, respectively, whereas plasma and intramyocellular glycerol, FFAs, triglycerides, and glucose levels were maintained at their basal levels by co-infusion of exogenous glycerol, FFAs, and triglycerides at fixed rates and glucose at varying rates. In obese rats, insulin suppressed incorporation of glycerol into the imcTG-glycerol moiety dose dependently (P < 0.01-P < 0.001) in gastrocnemius and tibialis anterior, but only the high insulin suppressed it in soleus (P < 0.05). The low insulin suppressed glucose incorporation into imcTG-glycerol in all three muscles (P = 0.01-P < 0.01). However, the low insulin did not affect (P > 0.05) and the high insulin suppressed (P < 0.05-P < 0.01) fatty acid incorporation into imcTG in all three muscles. Insulin also suppressed glycerol incorporation in lean rats (P < 0.01-P < 0.04). On the other hand, imcTG pool size was not affected by insulin (P > 0.05). These observations suggest that acute hyperinsulinemia inhibits imcTG synthesis and thus does not appear to promote imcTG accumulation via the synthetic pathway, at least in the short term.     

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.     

The influence of insulin and of contraction on glucose metabolism in the perfused diaphragm muscle from normal and streptozotocin-treated rats

1. The metabolism of [U-(14)C]glucose in perfused resting and contracting diaphragm muscle from normal rats and rats made diabetic with streptozotocin was studied in the presence and absence of insulin. 2. The incorporation of [U-(14)C]-glucose into glycogen and oligosaccharides was stimulated by insulin under all experimental conditions studied. 3. In the normal perfused resting diaphragm muscle the incorporation of radioactivity from [(14)C]glucose into lactate and CO(2) was not affected by insulin. 4. Periodic contractions, induced by electrical stimulation of the perfused diaphragm muscle in the absence of insulin, caused an increased incorporation of (14)C into glycogen and hexose phosphate esters, whereas incorporation of (14)C into lactate was greatly decreased. Production of (14)CO(2) in the contracting muscle was not significantly different from that in resting muscle. Addition of insulin to the perfusion liquid caused a further increase in formation of [(14)C]-glycogen in contracting muscle to values reached in the resting muscle in the presence of insulin. Formation of [(14)C]lactate was also stimulated by insulin, to values close to those found in the resting muscle in the presence of insulin. 5. In the diabetic resting muscle the rate of glucose metabolism was very low in the absence of insulin. Insulin increased formation of [(14)C]glycogen to the value found in normal muscle in the absence of insulin. Production of (14)CO(2) and formation of [(14)C]hexose phosphate remained unchanged. 6. In the diabetic contracting muscle production of (14)CO(2) was increased to values approaching those found in normal contracting muscle. Formation of [(14)C]lactate and [(14)C]glycogen was also increased by contraction, to normal values. Only traces of [(14)C]hexose phosphate were detectable. Addition of insulin to the perfusion medium stimulated formation of [(14)C]glycogen, to values found in normal contracting muscle. Production of [(14)C]hexose phosphate was stimulated by insulin, to approximately the values found in the normal contracting muscle. Production of (14)CO(2) and [(14)C]lactate, however, was not significantly affected by insulin. 7. These results indicate that the defects of glucose metabolism observed in perfused resting diabetic diaphragm muscle can be partially corrected by contraction, and in the presence of insulin the contracting diabetic muscle has a completely normal pattern of glycogen synthesis and lactate production, but CO(2) production remains impaired.     

Myocardial triglyceride turnover and contribution to energy substrate utilization in isolated working rat hearts

The objective of this study was to determine the contribution of myocardial triglycerides to overall ATP production in isolated working rat hearts. Endogenous lipid pools were initially prelabeled (pulsed) by perfusing hearts for 60 min with Krebs-Henseleit buffer containing 1.2 mM [1-14C]palmitate. During a subsequent 60-min period (chase), hearts were perfused with either no fat, low fat (0.4 mM [9,10-3H] palmitate), or high fat (1.2 mM [9,10-3H]palmitate). All buffers contained 11 mM glucose. During the "chase," 14CO2 production (a measure of endogenous fatty acid oxidation) and 3H2O production (a measure of exogenous fatty acid oxidation) were determined. Oxidative rates of endogenous fatty acids during the chase were 279 +/- 50, 88 +/- 14, and 88 +/- 8 nmol of [14C]palmitate oxidized per g dry weight.min in the no fat, low fat, and high fat groups, respectively, compared to exogenous palmitate oxidation rates of 0, 361 +/- 68, and 633 +/- 60 nmol of [3H]palmitate/g dry weight.min, in the no fat, low fat, and high fat groups, respectively. Endogenous [14C]palmitate oxidation rates were matched by loss of [14C]palmitate from endogenous myocardial triglycerides. Overall triglyceride content decreased during the no fat and low fat chase perfusion but did not change during the high fat chase. Loss of triglyceride [14C]palmitate during the high fat chase was matched by incorporation of exogenous [3H]palmitate in triglycerides. In a second series of perfusions, three groups of hearts were perfused under similar conditions, except that unlabeled palmitate was used during the "pulse" and that 11 mM [2-3H/U-14C]glucose and unlabeled palmitate was present during the chase. During the chase, both glycolysis (3H2O production) and glucose oxidation (14CO2 production) rates were measured. Rates of glucose oxidation were inversely related to the fatty acid concentration in the perfusate (1257 +/- 158, 366 +/- 40, and 124 +/- 26 nmol of glucose oxidized per min.g dry weight in the no fat, low fat, and high fat groups, respectively), while rates of glycolysis were not significantly different between these groups. Calculation of overall ATP production from both oxidative and glycolytic sources determined that even in the presence of high concentrations of fatty acids, myocardial triglyceride turnover can provide over 11% of steady state ATP production in the aerobically perfused heart. In the absence of fatty acids, myocardial triglyceride fatty acids can become the major energy substrate of the heart.(ABSTRACT TRUNCATED AT 400 WORDS)     

Myxospore coat synthesis in Myxococcus xanthus: in vivo incorporation of acetate and glycine.

Myxospore coat synthesis in Myxococcus xanthus was studied by incorporation of [(14)C]acetate into intermediates in the biosynthesis of coat polysaccharide and into acid-insoluble material during vegetative growth and after glycerol induction of myxospores. During short labeling periods at 27 degrees C, the radioactivity was shown to be located primarily in N-acetyl groups rather than sugar moieties. Two hours after glycerol induction, the pools of N-acetylglucosamine 6-phosphate and uridine 5'-diphosphate-N-acetylgalactosamine (UDPGalNAc) plus uridine 5'-diphosphate-N-glucosamine increased about twofold and were labeled at twice the rate measured for vegetative cells. The increased rate of synthesis of UDPGalNAc and its precursors could be correlated with increased enzyme activities measured in vitro. Controlled acid hydrolysis revealed that the galactosamine portion of the myxospore coat was N-acetylated. After glycerol induction, the incorporation of acetate into acid-insoluble material increased threefold. This enhanced incorporation was sensitive to neither penicillin nor d-cycloserine. In contrast, bacitracin inhibited the incorporation of [(14)C]acetate into acid-insoluble material more effectively 2 h after myxospore induction than during vegetative growth. Chloramphenicol added to cells 90 min after induction blocked further increase in the rate of [(14)C]acetate incorporation. Since the myxospore coat contains glycine, polymer synthesis was also measured by chloramphenicol-insensitive [(14)C]glycine incorporation into acid-insoluble material. Although protein synthesis decreased after glycerol induction, glycine incorporation increased. Two hours after induction, glycine incorporation was only 75% inhibited by chloramphenicol and rifampin. The chloramphenicol-insensitive rate of incorporation of [(14)C]glycine increased during the first hour after myxospore induction and reached a peak rate after 2 to 3 h. The chloramphenicol-resistant incorporation of [(14)C]glycine was resistant to penicillin but sensitive to bacitracin.     

The effect of noradrenaline on glyceride synthesis and oxidative metabolism in vitro in the brown fat of newborn rabbits

1. The effect of noradrenaline on the synthesis of glyceride from [U-¹⁴C]glucose and on gas exchange in the brown fat of newborn rabbits in vitro was investigated. 2. The specific radioactivity of l-glycerol 3-phosphate was lower than that of lactate, presumably because glycerol derived from glyceride was rephosphorylated by glycerokinase. 3. In the basal state more than 25% of the total respiration was due to pyruvate oxidation. Noradrenaline stimulated glyceride synthesis and total respiration without changing the proportion of the total respiration due to pyruvate oxidation. 4. The extra ADP released by noradrenaline stimulation of glyceride synthesis could not have supported more than 2% of the observed increase in substrate oxidation if mitochondria from brown-fat-cells remain fully coupled in the stimulated state, but could have supported about one-third of the observed increase if they become uncoupled in the presence of noradrenaline.     

Control of glucose metabolism in isolated acini of the lactating mammary gland of the rat. The ability of glycerol to mimic some of the effects of insulin.

Inhibition of glucose uptake by acetoacetate and relief of this inhibition by insulin found previously in slices of rat mammary gland [Williamson, McKeown & Ilic (1975) Biochem. J. 150. 145-152] was confirmed in acini, which represent a more homogeneous population of cells. Glycerol (1mM) behaved like insulin (50 minuits/ml) in its ability to relieve the inhibition of glucose (5 mM) utilization caused by acetoacetate (2 mM) in acini. Both glycerol and insulin reversed the increase in [citrate] and the decrease in [glycerol 3-phosphate] and the [lactate]/[pyruvate] ratio in the presence of acetoacetate. Lipogenesis from 3H2O, [3-14C] acetoacetate, [1-14C]- and [6-14C]-glucose was stimulated, whereas 14CO2 formation from [3-14C]acetoacetate was decreased. Neither insulin nor glycerol relieved the acetoacetate inhibition of glucose uptake when lipogenesis was inhibited by 5-(tetradecyloxy)-2-furoic acid. From measurements of [3-14C]acetoacetate incorporation into lipid in the various situations it is suggested that a cytosolic pathway for acetoacetate utilization may exist in rat mammary gland. In the absence of acetoacetate, glycerol inhibited glucose utilization by 60% and increased both [glycerol 3-phosphate] and the [lactate/[pyruvate] ratio. Possible ways in which glycerol may mimic the effects of insulin are discussed.     

Regulation by cytidine nucleotides of the acylation of sn-[14C]glycerol 3-phosphate. Regional and subcellular distribution of the enzymes responsible for phosphatidic acid synthesis de novo in the central nervous system of the rat

1. The regional and subcellular distribution of the incorporation of sn-[(14)C]glycerol 3-phosphate into rat brain lipids in vitro was investigated and compared with the relative specific activity of various chemical and enzyme markers. The similarity between the subcellular distribution of this incorporation and of NADPH-cytochrome c reductase activity indicated that the synthesis of phosphatidic acid via this route correlated with the presence of endoplasmic reticulum. 2. Experiments in which various amounts of the microsomal fraction were added to fixed amounts of nuclear, myelin, nerve-ending and mitochondrial preparations clearly demonstrated that the endoplasmic-reticulum contamination of these fractions was entirely responsible for the incorporation of sn-[(14)C]glycerol 3-phosphate. 3. The presence of CMP or CTP inhibited the incorporation of sn-[(14)C]glycerol 3-phosphate into the whole homogenate. Similar effects were observed with individual fractions, except for the mitochondria. With the mitochondrial fraction the effect of these cytidine nucleotides varied with the preparation, stimulating in some preparations and inhibiting with other preparations. The presence of CDP-choline stimulated the incorporation into the whole homogenate and to a lesser extent into the subcellular fractions. 4. These results indicate that the various organelles of the central nervous system are more dependent on endoplasmic reticulum for the production of glycerolipids de novo than has previously been appreciated.     

Effect of CO 2 concentration on phospholipid metabolism in the isolated perfused rat lung

Studies have been carried out on the incorporation of [U-(14)C]glucose, [2-(14)C]pyruvate, [2-(14)C]acetate, and [1-(14)C]-palmitate into the phospholipids of the isolated perfused rat lung in the presence of either 6 or 45 mm total CO(2) concentration in the perfusion medium. Incorporation of [U-(14)C]glucose into total phospholipid and into the phosphatidylcholine fraction was increased 19-53% over the 2-hr perfusion period in lungs perfused with medium containing 45 as compared with 6 mm CO(2). The incorporation of [2-(14)C]acetate, [2-(14)C]-pyruvate, and [1-(14)C]palmitate was not affected by the change in medium CO(2) concentration. Increased incorporation of [U-(14)C]glucose combined with a shift toward greater incorporation into the fatty acids of the phosphatidylcholine fraction produced a maximum increase of 90% in [U-(14)C]glucose incorporation into the fatty acids of phosphatidylcholine after 2 hr of perfusion in the presence of medium containing 45 mm CO(2) as compared with 6 mm CO(2). The increase in medium CO(2) concentration produced as much as a 150% increase in [U-(14)C]glucose incorporation into palmitate derived from the phosphatidylcholine fraction. The results provide evidence that glucose functions as an important precursor of palmitate in the phosphatidylcholine fraction of lung phospholipids and that the CO(2) concentration of the perfusion medium affects the incorporation of glucose into palmitate.     

Insulin insensitivity and altered glucose utilization in cultured rat adipose tissue

Glucose utilization was studied in isolated fat cells prepared from rat adipose tissue which had been cultured for 18 hr in TC 199 medium. When 1% bovine serum albumin (BSA) was in the culture medium, basal rates of (14)CO(2) and [(14)C]triglyceride production from [1-(14)C]glucose were markedly depressed and there was no effect of insulin. With 4% BSA, basal (14)CO(2) production was the same as in cells prepared from fresh tissue and basal triglyceride production was greatly increased. Insulin effect on these cells was minimal. One-minute uptake of [(14)C]2-deoxyglucose was stimulated by 800-1000% in fresh cells and 300-500% in cells cultured with either 1% or 4% BSA. Oxidation of [U-(14)C]glucose showed a much smaller impairment in cultured cells than for [1-(14)C]glucose, suggesting that the pentose phosphate shunt was more severely impaired than glycolysis. Glyceride-glycerol production was increased in cultured cells relative to preculture (fresh) cells. There was no effect of insulin in the culture medium in any of these systems. Rates of free fatty acid and glycerol release were markedly increased in cultured cells, especially when insulin was present in the culture medium. The acute antilipolytic effect of insulin was retained, so that insulin in the test incubation decreased lipolysis by 40-80%. Nevertheless, cell-associated fatty acids were increased in cultured cells and FFA/albumin ratios in the medium often reached potentially toxic levels. The reduction in pentose phosphate shunt activity, lipogenesis, and insulin effect resembles other models of insulin insensitivity. The impaired metabolism is probably due to an intracellular defect. A possible toxic role of either intracellular or extracellular fatty acids cannot be excluded. This system should be a useful model in which to study the cellular mechanisms of insulin insensitivity in adipocytes.-Bernstein, R. S. Insulin insensitivity and altered glucose utilization in cultured rat adipose tissue.     

Glycerol kinase activity and glycerol metabolism of rat granular pneumocytes in primary culture

Glycerol kinase activity and glycerol utilization by rat granular pneumocytes were determined in order to investigate the rate-limiting step for glycerol incorporation into lung lipids. Granular pneumocytes were isolated in primary culture following trypsinization of rat lungs. Glycerol kinase activity was 8.2 nmol/h per 10(6) cells. Incorporation of [1,3-14C]glycerol into total cell lipids was 0.29 nmol/h per 10(6) cells. In the presence of saturating glycerol concentration, production of 3H2O from [2-3H]glycerol was 13 times greater than incorporation of [14C]glycerol into lipids. Glycerol phosphate dehydrogenase activity in isolated cells was approximately 10 times glycerol kinase activity. In the presence of 5.6 mM glucose, glycerol incorporation into lipids was decreased 79% and detritiation of glycerol was decreased 34%. This effect of glucose was due to a 25% increase in cell glycerol 3-phosphate content, resulting in dilution of the precursor pool and possible inhibition of glycerol phosphorylation. These results indicate that the relatively limited incorporation of glycerol into surfactant phospholipids by lung epithelial cells reflects the relatively high rate of glycerol 3-phosphate oxidation.     

Studies on plasma free-fatty-acid metabolism and triglyceride synthesis of brown adipose tissue in vivo during cold-induced thermogenesis of the newborn rabbit.

Parameters of plasma free fatty acid metabolism (pool size, half time, disappearance rate, turnover time and absolute turnover rate), the influx of plasma free fatty acids into the glycerides of brown adipose tissue and the pathway of triglyceride synthesis in brown adipose tissue (glycerol-1-phosphate versus monoglyceride pathway) were examined after intravenous injection of [1-14C]palmitate in newborn rabbits. In the thermoneutral environment of 35 degrees C the turnover rate of plasma free fatty acids was 10.20 mumol/min per 100 g body weight and its flux into the glycerides of brown adipose tissue 0.367 mumol/min per 100 g body weight. Cold exposure at an ambient temperature of 20 degrees C caused a decrease to 5.84 mumol/min and 0.207 mumol/min per 100 g body weight, respectively. Both under basal conditions at an ambient temperature of 35 degrees C and under cold-induced thermogenesis at an ambient temperature of 20 degrees C triglyceride synthesis in brown adipose tissue ran through the glycerol 1-phosphate pathway.     

The incorporation of isotopic carbon (14C) into the cerebral glycogen of rabbits

1. The incorporation of (14)C into the brain glycogen of conscious rabbits with labelled glucose, bicarbonate and glutamate as precursors has been studied. 2. Substantial incorporation from all these precursors was demonstrated after an interval of 5hr. from their injection. 3. With [(14)C]glucose maximal incorporation occurred at about 8hr. from the time of injection. 4. Hydrocortisone led to increased incorporation of (14)C from labelled glucose. 5. Some comparisons between the turnover of brain glycogen and that of skeletal and cardiac muscle are reported.