Role of gluconeogenesis in epinephrine-stimulated hepatic glucose production in humans

To evaluate the contribution of gluconeogenesis to epinephrine-stimulated glucose production, we infused epinephrine (0.06 micrograms X kg-1 X min-1) for 90 min into normal humans during combined hepatic vein catheterization and [U-14C]alanine infusion. Epinephrine infusion produced a rise in blood glucose (50-60%) and plasma insulin (30-40%), whereas glucagon levels increased only at 30 min (19%, P less than 0.05). Net splanchnic glucose output transiently increased by 150% and then returned to base line by 60 min. In contrast, the conversion of labeled alanine and lactate into glucose increased fourfold and remained elevated throughout the epinephrine infusion. Similarly, epinephrine produced a sustained increase in the net splanchnic uptake of cold lactate (four- to fivefold) and alanine (50-80%) although the fractional extraction of both substrates by splanchnic tissues was unchanged. We conclude that a) epinephrine is a potent stimulator of gluconeogenesis in humans, and b) this effect is primarily mediated by mobilization of lactate and alanine from extrasplanchnic tissues. Our data suggest that the initial epinephrine-induced rise in glucose production is largely due to activation of glycogenolysis. Thereafter, the effect of epinephrine on glycogenolysis (but not gluconeogenesis) wanes, and epinephrine-stimulated gluconeogenesis becomes the major factor maintaining hepatic glucose production.     

Estimating gluconeogenesis by NMR isotopomer distribution analysis of [13C]bicarbonate and [1-13C]lactate

The gluconeogenic contribution to glucose production in livers isolated from rats fasted for 24 h was determined by 13C-NMR isotopomer distribution analysis of secreted glucose enriched from 99% [13C]bicarbonate (n = 4) and 99% [1-13C]lactate (n = 4). Experiments with 3% 2H2O were also performed, allowing the gluconeogenic contribution to be measured by the relative 2H enrichments at positions 5 and 2 of glucose. From 13C-NMR analyses, the contribution of gluconeogenesis to glucose output was estimated to be 93 +/- 3% for [13C]bicarbonate perfusion and 91 +/- 3% for [1-13C]lactate perfusion, in good agreement with the 2H-NMR analysis of the gluconeogenic contribution to glucose production (100 +/- 1% and 99 +/- 1%, respectively) and consistent with the expected negligible contribution from glycogenolysis. These results indicate that 13C-NMR analysis of glucose 13C-isotopomer distribution from either [13C]bicarbonate or [1-13C]lactate precursor provides realistic estimates of the gluconeogenic contribution to hepatic glucose output.     

Changes in glycemia induced by exercise in rats: contribution of hepatic glycogenolysis and gluconeogenesis

The participation of hepatic glycogenolysis and gluconeogenesis to the glycemic changes promoted by exercise was investigated. For this purpose, we employed swimming rats (2.5% body weight extra load attached to the tail, at 24 degrees C) using a favorable condition to measure hepatic glycogenolysis (fed rats) and a favorable condition to measure hepatic gluconeogenesis (fasted rats). This experimental approach permits us to compare the contribution of hepatic glycogenolysis and gluconeogenesis to glucose changes for a specific schedule of exercise. The animals were investigated at rest, after 5 minutes of swimming and after swimming to exhaustion. Our results show that hepatic glycogen has a crucial role to determine hyperglycemia during exercise. In contrast, hypoglycemia developed during exercise when glycogen was depleted. However, the ability of the liver to produce glucose from L-lactate, glycerol and L-glutamine was increased during exercise. Taken together, these findings suggest that the hepatic capacity to produce glucose from gluconeogenic substrates (except for L-alanine) was increased when hepatic glycogen stores were depleted. Thus, the increased capacity to produce glucose shown by livers from exercising rats must to be an important metabolic adaptation to protect against severe hypoglycemia.     

Effects of somatostatin on glucagon-stimulated glycogenolysis and gluconeogenesis in hepatocytes cultured in vitro

The effects of somatostatin (SS-14) on glycogenolysis and gluconeogenesis in rat hepatocytes cultured in vitro in a serum-free medium were investigated. Somatostatin (122 nmol 1^-1) did not significantly change the basal glucose production with or without pyruvate (10 mmol 1^-1). Glucagon strongly (over 100%) increased the glucose production in hepatocytes incubated in a medium supplemented with 10 mmol 1^-1 pyruvate. This increase in glucose production is the result of increased rates of gluconeogenesis and glycogenolysis. Somatostatin partially inhibited the glucagon stimulated increase in glucose production. Glucagon also significantly increased the glucose production in a glucose-free medium without pyruvate, which resulted from an increase of glycogenolysis. Somatostatin did not inhibit the increase in glucose production in these conditions. After a 4 h ‘fast’, glycogen in hepatocytes fell to a very low level. Glucose production was minimal. After the addition of pyruvate, there was a increase in gluconeogenesis and glucose production. Glucagon stimulated the rate of gluconeogenesis. Somatostatin completely inhibited this glucagon-stimulated increase in gluconeogenesis.     

On the inhibition of hepatic glycogenolysis by fructose. A 31P-NMR study in perfused rat liver using the fructose analogue 2,5-anhydro-D-mannitol.

Inhibition of hormone-stimulated hepatic glycogenolysis by fructose (Fru) has been attributed to accumulation of the competitive inhibitor Fru1P and/or to the associated depletion of the substrate phosphate (Pi). To evaluate the relative importance of either factor, we used the Fru analogue 2,5-anhydro-D-mannitol (aHMol). This analogue is avidly phosphorylated, traps Pi, and inhibits hormone-stimulated glycogenolysis, but it is not a gluconeogenic substrate, and hence does not confound glycogenolytic glucose production. Livers were continuously perfused with dibutyryl-cAMP (100 microM) to clamp phosphorylase in its fully activated a form. We administered aHMol (3.8 mM), and studied changes in glycogenolysis (glucose, lactate and pyruvate output) and in cytosolic Pi and phosphomonoester (PME), using in situ 31P-NMR spectroscopy (n = 4). Lobes of seven livers perfused outside the magnet were extracted for evaluation, by high-resolution 31P-NMR, of the evolution of aHMol1P and of aHMol(1,6)P2. After addition of aHMol, both glycogenolysis and the NMR Pi signal dropped precipitously, while the PME signal rose continuously and was almost entirely composed of aHMol1P. Inhibition of glycogenolysis in excess of the drop in Pi could be explained by continuing accumulation of aHMol1P. A subsequent block of mitochondrial ATP synthesis by KCN (1 mM) caused a rapid increase of Pi. Despite recovery of Pi to values exceeding control levels, glycogenolysis only recovered partially, attesting to the Pi-dependence of glycogenolysis, but also to inhibition by aHMol phosphorylation products. However, KCN resulted in conversion of the major part of aHMol1P into aHMol(1,6)P2. Residual inhibition of glycogenolysis was due to aHMol1P. Indeed, the subsequent withdrawal of aHMol caused a further gradual decrease in the proportion of aHMol1P (being converted into aHMol(1,6)P2, in the absence of de novo aHMol1P synthesis), and this resulted in a gradual de-inhibition of glycogenolysis, in the absence of marked changes in Pi. Glycogenolytic rates were consistently predicted by a model assuming non-saturated Pi kinetics and competition by aHMol1P exclusively: In conclusion, limited Pi availability and the presence of competitive inhibitors are decisive factors in the control of the in situ catalytic potential of phosphorylase a.     

Effects of somatostatin on glucagon-stimulated glycogenolysis and gluconeogenesis in hepatocytes cultured in vitro.

The effects of somatostatin (SS-14) on glycogenolysis and gluconeogenesis in rat hepatocytes cultured in vitro in a serum-free medium were investigated. Somatostatin (122 nmol l-1) did not significantly change the basal glucose production with or without pyruvate (10 mmol l-1). Glucagon strongly (over 100%) increased the glucose production in hepatocytes incubated in a medium supplemented with 10 mmol l-1 pyruvate. This increase in glucose production is the result of increased rates of gluconeogenesis and glycogenolysis. Somatostatin partially inhibited the glucagon stimulated increase in glucose production. Glucagon also significantly increased the glucose production in a glucose-free medium without pyruvate, which resulted from an increase of glycogenolysis. Somatostatin did not inhibit the increase in glucose production in these conditions. After a 4 h 'fast', glycogen in hepatocytes fell to a very low level. Glucose production was minimal. After the addition of pyruvate, there was a increase in gluconeogenesis and glucose production. Glucagon stimulated the rate of gluconeogenesis. Somatostatin completely inhibited this glucagon-stimulated increase in gluconeogenesis.     

Metabolism of [1,3-(13)C]glycerol-1,2,3-tris(methylsuccinate) and glycerol-1,2,3-tris(methyl[2,3-(13)C]succinate) in hepatocytes from Goto-Kakizaki rats

The metabolism of [1,3-(13)C]glycerol-1,2,3-tris(methylsuccinate) and glycerol-1,2,3-tris(methyl[2,3-(13)C] succinate) was examined in hepatocytes prepared from hereditarily diabetic Goto-Kakizaki rats. Over 120 min incubation in the presence of one of the two (13)C-labelled esters (2.5 mM), the output of (13)C-enriched glucose averaged 57.1 +/- 18.5 and 54.1 +/- 22.7 nmol per 10(6) cells, when expressed as [1,3-(13)C]glycerol and [2,3-(13)C] succinate equivalent, respectively. In the case of [1,3-(13)C]glycerol-1,2,3-tris(methyl-succinate), the molecules of glucose were symmetrically labelled. In the case of glycerol-1,2,3-tris(methyl[2,3-(13)C] succinate), however, both the single-labelled and double-labelled isotopomers of glucose contained more (13)C atoms in their C(6)-C(5)-C(4) than C(1)-C(2)-C(3) moiety. These findings indicate that glycerol-1,2,3-tris(methylsuccinate), recently proposed as a novel insulinotropic tool for the treatment of non-insulin-dependent diabetes mellitus, is efficiently metabolized in hepatocytes from diabetic rats, the high rate of gluconeogenesis coinciding with channelling of D-glyceraldehyde-3-phosphate between glyceraldehyde-3-phosphate dehydrogenase and phosphofructoaldolase.     

Carbohydrate metabolism of the rat C6 glioma. An in vivo 13C and in vitro 1H magnetic resonance spectroscopy study

Surface coil 13C nuclear magnetic resonance (NMR) spectroscopy was used to investigate the in vivo carbohydrate metabolism of rat C6 gliomas during and after infusion with [1-13C] glucose. In vivo 1H-decoupled 13C NMR spectra of the glioma following infusion with [1-13C]glucose revealed the direct production of [3-13C]lactic acid, [1-13C]glycogen, and [4-13C], [3-13C], and [2-13C]glutamate/glutamine. Lactate levels of in vivo gliomas increased and reached steady state levels during [1-13C]glucose infusion, and decreased following termination of infusion. Complementary in vitro studies using supernatant media collected from C6 glioma cells incubated with media containing [1-13C] or [6-13C]glucose and glutamine were examined by 1H NMR spectroscopy. The [3-(13C/12C)]lactate ratios obtained from 1H spectra of supernatant media containing [1-13C]glucose revealed the percentage of glucose metabolized through the hexose monophosphate shunt to be 10.01 +/- 0.85% (n = 3), while similar measurements of media containing [6-13C]glucose and glutamine showed that glutaminolysis contributed 9.0 +/- 1.0% of total lactate production under these conditions. Enzymatic analysis of media determined lactate production to be 139 +/- 9 nmol per 10(6) cells per h (n = 4). These measurements demonstrate the ability of NMR to monitor brain tumor carbohydrate metabolism both in vitro and in vivo.     

Splanchnic and renal vasoconstrictor and metabolic responses to neuropeptide Y in resting and exercising man.

The local clearance of neuropeptide Y (NPY) and whether NPY influences splanchnic and renal metabolism in man have not been investigated previously. The influence of NPY on splanchnic and renal blood flows at physiologically elevated levels has also not been investigated. The effects of a 40-min constant NPY infusion (3 pmol kg-1 min-1) at rest and during 130 min of exercise (50% of VO2max) were studied in six healthy subjects and compared with resting and exercising subjects receiving no NPY. Blood samples were drawn from arterial, hepatic and renal vein catheters for the determination of blood flows (indicators: cardiogreen and para-aminohippuric acid [PAH]), NPY, catecholamines, glucose, lactate and glycerol. NPY infusion was accompanied by: (1) significant fractional extraction of NPY-like immunoreactivity (NPY-Li) by splanchnic tissues at rest (58 +/- 5%) and during exercise (53 +/- 6%), while no arterial-venous differences could be detected across the kidney; (2) a reduction in splanchnic and renal blood flows of up to 18 and 13% respectively (P less than 0.01-0.001) at rest without any additional changes during exercise; and (3) metabolic changes as reflected in: (a) a more marked fall in arterial glucose during exercise compared to the reference group (P less than 0.05); (b) a 35% lower splanchnic glucose release (P less than 0.01) during exercise due to diminished glycogenolysis (P less than 0.01); and (c) a lower arterial lactate level (18% P less than 0.05) together with unchanged splanchnic lactate uptake during exercise, suggesting reduced lactate production by extrahepatic tissues. The disappearance of plasma NPY-Li after the infusions was biphasic with two similar half-lives at rest (4 and 39 min) and during exercise (3 and 43 min).     

Changes in liver gluconeogenesis during the development of Walker‐256 tumour in rats

Few studies have investigated liver gluconeogenesis in cancer and there is no agreement as to whether the activity of this pathway is increased or decreased in this disease. The aim of this study was to evaluate gluconeogenesis from alanine, pyruvate and glycerol, and related metabolic parameters in perfused liver from Walker‐256 tumour‐bearing rats on days 5 (WK5 group), 8 (WK8 group) and 12 (WK12 group) of tumour development. There was reduction (P < 0.05) of liver glucose production from alanine and pyruvate in WK5, WK8 and WK12 groups, which was accompanied by a decrease (P < 0.05) in oxygen consumption. Moreover, there was higher (P < 0.05) pyruvate and lactate production from alanine in the WK5 group and a marked reduction (P < 0.05) of pyruvate and urea production from alanine in the WK12 group. In addition, liver glucose production and oxygen consumption from glycerol were not reduced in WK5, WK8 and WK12 groups. Thus the, the results show inhibition of hepatic gluconeogenesis from alanine and pyruvate, but not from glycerol, on days 5, 8 and 12 of Walker‐256 tumour development, which can be attributed to the metabolic step in which the substrate enters the gluconeogenic pathway.     

Effects of age and fasting on gluconeogenesis from glycerol in dogs.

The extent of gluconeogenesis from glycerol was examined in pups and adult dogs. With use of the SAAM-26 program, a four compartment model was formulated from tracer data to calculate the kinetics of the glycerol:glucose system. In the postabsorptive state gluconeogenesis from glycerol declines with age: 13.8% of glucose carbon originated from glycerol in 0- to 4-day-old pups, 6% in adults. Approximately 50% of glycerol carbon is converted to glucose carbon independent of age. During fasting, a) the percentage of glucose carbon arising from glycerol carbon increased to 13.3% and 10.3% in adult dogs and pups 5-19 days old, respectively, in younger pups it declined to 3.4%; b) glycerol production increased in adults, but decreased in the youngest pups; c) glucose production and utilization decreased at all ages, and a smaller percentage of glycerol carbon was converted to glucose carbon, especially in the youngest pups. Thus in neonates fasting decreases gluconeogenesis from glycerol.     

Metabolism of alternative substrates and the bioenergetic status of EMT6 tumor cell spheroids

In order to evaluate the ability of EMT6/Ro multicellular spheroids to utilize various pathways of energy production, (13)C and (31)P MRS have been employed to monitor the metabolism of glucose, glutamine, acetate and propionate. EMT6/Ro spheroids perfused with culture medium containing 5.5 mM glucose maintain stable levels of nucleotide triphosphates (NTP) and phosphocreatine (PCr) for up to 48 h, even in the absence of glutamine. The metabolism of 1-(13)C-glucose was almost entirely to 3-(13)C-lactate (88 +/- 12%, n = 7), even though the perfusion medium was equilibrated with 95% O(2). Labeling was also observed in other glycolytic metabolites, primarily alanine and alpha-glycerolphosphate. A low level of (13)C labeling in glutamate, indicative of mitochondrial oxidative metabolism (TCA cycle), was consistently detected when spheroids were perfused with 1-(13)C-glucose, almost exclusively in the C4 position of glutamate. Labeling of glutamate C2 and C3 was always less than 20% of the labeling in C4 and was usually undetectable. No evidence of adjacent carbon labeling in individual glutamate molecules (indicative of multiple cycles of label incorporation) was found, even in high-resolution (13)C NMR spectra of extracts from cells or spheroids. Despite the predominantly glycolytic metabolism of glucose, the mitochondrial substrate glutamine (2 mM, in the presence of < or =0.5 mM glucose from fetal bovine serum), supported stable levels of NTP and PCr in the tumor cells for up to 12 h. In the presence of 2.5 mM acetate, the bioenergetic status of cells in EMT6 spheroids declined slowly but measurably, and no incorporation of label from 2-(13)C-acetate into other metabolites was detected either in intact perfused spheroids or in high-resolution spectra of extracts. In contrast, when the anaplerotic TCA cycle substrate 3-(13)C-propionate replaced acetate, the high-energy phosphate levels in EMT6/Ro spheroids were somewhat reduced, but stabilized at a new lower level. Incubation of spheroids with 3-(13)C-propionate (with natural abundance glucose and glutamine) resulted in label detectable in the C2 and C3 of glutamate, but the primary labeled compound was methylmalonate, an intermediate in propionate metabolism. Addition of vitamin B(12), a cofactor for methylmalonyl CoA reductase, to the growth medium 24 h prior to perfusion with propionate resulted in the elimination of the methylmalonate resonance. A variety of 2- and 3-labeled metabolites were detected, including succinate, malate and glutamate. Labeling of C2 and C3 of lactate implicated cytoplasmic malic enzyme activity.     

Metformin-induced suppression of glucose-6-phosphatase expression is independent of insulin signaling in rat hepatoma cells

Metformin is thought to decrease blood glucose levels by reducing hepatic glucose output. To elucidate the pharmacological action of metformin on hepatic glucose production, we examined its effect on the gene expression of glucose-6-phosphatase (G6Pase), a key enzyme of gluconeogenesis, in H4IIE rat hepatoma cell line by RT-PCR and quantitative real-time PCR. Metformin suppressed dexamethasone/cAMP-induced expression of G6Pase mRNA in a dose dependent manner, its maximum effect being observed at 2 mM (79.3% inhibition, P<0.05). Pretreatment with the PI3-kinase inhibitor wortmannin, the MEK-1 inhibitor PD98059 or the protein kinase C inhibitor GF109203X had no effect on suppressed G6Pase expression by metformin. Moreover, metformin did not stimulate Akt phosphorylation. In the present study, we demonstrate that metformin suppresses G6Pase mRNA expression by a mechanism that is independent of the activation of PI3-kinase, Akt, MAP kinase and protein kinase C pathway in hepatocytes.     

Carbohydrate metabolism in perfused livers of adrenalectomized and steroid-replaced rats.

In livers from fasted rats perfused with bicarbonate buffer containing bovine albumin and erythrocytes, adrenalectomy decreased glycogen levels and glucose production, impaired the incorporation of 14C from [14C]lactate into glucose or glycogen, and decreased the activity of the active (I) form of glycogen synthase. Cortisol treatment restored gluconeogenesis after 1 h and glycogen synthesis after 2 h. Adrenalectomy did not alter the production of glucose or lactate or the levels of gluconeogenic intermediates in livers from fasted rats perfused with fructose, but reduced the formation of glycogen from this substrate. Adrenalectomy increased the levels of lactate and decreased the levels of P-pyruvate and subsequent intermediates in the gluconeogenic pathway. These changes were reversed by cortisol treatment. It is concluded that glucocorticoids support gluconeogenesis and glycogen synthesis in livers from fasted rats primarily by facilitating a reaction(s) located between pyruvate and P-pyruvate in the gluconeogenic pathway and by promoting the conversion of inactive to active glycogen synthase.     

Combined administration of glucose precursors is more efficient than that of glucose itself in recovery from hypoglycemia

The purpose of the present study was to investigate the effect of the combined administration of hepatic gluconeogenic substrates (glycerol + L-lactate + L-alanine + L-glutamine) on glucose recovery during insulin induced hypoglycemia (IIH), in rats. IIH was obtained by an ip injection of regular insulin (1 U/kg). Thus, 150 min after insulin administration the rats received an ip injection of glycerol + L-lactate + L-alanine + L-glutamine (each 100 mg/kg). In these experiments control groups, which received saline, glucose or isolated precursors (100 mg/kg), were employed. Glycemia was measured 30 min later, i.e., 180 min after insulin injection. The results showed that the combined administration of gluconeogenic precursors is more efficient than that of glucose itself to promote glycemia recovery. Since, the blood levels of hepatic glucose precursors were decreased (glycerol, L-lactate and L-alanine) or maintained (L-glutamine) during IIH, the ability of the liver to produce glucose from these gluconeogenic substrates was investigated. The results showed that the maximal capacity of the liver to produce glucose from glycerol (2 mM), L-lactate (2 mM), L-alanine (5 mM) and L-glutamine (5 mM) was increased. To L-alanine and L-glutamine, not only the glucose production was increased (P < 0.05) but also the production of L-lactate, pyruvate and urea. Therefore, the results suggest that the decreased availability of glucose precursors, promoted by insulin administration, limits the participation of hepatic gluconeogenesis to glycemia recovery. However, the administration of gluconeogenic precursors could overcome this limitation and promote better glycemia recovery than glucose itself.     

Hepatic metabolism of genetically diabetic (db/db) mice. I. Carbohydrate metabolism.

Hepatic carbohydrate metabolism in genetically diabetic mice (db/db) and their normal littermates has been studied. In db/db mice, body water was below normal and declined with age. The liver of db/db mice was abnormally large in relation to the metabolic mass of the body at all ages studied. In db/db mice, hepatic glycogenolysis, glycogen synthesis, glycogen synthetase, and phosphorylase were markedly increased. Gluconeogenesis from alanine or lactate in perfused livers of db/db mice was greater than normal per 100 g body water. Activities of fructose-1, 6-biophosphatase, glucose-6-phosphatase, glucokinase + hexokinase, and pyruvate kinase were elevated in livers of db/db mice. Diabetic mouse livers perfused with lactate showed a markedly reduced concentration of P-enolpyruvate and clear "forward crossover" between fructose-1, 6-P2 and fructose-6-P. In vivo glucose clearance, measured with [3-3H]glucose, in db/db mice was 170% that of normal mice. Data presented indicate that in livers of db/db mice: 1) glucose production is elevated prior to hyperglycemia, 2) glycogen turns over more rapidly, and 3) glycolytic and gluconeogenic enzymes are elevated paradoxically. These abnormalities are discussed from the viewpoint of their etiology.     

Splanchnic and renal vasoconstrictor and metabolic responses to neuropeptide Y in resting and exercising man

The local clearance of neuropeptide Y (NPY) and whether NPY influences splanchnic and renal metabolism in man have not been investigated previously. The influence of NPY on splanchnic and renal blood flows at physiologically elevated levels has also not been investigated. The effects of a 40-min constant NPY infusion (3 pmol kg^-1 min^-1) at rest and during 130 min of exercise (50% of Vo_2max) were studied in six healthy subjects and compared with resting and exercising subjects receiving no NPY. Blood samples were drawn from arterial, hepatic and renal vein catheters for the determination of blood flows (indicators: cardiogreen and paraaminohippuric acid [PAH]), NPY, catecholamines, glucose, lactate and glycerol. NPY infusion was accompanied by: (1) significant fractional extraction of NPY-like immunoreactivity (NPY-Li) by splanchnic tissues at rest (58±5%) and during exercise (53±6%), while no arterial–venous differences could be detected across the kidney; (2) a reduction in splanchnic and renal blood flows of up to 18 and 13% respectively (P < 0.01–0.001) at rest without any additional changes during exercise; and (3) metabolic changes as reflected in: (a) a more marked fall in arterial glucose during exercise compared to the reference group (P < 0.05); (b) a 35% lower splanchnic glucose release (P < 0.01) during exercise due to diminished glycogenolysis (P < 0.01); and (c) a lower arterial lactate level (18%P < 0.05) together with unchanged splanchnic lactate uptake during exercise, suggesting reduced lactate production by extrahepatic tissues. The disappearance of plasma NPY-Li after the infusions was biphasic with two similar half-lives at rest (4 and 39 min) and during exercise (3 and 43 min).     

Hepatic carbohydrate metabolism in the spontaneously diabetic Bio-Breeding Worcester rat

The effects of diabetes on hepatic carbohydrate metabolism were investigated in spontaneously diabetic Bio-Breeding Worcester (BB/W) rats. The juvenile-onset-type syndrome displayed by these animals is characterized by beta-cell destruction with subsequent ketosis-prone insulinopenia. Livers from diabetic animals demonstrated increased adenosine 3',5'-cyclic monophosphate levels but subnormal total protein and glycogen content. Isolated perfused livers of diabetic BB/W rats demonstrated an increased rate of glucose production from [14C]lactate and an impaired rate of glycogen synthesis. These data were consonant with hepatic enzyme studies demonstrating markedly increased activities of component gluconeogenic (glucose-6-phosphatase, fructose-1,6-diphosphatase, phosphoenolpyruvate carboxykinase) and glycogenolytic (glycogen phosphorylase) enzymes with decreased activities of glycolytic (hexokinase, pyruvate kinase) and glycogenic (glycogen synthase) enzymes. These findings agree with previous studies using alloxan- and streptozotocin-induced diabetic animals and suggest that accelerated hepatic gluconeogenesis and impaired glucose utilization are pathognomonic of all insulin-deficient diabetic syndromes.     

Myocardial glucose uptake and breakdown during adenosine-induced vasodilation

Summary In isolated K⁺ (16.2 mM)-arrested cat hearts perfused at constant pressure adenosine infusions (0.8 moles · min^–1 · 100 g^–1 for 10 min) caused an increase in myocardial¹⁴C-glucose uptake and release of¹⁴CO₂+H¹⁴CO _– ³ and¹⁴C-lactate simultaneously with a rise in coronary flow. The ratio of the release of¹⁴CO₂+H¹⁴CO _– ³ to that of¹⁴C-lactate and the specific activity of lactate in the effluate were not altered. In K⁺-arrested hearts perfused with constant volume neither glucose uptake nor glucose breakdown were influenced by 0.8 or 100 moles · min^–1 · 100 g^–1 adenosine with 0.1–5 mM glucose in the perfusion medium. It is concluded that adenosine does not affect directly the myocardial glucose carrier system, aerobic or anaerobic glucose breakdown or glycogenolysis, but enhances glucose uptake secondarily by increasing coronary flow. This interpretation is substantiated by the finding that mechanically produced increases in perfusion volume caused similar increases in myocardial glucose uptake as were observed with comparable adenosine-induced coronary flow increments.Parts of this work were presented at the 44th Meeting of the German Physiological Society in Bochum, FRG, March 18–21, 1975 [Pflügers Arch.355, R 16 (1975)]     

Mobilization of glucose from the liver during exercise and replenishment afterward.

The liver is anatomically well situated to regulate blood glucose. It is positioned downstream from the pancreas, which releases the key regulatory hormones glucagon and insulin. It is also just downstream from the gut, permitting efficient extraction of ingested glucose and preventing large excursions in systemic glucose after a glucose-rich meal. The position of the liver is not as well situated from the standpoint of experimentation and clinical assessment, as its primary blood supply is impossible to access in conscious human subjects. Over the last 20 years, to study hepatic glucose metabolism during and after exercise, we have utilized a conscious dog model which permits sampling of the blood that perfuses (portal vein, artery) and drains (hepatic vein) the liver. Our work has demonstrated the key role of exercise-induced changes in glucagon and insulin in stimulating hepatic glycogenolysis and gluconeogenesis during exercise. Recently we showed that portal venous infusion of the pharmacological agent 5'-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside leads to a marked increase in hepatic glucose production. Based on this, we propose that the concentration of AMP may be a component of a physiological pathway for stimulating hepatic glucose production during exercise. Insulin-stimulated hepatic glucose uptake is increased following exercise by an undefined mechanism that is independent of liver glycogen content. The fate of glucose taken up by the liver is critically dependent on hepatic glycogen stores, however, as glycogen deposition is greatly facilitated by prior glycogen depletion.