Effect of L-alanine infusion on gluconeogenesis and ketogenesis in the rat in vivo.

1. In 48 h-starved 6-week-old rats the 14C incorporation in vivo into blood glucose from a constant-specific-radioactivity pool of circulating [14c]actateconfirmed that lactate is the preferred gluconeogenic substrate. 2. Increasing the blood [alanine] to that occurrring in the fed state increased 14C incorporation into blood glucose 2.3-fold from [14c]alanine and 1.7-fold from [14c]lactate. 3. When the blood [alanine] was increased to that in the fed state, the 14C incorporation into liver glycogen from circulating [14c]alanine or [14c]lactate increased 13.5- and 1.7-fold respectively. 4. The incorporation of 14C into blood acetoacetate and 3-hydroxybutyrate from a constant-specific-radioactivity pool of circulating [14c]oleate was virtually abolished by increasing the blood [alanine] to that existing in the fed state. However, the [acetoacetate] remained unchanged, whereas [3-hydroxybutyrate] decreased, although less rapidly than did its radiochemical concentration. 5. It is concluded that during starvation in 6-week-old rats, the blood [alanine] appears to influence ketogenesis for circulating unesterfied fatty acids and inversely affects gluconeogenesis from either lactate or alanine. A different pattern of gluconeogenesis may exist for alanine and lactate as evidenced by comparative 14C incorporation into liver glycogen and blood glucose.     

Glucose and lactate oxidation rates in the fetal lamb

Both glucose and lactate are nutrients of the ovine fetus. Each may be used by the fetus as a fuel for oxidation or as a source of carbon for energy storage and net tissue accretion. The present report describes the oxidation rates of glucose and lactate in vivo for the fetal lamb over a relatively short time period. The fraction of fetal glucose or lactate oxidized was defined as the ratio of 14CO2 excretion across the umbilical circulation to the net entry of [14C]glucose or [14C]lactate into fetal tissues. The fraction of glucose oxidized over a 3-hr study averaged 61.2%, accounting for 2.55 mg X min-1 X kg-1 of glucose oxidized and for 28% of the simultaneous net oxygen uptake. The fraction of lactate oxidized averaged 71.5%, accounting for 4.12 mg X min-1 X kg-1 of lactate oxidized. Oxidation fractions and rates for both glucose and lactate increased with their concentrations in fetal blood suggesting sparing of other fuels for oxidation at higher glucose and lactate concentrations.     

Increased gluconeogenesis in the rat at term gestation

In this study the contribution of maternal gluconeogenesis to the glucose homeostasis of the maternal-fetal unit has been studied in fed term pregnant rats. We have measured the activity of two gluconeogenic enzymes, the rates of lactate turnover and the rates of gluconeogenesis from lactate in fed term pregnant rats. A decrease in plasma glucose and liver glycogen concentrations, and an increase of plasma lactate and alanine concentrations were observed in fed 22-day pregnant rats compared to virgin controls. Also, liver and kidney phosphoenolpyruvate carboxykinase activities and liver lactate dehydrogenase and hexose bisphosphatase activities significantly increased in fed term pregnant rats compared to virgin rats. The lactate turnover rate and the rate of gluconeogenesis in vivo from L-[U14C] Lactate increased four- and two-fold respectively in fed pregnant rats compared to fed virgins.     

Perinatal onset of hepatic gluconeogenesis in the lamb

Hepatic gluconeogenesis does not occur in the unstressed fetal sheep. After birth, in addition to glycogenolysis, the newborn lamb must eventually initiate gluconeogenesis to maintain glucose homeostasis. The regulation and time course of this transition have not been defined. We studied six animals in an acute preparation before and after delivery to determine hepatic lactate and glucose uptake, hepatic gluconeogenesis from lactate, and plasma catecholamine and cortisol concentrations. After a priming dose, continuous infusion of [14C]lactate provided tracer substrate for calculations of gluconeogenesis in the fetus and then for ten hours after delivery in the newborn lamb. The radionuclide-labelled microsphere method was used to measure hepatic blood flow. Appreciable gluconeogenesis was not present during the fetal period. Following delivery, the newborn lambs began to produce significant quantities of glucose from lactate at 6 h of age (1.37 +/- 0.84 mg.min-1.100 g-1 min-1 x 100 g-1 liver), when gluconeogenesis from lactate accounted for 22% of hepatic glucose output. Despite the onset of gluconeogenesis, postnatal lambs had blood glucose concentrations that remained less than fetal levels of 23.4 +/- 12.1 mg/dl for the duration of the 10-h study. Plasma norepinephrine concentration was 1380 +/- 1145 pg/ml in the fetus and fell by 2 h after birth. Plasma epinephrine concentrations were highest at 15 min after birth (205 +/- 262 pg/ml), but remained quite low for the remainder of the study. Plasma cortisol concentrations did not vary over the course of study, ranging from 40 to 50 ng/ml.(ABSTRACT TRUNCATED AT 250 WORDS)     

Anabolic regulation of gluconeogenesis by insulin in isolated rat hepatocytes

The role of substrate availability in the regulation of gluconeogenesis in isolated rat hepatocytes was studied using [U-14C]alanine as a tracer in the presence of different concentrations of L-alanine in the incubation medium. At low alanine concentrations (0.5 mM) insulin decreased the 14C incorporation into the glucose pool and increased the incorporation of tracer carbons into the protein and lipid pools and into CO2. The net radioactivity lost from the glucose pool was only a small percentage of the total increase in the activity of the protein, lipid, CO2, or glycogen pools, supporting the notion that the effect of insulin in diminishing gluconeogenesis is secondary to its effects on pathways using pyruvate. At higher concentrations of alanine (2.5, 5.0, and 10.0 mM) in the incubation medium insulin increased the movement of alanine carbons into protein and glucose. This suggests that at higher substrate concentrations the ability of the liver to synthesize proteins is overwhelmed and the pyruvate carbons are forced into the gluconeogenesis pathway. These results were further confirmed by using [U-14C]lactate. The increases in observed specific activity of glucose following insulin administration would not be possible if insulin acted by affecting the activity of any enzyme directly involved in the formation or utilization of pyruvate, most of which have been proposed as sites of insulin action. Data presented show that insulin "inhibits" gluconeogenesis by affecting a change in substrate availability.     

A reexamination of the role of the cytosolic alanine aminotransferase in hepatic gluconeogenesis

The relative importance of the mitochondrial and cytosolic alanine aminotransferase isozymes for providing pyruvate from alanine for further metabolism in the mitochondrial compartment was examined in the isolated perfused rat liver. The experimental rationale employed depends upon the supposition that gluconeogenesis from alanine and the decarboxylation of infused [1-14C]alanine should be diminished by pyruvate transport inhibitors (e.g., alpha-cyanocinnamate) in proportion to the contribution of the cytosolic alanine aminotransferase for generating pyruvate. alpha-Cyanocinnamate inhibited the endogenous rate of glucose production in perfused livers derived from 24-h-fasted rats. The rate of [1-14C]alanine decarboxylation at low (1 mM) and high (10 mM) perfusate alanine concentrations was inhibited by 9.5 and 42%, respectively, in the presence of alpha-cyanocinnamate. In livers from fasted animals perfused with either 1 or 10 mM alanine, alpha-cyanocinnamate caused a substantial increase in the rates of both lactate and pyruvate production. Elevating the hepatic ketogenic rate during infusion of acetate in livers, perfused with alanine, stimulated both the rates of alanine decarboxylation and glucose production; the extent of stimulation of these two metabolic parameters was determined to be a function of the alanine concentration in the perfusate. The stimulation of the rate of alanine decarboxylation during acetate-induced ketogenesis was reversed by co-infusion of alpha-cyanocinnamate with simultaneous increases in the rates of lactate and pyruvate production. The results indicate that during rapid ketogenesis, cytosolic transamination of alanine contributes at least 19% (at 1 mM alanine) and 55% (at 10 mM alanine) of the pyruvate for gluconeogenesis.     

Placental Transfer of Lactate,Glucose and 2-deoxyglucose in Control and Diabetic Wistar Rats

Placental transfer of lactate, glucose and 2-deoxyglucose was examined employing the in situ perfused placenta. Control and streptozotocin induced diabetic Wistar rats were infused with [U¹⁴C]-glucose and [³H]-2-deoxyglucose (2DG). The fetal side of the placenta was perfuseci with a cell free medium and glucose uptake was calculated in the adjacent fetuses. Despite the 5-fold higher maternal plasma glucose concentration in the diabetic dams the calculated fetal glucose metabolic index was not significantly different between the 2 groups. Placental blood flow was reduced in the diabetic animals compared with controls but reduction of transfer of [U¹⁴C]-glucose and [³H]-2-deoxyglucose and endogenously derived [¹⁴C]-Lactate to the fetal compartment, could not be accounted for by reduced placental blood flow alone. There was no significant net production or uptake of lactate into the perfusion medium that had perfused the fetal side of the placenta in either group. The plasma lactate levels in the fetuses adjacent to the perfused placenta were found to be higher than in the maternal plasma and significantly higher in the fetuses of the diabetic group compared with control group. In this model the in situ perfused placenta does not secrete significant quantities of lactate into the fetal compartment in either the control or diabetic group.     

Extracellular Intermediates of Glucose Metabolism: Fluxes of Endogenous Lactate and Alanine Through Extracellular Pools in Embryonic Sympathetic Ganglia

The flux rates of lactate and alanine in and out of the cells of an intact tissue, which cannot be measured directly because some of the released materials are reabsorbed, were determined by computer analysis of uptakes and outputs by the whole tissue in the presence of various concentrations of these substances. The outputs of labeled lactate and alanine from [U-14C]glucose and the uptakes of [U-14C]lactate and [U-14C]alanine were measured on intact sympathetic ganglia excised from 15-day-old chicken embryos. The volume and time constant of the extracellular space were measured using labeled lactate, alanine, and sucrose. Models, which mathematically described the cellular uptakes and outputs as functions of the extracellular concentrations, were used to predict the exchanges that would be observed on the whole tissue, and their parameters were adjusted for best fit to the actual observations. The fitted models were then used to calculate the fluxes in and out of the cells and the concentrations in the extracellular space. The following results were obtained: (1) Cellular uptakes of lactate and alanine were both well described by familiar Michaelis-Menten kinetics. (2) The cellular output of [14C]-lactate from [14C]glucose declined with increase in the extracellular lactate concentration, whereas the cellular output of [14C]alanine from [14C]glucose rose with the extracellular alanine concentration. (3) Half-saturation values for cellular uptake, determined from the fitted equations, were 0.45 mM for lactate and 1.17 mM for alanine, both several-fold lower than less relevant estimates for the whole tissue made directly from the uptake observations. (4) As much as 45% of the carbon in the glucose consumed was released into the extracellular space as lactate and alanine, but much of this was reabsorbed. Implications for brain metabolism are discussed.     

Glucose turnover and hepatocyte glucose production of starved and toxaemic pregnant sheep

Ewes bearing twins were starved for 10 days during the last month of gestation to induce ovine pregnancy toxaemia (OPT). Glucose turnover was measured by a primed continuous infusion of [U-14C]- and [6-3H]glucose at the end of 10 days of starvation (non-susceptible), or earlier when ewes became recumbent with OPT (susceptible). All ewes were slaughtered at the end of the infusion and hepatocytes were prepared in order to measure glucose production from different substrates. Many of the ewes had dead foetuses when slaughtered. Glucose production rates by hepatocytes with the substrates propionate, lactate or alanine were significantly less from the susceptible ewes than were those from non-susceptible ewes. These low rates were not stimulated by incubation with glucagon (10(-8) M), glutamine or glycerol. Rates of glucose turnover and of hepatic glucose production from all substrates were higher for ewes with dead than with live foetuses. The data support the hypothesis that pathogenesis of OPT is related to an impairment of hepatic gluconeogenesis, and further suggest that, in starved pregnant ewes, maternal glucose production may be restrained in the presence of a live foetus.     

Use of 3H and 14C doubly labeled glucose and amino acids in the study of hormonal regulation of gluconeogenesis in rats.

Double isotope procedures (3H and 14C) were used in vivo to investigate a) slow long-term gluconeogenic actions of adrenal glucocorticoids, and b) rapid stimulation of gluconeogenesis by glucagon. [U-14C,6-3H]Glucose was administered to normal and adrenalectomized rats. No effect was observed on the [6-3H]glucose half-life suggesting the dicarboxylic acid shuttle is unaffected by adrenalectomy; the Cori cycle is also not influenced. Loads of [14C]aspartate, [14C]glutamate, or [14C]alanine were given to normal and adrenalectomized rats. Simultaneously, in vivo transaminase activity was studied by measuring the appearance of 3H2O in body water after administration of [2-3H]aspartate, [2-3H]glutamate, or [2-3H]alanine, Adrenalectomy has no influence on the incorporation of glutamate or aspartate into glucose or on their in vivo transaminases. Diminution of incorporation of [14C]alanine into glucose and alanine transaminase activities occurs only when rats are given unphysiological loads. These studies support the contention that glucocorticoid rate-limiting actions occur in extrahepatic tissues to produce an increased flow of glucose precursors to the liver. [U-14C,3-3H]Glucose was used to investigate the effect of glucagon on the hepatic fructose-6-phosphate (F-6-P) cycle. Glucagon administration resulted in a rapid drop in the 3H/14C ratio of circulating glucose, suggesting an increase in F-6-P recycling caused by activation of FDPase with little or no decrease in phosphofructokinase. Such a change would direct substrate flux toward gluconeogenesis.     

Gluconeogenesis in isolated intact lamb liver cells. Effects of glucagon and butyrate.

1. Isolated lamb liver cells were prepared from 24-h-starved animals by venous perfusion of the excised caudate lobe with buffer containing collagenase. On the basis of Trypan-Blue exclusion, rate of O2 uptake, adenine nucleotide content and retention of constitutive enzymes, these cells were judged to be intact. 2. Isolated caudate-lobe liver cells showed rates of gluconeogenesis from 10 mM-propionate and 10 mM-lactate that compared favourably with rates determined in isolated median-lobe cells and with rates determined with the isolated perfused lamb liver. 3. The gluconeogenic potential of substrates tested depended on the lamb's age. Cells prepared from suckling lambs (up to 20 days of age and essentially non-ruminant) showed highest rates from galactose, serine and alanine; those prepared from post-weaned lambs (older than 30 days of age and ruminant) showed highest rates from propionate, lactate and fructose. 4. Gluconeogenic rates from endogeneous precursors, 10 mM-propionate and 10mM-galactose, were linear for 1 h and were both stimulated by 1 muM-glucagon. Provided the endogenous rate of gluconeogenesis remained unchanged after substrate addition, glucagon caused a net stimulation of gluconeogenesis from each of these substrates. 5. Gluconeogenic capacity and glucagon sensitivity were examined in cells maintained in substrate-free oxygenated buffer at 37 degrees, 22 degrees and * degrees C. Even under the best of the three conditions of storage that were tested (i.e. at 22 degrees C in gelatin-containing buffer) deterioration of the lamb cells proceeded rapidly, and loss of glucagon responsiveness preceeded the loss of ability to convert precursor into glucose. 6. n-Butyric acid, 2-methylpropanoic acid and 3-methylbutanoic acid at concentrations comparable with those found in lamb portal-vein blood each stimulated gluconeogenesis from 10mM-galactose or 10mM-propionate; gluconeogenesis from galactose was stimulated to the greater extent. 7. The regulatory effects of glucagon and sodium butyrate on lamb liver-cell gluconeogenesis and glycogenolysis were compared. Glucagon (1 muM) and 2mM-butyrate accelerated the rate of glucose formation of liver cells of 24h-starved animals from lactate+pyruvate or fructose. Insulin (20nM) decreased both gluconeogenesis and the efficacy of 1 muM-glucagon. For lactate+pyruvate as substrate, the stimulatory effect of butyrate was additive to that of 1muM-glucagon and for both lactate+pyruvate and fructose the stimulatory effect of butyrate was not influenced by 20nM-insulin. In contrast with glucagon, which stimulated the rate of glycogenolysis in cells prepared from fed lambs, butyrate (0.1-20mM) had no effect. 8. It is concluded that glucagon and butyrate stimulate lamb liver-cell gluconeogenesis by different mechanisms.     

[14C]bicarbonate fixation into glucose and other metabolites in the liver of the starved rat under halothane anaesthesia. Metabolic channelling of mitochondrial oxaloacetate.

Previous attempts to account for the labelling in vivo of liver metabolites associated with the citrate cycle and gluconeogenesis have foundered because proper allowance was not made for the heterogeneity of the liver. In the basal state (anaesthetized after 24h starvation) this heterogeneity is minimal, and we show that labelling by [14C]bicarbonate can be interpreted unambiguously. [14C]Bicarbonate was infused to an isotopic steady state, and measurements were made of specific radioactivities of blood bicarbonate, alanine, glycerol and lactate, of liver alanine and lactate, and of individual carbon atoms in blood glucose and liver aspartate, citrate and malate. (Existing methods for several of these measurements were extensively modified.) The results were combined with published rates of gluconeogenesis, uptake of gluconeogenic precursors by the liver, and citrate-cycle flux, all measured under similar conditions, and with estimates of other rates made from published data. To interpret the results, three ancillary measurements were made: the rate of CO2 exchange by phosphoenolpyruvate carboxykinase (PEPCK; EC 4.1.1.32) under conditions that simulated those in vivo; the 14C isotope effect in the pyruvate carboxylase (EC 6.4.1.1) reaction (14C/12C = 0.992 +/- 0.008; S.E.M., n = 8); the ratio of labelling by [2-14C]- to that by [1-14C]-pyruvate of liver glutamate 1.5 min after injection. This ratio, 3.38, is a measure of the disequilibrium in the mitochondria between malate and oxaloacetate. The data were analysed with due regard to experimental variance, uncertainties in values of fluxes measured in vitro, hepatic heterogeneity and renal glucose output. The following conclusions were reached. The results could not be explained if CO2 fixation was confined to pyruvate carboxylase and there was only one, well-mixed, pool of oxaloacetate in the mitochondria. Addition of the other carboxylation reactions, those of PEPCK, isocitrate dehydrogenase (EC 1.1.1.42) and malic enzyme (EC 1.1.1.40), was not enough. Incomplete mixing of mitochondrial oxaloacetate had to be assumed, i.e. that there was metabolic channelling of oxaloacetate formed from pyruvate towards gluconeogenesis. There was some evidence that malate exchange across the mitochondrial membrane might also be channelled, with incomplete mixing with that in the citrate cycle. Calculated rates of exchange of CO2 by PEPCK were in agreement with those measured in vitro, with little or no activation by Fe2+ ions.(ABSTRACT TRUNCATED AT 400 WORDS)     

Regulation of gluconeogenesis from pyruvate and lactate in the isolated perfused rat liver

The effects of glucagon and the alpha-adrenergic agonist, phenylephrine, on the rate of 14CO2 production and gluconeogenesis from [1-14C]lactate and [1-14C]pyruvate were investigated in isolated perfused livers of 24-h-fasted rats. Both glucagon and phenylephrine stimulated the rate of 14CO2 production from [1-14C]lactate but not from [1-14C]pyruvate. Neither glucagon nor phenylephrine affected the activation state of the pyruvate dehydrogenase complex in perfused livers derived from 24-h-fasted rats. 3-Mercaptopicolinate, an inhibitor of the phosphoenolpyruvate carboxykinase reaction, inhibited the rates of 14CO2 production and glucose production from [1-14C]lactate by 50% and 100%, respectively. Furthermore, 3-mercaptopicolinate blocked the glucagon- and phenylephrine-stimulated 14CO2 production from [1-14C]lactate. Additionally, measurements of the specific radioactivity of glucose synthesized from [1-14C]lactate, [1-14C]pyruvate and [2-14C]pyruvate indicated that the 14C-labeled carboxyl groups of oxaloacetate synthesized from 1-14C-labeled precursors were completely randomized and pyruvate----oxaloacetate----pyruvate substrate cycle activity was minimal. The present study also demonstrates that glucagon and phenylephrine stimulation of the rate of 14CO2 production from [1-14C]lactate is a result of increased metabolic flux through the phosphoenolpyruvate carboxykinase reaction, and phenylephrine-stimulated gluconeogenesis from pyruvate is regulated at step(s) between phosphoenolpyruvate and glucose.     

Stimulation by alpha-adrenergic agonists of Ca2+ fluxes, mitochondrial oxidation and gluconeogenesis in perfused rat liver.

Glucose output from perfused livers of 48 h-starved rats was stimulated by phenylephrine (2 microM) when lactate, pyruvate, alanine, glycerol, sorbitol, dihydroxyacetone or fructose were used as gluconeogenic precursors. Phenylephrine-induced increases in glucose output were immediately preceded by a transient efflux of Ca2+ and a sustained increase in oxygen uptake. Phenylephrine decreased the perfusate [lactate]/[pyruvate] ratio when sorbitol or glycerol was present, but increased the ratio when alanine, dihydroxyacetone or fructose was present. Phenylephrine induced a rapid increase in the perfusate [beta-hydroxybutyrate]/[acetoacetate] ratio and increased total ketone-body output by 40-50% with all substrates. The oxidation of [1-14C]octanoate or 2-oxo[1-14C]glutarate to 14CO2 was increased by up to 200% by phenylephrine. All responses to phenylephrine infusion were diminished after depletion of the hepatic alpha-agonist-sensitive pool of Ca2+ and returned toward maximal responses after Ca2+ re-addition. Phenylephrine-induced increases in glucose output from lactate, sorbitol and glycerol were inhibited by the transaminase inhibitor amino-oxyacetate by 95%, 75% and 66% respectively. Data presented suggest that the mobilization of an intracellular pool of Ca2+ is involved in the activation of gluconeogenesis by alpha-adrenergic agonists in perfused rat liver. alpha-Adrenergic activation of gluconeogenesis is apparently accompanied by increases in fatty acid oxidation and tricarboxylic acid-cycle flux. An enhanced transfer of reducing equivalents from the cytoplasmic to the mitochondrial compartment may also be involved in the stimulation of glucose output from the relatively reduced substrates glycerol and sorbitol and may arise principally from an increased flux through the malate-aspartate shuttle.     

Hyperglycaemic activity and metabolic effects of 3-aminopicolinic acid

Administering 3-aminopicolinate to rats starved for 24h immediately initiated a progressive increase in blood glucose concentration. Hyperglycaemia was not the result of glycogenolysis, nor was it due to an inhibition of insulin release, since it caused marked hyperinsulinaemia. The rate of [6-³H]glucose disappearance from the blood of the intact rat was not altered by 3-aminopicolinate, indicating that it does not cause hyperglycaemia by inhibiting glucose utilization or by causing a redistribution of total body glucose. 3-Aminopicolinate increased the rate of fall in the specific radioactivity of blood [6-³H]-glucose, indicating dilution of the glucose pool by newly synthesized glucose. The rate of ¹⁴C incorporation into blood glucose from [¹⁴C]alanine and [¹⁴C]lactate was increased 90 and 35% respectively, whereas that from [¹⁴C]glycerol and [¹⁴C]xylitol was either unaffected or slightly decreased by 3-aminopicolinate administration. Liver phosphoenolpyruvate of rats was increased to four to seven times the normal concentration 10min to 1h after injections of 50–300mg of 3-aminopicolinate/kg body wt. and the amounts of 2-phosphoglycerate and 3-phosphoglycerate were increased to three to four times normal. The high concentrations of liver phosphoenolpyruvate, 2-phosphoglycerate and 3-phosphoglycerate, as well as the enhancement of gluconeogenesis from lactate and alanine, but not from glycerol or xylitol, is compatible with an enhancement of gluconeogenesis at a step between pyruvate and the triose phosphates. After injections of 3-aminopicolinate, liver malate, citrate, aspartate, alanine, lactate and pyruvate were also increased, but to lesser extents than was phosphoenolpyruvate. The increases in some of these metabolites were approximated after an intravenous infusion of glucose, so their elevated concentration after 3-aminopicolinate administration could have been, in part, a consequence of the hyperglycaemia. The possibility is considered that 3-aminopicolinate stimulates gluconeogenesis in vivo by facilitating Fe²⁺ activation of phosphoenolpyruvate carboxykinase as it does with the purified enzyme in vitro [MacDonald & Lardy (1978) J. Biol. Chem. 253, 2300–2307]. In this effect 3-aminopicolinate may simulate the physiological role of the naturally occurring ferroactivator protein [Bentle & Lardy (1977) J. Biol. Chem. 252, 1431–1440].     

Effect of cortisol on hepatic gluconeogenesis in the fetal sheep

To determine whether the prenatal surge in cortisol induces the onset of gluconeogenesis in the fetal sheep, we performed studies in eight fetal sheep of 124 +/- 3 days gestational age. Catheters were inserted chronically in the descending aorta, inferior vena cava, and hepatic and umbilical veins, allowing the measurement of substrate flux across the liver and placenta. Cortisol was infused over a 48-h period, raising plasma cortisol concentrations from 3.5 +/- 2.5 ng/ml to 78 +/- 22 ng/ml at 24 h and 111 41 ng/ml at 48 h. At 24 and 48 h, [14C]lactate was infused into the inferior vena cava, and blood samples were obtained to measure plasma concentrations and specific activities of glucose and lactate. Comparison of the cortisol-treated group with an untreated control group of animals revealed no differences in blood gases, haemoglobin concentrations, or glucose and lactate levels. Similarly, there were no differences between groups in liver oxygen consumption, glucose and lactate flux, or gluconeogenesis from lactate. In two animals we demonstrated hepatic glucose production from lactate. One of these was in active labor at the time of study, and one aborted within hours of the study. We conclude that the prenatal cortisol surge alone is not responsible for the onset of hepatic gluconeogenesis in the perinatal period. However, cortisol may have a permissive action, promoting hepatic gluconeogenesis in response to other hormonal stimuli.     

Glutamine metabolism in isolated incubated adipocytes of the rat.

1. Phosphate-dependent glutaminase activity in the epididymal fat-pad was 15.1 nmol/min per mg of protein. Glutaminase activity demonstrated differences with respect to adipose-tissue sites. Considerable variation was found in different sites of adipose tissue from lean control and Zucker obese animals. 2. Adipocytes incubated in the presence of 2 mM-glutamine utilized glutamine at a rate of 1.8 mumol/h per g dry wt., and glutamate, ammonia, lactate and alanine were produced. Addition of glucose plus insulin increased the rates of glutamine utilization and glutamate, ammonia, lactate and alanine production. Isoprenaline alone or plus glucose further stimulated the rate of glutamine utilization and formation of end products. 3. The rate of incorporation of 14C from glutamine into CO2 was similar to that of glucose, but the rate of incorporation into triacylglycerol was much less. Addition of unlabelled glucose or glucose plus insulin stimulated the rate of incorporation of [14C]glutamine into triacylglycerol, but had no effect on that of 14CO2 formation. Isoprenaline plus glucose increased the rate of incorporation of [14C]glutamine into CO2, but decreased the rate of incorporation into triacylglycerol. 4. In the absence of insulin, the rate of [14C]glutamine incorporation into triacylglycerol was related to the glucose concentration (0-10 mM). However, in the presence of insulin, the rate of incorporation of [14C]glutamine was maximal at 1 mM-glucose.     

Localization and role of pyruvate kinase isoenzymes in the regulation of carbohydrate metabolism and pyruvate recycling in rat kidney cortex

This work was performed to gain more information on the role of pyruvate kinase isoenzymes in the regulation of renal carbohydrate metabolism. Immunohistochemically, pyruvate kinase type L is shown to be localized in the proximal tubule of the nephron and pyruvate kinase type M2 in the distal tubule and the collecting duct. a tight relationship between gluconeogenesis and pyruvate recycling was found. The rate of gluconeogenesis (8 mumol/g wet wt. per 30 min) was of the same order of magnitude as the rate of pyruvate recycling (10.92 mumol/g wet wt. per 30 min). Stimulation of gluconeogenesis from 20 mM lactate in kidney cortex slices of 24-h-starved rats by dibutyryl-cAMP, alanine and parathyroid hormone was connected with a decrease in pyruvate recycling; inhibition of gluconeogenesis due to a lack of Ca2+ in the incubation medium was linked with an increase in pyruvate recycling. The degradation of [6-14C]glucose to lactate, pyruvate, ketone bodies and CO2 and of [2-14C]lactate was unaffected by dibutyryl-cAMP, alanine, epinephrine, vasopressin or the omission of Ca2+ from the incubation medium. 1 mM dibutyryl-cAMP or 5 mM alanine did not alter the activities of oxaloacetate decarboxylase, 'malic' enzyme and malate dehydrogenase from rat kidney cortex. Since aerobic glycolysis in the distal tubules and the collecting ducts is not influenced by hormones, dibutyryl-cAMP and Ca2+, pyruvate kinase type M2 residing in this tissue is unlikely to be a control point of glycolysis. Since this tissue degrades only one-seventh of the glucose formed via gluconeogenesis, it does not contribute significantly to pyruvate recycling. Therefore, the decrease of pyruvate recycling in the presence of dibutyryl-cAMP and alanine in rat kidney cortex slices, leading to increased renal gluconeogenesis, has to be ascribed to the regulation of pyruvate kinase type L.     

Effect of glycerol on gluconeogenesis in isolated rabbit kidney cortex tubules

In renal tubules isolated from fed rabbits glycerol is not utilized as a glucose precursor, probably due to the rate-limiting transfer of reducing equivalents from cytosol to mitochondria. Pyruvate and glutamate stimulated an incorporation of [14C]glycerol to glucose by 50- and 10-fold, respectively, indicating that glycerol is utilized as a gluconeogenic substrate under these conditions. Glycerol at concentration of 1.5 mM resulted in an acceleration of both glucose formation and incorporation of [14C]pyruvate and [14C]glutamate into glucose by 2- and 9-fold, respectively, while it decreased the rates of these processes from lactate as a substrate. In the presence of fructose, glycerol decreased the ATP level, limiting the rate of fructose phosphorylation and glucose synthesis. As concluded from the 'cross-over' plots, the ratios of both 3-hydroxybutyrate/acetoacetate and glycerol 3-phosphate/dihydroxyacetone phosphate, as well as from experiments performed with methylene blue and acetoacetate, the stimulatory effect of glycerol on glucose formation from pyruvate and glutamate may result from an acceleration of fluxes through the first steps of gluconeogenesis as well as glyceraldehyde-3-phosphate dehydrogenase. As inhibition by glycerol of gluconeogenesis from lactate is probably due to a marked elevation of the cytosolic NADH/NAD+ ratio resulting in a decline of flux through lactate dehydrogenase.     

Characteristics of glycogen synthesis from [1-14C]glucose and its labeled precursors in the piglet liver

The in vivo experiments have established that the rapid decrease in the glycogen content in the liver of piglets during the first 24 hours after birth is associated with the reduction of the degree of label inclusion from [1-14C]glucose into polysaccharide. The level of label inclusion from [1-14C]pyruvate and [1-14C]lactate into the liver glycogen in new-born piglets is higher than from [1-14C]alanine and [1-14C]glutamic acid. During the days immediately after birth the extension of the pool of glucogenic substrates occurs at the expense of alanine and other amino acids during catabolism of which pyruvate is formed. The degree of label inclusion from the investigated substrates into the liver glycogen of piglets of early age decreases in the series: [1-14C]glucose greater than [1-14C]lactate greater than [1-14C]pyruvate greater than [1-14C]alanine. Glutamic acid in the liver of piglets of early age is not a glucogenic substrate.