Glucose Precursors Research Articles

Effects of glucagon and insulin on net hepatic metabolism of glucose precursors in sheep.

The net hepatic metabolism of amino glycerol, lactate, and pyruvate was determined in conscious fed sheep by multiplying the venoarterial concentration differences by the hepatic blood or plasma flow. In each experiment several sets of control blood samples were taken; glucagon or insulin then was infused intraportally for 2 h during which additional samples were taken. Four types of experiments were performed: 1) glucagon infusion (150 mug/h) into normal sheep, 2) glucagon infusion (100 mug/h) into insulin-treated alloxanized sheep, 3) insulin infusion (1.17 U/h) into normal sheep, and 4) insulin plus glucose infusion (12.3 mmol/h) into normal sheep. The second group of experiments was performed to prevent reflex hyperinsulinemia, and the fourth was performed to prevent reflex hyperglucagonemia. Glucagon directly stimulated the net hepatic uptake of alanine, glycine, glutamine, arginine, asparagine, threonine, serine, and lactate. Glucagon also stimulated lipolysis in adipose tissue. Insulin, on the other hand, appeared to have a lipogenic effect on adipose tissue and to stimulate directly the uptake of valine, isoleucine, leucine, tyrosine, lysine, and alanine only at extrahepatic sites. The study showed that, in sheep, the effects of glucagon primarily are on liver, and insulin's effects primarily are on skeletal muscle and adipose tissue where it promotes protein and lipid synthesis.

Glucose formation by the R3230AC rat mammary adenocarcinoma

1. 1. The presence of d-fructose-1,6-diphosphate 1-phosphohydrolase (E.C.3.1.3.11) activity has been established in the mammary glands of late pregnant Fisher 344 rats and in the R3230AC rat mammary adenocarcinoma. 2. 2. Homogenates of the R3230AC tumour are capable of synthesizing glucose from fructose-1,6-diphosphate a and other gluconeogenic precursors. The rate of glucose synthesis from fructose-1,6-diphosphate is decreased by high substrate concentrations, estrogen administration to the tumour bearing rats and by the addition of 5′AMP or ATP to the incubation mixture. 3. 3. Starvation increases glucose formation from fructose-1,6-diphosphate in the estrogen-treated rats but has no effect in the non-treated animals. 4. 4. Tumour homogenates produced α-glycerophosphate from glucose and other glycolytic precursors. The latter finding points to the presence of the α-glycerophosphate shunt in the R3230AC adenocarcinoma.

Splanchnic and leg exchange of glucose, amino acids, and free fatty acids during exercise in diabetes mellitus.

The influence of exercise on leg and splanchnic exchange of substrates was examined in eight insulin-dependent diabetics 24 h after withdrawal of insulin and in eight healthy controls studied at rest and after 40 min of bicycle ergometer exercise at 55-60% of maximal capacity. In four of the diabetic subjects, basal arterial ketone acid levels were 3-4 mmol/ liter (ketotic diabetics) and in the remainder, below 1 mmol/liter (nonketotic diabetics). ,ree fatty acid (FFA) turnover and regional exchange were evaluated with 14-C- labeled oleic acid. Leg uptake of blood glucose rose 13-18 fold during exercise in both the diabetics and controls and accounted for a similar proportion of the total oxygen uptake by leg muscles (25-28%) in the two groups. In contrast, leg uptake of FFA corresponded to 39% of leg oxygen consumption in the diabetic group but only 27% in controls. Systemic turnover of oleic acid was similar in the two groups. Splanchnic glucose output increased during exercise 3-4 fold above resting levels in both groups. In the diabetics, splanchnic uptake of lactate, pyruvate, glycerol, and glycogenic amino acids rose more than twofold above resting levels and was fourfold greater than in exercising controls. Total precursor uptake could account for 30% of the splanchnic glucose output in the diabetic group. In contrast, in the controls, total splanchnic uptake of glucose precursors was no greater during exercise than in the resting state and could account for no more than 11% of splanchnic glucose output. The augmented precursor uptake during exercise in the diabetics was a consequence of increased splanchnic fractional extraction as well as increased peripheral production of gluconeogenic substrates. The arterial glucagon concentration was unchanged by exercise in both groups, but was higher in the diabetics. In the diabetic subjects with ketosis in the resting state, exercise elicited a rise in arterial glucose and FFA, an augmented splanchnic uptake of FFA, and a 2-3 fold increase in splanchnic output of 3-hydroxybutyrate. Uptake of 3-hydroxybutyrate by the exercising leg rose more rapidly than splanchnic production, resulting in a fall in arterial levels of 3-hydroxybutyrate. It is concluded that (a) glucose uptake by exercising muscle in hyperglycemic diabetics is no different from that of controls; (b) splanchnic glucose output rises during exercise to a similar extent in diabetics and controls, while uptake of gluconeogenic substrates is markedly higher in diabetics and accounts for a greater proportion of total splanchnic glucose output; (c) exercise in diabetic patients with mild ketosis is associated with a rise in blood glucose and FFA levels as well as augmented splanchnic production and peripheral uptake of ketone bodies.

Inhibition by salicylate of gluconeogenesis in the isolated perfused rat liver.

SUMMARY 1. Sodium salicylate has been used as a hypoglycaemic agent in the treatment of diabetic patients and severe hypoglycaemia has been reported in children treated with aspirin. The mechanism of this hypoglycaemic action has never been elucidated. 2. In the isolated perfused rat liver, salicylate inhibits gluconeogenesis from lactate, alanine and propionate. The degree of inhibition is related to salicylate concentration and occurs with 2 mM (30 mg/100 ml). 3. The inhibitory action probably results from the effect of salicylate on oxidative phosphorylation in that there was little or no inhibition of gluconeogenesis from precursors which do not have a high requirement for ATP in their metabolism to glucose. 4. In man, lactate and alanine are the main precursors of glucose, and the liver is the main site of gluconeogenesis. These results demonstrate a mechanism for salicylate induced hypoglycaemia in man, and in addition suggest that accumulation of lactate may account for the acidosis sometimes produced by overdosage with salicylate.

Splanchnic glucose and amino acid metabolism in obesity.

Arterial concentrations and splanchnic exchange of glucose, lactate, pyruvate, glycerol, free fatty acids, and individual acidic and neutral amino acids were determined in obese and nonobese control subjects in the basal state and during a 45 min infusion of glucose. Glucose was administered to the controls at a rate (2 mg/kg/min; 144 +/- 4 mg/min) known to inhibit splanchnic glucose output without influencing peripheral glucose utilization. The obese subjects received glucose at two dose levels (75 and 150 mg/min) which simulated either the rise in insulin or the inhibition in splanchnic glucose production observed in the controls. In the basal state splanchnic glucose production did not differ significantly between obese and control subjects. However splanchnic uptake of lactate, glycerol, alanine, free fatty acids, and oxygen was 50-160% greater in obese subjects. Splanchnic uptake of glucose precursors could account for 33% of hepatic glucose output in the obese group as compared to 19% in controls. The increase in alanine and lactate uptake was due in part, to a 50% increase in splanchnic fractional extraction. Administration of glucose to the control subjects 144 +/- 4 mg/min) resulted in a 50-60% increment in arterial insulin and a 75% reduction in splanchnic glucose output. In the obese group, infusion of glucose at a rate of 75 mg/min resulted in an equivalent rise in arterial insulin, but was accompanied by a less than 40% inhibition in splanchnic glucose output. Glucose infusion at a rate of 150 mg/min in the obese resulted in a 75% reduction in splanchnic glucose output which was equivalent to that observed in controls, but was accompanied by a significantly greater rise (100-200%) in arterial insulin. It is concluded that in obesity (a) despite basal hyperinsulinemia, splanchnic uptake of glucose precursors is increased, the relative contribution to total glucose release attributable to gluconeogenesis being 70% higher than in controls; (b) infusion of glucose at rates causing equivalent increases in arterial insulin induces a smaller inhibition in splanchnic glucose output than in controls; (c) infusion of glucose at rates causing comparable inhibition in splanchnic glucose output is accompanied by a disproportionately greater increase in endogenous insulin than in controls. These data are compatible with hepatic resistance to insulin in obesity.

Effect of elevated lactataemia on the rates of lactate turnover and oxidation in rats.

1 The L-Lactate14C(U) priming dose-perfusion technique was used to measure the rates of lactate turnover and oxidation in anesthetized and mechanically ventilated rats. Lactataemia ([L]) was normal (0.5–1mM) or elevated either through moderate hypoxaemia or after lactate loading in normoxic conditions. 2. Lactate turnover and oxidation rates increased linearly with [L] up to 6 mM and the values of the three groups stand on same lines. 3. The slope of the regression line of oxidation rate vs [L] was lower than the turnover one. Thus an increasing part of the lactate flow was oriented to other metabolic pathways in these hyperlactataemic conditions. 4. 15% of the metabolic rate was explained by lactate oxidation in basal condition (50% of the turnover rate oxidized). This participation of [L] to the total energy expenditure increased when [L] rose and reached 45% when [L] is 6 mM. 5. These results are discussed in the light of the physiological role of lactate as metabolic regulator, glucose precursor and energetic substrate.

Contribution of the Cytosol and Mitochondrial Pathways to Phosphoenolpyruvate Formation during Gluconeogenesis

Gluconeogenesis was compared in liver slices from guinea pigs and rats to assess the significance of the different distribution patterns of phosphoenolpyruvate carboxykinase. Preferential formation of phosphoenolpyruvate in the cytosol or mitochondria was determined by addition of compounds (tryptophan, quinolinic acid, n-butylmalonate and amino oxyacetic acid) which inhibit cytosolic phosphoenolpyruvate carboxykinase or decrease the availability of substrate precursors such as malate and aspartate. During active gluconeogenesis these inhibitors are all effective. In particular, quinolinic acid at a concentration of 10 mM inhibits glucose formation about 90% in rat liver and produces a 60% inhibition in guinea pig liver. A study of avian liver phosphoenolpyruvate carboxykinase which is predominantly a mitochondrial enzyme, revealed a cytosol component during embryogenesis when there is a requirement for active gluconeogenesis. These results indicate that during maximum gluconeogenesis, the formation of phosphoenolpyruvate in the cytosol assumes a significant proportion of the carbon pathway, even in those animals which have the capacity to synthesize large amounts of this important precursor of glucose in the mitochondria.

Splanchic glucose production and its regulation in healthy monozygotic twins of diabetics.

1. Splanchnic exchange of glucose, lactate, pyruvate, glycerol and individual plasma amino acids was determined in five clinically unaffected monozygotic twins of insulin-dependent diabetics and in a group of five age- and sex-matched controls. The studies were performed in the postabsorptive state and after stimulation of endogenous insulin secretion by the intravenous infusion of glucose at a rate of 2 mg/min per kg for 45 min. 2. In the basal state the mean splanchnic glucose output was 25% lower for the twins than for the controls while splanchnic uptake of glucose precursors was similar in the two groups. The concentration of circulating insulin (which rose by no more than 10–15 μunits/ml in either group) was lower during the infusion in the twins than in the controls. Despite the lower insulin concentrations, infusion of glucose resulted in a complete shut off of splanchnic glucose production in the twins, while in the controls only a 50% inhibition of basal glucose output was observed. In both the twins and controls estimated peripheral glucose utilization was unchanged during the glucose infusion. 3. It is concluded that: (a) in clinically unaffected identical twins of known diabetics basal splanchnic glucose production is reduced while uptake of glucose precursors is unchanged, suggesting a diminished contribution to basal glucose output from hepatic glycogenolytic processes; (b) in the twins as well as in controls, the liver is the primary site of action of small increments in endogenous insulin; (c) hepatic sensitivity to endogenous insulin is augmented in twins as compared with controls.

Glucose utilization and production by the dog kidney in vivo in metabolic acidosis and alkalosis.

Using D-[1-(14)C]glucose as a tracer, renal glucose utilization and production was measured in chronic metabolic acidosis and alkalosis in dog kidney in vivo. In six experiments in acidosis, mean total renal glucose production was 4.447+/-1.655 SE mumol/min and glucose utilization was 4.187+/-0.576 SE mumol/min. In five alkalotic experiments it was found that mean total glucose production was 12.227+/-2.026 SE mumol/min and glucose utilization was 18.186+/-2.054 SE mumol/min. Renal glucose utilization and production are therefore significantly higher in alkalosis than in acidosis in vivo. Since glucose production is maximal under conditions when glutamine extraction is minimal (i.e. alkalosis), it is apparent that in alkalosis glutamine is not a major precursor of glucose.

Splanchnic and peripheral glucose and amino acid metabolism in diabetes mellitus.

Splanchnic and leg exchange of glucose, lactate, pyruvate, and individual plasma amino acids was studied in diabetics 24 hr after withdrawal of insulin and in healthy controls. Measurements were made in the basal postabsorptive state and during the administration of glucose at a rate of 2 mg/kg per min for 45 min. In the basal state, net splanchnic glucose production did not differ significantly between diabetics and controls. However, splanchnic uptake of alanine and other glycogenic amino acids was 1(1/2)-2 times greater in the diabetics, while lactate and pyruvate uptake was increased by 65-115%. Splanchnic uptake of these glucose precursors could account for 32% of hepatic glucose output in the diabetics, as compared to 20% in the controls. This increase in precursor uptake was a consequence of a two- to threefold increment in fractional extraction of these substrates inasmuch as arterial levels of alanine, glycine, and threonine were reduced in the diabetics, while the levels of the remaining substrates were similar in the two groups. Peripheral output of alanine and other glycogenic amino acids as reflected in arterio-femoral venous differences was similar in both groups. An elevation in arterial valine, leucine, and isoleucine was observed in the diabetics, but could not be accounted for on the basis of alterations in splanchnic or peripheral exchange of these amino acids. Administration of glucose (2 mg/kg per min) for 45 min resulted in an 80% reduction in splanchnic glucose output in controls, but failed to inhibit hepatic glucose release in the diabetics despite a twofold greater increment in arterial glucose levels. In both groups no consistent changes in arterial glucagon were observed during the infusion. It is concluded that in nonketotic diabetics (a) total splanchnic output of glucose is comparable to controls, but the relative contribution of gluconeogenesis may be increased by more than 50%; (b) accelerated splanchnic uptake of glucose precursors is a consequence of increased hepatic extraction of available substrates rather than a result of augmented substrate supply; and (c) the failure of glucose infusion to inhibit hepatic glucose output suggests that the exquisite sensitivity of the liver to the infusion of glucose in normal man is a consequence of glucose-induced insulin secretion.

Glucagon and the insulin: glucagon ratio in diabetes and other catabolic illnesses.

The biologic opposition which insulin and glucagon exert upon the liver and adipose tissue suggests that the relative concentrations of these two hormones which perfuse these tissues may determine their net nutrient balance. A review of recent studies of plasma glucagon and insulin levels reveals that the insulin:glucagon ratio (I/G) varies inversely with need for endogenous glucose production, being lowest in total starvation and highest during loading with exogenous carbohydrate. Similarly, the infusion of the glucose precursor, alanine, which in the fasting state causes a fall in I/G, a “catabolic response,” increases I/G during a glucose infusion, an ”anabolic response,/ which must spare alanine from gluconeogenesis. The same bihormonal relationship to need for glucose production has been observed after a protein load; normally after an overnight fast I/G rises in response to a beef meal, an anabolic response, while in the carbohydrate-deprived subject, the I/G does not rise, remaining at a catabolic level; during a glucose infusion the ingestion of a beef meal induces a greatly exaggerated anabolic rise in I/G. These findings are believed to account for certain previously poorly understood clinical observations. They may explain the so-called “protein sparing action” of glucose and the salutory nutritional effect of intravenously administered amino acid preparations when glucose is also provided in large quantities. The inability of diabetics to increase their I/G generally parallels their catabolic state, and, by stimulating glucagon but not insulin, tissue breakdown due to infection or trauma may further reduce the I/G and cause metabolic deterioration. The possible role of the I/G in the response to burns, trauma, infection, and malignancy is considered.

Relationships between the Concentration of Liver Metabolites and Ketogenesis in Chickens Fed “Carbohydrate-free” Diets

The effects on ketogenesis of feeding growing chickens diets in which all nonprotein calories were supplied by either soybean oil or soybean oil fatty acids were studied. A glucose reference diet was fed for comparison. Concentrations of several metabolites in liver, frozen in situ with the freeze-clamp technique, showed a general increase in those compounds associated with ketogenesis, and a general decrease in precursors of glucose, including citrate, when the “carbohydrate-free” diets were fed. Deletion of dietary carbohydrate decreased the NAD/NADH ratio in liver but oxaloacetate content was not decreased proportionately. In the absence of any significant dietary effect on the specific activity of the acetoacetate synthesis system, citrate synthase or citrate-cleavage enzyme, it is proposed that the utilization of acetyl-CoA shifts from the normal oxidative pathway to ketogenesis when the “carbohydrate-free” diets were fed, because acetyl-CoA is transported out of the mitochondria to the cytoplasm, where acetoacetate synthesis occurs in chicken liver.

Studies of liver glycogenoses, with particular reference to the metabolism of intravenously administered glycerol.

Abstract To investigate the pathophysiology of glycogenoses of the liver, glycerol, an endogenous precursor of glucose, was administered intravenously to eight patients and to 22 control subjects. ...

The Pattern of Radioactivity in Glucose and Glucose 6-Phosphate following Intraportal Injection of Either Fructose-1-14C or Fructose-6-14C in Rats

Following a brief intraportal injection of fructose-1-14C or fructose-6-14C in rats, the liver was promptly extirpated and homogenized, and glucose as well as glucose 6-phosphate was isolated. Examination of these metabolic products for the distribution of radioactivity in carbon atoms 1 and 6 showed that the patterns were dissimilar. Fructose-1-14C gave glucose labeled preponderantly in carbon 1, while the label in glucose 6-phosphate was equal in carbon atoms 1 and 6. Following fructose-6-14C injection, the glucose was labeled nearly equally in carbon atoms 1 and 6, while the glucose 6-phosphate was labeled chiefly in carbon 6. These results support the view that glucose can be formed from fructose by another pathway operating in conjunction with the currently accepted one. They further suggest that glucose 6-phosphate is not the immediate precursor of glucose in this alternate pathway.

Ethanol-induced hypoglycemia. II. Mechanism of suppression of hepatic gluconeogenesis.

The postulation that the increased NADH2/NAD ratio generated in the liver cell during ethanol metabolism causes the suppression of hepatic gluconeogenesis has been tested in several ways in twenty-eight fasted glycogendepleted dogs in whom hepatic gluconeogenesis was inhibited by infusions of ethanol. First, it was shown that fructose, a non-NAD-dependent precursor of glucose, produced a rapid restoration of hepatic glucose output during ethanol-induced suppression of hepatic gluconeogenesis. Second, in contrast, the infusion of glutamate and α-ketoglutarate, both NAD-dependent precursors of glucose, failed to augment the depressed rate of hepatic gluconeogenesis induced by ethanol. Finally, the administration of methylene blue, a redox dye which oxidizes NADH2 to NAD, not only prevented the expected fall in hepatic glucose output when infused simultaneously with ethanol, but also produced a rapid restoration of hepatic glucose output previously depressed by ethanol administration in fasting dogs. These data are consonant with the thesis that the increased NADH2/NAD ratio, which characterizes ethanol oxidation by the liver cell, causes a partial block at several points in the gluconeo genic pathway and is responsible for the ethanol-induced suppression of hepatic gluconeogenesis.

Control of Glycolysis and Gluconeogenesis in Liver and Kidney Cortex

This chapter discusses the control of glycolysis and gluconeogenesis in liver and kidney cortex. Liver and kidney cortex tissues appear to be unique in higher animals—that is, they possess the enzymatic potential for both glucose synthesis from noncarbohydrate precursors (gluconeogenesis), and glucose degradation by the glycolytic pathway. The functioning of both these pathways is demonstrated unequivocally in the perfused kidney, kidney cortex slices, the perfused liver, and liver slices. Gluconeogenesis is important when the dietary supply of glucose does not satisfy the metabolic demands of the animal; under these conditions glucose is required by the central nervous system, the red blood cells, and other tissues which cannot obtain all their energy requirements from fatty acid or ketone body oxidation. The gluconeogenesis is also important in the removal of excessive quantities of glucose precursors from the blood. The experimental approach to the study of gluconeogenesis and the theories of metabolic control are based on the assumption that the rate of glycolysis in these tissues is low, so that there is little impedance to the synthesis of glucose. The chapter also discusses the regulatory factors that change enzyme activity rapidly.

Studies on the origin of the methyl groups of choline in Ochromonas malhamensis.

SYNOPSIS. Five‐ to 6‐day‐old resting cells of Ochromonas malhamensis were incubated at pH 6.5 with glucose and appropriate C14 precursors of the methyl groups of phospholipid‐choline. Under the experimental conditions L‐methionine‐C14H3 was the most efficient source of choline‐methyl groups, followed by formate‐C14, formaldehyde‐C14 and DL‐serine‐3‐C14, respectively. Glycine‐2‐C14 was not incorporated into choline. Both L‐methionine‐C14H3 and formate‐C14 served as precursors for the methyl groups of monomethylethanolamine, dimethylethanolamine and choline. Addition of non‐radio‐active L‐methionine depressed the incorporation of formate‐C14 into choline‐methyl groups by 50%. The results support the hypothesis that methionine can be the source of all 3 methyl groups of choline, and that formate is probably converted to the methyl group of methionine before transmethylation to choline. However, an alternate pathway from single‐carbon sources cannot be excluded.

Effect of Cortisone on Liver Phosphorylase Activity.

SummaryRate of liver phosphorylase activity is decreased in fasted, adrenalectomized rats. Cortisone acetate administered at levels of 5, 2.5 or 1.25 mg/100 g body weight increased this activity above that of controls, the latter dosage only slightly above control level. Since glucose-6-phosphatase activity is also reported to be increased it is suggested that these 2 enzymatic effects may represent response to increased production of glucose precursors from peripheral protein.

Galactose metabolism and galactosemia

1. 1. Galactose forms an integral part of a number of biologically important compounds, such as cerebrosides, mucopolysaccharides and lactose. Most of the galactose present in these substances is not derived directly from ingested galactose but rather is synthesized from glucose and other precursors. 2. 2. Ingested galactose after absorption is transported to the liver, converted to glucose derivatives, and then enters the general “glucose pool.” The enzymatic reactions involved in galactoseglucose interconversion are reviewed. 3. 3. Impairment of galactose utilization occurs in liver disease, thyrotoxicosis, after ingestion of ethanol, and in galactosemia. 4. 4. Galactosemia is a congenital disorder of galactose metabolism which is transmitted by a single recessive gene and is due to the deficiency of the enzyme, galactose-1-phosphate uridyl transferase. The clinical manifestations consist of nutritional failure, hepatosplenomegaly, cataracts and mental retardation. The urine contains galactose, varying types and amounts of aminoacids, and albumin. Most of the clinical manifestations, except the mental changes, usually improve when galactose is removed from the diet. The toxic manifestations may be related to the accumulation of galactose-1-phosphate in the tissues. The presence of galactose-1-phosphate uridyl transferase in normal erythrocytes and its absence in galactosemic cells has provided the basis for a rapid and specific diagnostic test for the disease. 5. 5. Some utilization of galactose occurs in patients with galactosemia and increases as they mature. This may occur by an alternate pathway of galactose metabolism mediated by the enzyme, uridine diphosphogalactose pyrophosphorylase.

Developmental changes in the respiratory activity of rabbit ova

Oxygen consumption of fertilized rabbit ova in Ca ++-free Ringer-Phosphate, pH 7.4 (RP) at various stages of development has been measured in the Cartesian diver in the presence of certain metabolites and inhibitors. Cyanide exerted a marked inhibitory effect on respiration presumably through the cytochrome oxidase system. Fluoride, phlorizin, malonate, malonatefumarate combinations, pyruvate and glucose had no marked effect on respiratory rate. The presence of homologous serum did not increase the rate of respiration. Indeed, in one out of three cases there appeared to be a definite inhibition of O 2 uptake. The mucin coat of large blastocysts does not seem to be a limiting barrier to O 2 consumption by blastocyst tissue. Pre-incubation of ova, morulae and blastocysts in RP for 1 1 2 to 3 hours resulted in no apparent decrease in endogenous respiratory rate. After such a pre-incubation, the presence of metabolites such as glucose or pyruvate had no significant effect on O 2 consumption. The results suggest the availability of an intracellular store of substrate in the fertilized developing egg which cannot be exhausted by the pre-incubations tested. That this store is not glucose or a glucose precursor in late morulae and in blastocysts is indicated by the following: preliminary experiments have shown that late morulae and blastocysts, but not freshly shed ova, display glycolytic activity only in the presence of exogenous glucose. No evidence for glycolytic activity in early ova (1 to 16 cells) has been found. These data probably reflect the emergence of an enzyme complex during development in the fertilized rabbit ovum.