Localization of prostaglandin F in the ovine uterus during early pregnancy

As part of a study on the anti-luteolytic action of the sheep conceptus, the distribution of prostaglandin F (PGF) within the uteri of 19 pregnant and 14 non-pregnant ewes was studied using three antisera to PGF_2α and fluorescent antibody tracing. In the uteri of seven non-pregnant ewes up to Day 11 after estrus (Day 0) and in uteri of $\text{18}\text{19}$ pregnant ewes up to Day 50, PGF was localized mainly in the lamina propria, with very little on epithelial cells lining the uterine lumen. After Day 11, in the uteri of seven non-pregnant ewes, PGF was localized on the surface of luminal epithelial cells and throughout their cytoplasm, and to a lesser extent in the lamina propria. The distribution of PGF on Days 14 and 17 in the gravid and non-gravid horns of the uteri of 13 ewes made unilaterally pregnant (conceptus confined to one uterine horn) was similar to that described above for normal pregnant and non-pregnant ewes after Day 11 respectively.These results are interpreted to indicate a change in the distribution of PGF in sheep uteri due to the presence of a conceptus.     

Kinetics of glucose metabolism in sheep

The kinetics of glucose cycling in 24 ewes bearing twins were studied 1 month before term by bolus injections of [6-3H]- and [U-14C]glucose. The function representing glucose carbon recycling was determined by deconvolution of the [3H]glucose from the [14C]glucose decay curves in plasma by using the SAAM and CONSAM programs, and a model for kinetics of glucose cycling was developed. The [3H]glucose data were fitted by four compartments, and an additional three compartments were required to explain recycling. The results show that labelled carbon was still recycling to plasma 2 days after the injection of tracer. By contrast, a similar analysis on a non-pregnant sheep, with data taken from the literature, showed that no more material was recycled after 1 day. It appears that a larger fraction (20 v. 5%) of the carbon 6 of glucose recycles in pregnant than in non-pregnant sheep. This presumably reflects the metabolism by the feto-placental unit and the increased rate of glucose metabolism during pregnancy.     

Lactate Metabolism and Its Effects on Glucose Metabolism in an Excised Neural Tissue

Abstract: Chains of lumbar sympathetic ganglia, excised from 15-day-old chicken embryos, were incubated for 4 h at 36°C in a bicarbonate-buffered physiological salt solution containing 5.5 mM glucose and equilibrated with 5% CO₂–95% O₂. [U-¹⁴C]Glucose and [U-¹⁴C]lactate were used as tracers to measure the products of glucose and lactate metabolism, respectively, including CO₂, lactate, and constituents of the tissue. When 5 mM lactate was added to bathing solution containing 5.5 mM glucose, lactate carbon displaced 50–70% of the glucose carbon otherwise used for CO₂ production and provided about three times as much carbon for CO₂ as did glucose. The lactate addition increased the total carbon incorporated into CO₂ and into constituents of the tissue above those observed with glucose alone and also increased the lactate released to the bathing solution from [U-¹⁴C]-glucose. The latter increase was evidently due to an interference with reuptake of the lactate released from the ganglion cells, not to an increase in the cellular release itself. When the volume of bathing solution was increased 10-fold relative to that of the tissue, the average output of CO₂ from [U-¹⁴C]glucose during a 4-h incubation was decreased by 50% when 5 mM lactate was present but was not affected significantly in the absence of added lactate. It is concluded that the effect of changing volume in the presence of lactate was due to the effects of lactate on glucose metabolism described above and resulted from a lower average lactate concentration in the smaller volume than in the larger one, due to metabolic depletion of the added lactate. Consumable substrates other than lactate, such as glutamine and certain amino acids, also affected glucose metabolism.     

Rat brain slices oxidize glucose at high rates: a (13)C NMR study

Since glucose is the main cerebral substrate, we have characterized the metabolism of various ¹³C glucose isotopomers in rat brain slices. For this, we have used our cellular metabolomic approach that combines enzymatic and carbon 13 NMR techniques with mathematical models of metabolic pathways. We identified the fate and the pathways of the conversion of glucose carbons into various products (pyruvate, lactate, alanine, aspartate, glutamate, GABA, glutamine and CO₂) and determined absolute fluxes through pathways of glucose metabolism. After 60 min of incubation, lactate and CO₂ were the main end-products of the metabolism of glucose which was avidly metabolized by the slices. Lactate was also used at high rates by the slices and mainly converted into CO₂. High values of flux through pyruvate carboxylase, which were similar with glucose and lactate as substrate, were observed. The addition of glutamine, but not of acetate, stimulated pyruvate carboxylation, the conversion of glutamate into succinate and fluxes through succinate dehydrogenase, malic enzyme, glutamine synthetase and aspartate aminotransferase. It is concluded that, unlike brain cells in culture, and consistent with high fluxes through PDH and enzymes of the tricarboxylic acid cycle, rat brain slices oxidized both glucose and lactate at high rates.     

Early identification of non-pregnant and pregnant ewes in the field using circulating progesterone concentration

A method for early indentification of non-pregnant and pregnant ewes is described. It is applicable to field research situations where mating data for individual ewes cannot be collected and requires three plasma progesterone measurements from each ewe over a 12-days period.Ewes were diagnosed non-pregnant according to whether their lowest progesterone concentration (p) was below or above a “discriminatory value”. This value was chosen after examining the overall frequency distribution of values of log₁₀p.In experiment 1, $\text{25}\text{27}$ non-pregnant ewes and $\text{64}\text{65}$ pregnant ewes (20 to 31 days post-mating) were diagnosed correctly (96.7% accuracy). In experiment 2, $\text{40}\text{41}$ non-pregnant ews and $\text{45}\text{48}$ pregnant ewes (21 to 34 days post-mating) were diagnosed correctly (95.5% accuracy).     

Characterization of gluconeogenesis in hepatocytes isolated from the American eel,Anguilla rostrata LeSueur,

Summary Isolated hepatocyte preparations from fed immature American eels,Anguilla rostrata Le Sueur, were used to study gluconeogenic, lipogenic, glycogenic and oxidative rates of radioactively labelled lactate, glycerol, alanine and aspartate. Eel hepatocytes maintain membrane integrity and energy charge during a 2 h incubation period and are considered a viable preparation for studying fish liver metabolism.Incubating eel hepatocytes with 10 mM substrates, the following results were obtained: glycerol, alanine and lactate, in that order, were effective gluconeogenic substrates; these three substrates reduced glucose release from glycogen stores, while aspartate had no such effect; lactate, alanine and aspartate led to high rates of glycerol production, with subsequent incorporation into lipid; incorporation into glycogen was low from all substrates; and, alanine oxidation was seven times higher than that observed with other substrates.When eel hepatocytes were incubated with low or physiological substrate concentrations gluconeogenic rates from lactate were twice those from alanine; rates from aspartate were very low. Glucagon stimulated lactate gluconeogenesis, but not amino acid gluconeogenesis, and had no significant effect on glycogenolysis. Cortisol increased gluconeogenic rates from 1 mM lactate.Thus, in the presence of adequate substrate, eel liver gluconeogenesis is preferentially stimulated relative to glycogenolysis to produce plasma glucose. These data support three important roles for gluconeogenesis: the recycling of muscle lactate, the synthesis of glucose from dietary amino acids to supplement glucose levels, and the production of glycerol for lipogenesis.This work was supported from operating grants to TWM from the National Research Council of Canada (A6944)     

Jugular levels of 13,14-dihydro-15-keto-prostaglandin F and progesterone around luteolysis and early pregnancy in the ewe

Six non-pregnant ewes at day 12 of the estrous cycle each had a day-12 embryo transferred into the uterine horn ipsilateral to the corpus luteum, and 4 non-pregnant ewes at day 13 each had a day-13 embryo similarly transferred. Four control ewes, 2 at day 12 and 2 at day 13 received sheep serum into the uterine horn ipsilateral to the corpus luteum. Jugular blood samples were taken at 2-hourly intervals for 3 days post-surgery, then twice-daily for a further 4 days, and the plasma radioimmunoassayed for progesterone and 13,14-dihydro-15-keto-prostaglandin F. All control ewes exhibited estrus within the expected time range and pulsatile peaks of 13,14-dihydro-15-keto-prostaglandin F occurred coincident with declining progesterone levels. With one exception, the recipient ewes had prolonged cycles and those ewes found pregnant at necropsy, 30 days after transfer, showed no progesterone decline and no pulsatile peaks of prostaglandin during days 12 to 16 after estrus. These observations suggest that the presence of the embryo at a critical stage after mating suppresses the release of uterine prostaglandin F_2α.     

Activities of enzymes concerned with pyruvate and oxaloacetate metabolism in the heart and liver of developing sheep.

1. In order to assess whether the potential ability of heart ventricular muscle and liver to metabolise substrates such as alanine, aspartate and lactate varies as the sheep matures and its nutrition changes, the activities of the following enzymes were determined in tissues of lambs obtained at varying intervals between 50 days after conception to 16 weeks after birth and in livers from adult pregnant ewes: lactate dehydrogenase (EC 1.1.1.27), alanine aminotransferase (EC 2.6.1.2), pyruvate kinase (EC 2.7.1.40), pyruvate carboxylase (EC 6.4.1.1), phosphoenolpyruvate carboxykinase (GTP)(EC 4.1.1.32), malate dehydrogenase (EC 1.1.1.37), aspartate aminotransferase (EC 2.6.1.1) and citrate (si)-synthase (EC 4.1.3.7). 2. In the heart a most marked increase in alanine aminotransferase activity was found throughout development. During this period the activities of citrate (si)-synthase, lactate dehydrogenase and pyruvate carboxylase also increased. There were no substantial changes in the activities of aspartate aminotransferase, malate dehydrogenase or pyruvate kinase. Pyruvate kinase activities were five times greater in the heart compared with those found in the liver. No significant activity of phosphoenolpyruvate carboxykinase (GTP) was detected in heart muscle. 3. In the liver the activities of both alanine aminotransferase and aspartate aminotransferase increased immediately following birth although the activity of alanine aminotransferase was lower in livers of pregnant ewes than in any of the lambs. As with alanine aminotransferase the highest activities of lactate dehydrogenase were found during the period of postnatal growth. No marked changes were observed in malate dehydrogenase or citrate (si)-synthase activities during development. A small decline in pyruvate kinase activity occurred whilst the activities of pyruvate carboxylase and phosphoenolpyruvate carboxykinase (GTP) tended to rise during development.     

METABOLISM OF HEXOSES IN RAT CEREBRAL CORTEX SLICES

Abstract—

• 1 The metabolism of two ¹⁴C-labelled hexoses and one hexose analogue, viz. mannose, fructose and glucosamine, has been compared with that of glucose for slices of rat cerebral cortex incubated in vitro.
• 2 The metabolism of [U-¹⁴C]mannose was essentially identical to that of glucose; oxygen consumption and CO₃ production were similar and maximal at a substrate concentration of 2·75 mM. Incorporation of label into lactate, aspartate, glutamate and GABA was similar for the two substrates at 5·5 mM substrate concentration.
• 3 With [U-¹⁴C]fructose, maximal oxygen consumption and CO₃ production were obtained at a substrate concentration of 11 mM. At 5·5 mM, incorporation into lactate was 5 per cent, into glutamate and GABA 30 per cent, into alanine 63 per cent and into aspartate 152 per cent of that from glucose. Increasing substrate concentration to 27·5 mm was without effect on incorporation into amino acids from glucose and raised incorporation from fructose into glutamate, GABA and alanine to a level similar to that found with glucose; at the higher substrate concentration aspartate incorporation from fructose was 200 per cent and lactate 42 per cent of that with glucose. Unlabelled fructose was without effect on incorporation of radioactivity from [3-¹⁴C]pyruvate into CO₂ or amino acids; it increased incorporation into lactate by 36 per cent. Unlabelled glucose diminished incorporation into CO₂ from [U-¹⁴C]fructose to 35 per cent; incorporation into lactate was stimulated 178 per cent at 5·5 mM fructose; at 27·5 mM it was diminished to 75 per cent.
• 4 By comparison with [1-¹⁴C]glucose, incorporation of radioactivity from [1-¹⁴C]-glucosamine into lactate, CO₂, alanine, GABA and glutamine was very low; incorporation into aspartate was similar to glucose. Thus the metabolism of glucosamine resembled that of fructose. Glucosamine-1-phosphate, glucosamine-6-phosphate, and an unidentified metabolite, all accumulated.

     

Partitioning of nutrients in merino ewes. II. Glucose utilization by skeletal muscle, the pregnant uterus and the lactating mammary gland in relation to whole body glucose utilization

The net uptake and oxidation of glucose by leg muscle, pregnant uterus, and lactating mammary gland, together with the rate of irreversible loss and oxidation of glucose in the whole body of Merino ewes are reported. The ewes were fed on either chaffed oaten hay (OH), chaffed lucerne hay (L), or a mixture of chaffed oaten and lucerne hays (OHL). Measurements were made during five different physiological states: dry (nonpregnant), at 94 and 125 days of pregnancy, and at 20 and 50 days after lambing. Whole body glucose irreversible loss was related significantly to intake of metabolizable energy and fleece-free maternal body weight and this relation was the same in dry, pregnant and lactating ewes. The proportion of glucose oxidized in the whole body was unaffected by diet, but was lower in pregnant than in dry or lactating ewes. Some 6% of whole body carbon dioxide (CO2) production was derived from oxidation of glucose, and in ewes eating the OH diet this proportion was lower than for ewes fed on other diets. The proportion of CO2 derived from glucose was lower in pregnant ewes than in dry and lactating ewes. Leg (muscle) glucose uptake was lower in ewes fed on the OH diet than in ewes given the other diets. This arose partly because of decreased blood flow to the leg in ewes fed OH. Muscle glucose uptake, corrected for lactate output, accounted for 20, 44 and 34% of glucose irreversible loss in ewes fed OH, OHL and L respectively. There was no significant effect of physiological state on glucose uptake by leg muscle. The maximum contribution glucose uptake, corrected for output of lactate, could make to leg muscle oxygen consumption was 31% and there were no differences due to diet or physiological state. Uterine glucose uptake was 10.5 mg min-1 kg-1, and was unaffected by diet and stage of pregnancy. Glucose uptake was maintained, despite a decline in blood flow per kilogram of uterus from 399 to 237 ml min-1 kg-1, between 94 and 125 days of pregnancy by an increase in arteriovenous difference of glucose over the same period from 2.8 to 4.4 mg 100 ml-1. Total uptake of glucose by the uterus increased from 26 to 47 mg min-1 between 94 and 125 days of pregnancy. The proportion of glucose irreversible loss accounted for by uterine uptake increased from 46 to 65% between 94 and 125 days, and was greater for ewes fed OH (84%) than L (46%) at 125 days of pregnancy. A maximum of 71% of milk lactose could have been derived directly from glucose; 17% of glucose taken up by the mammary gland was oxidized, contributing to 20% of mammary CO2 output. Mammary glucose uptake was lower in ewes fed OH than in ewes fed the other diets.(ABSTRACT TRUNCATED AT 400 WORDS)     

Pyruvate kinase and alanine synthesis in skeletal muscle

L-Phenylalanine is an allosteric inhibitor of M₁-type pyruvate kinase. Accordingly, the effects were studied of 20 mM phenylalanine on the metabolism of 5 mM [U-¹⁴C]glucose and 3 mM L-[U-¹⁴C]glutamate by isolated hemidiaphragms from starved rats. Phenylalanine inhibited lactate and¹⁴CO₂ production from both substrates and stimulated alanine release. It is concluded that pyruvate kinase may have a dual role in intermediary metabolism in skeletal muscle: the enzyme is a component of the lower glycolytic pathway and is implicated in a pathway of amino acid oxidation and alanine synthesis.     

Early pregnancy diagnosis in the ewe, based on milk progesterone levels.

Plasma and milk progesterone concentrations in pregnant sheep (18--22 days after mating) were similar, about 3.7 ng/ml whereas values in non-pregnant sheep were less than 1 ng/ml. Lambing results indicated identical accuracy for both methods (82 and 84% in 2 flocks). The accuracy was 92--100% for ewes diagnosed non-pregnant in the breeding season, but for ewes tested in the non-breeding season the diagnosis of non-pregnancy according to milk progesterone levels was only 50% accurate.     

Amino acid and glucose fermentation by Treponema denticola

Summary Treponema denticola was grown in serum-containing media to which ¹⁴C-labelled compounds were added. Determinations of radioactivity in the products formed indicated that the organism fermented alanine, cysteine, glycine, serine, and glucose. Fermentation products included acetate, lactate, succinate, formate, pyruvate, ethanol, CO₂, H₂S, and NH₃. The products formed from glucose constituted a small portion of the total products. Assays of enzymatic activities in cell extracts indicated that the organism degraded glucose via the Embden-Meyerhof pathway. T. denticola possessed a coenzyme A-dependent CO₂-pyruvate exchange activity associated with a clostridial-type clastic system for pyruvate metabolism. Phosphotransacetylase and acetate kinase activities were present in cell extracts. Acetyl phosphate formation and benzyl viologen reduction were detected when cell extracts were incubated with pyruvate, serine or cysteine. The data indicate that T. denticola is an amino acid fermenter and that it possesses the enzymes needed for the fermentation of glucose. However, glucose does not serve as the primary substrate when the organism grows in media including both this carbohydrate and amino acids.     

Effect of restriction of placental growth on fetal and utero-placental metabolism

The effect of restriction of placental growth on the supply of glucose to the gravid uterus and fetus and on fetal and utero-placental metabolism of glucose and lactate was examined in this study. Endometrial caruncles were removed from 13 sheep (caruncle sheep) prior to mating, which restricted placental growth in the subsequent pregnancy. Half the fetuses of caruncle sheep were small or growth retarded, with the remainder normal in size. After insertion of vascular catheters at 110 days gestation, the caruncle sheep, together with 16 control sheep, were studied between 121 and 130 days of gestation. Glucose delivery to and consumption by the gravid uterus and its contents, both as a total and per kg of tissue mass, was significantly lower in caruncle ewes with small fetuses, although glucose extraction was similar to that in controls. Utero-placental glucose consumption was significantly lower in caruncle ewes carrying small fetuses compared to that in control ewes, both as a total and per kg of placenta. Small caruncle fetuses were hypoxaemic and hypoglycaemic and the lactate concentration in the common umbilical vein was significantly higher than in control sheep. Glucose delivery to and consumption by the fetus was significantly lower in normal-sized and in small caruncle fetuses compared to controls. Fetal glucose consumption per kg of fetus was similar in control and caruncle sheep. Fetal glucose extraction increased as fetal weight decreased. Utero-placental production of lactate was similar in control and caruncle ewes. However, uterine output of lactate decreased as placental weight fell. Utero-placental production of lactate per kg of placenta was significantly higher in caruncle ewes compared to controls and increased as oxygen content in blood from the fetal femoral artery decreased. Fetal lactate consumption per kg of fetus increased as the concentration of lactate in blood from the common umbilical vein increased. It is concluded that intrauterine growth retardation due to restriction of placental growth is associated with a reduced supply of glucose to both the pregnant uterus and fetus and a redistribution of glucose therein to the fetus, both directly as glucose and indirectly as lactate. This reflects the disproportionate maintenance of fetal weight relative to that of the placenta, reduced utero-placental consumption of glucose per kg of placenta, conversion of a greater proportion of that glucose or other substrate(s) to lactate by the placenta and an increase in the fraction of the lactate produced by utero-placental tissues that is secreted into the fetal circulation.     

Distribution of Methylene Blue after Injection into the Epidural Space of Anaesthetized Pregnant and Non-Pregnant Sheep

The aim of the study was to determine the distribution of different volumes of methylene blue solution injected into the epidural space in anaesthetized pregnant and non-pregnant sheep, to evaluate its cranial distribution and to compare between them. Fifteen pregnant and fifteen non-pregnant sheep were included in the study. Sheep were anaesthetized and received 0.05, 0.1, or 0.2 mL/kg of a lumbosacral epidural solution containing 0.12% methylene blue in 0.9% saline. Thirty minutes after the epidural injection, the ewes were euthanized. The extension of the dye within the epidural space was measured, and the correlation between the volume of the dye injected and the number of stained vertebrae was evaluated. The cranial migration of the dye between pregnant and non-pregnant sheep was also compared. The results show that the volume of methylene blue injected epidurally into pregnant and non-pregnant sheep correlated directly with its cephalic distribution into the epidural space; and a volume of 0.1 mL/kg or 0.2 mL/kg stained up to the first lumbar segment in pregnant and non-pregnant sheep, respectively. Also, the results suggest that the volume of drugs administered into the epidural space of pregnant sheep should be half the volume that would be used in non-pregnant sheep.     

Metabolic Interactions Between Glucose, Glycerol, Alanine and Acetate in Leishmania braziliensis panamensis Promastigotes

¹³C-nuciear magnetic resonance (NMR) spectroscopy was used to investigate the products of glycerol and acetate metabolism released by Leishmania braziliensis panamensis promastigotes and also to examine the interaction of each of these substrates with glucose or alanine. The NMR data were supplemented by measurements of the rates of oxygen consumption and substrate utilization, and of ¹⁴CO₂ production from ¹⁴C-labeIed substrate. Cells incubated with [2-¹³C]glycerol released acetate, succinate and D-lactate in addition to CO₂. Cells incubated with acetate released only CO₂. More succinate C-2/C-3 than C-l/C-4 was released from both [2-¹³C]glycerol and [2-¹³C]glucose, indicating that succinate was formed predominantly by CO₂ fixation followed by reverse flux through part of the Krebs cycle. Some redistribution of the position of labeling was also seen in alanine and pyruvate, suggesting cycling through pyruvate/oxaloacetate/phosphoenolpyruvate. Cells incubated with combinations of 2 substrates consumed oxygen at the same rate as cells incubated with 1 or no substrate, even though the total substrate utilization had increased. When promastigotes were incubated with both glycerol and glucose, the rate of glucose consumption was unchanged but glycerol consumption decreased about 50%, and the rate of ¹⁴CO₂ production from [l,(3)-¹⁴C]glycerol decreased about 60%. Alanine did not affect the rates of consumption of glucose or glycerol, but decreased ¹⁴CO₂ production from these substrates by increasing flow of label into alanine. Although glucose decreased alanine consumption by 70%, it increased the rate of ¹⁴CO₂ production from [U-¹⁴C]- and [l-¹⁴C]alanine by about 20%. This is consistent with rapid equilibration of alanine with pyruvate derived from glucose and yet little decrease in the specific activity of the large alanine pool.     

Alanine metabolism in skeletal muscle in tissue culture

Alanine production by skeletal muscle in tissue culture was studied using an established myogenic line (L₆) of rat skeletal muscle cells. Correlation analyses were performed on rates of metabolism of alanine, glucose, lactate and pyruvate over incubation periods up to 96 h. Alanine production did not correlate significantly with glucose utilization (r = 0.24, P < 0.20). Alanine production, however, did correlate with lactate production (r = 0.72, P < 0.0005) as well as medium (r = 0.50, P < 0.025) and intracellular (r = 0.85, P < 0.0005) pyruvate concentrations. The intercepts of the latter two correlation analyses indicated that when medium or cell pyruvate fell below 0.28 mM or 1 nmol/mg protein, respectively, net alanine consumption occurred. Alanine synthesis also correlated (r = 0.71, P < 0.0005) with the percent change in the cell mass action ratio for the sum of the alanine and aspartate aminotransferase reactions, i.e., [alanine] [malate]/[aspartate] [lactate]. These results suggest that alanine production is not necessarily linked to the rate of glucose utilization but rather to pyruvate overflow above a critical intracellular level; under conditions of pyruvate overflow, alanine synthesis is driven by the tendency to establish equilibrium between metabolites of the linked amino acid transaminases in skeletal muscle.     

Comparative energy metabolism in cultured heart muscle and hela cells

The yields of energy from oxidation of fatty acids, glucose, and glutamine were compared in cultures of chick embryo heart muscle (heart) and HeLa cells. Aerobic energy production, as measured by oxygen utilization, was comparable in the two cell types. In media containing dialyzed sera, the rates of incorporation of fatty acids directly into lipids were similar in both cells and accounted for > 97% of fatty acid metabolism in HeLa cells. However, in heart cells only 45% ended in lipid, 42% in protein, and 13% was released as CO₂; the latter two products probably reflect the oxidation of fatty acids to acetyl-coenzyme A (-CoA) and its subsequent metabolism in the citrate cycle. Increased serum concentration in the medium did not affect fatty acid metabolism in HeLa cultures, but resulted in greater oxidation by heart cells (> 100 times that by HeLa cells). The metabolisms of both glucose and glutamine were similar in heart and HeLa cells with ? 60% of glucose carbon ending as medium lactate and only 3–5% converted to acetyl-CoA. About 25% of glutamine carbon ended as CO₂ and increased utilizations with increasing serum concentrations was accountable in both cells by increased lactate from glucose and glutamate from glutamine. CO₂ production (and energy) from glutamine was independent of glutamine concentration within a tenfold range of physiological concentrations. The yields of energy have been calculated. In 10% dialyzed calf serum, oxidation of glutamine carbon provided about half of the total energy in heart cells, glucose about 35–45%, with most coming from glycolysis; oxidation of fatty acid carbon provided only 5–10%. That > 90% of the aerobic energy comes from glutamine in both cells can account for the comparable rates of oxygen utilization. HeLa cells derived little or no energy from fatty acids.     

Glucose Metabolism in Rat Retinal Pigment Epithelium

The retinal pigment epithelium (RPE) is the major transport pathway for exchange of metabolites and ions between choroidal blood supply and the neural retina. To gain insight into the mechanisms controlling glucose metabolism in RPE and its possible relationship to retinopathy, we studied the influence of different glucose concentrations on glycogen and lactate levels and CO₂ production in RPE from normal and streptozotocin-treated diabetic rats. Incubation of normal RPE in the absence of glucose caused a decrease in lactate production and glycogen content. In normal RPE, increasing glucose concentrations from 5.6 mM to 30 mM caused a four-fold increase in glucose accumulation and CO₂ yield, as well as reduction in lactate and glycogen production. In RPE from diabetic rats glucose accumulation did not increase in the presence of high glucose substrate, but it showed a four- and a seven-fold increase in CO₂ production through the mitochondrial and pentose phosphate pathways, respectively. We found high glycogen levels in RPE which can be used as an energy reserve for RPE itself and/or neural retina. Findings further show that the RPE possesses a high oxidative capacity. The large increase in glucose shunting to the pentose phosphate pathway in diabetic retina exposed to high glucose suggests a need for reducing capacity, consistent with increased oxidative stress.