Liver Glucose Production Research Articles

Net nutrient absorption and liver metabolism in lactating dairy cows fed supplemental dietary biotin

The effect of feeding supplemental biotin on net absorption and metabolism of nutrients by the portal-drained viscera (PDV; the gut, pancreas, spleen and associated fat) and liver of lactating dairy cows was measured. Three cows in early to mid-lactation catheterised for measurements of net nutrient absorption and metabolism by the PDV and liver were fed a total-mixed ration with or without supplemental biotin at 20 mg/day using a switch-back design (ABA v. BAB) with three 2-week periods. There were no effects of feeding biotin on dry matter intake (22.2 kg/day), milk yield (29.5 kg/day) or milk composition. There was also no effect of feeding biotin on net release of glucose by the liver, net liver removal of glucose precursors (propionate, alanine, lactate) or net liver release of β-hydroxybutyrate. Feeding biotin increased net PDV release of ammonia. Reasons for the response are not certain, but a numerical increase in net PDV release of acetate suggests that rumen or hindgut fermentation was altered. Results of the present study do not support the hypothesis that supplemental biotin increases liver glucose production in lactating dairy cows.

Quantification of Liver Glucose Metabolism by Positron Emission Tomography: Validation Study in Pigs

The liver is inaccessible to organ balance measurements in humans. To validate [(18)F]fluorodeoxyglucose ([(18)F]FDG) positron emission tomography (PET) in the quantification of hepatic glucose uptake (HGU), we determined [(18)F]FDG modeling parameters, lumped constant (LC), and input functions (single arterial versus dual). Anesthetized pigs were studied during fasting (n = 6), physiologic (n = 4), and supraphysiologic (n = 4) hyperinsulinemia. PET was performed with C(15)O (blood pool) and [(18)F]FDG (glucose uptake). 6,6-Deuterated glucose ([(2)H]G) was coinjected with [(18)F]FDG and blood collected from the carotid artery and portal and hepatic veins to compute LC as ratio between tracers fractional extraction. HGU was estimated from PET images and ex vivo from high-performance liquid chromatography measurements of liver [(18)F]FDG versus [(18)F]FDG-6-phosphate and [(18)F]-glycogen. Endogenous glucose production was measured with [(2)H]G and hepatic blood flow by flowmeters. HGU was increased in hyperinsulinemia versus fasting (P < .05). Fractional extraction of [(18)F]FDG and [(2)H]G was similar (not significant), intercorrelated (r = 0.98, P < .0001), and equally higher during hyperinsulinemia than fasting (P <or= .05), with an LC of 0.98 +/- 0.10 and 1.18 +/- 0.26, respectively. [(18)F]FDG-PET modeling provided HGU values that did not differ from, and were correlated with, those from ex vivo measurements (r = 0.61, P <or= .02); proportional estimates of liver perfusion and endogenous glucose production were also obtained. Single and dual input functions produced strongly intercorrelated results (r > 0.95, P < .0001), with a modest underestimation of HGU by the former. [(18)F]FDG-PET-derived parameters provide accurate quantification of HGU and estimates of liver perfusion and glucose production. In the liver, LC of [(18)F]FDG is nearly unitary. Using a single arterial input introduces only a small error in estimation of HGU.

Adipose tissue, hepatic, and skeletal muscle insulin sensitivity in extremely obese subjects with acanthosis nigricans

We evaluated insulin action in skeletal muscle (glucose disposal), liver (glucose production), and adipose tissue (lipolysis) in 5 extremely obese women with acanthosis nigricans (AN), who had normal oral glucose tolerance, and 5 healthy lean subjects, by using a 5-stage pancreatic clamp and stable isotopically labeled tracer infusion. Basal plasma insulin concentration was much greater in obese subjects with AN than lean subjects (54.8 ± 4.5 vs 8.0 ± 1.3 μU/mL, P < .001), but basal glucose and free fatty acid concentrations were similar in both groups. During stage 1 of the clamp, glucose rate of appearance ( R a) (2.6 ± 0.3 vs 3.7 ± 0.3 μmol · kg FFM −1 · min −1, P = .02) and palmitate R a (2.4 ± 0.6 vs 7.0 ± 1.5 μmol · kg FFM −1 · min −1, P < .05) were greater in obese subjects with AN than lean subjects despite slightly greater plasma insulin concentration in subjects with AN (3.0 ± 0.7 vs 1.1 ± 0.4 μU/mL, P < .05). The area under the curve for palmitate R a (1867 ± 501 vs 663 ± 75 μmol · kg FFM −1 · 600 min −1, P = .03) and glucose R a (1920 ± 374 vs 1032 ± 88 μmol · kg FFM −1 · 600 min −1, P = .02) during the entire clamp procedure was greater in subjects with AN than lean subjects. During intermediate insulin conditions (plasma insulin, ~35 μU/mL), palmitate R a was 5-fold greater in subjects with AN than in lean subjects (2.6 ± 1.1 vs 0.5 ± 0.2 μmol · kg FFM −1 · min −1, P = .05). Maximal glucose disposal was markedly lower in obese subjects with AN than in lean subjects (13.0 ± 0.8 vs 23.4 ± 1.8 mg · kg FFM −1 · min -1, P = .01) despite greater peak plasma insulin concentration (1842 ± 254 vs 598 ± 38 μU/mL, P < .05). These data demonstrate obese young adults with AN have marked insulin resistance in multiple tissues. However, marked insulin hypersecretion can compensate for impaired insulin action, resulting in normal glucose and fatty acid metabolism during basal conditions.

Effects of the Antidiabetic Drug Metformin on Maximal Aerobic Capacity

Metformin is a commonly prescribed antidiabetic drug that promotes normoglycemia mainly by decreasing liver glucose production. Metformin has been shown to inhibit Complex 1 of the electron transport system, reducing mitochondrial oxidative phosphorylation (OxPhos). Lower capacity for OxPhos is expected to constrain maximal aerobic capacity (VO2max). This impact on VO2max is clinically relevant given that the prediabetic and diabetic individuals prescribed metformin are also encouraged to be physically active. PURPOSE: To examine the effects of metformin on VO2max as well as maximal heart rate (HRmax) and ventilation (VEmax). METHODS: Using a randomized, counterbalanced, placebo-controlled design, 7 healthy, normoglycemic men and women (33.6 ± 3.9 yrs) underwent an initial VO2max test after at least 3 days stabilization at the typical clinical dose of metformin (2000 mg/day) or placebo (single-blind). Following at least 7 days of the alternate treatment, subjects underwent a second VO2max test. Each continuous exercise test was conducted on a cycle ergometer or treadmill and commenced at a low work rate with incremental increases in treadmill grade or ergometer resistance until a maximal voluntary effort was achieved. Gas exchange measurements were obtained continuously throughout the test by open-circuit spirometry. HR was measured by telemetry using a Polar monitor. RESULTS: After treatment with metformin, absolute (L/min−1) and relative (mL/kg−1/min−1) VO2max were consistently lower relative to placebo (−4.7 ± 1.6% and−4.6 ± 1.5%, respectively). In addition, HRmax (bpm−1) and VEmax (L/min−1) were lower after metformin treatment compared to placebo (−4.0 ± 0.8 and−7.7 ± 1.8%, respectively). Maximal work output achieved was the same across conditions and mean test duration was 15.2 ± 1.1 min−1 on placebo versus 14.5 ± 0.7 min−1 with metformin treatment. CONCLUSION: Treatment with metformin appears to lower VO2max, HRmax, and VEmax. We hypothesized that VO2max would be lower with metformin treatment due to reduced OxPhos from the inhibition of Complex 1. However, because HRmax was also lower with metformin, it is unclear whether the decrease in VO2max is a result of central or peripheral factors, or both. These findings may have clinical relevance given the millions of people who are prescribed metformin and also encouraged to be physically active. This study was supported by a grant from the American Diabetes Association.

Mechanisms regulating energy metabolism by adiponectin in obesity and diabetes

Nutritional control of molecular events has become of great interest given the increased incidence of diet-induced obesity, and consequently Type 2 (non-insulin-dependent) diabetes, in recent years. The altered adipose tissue content in obese individuals results in an altered profile of circulating adipokines, and here we focus on adiponectin, whose circulating levels decrease in obese individuals. Adiponectin is a 30 kDa protein but circulates primarily as hexameric, oligomeric and, to a lesser extent, trimeric forms. Full-length adiponectin can also be cleaved to produce a fragment containing the globular domain that exerts potent metabolic effects. Adiponectin has insulin-mimetic and -sensitizing actions including stimulation of glucose uptake in skeletal muscle and suppression of glucose production in liver. Hence, adiponectin has attracted great interest as an antidiabetic agent. Adiponectin acts via two receptor isoforms, AdipoR1 (adiponectin receptor 1) and AdipoR2, which have distinct tissue distributions and affinities for recognition of the various adiponectin forms. Expression of AdipoR isoforms can be regulated by hyperinsulinaemia and hyperglycaemia with the consequence of increased sensitivity or resistance to specific forms of adiponectin. In summary, regulation of adiponectin or AdipoR expression may be of great importance in the development of metabolic perturbations characteristic of Type 2 diabetes in obese individuals.

Medication-Induced Weight Gain and Dyslipidemia in Patients With Schizophrenia

Our friend and colleague Wayne Fenton asked to write this article for the Journal because of his desire to educate other psychiatrists about the treatment of schizophrenia, including what he recognized to be a growing problem with the metabolic syndrome. This lifelong passion, which he pursued from his psychiatry residency at Yale, through his directorship of Chestnut Lodge, to his position at NIMH as Director of the Division of Adult Translational Research and Associate Director for Clinical Affairs, ended tragically with his killing during an evaluation of a psychotic young man. Wayne had worked tirelessly to secure support for new drug discovery in the NIMH programs that he directed. The Journal will be initiating in 2007 a series of articles on the discovery of new mental illness treatments. We will dedicate this series to Wayne's memory and include with it a memorial of his life and contributions to the treatment of mental illness.

Identification of the tyrosine phosphatase PTP-MEG2 as an antagonist of hepatic insulin signaling

Insulin resistance is a primary defect in type 2 diabetes characterized by impaired peripheral glucose uptake and insufficient suppression of hepatic glucose output. Insulin signaling inhibits liver glucose production by inducing nuclear exclusion of the gluconeogenic transcription factor FOXO1 in an Akt-dependent manner. Through the concomitant application of genome-scale functional screening and quantitative image analysis, we have identified PTP-MEG2 as a modulator of insulin-dependent FOXO1 subcellular localization. Ectopic expression of PTP-MEG2 in cells inhibited insulin-induced phosphorylation of the insulin receptor, while RNAi-mediated reduction of PTP-MEG2 transcript levels enhanced insulin action. Additionally, adenoviral-mediated depletion of PTP-MEG2 in livers of diabetic (db/db) mice resulted in insulin sensitization and normalization of hyperglycemia. These data implicate PTP-MEG2 as a mediator of blood glucose homeostasis through antagonism of insulin signaling, and suggest that modulation of PTP-MEG2 activity may be an effective strategy in the treatment of type 2 diabetes.

Mineralocorticoid receptor is involved in the regulation of genes responsible for hepatic glucose production

The mineralocorticoid receptor (MR) is expressed in kidney and plays a central role in the control of sodium, homeostatic fluid, and blood pressure. It has also been implicated in other functions in cardiovascular system, central nervous system, and adipose tissue. This study revealed a novel role of MR in the gene regulation related to hepatic glucose production. RNAi-mediated MR silencing led to a decrease in the expression of glucose-6-phosphatase (G6Pase), phosphoenolpyruvate carboxykinase, and fructose-1,6-bisphosphatase 1, the enzymes known to be involved in glucose production in liver. The MR-specific antagonists also down-regulated the expression of G6Pase, while the specific agonist enhanced G6Pase expression. These observations, for the first time, revealed a novel role for MR and its ligands in the regulation of de novo glucose synthesis in hepatocytes. It also suggests the potential of liver-specific MR modulation for the treatment of hyperglycemia.

The tangled path to glucose production

Liver glucose production is crucial to survival during fast and is abnormally elevated in diabetes. Studies of the transcriptional coactivator Torc2 redefine the mechanism by which cAMP signaling affects fasting-induced glucogenesis.

17β-Estradiol Supplementation Decreases Glucose Rate of Appearance and Disappearance with No Effect on Glycogen Utilization during Moderate Intensity Exercise in Men

Women use less carbohydrate during endurance exercise, as compared with men. In rodents, 17beta-estradiol (E2) supplementation robustly increases lipid use and lowers muscle and liver glycogen use during exercise. E2 supplementation has been found to influence substrate selection by decreasing glucose rate of appearance (Ra), disappearance (Rd), and metabolic clearance rate during exercise in humans; however, neither a change in total carbohydrate use nor a sparing of muscle glycogen was demonstrated. We investigated the effect of 8 d of E2 (2 mg/d) supplementation on glucose turnover and net muscle glycogen use in 11 men using a randomized, double-blind, placebo-controlled, crossover design. Subjects underwent primed constant infusion of [6,6-(2)H]glucose, and muscle biopsies were taken before and after 90 min of cycling at 65% maximal oxygen uptake. E2 supplementation decreased the respiratory exchange ratio (P = 0.03) and glucose Ra and Rd (both P = 0.04) during exercise, as compared with placebo. E2 supplementation lowered proglycogen (P < 0.05) and total glycogen (P = 0.04) concentration, as compared with placebo; however, there was no effect of E2 on net muscle glycogen use during exercise. These findings show that E2 supplementation alters fuel selection in exercising men by increasing lipid use and reducing carbohydrate use, glucose Ra (primarily liver glucose production), and Rd (primarily muscle glucose uptake). Furthermore, E2 reduces the basal level of total muscle glycogen, particularly the proglycogen form.

The Ultimate Glucose Monitor

The brain, particularly the hypothalamus, controls liver glucose production, but the cellular and molecular mechanisms by which the brain senses glucose levels have been unclear. Lam et al. now show that, in rats, this process requires the conversion of glucose in the hypothalamus to lactate, which in turn stimulates pyruvate metabolism and adenosine triphosphate (ATP) production. Alterations in ATP levels control neuronal excitability through effects on ATP-sensitive potassium channels, which have been implicated in glucose output by the liver. The elucidation of this metabolic regulatory loop may lead to new pharmacological interventions that could restore regulation of blood glucose in diabetes. T. K. T. Lam, R. Gutierrez-Juarez, A. Pocai, L. Rossetti, Regulation of blood glucose by hypothalamic pyruvate metabolism. Science 309 , 943-947 (2005). [Abstract] [Full Text]

Thymoquinone reduces hepatic glucose production in diabetic hamsters

The aim of this study was to elucidate the mechanisms underlying the glucose lowering effects of thymoquinone in streptozotocin (STZ)-induced diabetic hamsters in terms of hepatic glucose production. Diabetes was induced by intraperitoneal injection of 65 mg/kg body weight of STZ. Treatment with thymoquinone commenced 4 weeks after induction of diabetes at a daily dose of 50 mg/kg body weight by gastric gauge. Blood glucose and glycated hemoglobin levels were significantly reduced depending on periods of the treatment. Thirty days after commencement of thymoquinone-treatment, hepatocytes were isolated using collagenase to determine liver glucose production. Glucose production after 2 h incubation of the isolated hepatocytes with gluconeogenic precursors (alanine, glycerol and lactate) was significantly lower in hamsters treated with thymoquinone as compared to that in vehicle controls. The results of this study demonstrate that the antidiabetic action of thymoquinone is at least partially mediated through a decrease in hepatic gluconeogenesis.

Hypothalamic sensing of circulating fatty acids is required for glucose homeostasis

Increased glucose production is a hallmark of type 2 diabetes and alterations in lipid metabolism have a causative role in its pathophysiology. Here we postulate that physiological increments in plasma fatty acids can be sensed within the hypothalamus and that this sensing is required to balance their direct stimulatory action on hepatic gluconeogenesis. In the presence of physiologically-relevant increases in the levels of plasma fatty acids, negating their central action on hepatic glucose fluxes through (i) inhibition of the hypothalamic esterification of fatty acids, (ii) genetic deletion (Sur1-deficient mice) of hypothalamic K(ATP) channels or pharmacological blockade (K(ATP) blocker) of their activation by fatty acids, or (iii) surgical resection of the hepatic branch of the vagus nerve led to a marked increase in liver glucose production. These findings indicate that a physiological elevation in circulating lipids can be sensed within the hypothalamus and that a defect in hypothalamic lipid sensing disrupts glucose homeostasis.

Swim training increases glucose output from liver perfused in situ with glucagon in fed and fasted rats

The effect of endurance swim training (3 hours per day, 5 days/week, for 10 weeks) on hepatic glucose production (HGP) in liver perfused in situ for 60 minutes with glucagon and insulin was studied in Sprague-Dawley rats. The experiments were performed in fed rats and in rats fasted for 24 hours, but with lactate (8 mmol/L) added to the perfusion medium. Liver glycogen content was significantly lower in fasted than fed rats (fasted untrained and trained: 14 ± 4 and 11 ± 3 μmol glycosyl U/g of liver wet weight (WW); fed untrained and trained: 205 ± 11 and 231 ± 11 μmol glycosyl U/g of liver WW; not significantly different in trained and untrained rats). Glucagon increased HGP in the 4 experimental groups, but the increases were more rapid and pronounced in trained than in untrained rats in both fed and fasted states. HGP values (area under the curve [AUC] in μmol/g of liver WW) were significantly higher in trained fed (112.1 ± 7.1 v 85.9 ± 12.2 in untrained rats) than in trained fasted rats (50.8 ± 4.4 v 34.7 ± 3.6 in untrained rats). When compared with untrained rats, the total amount of glucose released by the liver in response to glucagon in trained rats was approximately 30% higher in the fed state and approximately 45% larger in the fasted state. These results indicate that endurance training increases the response of both glycogenolysis and gluconeogenesis to glucagon.

Mechanisms of the hypoglycaemic and immunopotentiating effects of Nigella sativa L. oil in streptozotocin-induced diabetic hamsters

The aim of this study was to elucidate the mechanisms underlying the hypoglycaemic effect of N. sativa oil ( Nigella sativa oil) in streptozotocin (STZ)-induced diabetic hamsters, in terms of hepatic glucose production, and to investigate the possible immunopotentiating effect of N. sativa oil on peritoneal macrophages. Diabetes was induced by intraperitoneal injection of 65 mg/kg body weight of STZ. Treatment with N. sativa oil commenced 6 weeks after induction of diabetes at a dose of 400 mg/kg body weight by gastric gavage. Isolated hepatocytes were collected using collagenase to determine liver glucose production. Phagocytic activity was evaluated by injection of fluorescent latex (2 μm diameter) intraperitoneally, followed 24 h later by collection of peritoneal macrophages. N. sativa oil reduced blood glucose from 391 ± 3.0 mg/dl before treatment to 325 ± 4.7, 246 ± 5.9, 208 ± 2.5 and 179 ± 3.1 mg/dl after the first, second, third and fourth weeks of treatment, respectively. Hepatic glucose production from gluconeogenic precursors (alanine, glycerol and lactate) was significantly lower in treated hamsters. Treatment with N. sativa oil significantly increased the phagocytic activity and phagocytic index of peritoneal macrophages and lymphocyte count in peripheral blood compared with untreated diabetic hamsters. Our data indicate that the hypoglycaemic effect of N. sativa oil is due to, at least in part, a decrease in hepatic gluconeogenesis, and that the immunopotentiating effect of N. sativa oil is mediated through stimulation of macrophage phagocytic activity either directly or via activation of lymphocytes.

Modeling the intra-venous glucose tolerance test: A global study for a single-distributed-delay model

The Intra Venous Glucose Tolerance Test (IVGTT) is a simple and established experimental procedure in which a challenge bolus of glucose is administered intra-venously and plasma glucose and insulin concentrations are then frequently sampled. The modeling of the measured concentrations has the goal of providing information on the state of the subject's glucose/insulin control system: an open problem is to construct a model representing simultaneously the entire control system with a physiologically believable qualitative behavior. A previously published single-distributed-delay differential model was shown to have desirable properties (positivity, boundedness, global stability of solutions) under the hypothesis of a specific, square-wave delay integral kernel. The present work extends the previous results to a family of models incorporating a generic non- negative, square integrable normalized kernel. Every model in this family describes the rate of glucose concentration variation as due to both insulin-dependent and insulin-independent net glucose tissue uptake, as well as to constant liver glucose production. The rate of variation of plasma insulin concentration depends on insulin catabolism and on pancreatic insulin secretion. Pancreatic insulin secretion at time $t$ is assumed to depend on the earlier effects of glucose concentrations, up to time $t$ (distributed delay). We consider a non-negative, square integrable normalized weight function $\omega$ on $R^+ =[0, \infty)$ as the fraction of maximal pancreatic insulin secretion at a given glucose concentration. No change in local asymptotic stability is introduced by the time delay. Considering an appropriate Lyapunov functional, it is found that the system is globally asymptotically stable if the average time delay has a parameter- dependent upper bound. An example of good model fit to experimental data is shown using a specific delay kernel.

Milrinone, a selective phosphodiesterase 3 inhibitor, stimulatesl lipolysis, endogenous glucose production, and insulin secretion

In vivo effects of milrinone, a selective phosphodiesterase 3 (PDE-3) inhibitor, on plasma free fatty acids (FFA), glucose, and insulin levels were examined in alert rats. In dose response studies, intravenous injection of 1, 5 or 25 μmol/kg of milrinone provoked an immediate increase in plasma concentrations of FFA and insulin, while glucose levels rose only in response to the 5- and 25-μmol/kg doses. During euglycemic-hyperinsulinemic (∼450 pmol/L) clamps, intravenous injection of milrinone (25 μmol/kg) completely inhibited insulin suppression of lipolysis and of endogenous glucose production, while having no effect on insulin-stimulated glucose uptake (ISGU). To explore the reason why ISGU was not affected, we performed reverse-transcriptase polymerase chain reaction (RT-PCR) with RNA from skeletal muscle, fat, and liver. The results showed that PDE-3B mRNA was expressed in adipose tissue and liver, but it was not detected in skeletal muscle. We conclude that PDE-3 plays a major role in the inhibitory action of insulin on lipolysis in fat and on glucose production in liver and, in addition, seems to be involved in insulin secretion in pancreatic β cells.

The insulin-sensitizing role of the fat derived hormone adiponectin.

Adiponectin is an insulin-sensitizing hormone whose blood concentration is reduced in obesity and type 2 diabetes. Administration of recombinant adiponectin in rodents increases glucose uptake and increases fat oxidation in muscle, reduces fatty acid uptake and hepatic glucose production in liver, and improves whole body insulin resistance. The exact receptor and signaling systems are unknown, however, recent studies suggest adiponectin activates AMPK, a putative master metabolic regulator. Thus, excitement surrounds the potential for adiponectin, or a homologue of adiponectin, as pharamacotherapy agents for patients suffering from the metabolic syndrome and more particularly for individuals with insulin resistance and type 2 diabetes.

TRB3: a tribbles homolog that inhibits Akt/PKB activation by insulin in liver.

Insulin resistance is a major hallmark in the development of type II diabetes, which is characterized by the failure of insulin to promote glucose uptake in muscle and to suppress glucose production in liver. The serine-threonine kinase Akt (PKB) is a principal target of insulin signaling that inhibits hepatic glucose output when glucose is available from food. Here we show that TRB3, a mammalian homolog of Drosophila tribbles, functions as a negative modulator of Akt. TRB3 expression is induced in liver under fasting conditions, and TRB3 disrupts insulin signaling by binding directly to Akt and blocking activation of the kinase. Amounts of TRB3 RNA and protein were increased in livers of db/db diabetic mice compared with those in wild-type mice. Hepatic overexpression of TRB3 in amounts comparable to those in db/db mice promoted hyperglycemia and glucose intolerance. Our results suggest that, by interfering with Akt activation, TRB3 contributes to insulin resistance in individuals with susceptibility to type II diabetes.

Developmental disorders of glucose metabolism in infants.

Developmental failures to adequately control postnatal blood glucose levels are common in the transition from fetal to infant life and can persist for many months. The standard method of functionally measuring hepatic glucose production and/or disordered glucose production is the response to a glucagon tolerance test. We adapted the standard glucagon tolerance test used for children and adults for use in preterm infants. 79 consecutive preterm infants gestational age range 25-36 weeks (mean 32.2 weeks), mean birth weight 1.66 kg admitted to the Neonatal Intensive Care Unit, Ninewells Hospital, Dundee and who survived to discharge home were recruited into the study. At the time of discharge home the characteristics of the group were as follows: adjusted mean gestational age 36.7 weeks, mean discharge weight 2.23 kg. In this study of preterm infants the maximal increase in plasma glucose following administration of a glucagon tolerance test is 1.39 +/- 07 mmol/L, n = 78 (range 0-3.98 mmol/L). An increase in plasma glucose of less than 4 mmol/L is considered abnormal in adults following administration of a fasting glucagon tolerance test. The responses of preterm infants and adults to glucagon are clearly different. The attenuated response to glucagon in the preterm infants is consistent with the low levels of hepatic glucose-6-phosphatase activity in premature infants as glucose-6-phosphatase is the terminal step of the two main pathways of liver glucose production.