Studies on regional blood flow of the mouse using whole-body autoradiography of 14C-iodoantipyrine

Summary In order to visualize regional blood flow in various tissues of the mouse at the same time, the distribution of radioactive carbon from ¹⁴C-iodoantipyrine was studied by whole-body autoradiography. The mice were frozen with Dry-Ice-hexane at 1, 10, 30 min, and 1 h and 3 h after intravenous injection of ¹⁴C-iodoantipyrine. Whole-sagittal sections of the frozen mouse, obtained by using a cryostat microtome, were dried in a cryostat and subjected to autoradiography. The resulting dry autoradiographs are called untreated autoradiographs in the present work. The sections were then fixed in cold 6% (w/v) HClO₄, dried at room temperature and again subjected to autoradiography. Autoradiographs thus obtained are referred to as treated autoradiographs. It was found that the method could be suitable for the estimation of regional blood flow of the renal cortex, spleen, lung, skeletal muscle, bone marrow, thymus, testes, and brain.     

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.     

Distribution of 35S-taurine in rat neonates and adults

Summary The distribution of ³⁵S-taurine in rat neonates and adults was investigated by wholy-body autoradiography. The neonates (4-day-old) were frozen in dry-ice hexane at 30 min, 1, 3 and 6 h after an intraperitoneal injection of ³⁵S-taurine, whereas survival intervals for adult rats were 1 and 3 h. Whole-sagittal sections of the frozen rat, obtained by using a cryostat microtome were dried in situ and autoradiographed. In rat neonates and adults, ³⁵S-taurine was mainly accumulated in the renal cortex, urine, feces, liver, eye (lens, vitreous fluid, retina), hypophysis, thymus, adrenal glands, nasal mucous membrane, salivary glands, gastric mucosa, small and large intestinal mucosa, choroid plexus, myocardium and sebaceous glands. In the rat neonate, such regions as the olfactory bulb, cerebrum, and cerebellum showed relatively high optical density.     

Liver and kidney cortex gluconeogenesis from L-alanine in fed and starved rats

Circulating [14C]glucose 2, 5 and 10 min after intravenous injection of [U-14C]-L-alanine was greater in 24 hr starved than in fed rats. In vitro uptake of [14C]alanine by liver and kidney cortex slices from 24 hr starved and fed rats rose in parallel with increased medium substrate concentration. Formation of [14C]glucose from 1mM [14C]alanine was similar in liver and kidney cortex slices and increased in tissues from 24 hr starved compared with fed rats. With 5 mM [14C]alanine more [14C]glucose was produced by liver than by kidney cortex slices from 24 hr starved rats. Liver slices always produced more [14C]lactate and less [14C]-CO2 from [14C]alanine than kidney cortex slices. It is proposed that under physiological conditions, the kidneys cortex actively participates in glucose production from alanine.     

Pancreatic fate of 14C-labelled hexoses

In order to assess the respective contribution of the exocrine and endocrine moieties of the pancreas to the overall net uptake of selected monosaccharides by the pancreatic gland, the apparent distribution space of L-[1-14C]glucose, 3-O-[14C-methyl]-D-glucose, D-[U-14C]glucose, D-[U-14C]mannose and D-[U-14C]fructose was measured in pieces of pancreas obtained from either control rats or animals injected with streptozotocin. Although the time course for the uptake of 3-O-[14C-methyl]-D-glucose, D-[U-14C]glucose, D-[U-14C]mannose and D-[U-14C]fructose was much slower in the pieces of pancreas than that previously documented in isolated pancreatic islets, no significant difference could, as a rule, be detected between the results obtained in pancreatic pieces of control and streptozotocin rats. A comparable situation prevailed in the pancreas of animals examined 3 min after the intravenous injection of 3-O-[14C-methyl]-D-glucose. D-Glucose inhibited the uptake of 3-O-[14C-methyl]-D-glucose and that of D-[U-14C]fructose. Likewise, 3-O-methyl-D-glucose inhibited the uptake of D-[U-14C]glucose. Cytochalasin B (20 microm) also inhibited the uptake of 3-O-[14C-methyl]-D-glucose and D-[U-14C]glucose, but not that of D-[U-14C]fructose. D-Mannoheptulose hexaacetate, but not the unesterified heptose, inhibited the metabolism of tritiated and 14C-labelled D-glucose, as well as the net uptake of D-[U-14C]glucose and D-[U-14C]mannose and, to a lesser extent, that of D-[U-14C]fructose. These findings indicate that despite marked differences between endocrine and exocrine pancreatic cells in terms of both the time course for the uptake of several hexoses and the inhibition of their phosphorylation by D-mannoheptulose, little or no preferential labelling of the endocrine moiety of the pancreas by the 14C-labelled hexoses is observed, at least when judged from their distribution space in pancreatic pieces or the whole pancreatic gland. Nevertheless, the findings made with D-mannoheptulose and its hexaacetate ester raise the view that this heptose could conceivably be used to achieve a sizeable preferential labelling of the endocrine pancreas under the present experimental conditions.     

The distribution of glucose and [14C]glucose between erythrocytes and plasma in the rat

1. Of the glucose in rat blood 79.8+/-3.3% (s.d.) was in the plasma. The variance was mostly due to differences between rats. 2. The concentration of glucose in erythrocyte water was 51+/-8% (s.d.) of that in plasma water. 3. The ratio (specific radioactivity in plasma)/(specific radioactivity in whole blood), i.e. the P/B ratio, was estimated for glucose at intervals after intravenous injection of [U-(14)C]glucose and [U-(14)C]fructose. The ratio differed from unity by more than the standard error of a single determination of the specific radioactivity of blood or plasma glucose except from 10 to 17min. after injection of [(14)C]glucose and from 22 to 30min. after injection of [(14)C]fructose. At all other times specific radioactivities in blood had to be corrected to give specific radioactivities in plasma. How to do so is described. 4. The P/B ratios were accounted for by a turnover of glucose in erythrocytes of 0.14mumole/min./ml. of erythrocytes. 5. Metabolism of glucose in rat erythrocytes is unlikely to be a major source of lactate.     

Mechanism of avian estrogen-induced hypertriglyceridemia: evidence for overproduction of triglyceride.

Relying on methods other than the determination of turnover rate of triglyceride from the curve of plasma triglyceride radioactivity after administration of labeled precursor, we have confirmed that the endogenous hypertriglyceridemia induced by estrogenization of the chick is accompanied by increased production of triglyceride. Chicks estrogenized with diethylstilbestrol became grossly hypertriglyceridemic and had elevated levels of plasma free fatty acid. Within 5 min of administration of labeled palmitate, estrogenized hypertriglyceridemic birds converted approximately 10 times more plasma free fatty acid to hepatic triglyceride than did controls. In addition, 2 hr after intraperitoneal injection of [14-C]acetate or [U-14-C]glucose, the specific activity of very low density lipoprotein triglyceride (VLDL-TG) of estrogenized birds reached or exceeded that of the untreated controls, and the rapid enrichment of the vastly expanded plasma VLDL-TG pool with labeled triglyceride further indicated that increased production of triglyceride occurs with estrogenization. Furthermore, [14-C]acetate incorporation into VLDL-TG was calculated to be 1.6 and 6.6% of the injected dose in estrogenized birds compared with 0.1 and 0.2% in untreated birds. Increased production of plasma VLDL-TG was confirmed by a kinetic study of VLDL-TG metabolism, employing reinjected, endogenously prepared [14-C]triglyceride-labeled VLDL. The fractional turnover rate of VLDL-TG in estrogenized hypertriglyceridemic birds was substantially less than that in untreated controls (0.32 plus or minus 0.03 vs 0.71 plus or minus 0.03/hr), but the total turnover rate was nearly 50 times greater (244 plus or minus 52 vs. 5 plus or minus 1 mg/hr).     

Comparative utilization in vivo of [U-14C] glycerol, [2-3H] glycerol, [U-14C] glucose and [1-C14] palmitate in the rat

Female rats were injected i.v. with comparable trace amounts of [U-14C] glycerol, [2-3H] glycerol, [U-14C] glucose, or [1-14C] palmitate, and killed 30 min afterwards. The radioactivity remaining in plasma at that time was maximal in animals receiving [U-14C] glucose while the appearance of radioactive lipids was higher in the [U-14C] glycerol animals than in other groups receiving hydrosoluble substrates. The carcass, more than the liver, was the tissue where the greatest proportion of radioactivity was recovered, while the greatest percentage of radioactivity appeared in the liver in the form of lipids. The values of total radioactivity found in different tissues were very similar when using either labelled glucose or glycerol but the amount recovered as lipids was much greater in the latter. The maximal proportion of radioactive lipids appeared in the fatty-acid form in the liver, carcass, and lumbar fat pads when using [U-14C] glycerol as a hydrosoluble substrate, and the highest lipidic fraction appeared in adipose tissue as labelled, esterified fatty acids. In the spleen, heart, and kidney, most of the lipidic radioactivity from any of the hydrosoluble substrates appeared as glyceride glycerol. The highest proportion of radioactivity from [1-14C] palmitate appeared in the esterified fatty acid in adipose tissue, being followed in decreasing proportion by the heart, carcass, liver, kidney, and spleen. Thus at least in part, both labelled glucose and glycerol are used throughout different routes for their conversion in vivo to lipids. A certain proportion of glycerol is directly utilized by adipose tissue. The fatty acids esterification ability differs among the tissues and does not correspond directly with the reported activities of glycerokinase, suggesting that the alpha-glycerophosphate for esterification comes mainly from glucose and not from glycerol.     

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.     

Insulin effect on in vivo gluconeogenesis from [3-14C]pyruvate in the starved rat

During a 24 hr fast rats received 4 subcutaneous injections of insulin, and 15 min after the last injection they were given an intravenous pulse of [3-14C]pyruvate. The amount of [14C]glucose in blood 2 min after the tracer did not differ between insulin treated and control animals, whereas at 5 and 10 min values were significantly lower in the former group. At 10 min after the tracer, liver [14C]glycogen specific activity and [14C]fatty acid amount were higher in the insulin treated animals than in controls while plasma concentration of gluconeogenic amino acids was lower in the first group. Similar changes but less pronounced and more retarded were found in 24 hr fasted rats given only one insulin dose 15 min before the [3-14C]pyruvate pulse. Results indicate that gluconeogenesis from pyruvate is not directly modified by insulin treatment. Effects found at 5 and/or 10 min after the tracer and reported effects after prolonged insulin treatments may be caused by one or all of the following possibilities: enhanced utilization of the new-formed glucose, reduced availability of gluconeogenic substrates, and counteracting action on gluconeogenic hormones.     

Contribution of propionate to glucose synthesis in sheep

1. The production rate of propionate in the rumen and the entry rate of glucose into the body pool of glucose in sheep were measured by isotope-dilution methods. Propionate production rates were measured by using a continuous infusion of specifically labelled [(14)C]propionate. Glucose entry rates were estimated by using either a primed infusion or a continuous infusion of [U-(14)C]glucose. 2. The specific radioactivity of plasma glucose was constant between 4 and 9hr. after the commencement of intravenous infusion of [U-(14)C]glucose and between 1 and 3hr. when a primed infusion was used. 3. Infusion of [(14)C]propionate intraruminally resulted in a fairly constant specific radioactivity of rumen propionate between about 4 and 9hr. and of plasma glucose between 6 and 9hr. after the commencement of the infusion. Comparison of the mean specific radioactivities of glucose and propionate during these periods allowed estimates to be made of the contribution of propionate to glucose synthesis. 4. Comparisons of the specific radioactivities of plasma glucose and rumen propionate during intraruminal infusions of one of [1-(14)C]-, [2-(14)C]-, [3-(14)C]- and [U-(14)C]-propionate indicated considerable exchange of C-1 of propionate on conversion into glucose. The incorporation of C-2 and C-3 of propionate into glucose and lactate indicated that 54% of both the glucose and lactate synthesized arose from propionate carbon. 5. No differences were found for glucose entry rates measured either by a primed infusion or by a continuous infusion. The mean entry rate (+/-s.e.m.) of glucose estimated by using a continuous infusion into sheep was 0.33+/-0.03 (4) m-mole/min. and by using a primed infusion was 0.32+/-0.01 (4) m-mole/min. The mean propionate production rate was 1.24+/-0.03 (8) m-moles/min. The conversion of propionate into glucose was 0.36 m-mole/min., indicating that 32% of the propionate produced in the rumen is used for glucose synthesis. 6. It was indicated that a considerable amount of the propionate converted into glucose was first converted into lactate.     

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.     

Evaluation of three isotope-dilution techniques for studying the kinetics of glucose metabolism in sheep

1. Comparisons have been made of three isotope-dilution techniques for measuring parameters of glucose metabolism in sheep given their daily ration in 12 equal amounts (i.e. from 07.00 to 18.00hr.) 2. [U-(14)C]Glucose was used in all experiments. After a single injection the specific radioactivity of plasma glucose was measured at specific times for up to 24hr. Primed infusions were made with various ratios of P, priming injection (nc), to F, infusion rate (nc/min.) (P/F ratios varying from 23:1 to 147:1) and the specific radioactivity of plasma glucose was measured at 60, 120, 150, 180, 210 and 240min. In continuous infusions the specific radioactivity of plasma glucose was followed for 9hr.; a constant specific radioactivity was observed after approximately 180min. 3. A computer programme was used to fit a multi-exponential equation to the log(specific radioactivity)-time curve after a single injection. A second- or third-order exponential equation was found to fit the results. 4. Conventional analyses of all results showed that similar estimates of the irreversible loss of glucose were obtained by using all three techniques. Estimates of glucose pool size and space by using the primed infusion technique were both significantly higher than estimates obtained by the single injection technique. In these experiments total entry rate could only be determined from the single-injection results and a wide variation in estimates was obtained. 5. Comparisons of the specific radioactivity-time relationships after a single injection of [U-(14)C]glucose in sheep given their ration either once daily or as a proportion at hourly intervals indicated that there were fluctuations in glucose synthesis in the former over the period of the experiment. The multi-exponential curves fitted to these results had larger residual variances than in sheep given food at hourly intervals. All parameters of glucose metabolism estimated were similar under both feeding regimes. 6. A number of methods of analysis are discussed and a model for glucose metabolism in sheep in suggested.     

A method for preparing whole-body sections suitable for autoradiographic, histological and histochemical studies

A method for the preparation of whole-body sections suitable for autoradiographic and histochemical study is described. Radioactive calcium chloride or [14C]proline was injected into the abdominal cavity of a rat. Thirty-five minutes after injection of calcium chloride or 40 min after injection of proline the rat was frozen in a mixture of hexane and solid carbon dioxide and blocked in 5% sodium carboxymethyl cellulose. The carboxymethyl cellulose block was trimmed and a piece of copy paper was attached to the surface of the block with cellulose tape. Cryotome sections cut from the block were transferred from the paper to a glass slide coated with synthetic rubber adhesive. For whole-body autoradiography, sections were freeze-dried for 2 days and then placed against X-ray film. For light microscopic autoradiography, the freeze-dried sections were covered with a dried film of photographic emulsion. For histochemical use, the sections were fixed by raising the temperature to 4 C after immersion in 100% ethanol below -10 C. For histological observation, sections were postfixed with 2.5% glutaraldehyde and stained. Whole-body and light microscopic autoradiographs showed that sections so prepared could be used for the demonstration of soluble substances in whole-body sections and for detailed autoradiography at the light microscopic level, and the stained sections could be used for histological and histochemical studies.     

Long-term metabolism of [U-14C]glucose in the whole rat brain

The experiment was performed on rats to which a single injection of [U-14C]glucose had been administered. Results were observed from the 7th to the 281st day following contamination. At 280 days only the lipids in the brain contained radioactivity, the highest degree of specific activity being found in the cerebrosides.     

Gluconeogenesis from acetone in starved rats.

To non-anaesthetized rats starved for 3 days, [U-14C]acetone, NaH14CO3, L-[U-14C]lactate, [2-14C]acetate or D-[U-14C]- plus D-[3-3H]-glucose was injected intravenously. From the change in the plasma concentration of labelled acetone versus time after the injection, the metabolic clearance rate of acetone was calculated as 2.25 ml/min per kg body wt., and its rate of turnover as 0.74 mumol/min per kg. The extent and time course of the labelling of plasma glucose, lactate, urea and acetoacetate were followed and compared with those observed after the injection of labelled lactate, acetate and NaHCO3. The labelling of plasma lactate was rapid and extensive. Some 1.37% of the 14C atoms of circulating glucose originated from plasma acetone, compared with 44% originating from lactate. By deconvolution of the Unit Impulse Response Function of glucose, it was shown that the flux of C atoms from acetone to glucose reached a peak at about 100 min after injection of labelled acetone. In comparable experiments the transfer from lactate reached a peak at 14 min after the injection of labelled lactate. It was concluded that acetone is converted into lactate to a degree sufficient to account for the labelling of plasma glucose and is thus a true, albeit minor, substrate of glucose synthesis in starved rats.     

Metabolism of glucose into glutamate via the hexose monophosphate shunt and its inhibition by 6-aminonicotinamide in rat brain in vivo

The treatment of rats for 4 h with 6-aminonicotinamide (60 mg kg-1) resulted in an 180-fold increase in the concentration of 6-phosphogluconate in their brains; glucose increased 2.6-fold and glucose 6-phosphate, 1.7-fold. Moreover, lactate decreased by 20%, glutamate by 8% and gamma-aminobutyrate by 12%, and aspartate increased by 10%. No significant changes were found in glutamine and citrate. In blood, 6-phosphogluconate increased 5-fold; glucose, 1.4-fold and glucose 6-phosphate, 1.8-fold. The metabolism of glucose in the rat brain, via both the Embden-Meyerhof pathway and the hexose monophosphate shunt, was investigated by injecting [U-14C]glucose or [2-14C]glucose, and that via the hexose monophosphate shunt alone by injecting [3,4-14C]glucose. The total radioactive yield of amino acids in the rat brain was 5.63 mumol at 20 min after injection of [U-14C]glucose, or 5.82 mumol after injection of [2-14C]glucose; by contrast, it was 0.62 mumol after injection of [3,4-14C]glucose. The treatment of rats with 6-aminonicotinamide showed significant decreases in these values, owing to decreases in the radioactive yields of glutamate, glutamine, aspartate, gamma-aminobutyrate, and alanine+glycine+serine. Glutamate isolated from the brain contained approximately 43% of its radioactivity in carbon 1 after injection of [3,4-14C]glucose, in contrast to 13% and 18% after injection of [U-14C]glucose and [2-14C]glucose, respectively, in both the control and treated rats. The calculations based on these findings showed that approximately 69% of the 14C-labelled glutamate was formed from [14C]acetyl coenzyme A (acetyl CoA) and the residual 31% by 14CO2 fixation of pyruvate after injection of [3,4-14C]glucose in both control and treated rats. The results gave direct evidence that glutamate and gamma-aminobutyrate in the brain were formed by metabolism of glucose via the hexose monophosphate shunt as well as via the Embden-Meyerhof pathway. From the radioactive yields of glutamate formed via [14C]acetyl CoA it was estimated that approximately 7.8% of the total glucose utilized was channelled via the hexose monophosphate shunt. Assuming that [14C]glutamate formed by carbon-dioxide fixation of pyruvate was also dependent on the metabolism of glucose through the hexose monophosphate shunt, the estimated value was approximately 9.5% of the total glucose converted into glutamate. The results of the present investigation, taken in conjunction with other findings, suggest that the utilization of glucose via the hexose monophosphate shunt is functionally important in the rat brain.     

Studies on lipogenesis in vivo. Lipogenesis during extended periods of re-feeding after starvation

1. Lipogenesis was studied in mice re-fed for up to 21 days after starvation. At appropriate times [U-(14)]glucose was given by stomach tube and incorporation of (14)C into various lipid fractions measured. 2. In mice starved for 48hr. and then re-fed for 4 days with a diet containing 1% of corn oil, incorporation of (14)C from [U-(14)C]glucose into liver fatty acids and cholesterol was respectively threefold and eightfold higher than in controls fed ad libitum. The percentages by weight of fatty acids and cholesterol in the liver also increased and reached peaks after 7 days. Both the radioactivity and weights of the fractions returned to control values after 10-14 days' re-feeding. These changes could be diminished by re-feeding the mice with a diet containing 20% of corn oil. Incorporation of (14)C from [U-(14)C]glucose into extrahepatic fatty acids (excluding those of the epididymal fat pads) was not elevated during re-feeding with a diet containing either 1% or 20% of corn oil. However, incorporation of (14)C from [U-(14)C]glucose into the fatty acids of the epididymal fat pads was increased in mice re-fed with either diet, as compared with non-starved controls. 3. Lipogenesis was also studied in mice alternately fed and starved. Mice given a diet containing 1% of corn oil for 6hr./day for 4 weeks lost weight initially and never attained the weight or carcass fat content of controls fed ad libitum. Incorporation of (14)C from dietary [U-(14)C]-glucose into the fatty acids of the epididymal fat pads was elevated threefold in the mice allowed limited access to food, although the incorporation into the remainder of the extrahepatic fatty acids was not different from that found for controls. Mice given a diet containing 20% of corn oil for 6hr./day adapted to the limited feeding regimen quicker and in 4 weeks did attain the weight and carcass fat content of controls. Incorporation of (14)C from [U-(14)C]glucose into the fatty acids of the epididymal fat pads and the remainder of the extrahepatic fatty acids was respectively fivefold and threefold higher than in controls fed ad libitum. 4. The elevation in liver lipogenesis during re-feeding was greatest on a diet containing 1% of corn oil, whereas in extrahepatic tissues the increase in lipogenesis was greater when the mice were re-fed or were allowed limited access to a diet containing 20% of corn oil. These results suggest that the causes of the increased rate of incorporation of (14)C from [U-(14)C]glucose into fatty acids during re-feeding may be different in liver from that in extrahepatic tissues.     

Comparative utilization of glycerol and alanine as liver gluconeogenic substrates in the fed late pregnant rat

The appearance of plasma [14C]glucose in the inferior cava vein after a pulse of 0.2 mmol of [U-14C]L-alanine or [U-14C]glycerol/200 g body wt given through the portal vein was studied in fed 21 day pregnant rats and virgin controls under pentobarbital anesthesia. In both groups values were much higher when [U-14C]glycerol was the administered tracer than when [U-14C]L-alanine, and they were augmented in pregnant versus virgin animals at 1 min when receiving [U-14C]glycerol and at 2 min when receiving [U-14C]L-alanine. 20 min after the tracers rats receiving [U-14C]glycerol showed much higher liver [14C]glycogen and [14C]glyceride glycerol than those receiving [U-14C]L-alanine. Radioactivity present in liver as [14C]glyceride glycerol was greater in pregnant than in virgin rats receiving [U-14C]glycerol whereas radioactivity corresponding to [14C]fatty acids was lower in the former group receiving either tracer. At 20 min after maternal treatments fetuses showed lower plasma [14C]glycerol than [14C]alanine values but plasma [14C]glucose and liver [14C]glycogen values were much greater in fetuses from mothers receiving [U-14C]glycerol than [U-14C]L-amine. Besides showing the higher gluconeogenic efficiency in pregnant than in virgin rats, results indicate that at late gestation glycerol is used as a preferential substrate for both glucose and glyceride glycerol synthesis in liver.     

Respiratory metabolism of D[U-14C]glucose in hens and cocks during prolonged treadmill exercise

1. The specific activity of expired 14CO2 was measured at rest and during 90 min treadmill exercise following an initial intravenous injection of D[U-14C]glucose. 2. The rate of CO2 production rose 4.5-fold during exercise in cocks but only 2.5-fold in females. The mean respiratory quotient was close to unity at rest and during exercise. 3. Estimated glucose turnover rate rose approximately 3.5-fold during exercise in cocks. Turnover rate did not increase in hens but the fraction of the glucose turnover oxidized to provide energy for the working muscles was increased. 4. It is concluded that carbohydrate sources account for the major fraction of energy expenditure during exercise of this magnitude and duration.