Influence of physiologic hyperglucagonemia on basal and insulin-inhibited splanchnic glucose output in normal man

Glucagon, from past to present: a century of intensive research and controversies

2022 corresponds to the 100th anniversary of the discovery of glucagon. This TimeCapsule aims to recall the main steps leading to the discovery, characterisation, and clinical importance of the so-called second pancreatic hormone. We describe the early historical findings in basic research (ie, discovery, purification, structure, α-cell origin, radioimmunoassay, glucagon gene [GCG], and glucagon receptor [GLR]), in which three future Nobel Prize laureates were actively involved. Considered as an anti-insulin hormone, glucagon was rapidly used to treat insulin-induced hypoglycaemic coma episodes in people with type 1 diabetes. A key step in the story of glucagon was the discovery of its role and the role of α cells in the physiology and pathophysiology (ie, paracrinopathy) of type 2 diabetes. This concept led to the design of different strategies targeting glucagon, among which GLP-1 receptor (GLP1R) agonists were a major breakthrough, and combination of inhibition of glucagon secretion with stimulation of insulin secretion (both in a glucose-dependent manner). Taking advantage of the glucagon-induced increase in energy metabolism, biased coagonists were developed. Besides the GLP-1 receptor, these coagonists also target the glucagon receptor to further promote weight loss. Thus, the 100-year story of glucagon has most probably not come to an end.

Glucagon's Metabolic Action in Health and Disease

Discovered almost simultaneously with insulin, glucagon is a pleiotropic hormone with metabolic action that goes far beyond its classical role to increase blood glucose. Albeit best known for its ability to directly act on the liver to increase de novo glucose production and to inhibit glycogen breakdown, glucagon lowers body weight by decreasing food intake and by increasing metabolic rate. Glucagon further promotes lipolysis and lipid oxidation and has positive chronotropic and inotropic effects in the heart. Interestingly, recent decades have witnessed a remarkable renaissance of glucagon's biology with the acknowledgment that glucagon has pharmacological value beyond its classical use as rescue medication to treat severe hypoglycemia. In this article, we summarize the multifaceted nature of glucagon with a special focus on its hepatic action and discuss the pharmacological potential of either agonizing or antagonizing the glucagon receptor for health and disease. © 2021 American Physiological Society. Compr Physiol 11:1759-1783, 2021.

The kinetics of glucagon action on the liver during insulin-induced hypoglycemia

Glucagon's effect on hepatic glucose production (HGP), under hyperglycemic conditions, is time dependent such that after an initial burst of HGP, it slowly wanes. It is not known whether this is also the case under hypoglycemic conditions, where an increase in HGP is essential. This question was addressed using adrenalectomized dogs to avoid the confounding effects of other counterregulatory hormones. During the study, infusions of epinephrine and cortisol were given to maintain basal levels. Somatostatin and insulin (800 µU·kg^-1·min^-1) were infused to induce hypoglycemia. After 30 min, glucagon was infused at a basal rate (1 ng·kg^-1·min^-1, baGGN group, n = 5 dogs) or a rate eightfold basal (8 ng·kg^-1·min^-1, hiGGN group, n = 5 dogs) for 4 h. Glucose was infused to match the arterial glucose levels between groups (≈50 mg/dL). Our data showed that glucagon has a biphasic effect on the liver despite hypoglycemia. Hyperglucagonemia stimulated a rapid, transient peak in HGP (4-fold basal production) over ~60 min, which was followed by a slow reduction in HGP to a rate 1.5-fold basal. During the last 2 h of the experiment, hiGGN stimulated glucose production at a rate fivefold greater than baGGN (2.5 vs. 0.5 mg·kg^-1·min^-1, respectively), indicating a sustained effect of the hormone. Of note, the hypoglycemia-induced rises in norepinephrine and glycerol were smaller in hiGGN compared with the baGGN group despite identical hypoglycemia. This finding suggests that there is reciprocity between glucagon and the sympathetic nervous system such that when glucagon is increased, the sympathetic nervous response to hypoglycemia is downregulated.

Metabolic effects of glucagon in humans

Diabetes is a common metabolic disorder that involves glucose, amino acids, and fatty acids. Either insulin deficiency or insulin resistance may cause diabetes. Insulin deficiency causes type 1 diabetes and diabetes associated with total pancreatectomy. Glucagon produces insulin resistance. Glucagon-induced insulin resistance promotes type 2 diabetes and diabetes associated with glucagonoma. Further, glucagon-induced insulin resistance aggravates the metabolic consequences of the insulin-deficient state. A major metabolic effect of insulin is the accumulation of glucose as glycogen in the liver. Glucagon opposes hepatic insulin action and enhances the rate of gluconeogenesis, increasing hepatic glucose output. In order to support gluconeogenesis, glucagon promotes skeletal muscle wasting to supply amino acids as gluconeogenic precursors. Glucagon promotes hepatic fatty acid oxidation to supply energy required to sustain gluconeogenesis. Hepatic fatty acid oxidation generates β-hydroxybutyrate and acetoacetate (ketogenesis). Prospective studies reveal that elevated glucagon secretion at baseline occurs in healthy subjects who develop impaired glucose tolerance at follow-up compared with subjects who maintain normal glucose tolerance, suggesting a relationship between elevated glucagon secretion and development of impaired glucose tolerance. Prospective studies have identified animal protein consumption as an independent risk factor for type 2 diabetes and cardiovascular disease. Animal protein intake activates glucagon secretion inducing sustained elevations in plasma glucagon. Glucagon is a major hormone that causes insulin resistance. Insulin resistance is an established cardiovascular risk factor additionally to its pathogenic role in diabetes. Glucagon may be a potential link between animal protein intake and the risk of developing type 2 diabetes and cardiovascular disease.

Evaluation of the specific effects of intranasal glucagon on glucose production and lipid concentration in healthy men during a pancreatic clamp

AIM

To investigate the specific effects of intranasal glucagon (ING) on plasma glucose, endogenous glucose production (EGP) and lipid concentration.

METHODS

We conducted a single-blind, randomized, crossover study at our academic investigation unit. Under pancreatic clamp conditions with tracer infusion, 1 mg ING or intranasal placebo (INP) was administered to 10 healthy men. As pilot studies showed that ING transiently increased plasma glucagon, we infused intravenous glucagon for 30 minutes along with INP to ensure similar plasma glucagon concentrations between interventions. The main outcome measures were plasma glucose, EGP, free fatty acid (FFA) and triglyceride (TG) concentrations.

RESULTS

In the presence of similar plasma glucagon concentrations, the increase in plasma glucose under these experimental conditions was attenuated with ING (mean plasma glucose analysis of variance P < .001) with reduction in EGP (P = .027). No significant differences were seen in plasma FFA and TG concentrations.

CONCLUSION

ING raises plasma glucose but this route of administration attenuates the gluco-stimulatory effect of glucagon by reducing EGP. This observation invites speculation about a potential central nervous system effect of glucagon, which requires further investigation. If ING is developed as a treatment for hypoglycaemia, this attenuated effect on plasma glucose should be taken into account.

Effects of 13-Hour Hyperglucagonemia on Energy Expenditure and Hepatic Glucose Production in Humans

Glucagon (GCG) acutely stimulates energy expenditure (EE) and hepatic glucose production (HGP) in humans, but whether these effects persist during hyperglucagonemia of longer duration is unclear. Using a prospective, randomized, single-blind, crossover study design, we therefore measured EE and rates of glucose appearance (glucose RA) during three separate infusion protocols in healthy lean males: A) 10-h overnight GCG infusion (6 ng/[kg × min]) followed by 3-h infusion of GCG, octreotide (OCT), and insulin (INS) for basal replacement; B) overnight saline (SAL) infusion followed by GCG/OCT/INS infusion; and C) overnight SAL infusion followed by SAL/OCT/INS infusion. Sleep EE, measured at 6 to 7 h of the overnight infusion, was increased 65-70 kcal/24 h in A compared with B and C. During the 3-h infusion, mean resting EE remained significantly increased in A versus C by ∼50 kcal/24 h; in B, resting EE increased with a statistical trend but was not significantly greater than in C. Glucose RA increased to comparable levels in A and B. We conclude that in healthy lean males, stimulation of EE and HGP is sustained during hyperglucagonemia of longer duration when insulin secretion is inhibited. The increase in EE at the present GCG dose was of marginal clinical significance.

Modeling Glucagon Action in Patients With Type 1 Diabetes

The dual-hormone artificial pancreas is an emerging technology to treat type 1 diabetes (T1D). It consists of a glucose sensor, infusion pumps, and a dosing algorithm that directs hormonal delivery. Preclinical optimization of dosing algorithms using computer simulations has the potential to accelerate the pace of development for this technology. However, current simulation environments consider glucose regulation models that either do not include glucagon action submodels or include submodels that were proposed without comparison to other candidate models. We consider here nine candidate models of glucagon action featuring a number of possible characteristics: insulin-independent glucagon action, insulin/glucagon ratio effect on hepatic glucose production, insulin-dependent suppression of glucagon action, and the effect of rate of change of glucagon. To assess the models, we use measurements of plasma insulin, plasma glucagon, and endogenous glucose production collected from experiments involving eight subjects with T1D who receive four subcutaneous glucagon boluses. We estimate each model's parameters using a Bayesian approach, and the models are contrasted based on the deviance information criterion. The model achieving the best fit features insulin-dependent suppression of glucagon action and incorporates effects of both glucagon levels and its rate of change.

Glucagon action in the brain

In recent years, novel discoveries have reshaped our understanding of the biology of brain glucagon in the regulation of peripheral homeostasis. Here we compare and contrast brain glucagon action in feeding vs glucose regulation and depict the physiological relevance of brain glucagon by reviewing their actions in two key regions of the central nervous system: the mediobasal hypothalamus and the dorsal vagal complex. These novel findings pave the way to future therapeutic strategies aimed at enhancing brain glucagon action for the treatment of diabetes and obesity. This review summarises a presentation given at the 'Novel data on glucagon' symposium at the 2015 annual meeting of the EASD. It is accompanied by two other reviews on topics from this symposium (by Young Lee and colleagues, DOI: 10.1007/s00125-016-3965-9 ), and by Russell Miller and Morris Birnbaum, DOI: 10.1007/s00125-016-3955-y ) and an overview by the Session Chair, Isabel Valverde (DOI: 10.1007/s00125-016-3946-z ).

Insulin and glucagon signaling in the central nervous system

The prevalence of the obesity and diabetes epidemic has triggered tremendous research investigating the role of the central nervous system (CNS) in the regulation of food intake, body weight gain and glucose homeostasis. This invited review focuses on the role of two pancreatic hormones--insulin and glucagon--that trigger signaling pathways in the brain to regulate energy and glucose homeostasis. Unlike in the periphery, insulin and glucagon signaling in the CNS does not seem to have opposing metabolic effects, as both hormones exert a suppressive effect on food intake and weight gain. They signal through different pathways and alter different neuronal populations suggesting a complementary action of the two hormones in regulating feeding behavior. Similar to its systemic effect, insulin signaling in the brain lowers glucose production. However, the ability of glucagon signaling in the brain to regulate glucose production remains unknown. Future studies that aim to dissect insulin and glucagon signaling in the CNS that regulate energy and glucose homeostasis could unveil novel signaling molecules to lower body weight and glucose levels in obesity and diabetes.

The role of dysregulated glucagon secretion in type 2 diabetes

Excessive production of glucose by the liver contributes to fasting and postprandial hyperglycaemia, hallmarks of type 2 diabetes. A central feature of this pathologic response is insufficient hepatic insulin action, due to a combination of insulin resistance and impaired β-cell function. However, a case can be made that glucagon also plays a role in dysregulated hepatic glucose production and abnormal glucose homeostasis. Plasma glucagon concentrations are inappropriately elevated in diabetic individuals, and α-cell suppression by hyperglycaemia is blunted. Experimental evidence suggests that this contributes to greater rates of hepatic glucose production in the fasting state and attenuated reduction after meals. Recent studies in animal models indicate that reduction of glucagon action can have profound effects to mitigate hyperglycaemia even in the face of severe hypoinsulinaemia. While there are no specific treatments for diabetic patients yet available that act specifically on the glucagon signalling pathway, newer agents including glucagon-like peptide-1 (GLP-1) receptor agonists and dipeptidyl peptidase-4 (DPP-4) inhibitors reduce plasma glucagon and this is thought to contribute to their action to lower blood glucose. The α-cell and glucagon receptor remain tempting targets for novel diabetes treatments, but it is important to understand the magnitude of benefit new strategies would provide as preclinical models suggest that chronic interference with glucagon action could entail adverse effects as well.

The role of glucagon, catecholamines and cortisol in counterregulation of insulin-induced hypoglycemia in normal man

To study the response of glucose counterregulation to insulin-induced hypoglycemia, six normals were given a 4-hour infusion of insulin (2.4 U/h) +/- somatostatin (50 micrograms/h). Supplementary glucagon (1.5 or 3.0 ng/kg/min) was given in additional experiments. In a separate study, glucagon was supplemented for 4 hours as a constant rate infusion (3.25 ng/kg/min) or at rates stepwise increasing from 1.5 to 5.0 ng/kg/min. Insulin decreased blood glucose by 1.5 mmol/l and simultaneous suppression of glucagon resulted in a more pronounced hypoglycemia enhancing the adrenaline and cortisol responses. The hyperglycemic effect of glucagon substitution (3 ng/kg/min) faded out after about 2 hours, whereafter exaggerated adrenaline and cortisol responses to hypoglycemia were seen. A comparison between the effects of steady state hyperglucagonemia and gradually appearing hyperglucagonemia on the counterregulation of hypoglycemia revealed no significant differences in glucose, adrenaline and cortisol responses to insulin. It is concluded that the glycemic effect of glucagon is transient in the hypoglycemic state. When the hepatic responsiveness to this hormone is decreased during hypoglycemia, adrenaline becomes the essential protective factor.

Effects of free fatty acids, insulin, glucagon and adrenaline on ketone body production in humans

In normal human subjects, when plasma insulin, glucagon and growth hormone were 'clamped' at basal concentrations (by infusion of somatostatin plus replacement infusion of these hormones), infusion of Intralipid and heparin increased plasma free fatty acid (FFA) concentrations to approx. 1.3 mM, and ketone body production increased 4-5 fold to approx. 11 mumol . kg -1 . min-1. Hyperglucagonaemia did not further increase ketogenesis. In conditions of combined insulin and glucagon deficiency (by infusion of somatostatin without insulin and glucagon), administration of Intralipid and heparin increased plasma FFA concentrations to approx. 2.2 mM but a further increase in ketone body production did not accompany this increase. In these conditions hyperglucagonaemia increased ketogenesis by 2-3 fold the increment seen in control studies. Infusion of adrenaline (epinephrine) in conditions in which insulin secretion was not inhibited caused only a transient increase in plasma FFA concentrations and in ketone body production. These data indicate: (1) that in humans increased FFA availability can markedly augment ketogenesis in the absence of insulin deficiency and without hyperglucagonaemia; (2) that glucagon can increase ketone body production during insulin deficiency but not in its absence; and (3) that insulin deficiency may be accompanied by increased ketogenesis only because of a lack of its restraint on lipolysis and because of the action of glucagon. Glucagon may be important in determining the magnitude of ketone body production for a given degree of FFA availability and insulin deficiency, and may be necessary for attainment of maximal rates of ketogenesis. Adrenaline increases ketone body production in humans, but whether this is primarily due to a direct effect on the liver or is mediated through enhancement of lipolysis remains to be determined.

Impaired fasting glycaemia vs impaired glucose tolerance: similar impairment of pancreatic alpha and beta cell function but differential roles of incretin hormones and insulin action

AIMS/HYPOTHESIS

The impact of strategies for prevention of type 2 diabetes in isolated impaired fasting glycaemia (i-IFG) vs isolated impaired glucose tolerance (i-IGT) may differ depending on the underlying pathophysiology. We examined insulin secretion during OGTTs and IVGTTs, hepatic and peripheral insulin action, and glucagon and incretin hormone secretion in individuals with i-IFG (n = 18), i-IGT (n = 28) and normal glucose tolerance (NGT, n = 20).

METHODS

Glucose tolerance status was confirmed by a repeated OGTT, during which circulating insulin, glucagon, glucose-dependent insulinotrophic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) levels were measured. A euglycaemic-hyperinsulinaemic clamp with [3-3H]glucose preceded by an IVGTT was performed.

RESULTS

Absolute first-phase insulin secretion during IVGTT was decreased in i-IFG (p = 0.026), but not in i-IGT (p = 0.892) compared with NGT. Hepatic insulin sensitivity was normal in i-IFG and i-IGT individuals (p > or = 0.179). Individuals with i-IGT had peripheral insulin resistance (p = 0.003 vs NGT), and consequently the disposition index (DI; insulin secretion x insulin sensitivity) during IVGTT (DI(IVGTT))) was reduced in both i-IFG and i-IGT (p < 0.005 vs NGT). In contrast, the DI during OGTT (DI(OGTT)) was decreased only in i-IGT (p < 0.001), but not in i-IFG (p = 0.143) compared with NGT. Decreased levels of GIP in i-IGT (p = 0.045 vs NGT) vs increased levels of GLP-1 in i-IFG (p = 0.013 vs NGT) during the OGTT may partially explain these discrepancies. Basal and post-load glucagon levels were significantly increased in both i-IFG and i-IGT individuals (p < or = 0.001 vs NGT).

CONCLUSIONS/INTERPRETATION

We propose that differentiated preventive initiatives in prediabetic individuals should be tested, targeting the specific underlying metabolic defects.

Impaired fasting glycaemia vs impaired glucose tolerance: similar impairment of pancreatic alpha and beta cell function but differential roles of incretin hormones and insulin action

AIMS/HYPOTHESIS

The impact of strategies for prevention of type 2 diabetes in isolated impaired fasting glycaemia (i-IFG) vs isolated impaired glucose tolerance (i-IGT) may differ depending on the underlying pathophysiology. We examined insulin secretion during OGTTs and IVGTTs, hepatic and peripheral insulin action, and glucagon and incretin hormone secretion in individuals with i-IFG (n = 18), i-IGT (n = 28) and normal glucose tolerance (NGT, n = 20).

METHODS

Glucose tolerance status was confirmed by a repeated OGTT, during which circulating insulin, glucagon, glucose-dependent insulinotrophic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) levels were measured. A euglycaemic-hyperinsulinaemic clamp with [3-3H]glucose preceded by an IVGTT was performed.

RESULTS

Absolute first-phase insulin secretion during IVGTT was decreased in i-IFG (p = 0.026), but not in i-IGT (p = 0.892) compared with NGT. Hepatic insulin sensitivity was normal in i-IFG and i-IGT individuals (p > or = 0.179). Individuals with i-IGT had peripheral insulin resistance (p = 0.003 vs NGT), and consequently the disposition index (DI; insulin secretion x insulin sensitivity) during IVGTT (DI(IVGTT))) was reduced in both i-IFG and i-IGT (p < 0.005 vs NGT). In contrast, the DI during OGTT (DI(OGTT)) was decreased only in i-IGT (p < 0.001), but not in i-IFG (p = 0.143) compared with NGT. Decreased levels of GIP in i-IGT (p = 0.045 vs NGT) vs increased levels of GLP-1 in i-IFG (p = 0.013 vs NGT) during the OGTT may partially explain these discrepancies. Basal and post-load glucagon levels were significantly increased in both i-IFG and i-IGT individuals (p < or = 0.001 vs NGT).

CONCLUSIONS/INTERPRETATION

We propose that differentiated preventive initiatives in prediabetic individuals should be tested, targeting the specific underlying metabolic defects.

The role of alpha-cell dysregulation in fasting and postprandial hyperglycemia in type 2 diabetes and therapeutic implications

The hyperglycemic activity of pancreatic extracts was encountered some 80 yr ago during efforts to optimize methods for the purification of insulin. The hyperglycemic substance was named "glucagon," and it was subsequently determined that glucagon is a 29-amino acid peptide synthesized and released from pancreatic alpha-cells. This article begins with a brief overview of the discovery of glucagon and the contributions that somatostatin and a sensitive and selective assay for pancreatic (vs. gut) glucagon made to understanding the physiological and pathophysiological roles of glucagon. Studies utilizing these tools to establish the function of glucagon in normal nutrient homeostasis and to document a relative glucagon excess in type 2 diabetes mellitus (T2DM) and precursors thereof are then discussed. The evidence that glucagon excess contributes to the development and maintenance of fasting hyperglycemia and that failure to suppress glucagon secretion contributes to postprandial hyperglycemia is then reviewed. Although key human studies are emphasized, salient animal studies highlighting the importance of glucagon in normal and defective glucoregulation are also described. The past eight decades of research in this area have led to development of new therapeutic approaches to treating T2DM that have been shown to, or are expected to, improve glycemic control in patients with T2DM in part by improving alpha-cell function or by blocking glucagon action. Accordingly, this review ends with a discussion of the status and therapeutic potential of glucagon receptor antagonists, alpha-cell selective somatostatin agonists, glucagon-like peptide-1 agonists, and dipeptidyl peptidase-IV inhibitors. Our overall conclusions are that there is considerable evidence that relative hyperglucagonemia contributes to fasting and postprandial hyperglycemia in patients with T2DM, and there are several new and emerging pharmacotherapies that may improve glycemic control in part by ameliorating the hyperglycemic effects of this relative glucagon excess.

Neutral endopeptidase 24.11 is important for the degradation of both endogenous and exogenous glucagon in anesthetized pigs

Glucagon has a short plasma t(1/2) in vivo, with renal extraction playing a major role in its elimination. Glucagon is degraded by neutral endopeptidase (NEP) 24.11 in vitro, but the physiological relevance of NEP 24.11 in glucagon metabolism is unknown. Therefore, the influence of candoxatril, a selective NEP inhibitor, on plasma levels of endogenous and exogenous glucagon was examined in anesthetized pigs. Candoxatril increased endogenous glucagon concentrations, from 6.3 +/- 2.5 to 20.7 +/- 6.3 pmol/l [COOH-terminal (C)-RIA, P < 0.05]. During glucagon infusion, candoxatril increased the t(1/2) determined by C-RIA (from 3.0 +/- 0.5 to 17.0 +/- 2.5 min, P < 0.005) and midregion (M)-RIA (2.8 +/- 0.5 to 17.0 +/- 3.0 min, P < 0.01) and reduced metabolic clearance rates (MCR; 19.1 +/- 3.2 to 9.4 +/- 2.0 ml.kg(-1).min(-1), P < 0.02, C-RIA; 19.2 +/- 4.8 to 9.0 +/- 2.3 ml.kg(-1).min(-1), P < 0.05, M-RIA). However, neither t(1/2) nor MCR determined by NH2-terminal (N)-RIA were significantly affected (t(1/2), 2.7 +/- 0.4 to 4.5 +/- 1.6 min; MCR, 30.3 +/- 6.4 to 28.5 +/- 9.0 ml.kg(-1).min(-1)), suggesting that candoxatril had no effect on NH2-terminal degradation but leads to the accumulation of NH2-terminally truncated forms of glucagon. Determination of arteriovenous glucagon concentration differences revealed that renal glucagon extraction was reduced (but not eliminated) by candoxatril (from 40.4 +/- 3.8 to 18.6 +/- 4.1%, P < 0.02, C-RIA; 29.2 +/- 3.1 to 14.7 +/- 2.2%, P < 0.02, M-RIA; 26.5 +/- 4.0 to 19.7 +/- 3.5%, P < 0.06, N-RIA). Femoral extraction was reduced by candoxatril when determined by C-RIA (from 22.7 +/- 2.4 to 8.0 +/- 5.1%, P < 0.05) but was not changed significantly when determined using M- or N-RIAs (10.0 +/- 2.8 to 4.7 +/- 3.7%, M-RIA; 10.5 +/- 2.5 to 7.8 +/- 4.2%, N-RIA). This study provides evidence that NEP 24.11 is an important mediator of the degradation of both endogenous and exogenous glucagon in vivo.

An increase in dietary protein improves the blood glucose response in persons with type 2 diabetes

BACKGROUND

In single-meal studies, dietary protein does not result in an increase in glucose concentrations in persons with or without type 2 diabetes, even though the resulting amino acids can be used for gluconeogenesis.

OBJECTIVE

The metabolic effects of a high-protein diet were compared with those of the prototypical healthy (control) diet, which is currently recommended by several scientific organizations.

DESIGN

The metabolic effects of both diets, consumed for 5 wk each (separated by a 2-5-wk washout period), were studied in 12 subjects with untreated type 2 diabetes. The ratio of protein to carbohydrate to fat was 30:40:30 in the high-protein diet and 15:55:30 in the control diet. The subjects remained weight-stable during the study.

RESULTS

With the fasting glucose concentration used as a baseline from which to determine the area under the curve, the high-protein diet resulted in a 40% decrease in the mean 24-h integrated glucose area response. Glycated hemoglobin decreased 0.8% and 0.3% after 5 wk of the high-protein and control diets, respectively; the difference was significant (P < 0.05). The rate of change over time was also significantly greater after the high-protein diet than after the control diet (P < 0.001). Fasting triacylglycerol was significantly lower after the high-protein diet than after the control diet. Insulin, C-peptide, and free fatty acid concentrations were not significantly different after the 2 diets.

CONCLUSION

A high-protein diet lowers blood glucose postprandially in persons with type 2 diabetes and improves overall glucose control. However, longer-term studies are necessary to determine the total magnitude of response, possible adverse effects, and the long-term acceptability of the diet.

Effect of insulin and energy restriction on the thermic effect of protein in type 2 diabetes mellitus

OBJECTIVE

The objective of this study was to test whether the thermic effect of oral protein is blunted in poorly controlled type 2 diabetes and is corrected by normalization of glycemia with insulin and 28 days of a very-low-energy diet.

RESEARCH METHODS AND PROCEDURES

Resting energy expenditure (REE) and the thermic effect of 90 g of oral protein were measured, using indirect calorimetry, in nine (five women and four men) obese diabetic people [weight, 108 +/- 10 kg; waist circumference, 123 +/- 8 cm; body mass index, 40 +/- 3 kg/m(2)] who were hyperglycemic on day 8 or euglycemic with insulin on day 16 of a weight-maintaining diet and euglycemic on day 28 of a very low energy diet (VLED). Results were compared with those of seven (six women and one man) weight- and body mass index-matched obese nondiabetic subjects with a waist circumference of 111 +/- 6 cm. Substrates and hormonal responses were determined concurrently.

RESULTS

Fasting glucose was normalized in the diabetic subjects with insulin from day 9 of VLED onward. Weight decreased in both groups by 9.9 +/- 0.9 kg with VLED. REE was 8 +/- 2% lower with insulin treatment and decreased by another 14 +/- 3% with VLED in the diabetic and by 15 +/- 1% in the nondiabetic subjects by week 4. After the protein meal, the thermic response was significantly (p < 0.05) less with hyperglycemia than with insulin-induced euglycemia, as percentage above REE (15.3 +/- 1.4 compared with 21.2 +/- 1.5%), as percentage of the energy content of the meal (19.5 +/- 1.5 compared with 25.2 +/- 1.7%), as kilocalories per 405 minutes (86 +/- 5 compared with 110 +/- 7), and less than in nondiabetic obese controls (21.0 +/- 2.2% above REE, 24.4 +/- 1.7% of energy of meal). After the VLED, the thermic effect of protein was significantly higher in both groups only as percentage above REE. The initial glucagon response was greater with hyperglycemia compared with euglycemia and post-VLED but not compared with the nondiabetic subjects. Hyperglycemia was associated with 21 +/- 4% greater urinary urea nitrogen excretion and urinary glucose losses of 134 +/- 50 mmol/d.

DISCUSSION

This study shows a blunted thermic effect of protein in obese hyperglycemic type 2 diabetic subjects compared with matched nondiabetic subjects that can be corrected with insulin- or energy restriction-induced euglycemia.

Glucagon increases glutamine uptake without affecting glutamine release in humans

Glucagon causes transient hyperglycemia and persistent hypoaminoacidemia, but the mechanisms of this action are unclear. To address this question, the present study measured the effects of glucagon on glucose, leucine, phenylalanine, and glutamine kinetics. Seven healthy subjects each underwent three pancreatic clamp studies (octreotide 30 ng/kg/min, insulin 0.15 mU/kg/min, and glucagon 1.4 ng/kg/min) lasting 7 hours. During the last 3.5 hours of the studies, glucagon infusion was either unchanged (study 0) or increased to 4 and 7 ng/kg/min (studies 1 and 2). The higher glucagon infusion rates increased the glucagon concentration by 50% and 100%, respectively. [6,6-(2)H2]glucose, [2-(15)N]glutamine, 2H5-phenylalanine, and 2H3-leucine were infused to quantify the respective fluxes. Glucagon transiently increased glucose concentrations by stimulating glucose production, which peaked in 15 minutes to 3.82 +/- 0.36 and 4.21 +/- 0.33 mg/kg/min in studies 1 and 2 and then returned to the postabsorptive levels. Glucagon decreased the glutamine concentration (-10% +/- 2% and -22% +/- 2% in studies 1 and 2 v study 0, P < .05), because glutamine uptake became greater than glutamine release (balance from -1.9 +/- 0.9 in study 0 to -8.1 +/- 1.1 and -13.6 +/- 1.0 micromol/kg/h in studies 1 and 2, P < .01). Glucagon decreased the leucine concentration (-11% +/- 3% in study 2 v study 0, P < .02) and caused a small increment in proteolysis (+6% in study 2 v study 0, P < .01) that was related to the decrement in glutamine concentrations. Phenylalanine kinetics were not significantly affected. These results show that glucagon promotes the uptake of gluconeogenic substrates but does not increase their release, suggesting that glucagon-induced hyperglycemia is short-lived because glucagon fails to provide more fuel for gluconeogenesis. The small increase in proteolysis and the depletion of circulating glutamine prove that physiologic hyperglucagonemia can contribute to protein catabolism.

Effects of glucagon on postprandial carbohydrate metabolism in nondiabetic humans

The present experiments sought to determine whether glucagon concentrations mimicking those observed in people with diabetes mellitus alter postprandial carbohydrate metabolism in nondiabetic humans. We measured the gastric emptying of solids and liquids, the systemic rate of appearance of ingested glucose, and endogenous glucose production either when postprandial suppression of glucagon was prevented by infusing glucagon at a rate of 0.65 ng/kg/min, when postprandial glucagon concentrations were elevated by infusing glucagon at a rate of 3.0 ng/kg/min, or when postprandial suppression of glucagon was permitted by infusion of saline. Despite marked differences in glucagon concentrations, postprandial glucose and insulin concentrations did not differ on any occasion. Although gastric emptying of liquids and solids was comparable on all three occasions, the high-dose, but not the low-dose, glucagon infusion caused a slight delay in the systemic appearance of ingested glucose and a significant decrease (P < .01) in postprandial D-xylose concentrations, suggesting a delay in carbohydrate absorption. However, this was offset by an increase (P < .05) in endogenous glucose production, resulting in no difference in postprandial glucose appearance. We conclude that in the absence of insulin deficiency, neither a lack of suppression of glucagon nor an elevation of glucagon to levels encountered in uncontrolled diabetes mellitus cause postprandial hyperglycemia in nondiabetic humans.

Clinical spectrum of hyperglucagonemia associated with malignant neuroendocrine tumors

OBJECTIVE

To review the clinical features associated with hyperglucagonemia in malignant neuroendocrine tumors.

MATERIAL AND METHODS

We retrospectively reviewed the medical records of patients with hyperglucagonemia encountered at our institution from Oct. 17, 1988, through February 1993 who had a fasting serum glucagon level of at least 120 pg/mL (twice the normal value). The 71 study patients also had no evidence of a secondary cause of hyperglucagonemia and had pathologic confirmation of a neuroendocrine tumor.

RESULTS

The study group consisted of 46 men and 25 women with a median age of 57 years. Two patients had multiple endocrine neoplasia. Forty-nine patients had biochemically polyfunctional tumors, and 22 had hyperglucagonemia only. The most common initial symptoms were weight loss, abdominal pain, diarrhea, nausea, peptic ulcer disease, diabetes, and necrolytic migratory erythema (NME). Diabetes eventually developed in 25 patients and was associated with NME in 11. The highest median serum glucagon values occurred in patients with the glucagonoma syndrome or insulinomas, and the lowest median values were in those with carcinoid syndrome, Zollinger-Ellison syndrome, or diabetes without NME. Fasting glucagon and glucose measurements were not correlated. The most common hormonal syndromes were the Zollinger-Ellison syndrome and the glucagonoma syndrome. All the neuroendocrine tumors were malignant. Several methods of treatment, including surgical debulking, chemotherapy, somatostatin, and hepatic artery embolization, were used. Death occurred in 29 patients at a median of 2.79 years after diagnosis; 42 patients were alive at a median of 2.86 years after diagnosis.

CONCLUSION

A mild degree of hyperglucagonemia can commonly be associated with multifunctional neuroendocrine tumors. The glucagonoma syndrome occurs in a few patients with malignant neuroendocrine tumors and hyperglucagonemia and is associated with very high serum glucagon levels. The correlation between serum glucagon levels and the development of diabetes is limited, and other factors such as insulin may be more important than hyperglucagonemia in the development of diabetes.

The glucagonoma syndrome. Clinical and pathologic features in 21 patients

The glucagonoma syndrome is a rare disorder characterized by weight loss, necrolytic migratory erythema (NME), diabetes, stomatitis, and diarrhea. We identified 21 patients with the glucagonoma syndrome evaluated at the Mayo Clinic from 1975 to 1991. Although NME and diabetes help identify patients with glucagonomas, other manifestations of malignant disease often lead to the diagnosis. If the diagnosis is made after the tumor is metastatic, the potential for cure is limited. The most common presenting symptoms of the glucagonoma syndrome were weight loss (71%), NME (67%), diabetes mellitus (38%), cheilosis or stomatitis (29%), and diarrhea (29%). Although only 8 of the 21 patients had diabetes at presentation, diabetes eventually developed in 16 patients, 75% of whom required insulin therapy. Symptoms other than NME or diabetes mellitus led to the diagnosis of an islet cell tumor in 7 patients. The combination of NME and diabetes mellitus led to a more rapid diagnosis (7 months) than either symptom alone (4 years). Ten patients had diabetes mellitus before the onset of NME. No patients had NME clearly preceding diabetes mellitus. Increased levels of secondary hormones, such as gastrin (4 patients), vasoactive intestinal peptide (1 patient), serotonin (5 patients), insulin (6 patients, clinically significant in 1 only), human pancreatic polypeptide (2 patients), calcitonin (2 patients) and adrenocorticotropic hormone (2 patients), contributed to clinical symptoms leading to the diagnosis of an islet cell tumor before the onset of the full glucagonoma syndrome in 2 patients. All patients had metastatic disease at presentation. Surgical debulking, chemotherapy, somatostatin, and hepatic artery embolization offered palliation of NME, diabetes, weight loss, and diarrhea. Despite the malignant potential of the glucagonomas, only 9 of 21 patients had tumor-related deaths, occurring an average of 4.91 years after diagnosis. Twelve patients were still alive, with an average age follow-up of 3.67 years.

Effect of danazol-induced chronic hyperglucagonaemia on glucose tolerance and turnover

It has been shown that danazol (14-ethinyltestosterone) induces hyperglucagonaemia. To investigate the effect of chronic glucagon excess on carbohydrate metabolism, we studied six patients before and after treatment with danazol for immunothrombopenia. Glucose tolerance and insulin, C-peptide and glucagon secretion during an oral glucose tolerance test (oGTT) as well as peripheral and hepatic insulin sensitivity were determined by means of euglycaemic clamp technique (40 mU m-2 min-1) before and after 3 months of danazol therapy. Overall glucose turnover (Rd) was assessed radioisotopically. (1) Plasma glucagon levels rose significantly from 88 +/- 16 pg mL-1 before to 683 +/- 148 pg mL-1 after therapy (P < 0.01). (2) Glucose levels during an oGTT were not significantly different before and after therapy. Glucose-stimulated insulin secretion at 60 and 120 min and the area under the curve (AUC) for insulin during the oGTT, were significantly increased after danazol treatment compared with pre-treatment values (P < 0.05), whereas glucagon secretion showed a similar decrease at both time points of investigation (NS). (3) Rd during steady state showed a significant decrease during the entire period of euglycaemic clamp following therapy (after 240 min, 3.8 +/- 0.6 vs. 5.3 +/- 0.7 mg kg-1 min-1, P < 0.05). The decline in glucagon during the clamp was similar during steady state before and after therapy. (4) Basal hepatic glucose output did not differ significantly before and after therapy (1.74 +/- 0.41 vs. 1.45 +/- 0.22 mg kg-1, NS), whereas hepatic glucose output during the clamp was significantly less suppressed after danazol therapy. The authors conclude that chronic glucagon excess leads to a decrease in peripheral and hepatic insulin action which is accompanied by an increase in insulin secretion.

Action of insulin on glucose metabolism in vivo

Insulin plays a key role in the maintenance of normal glucose tolerance by suppressing endogenous glucose production during a meal. Insulin is not, however, involved in the regulation of splanchnic glucose uptake. The latter process appears, based on studies performed in dogs, to be regulated primarily by the arterial-portal glucose gradient and to a smaller extent by glucose mass-action. Regarding peripheral glucose utilization, insulin is not needed to maintain a normal rate of glucose utilization since this can also be achieved by hyperglycaemia and glucose mass-action. Insulin is, however, necessary for the maintenance of normal rates of glucose oxidation and storage in insulin-sensitive tissues, and for the prevention of excessive gluconeogenic substrate production.

Evidence against important catecholamine compensation for absent glucagon counterregulation

To assess the counterregulatory role of glucagon and to test the hypothesis that catecholamines can largely compensate for an impaired glucagon response, four studies were performed in seven normal volunteers. In all studies, insulin was infused subcutaneously (15 mU.m-2.min-1) and increased circulating insulin approximately twofold to levels (26 +/- 1 microU/ml) observed with intensive insulin therapy. In study 1, plasma glucose fluxes (D-[3-3H]glucose) and plasma substrate and counterregulatory hormone concentrations were simply monitored; plasma glucose decreased from 87 +/- 2 mg/dl and plateaued at 51 +/- 2 mg/dl for 3 h. In study 2 [pituitary-adrenal-pancreatic (PAP) clamp], secretion of insulin and counterregulatory hormones (except for catecholamines) was prevented by somatostatin (0.5 mg/h i.v.) and metyrapone (0.5 g/4 h per os), and glucagon, cortisol, and growth hormone were reinfused to reproduce the concentrations of study 1. In study 3 (lack of glucagon response), the PAP clamp was performed with maintenance of plasma glucagon at basal levels, and glucose was infused whenever needed to reproduce plasma glucose concentration of study 2. Study 4 was identical to study 3, but exogenous glucose was not infused. The PAP clamp (study 2) reproduced glucose concentrations and fluxes observed in study 1. In studies 3 and 4, isolated lack of glucagon response did not affect glucose utilization but caused an early and persistent decrease in hepatic glucose production (approximately 60%) that caused plasma glucose to decrease to 38 +/- 2 mg/dl (P less than 0.01 vs. control 62 +/- 2 mg/dl), despite compensatory increases in plasma epinephrine. We conclude that, in a model of clinical hypoglycemia, glucagon's effect on hepatic glucose production is a dominant counterregulatory factor in humans and that its absence cannot be compensated for by increased epinephrine secretion.

Interactions of glucagon and free fatty acids with insulin in control of glucose metabolism

To study the interactions of physiological glucagon and free fatty acids (FFA) concentrations with insulin in the control of glucose metabolism, we determined in normal subjects the response of endogenous glucose production (EGP) and glucose utilization (Rd) to a progressive and moderate increase of insulinemia in the presence of glucagon and FFA levels either decreased (somatostatin [SRIF] and insulin infusion, C test) or maintained to normal postabsorptive values isolated (SRIF + insulin + glucagon infusion, G test; SRIF + insulin + Intralipid infusion, IL test) or in association (SRIF + insulin + glucagon + Intralipid infusion, IL + G test). Compared with the C test, maintenance of glucagon level had only small and inconsistent effects on glucose Rd, but induced a shift to the right of the dose-response curve to insulin of EGP (apparent ED50: C test, 10.9 mU.L-1; G test, 15.2 mU.L-1). Intralipid infusion resulted, whether glucagon was substituted or not, in a near total suppression of the insulin-induced increase of glucose Rd (Rd at the end of the tests: C test, 6.13 +/- 0.85 mg.kg-1.min-1; G test, 7.29 +/- 0.87 mg.kg-1.min-1; IL test, 3.30 +/- 0.65 mg.kg-1.min-1; IL + G test, 3.57 +/- 0.42 mg.kg-1.min-1). In the absence of glucagon, substitution Intralipid infusion also antagonized the action of insulin on EGP. However, this effect was no longer apparent when glucagon was replaced (dose-response curve to insulin of EGP during the G and the IL + G test were comparable).(ABSTRACT TRUNCATED AT 250 WORDS)

Role of glucagon in disposal of an amino acid load

Amino acids stimulate the release of glucagon and insulin. To assess the role of aminogenic hyperglucagonemia, we have studied, in healthy young males, the effects of basal (less than 100 pg/ml) and high (200-400 pg/ml) plasma glucagon concentrations on amino acid metabolism during intravenous infusion (0.5 g.h-1.4 h) of a mixture of 15 amino acids. Basal plasma glucagon concentrations were obtained by infusion of somatostatin (0.5 mg/h) plus glucagon (0.25 ng.kg-1.min-1) and high plasma glucagon concentrations by infusion of somatostatin plus glucagon (3.0 ng.kg-1.min-1) or by infusion of amino acids alone. All studies were performed under conditions of euglycemic (83-91 mg/dl) hyperinsulinemia (50-80 microU/ml). Hyperglucagonemia significantly increased 1) net amino acid transport from the extracellular into the intracellular space (by approximately 4%), 2) net degradation of amino acids entering the intracellular space (by approximately 40%), and 3) conversion of degraded amino acids into glucose from 0-10% (basal glucagon) to 70-100% (high glucagon). Hyperglucagonemia did not affect the amount of amino acids excreted in the urine (approximately 4%). We conclude that glucagon plays an important role in the disposition of amino acids by increasing their inward transport, their degradation, and their conversion into glucose.

The acute splanchnic and peripheral tissue metabolic response to endotoxin in humans

The in vivo alterations in organ-specific substrate processing and endogenous mediator production induced by endotoxin were investigated in healthy volunteers. An endotoxin bolus (20 U/kg) produced increased energy expenditure, hyperglycemia, hypoaminoacidemia, and an increase in circulating free fatty acids. These changes included increased peripheral lactate and free fatty acid output, along with increased peripheral uptake of glucose. Coordinately, there were increased splanchnic uptake of oxygen, lactate, amino acids, and free fatty acids, and increased splanchnic glucose output. There were no changes in circulating glucagon, or insulin and transient changes in epinephrine and cortisol were insufficient to explain the metabolic changes. Plasma cachectin levels peaked 90 min after the endotoxin infusion, and hepatic venous (HV) cachectin levels (peak 250 +/- 50 pg/ml) were consistently higher than arterial levels (peak 130 +/- 30 pg/ml, P less than 0.05 vs. HV). No interleukin 1 alpha or 1 beta was detected in the circulation. Circulating interleukin 6, measured by B.9 hybridoma proliferation, peaked 2 h after the endotoxin challenge (arterial, 16 +/- 2 U/ml; HV, 21 +/- 3 U/ml). The net cachectin efflux (approximately 7 micrograms) from the splanchnic organs demonstrates that these tissues are a major site for production of this cytokine. Hence, splanchnic tissues are likely influenced in a paracrine fashion by regional cachectin production and may also serve as a significant source for systemic appearance of this cytokine.

Oral and intravenous glucose-induced insulin secretion in hyperthyroid patients

To elucidate glucose intolerance in hyperthyroidism, insulin response to oral (75 g) and intravenous (IV) (20 g) glucose administration was investigated in 18 hyperthyroid patients and six normal control subjects. In oral glucose tolerance tests (OGTT), plasma insulin and C-peptide levels in hyperthyroid patients were not significantly different from that in controls; however, an impaired blood sugar response was observed in hyperthyroid patients. In IVGTT, blood sugar, plasma insulin, and C-peptide levels were significantly higher in hyperthyroid patients than in controls. Insulin secretion in proportion to blood sugar stimulus (the sum of increment in insulin divided by the sum of increment in blood sugar after glucose load, sigma delta IRI/sigma delta BS) in IVGTT was similar in hyperthyroid patients and controls; however, that in OGTT was significantly lower in hyperthyroid patients. After thyroid function tests had returned to normal by treatment with thiamazole, glucose tolerance and sigma delta IRI/sigma delta BS in OGTT were almost normalized. These results indicate that decreased insulin secretion after oral glucose may have an important role in abnormal oral glucose metabolism in hyperthyroidism.

Glucagon-cortisol interactions on glucose turnover and lactate gluconeogenesis in normal humans

To determine the mechanism for cortisol enhancement of glucagon-stimulated overall hepatic glucose output (OHGO), we employed the glucose-insulin clamp technique with infusions of [6-3H]glucose and [U-14C]lactate and measured OHGO, glucose utilization, and the turnover and incorporation of lactate in plasma glucose in normal volunteers under four experimental conditions: 1) normoglucagonemia (approximately 150 pg/ml)- normocortisolemia (approximately 14 micrograms/dl); 2) isolated hyperglucagonemia (approximately 550 pg/ml); 3) isolated hypercortisolemia (approximately 32 micrograms/dl); and 4) combined hyperglucagonemia-hypercortisolemia. Isolated hyperglucagonemia caused initial increases in OHGO and lactate gluconeogenesis, which were maximal at 1 h (23.9 +/- 1 and 2.7 +/- 0.4 mumol.kg-1.min-1, respectively) but remained significantly above values in control experiments through 5 h (10.3 +/- 0.7 vs. 8.2 +/- 1.1, P less than 0.03; 2.2 +/- 0.4 vs. 1.2 +/- 0.3, mumol.kg-1.min-1, P less than 0.04, respectively). Hypercortisolemia has no effect on OHGO but increased lactate gluconeogenesis after 3 h. Superimposition of hypercortisolemia on hyperglucagonemia did not further increase OHGO (11.1 +/- 0.7 vs. 10.3 +/- 0.7 mumol.kg-1.min-1, P = NS) but augmented lactate gluconeogenesis additively (isolated hyperglucagonemia = 0.96, isolated hypercortisolemia = 0.98; combined = 2.02 mumol.kg-1.min-1). Neither glucagon nor cortisol affected lactate turnover or glucose utilization. We conclude that glucagon has a persistent effect on OHGO largely accounted for by increased gluconeogenesis. Cortisol augments glucagon-stimulated gluconeogenesis in an additive manner best explained by changes in gluconeogenic enzymes rather than in substrate availability. Finally, the fact that cortisol increased gluconeogenesis without affecting glucose utilization suggests that the liver is more sensitive to the diabetogenic effects of cortisol than are peripheral tissues.

Effect of somatostatin on plasma amino acid uptake by human pancreas

We studied the effect of somatostatin on amino acid uptake by pancreatic acinar cells in 12 healthy male volunteers, aged 20-48 yr. Pancreatic amino acid uptake was assessed by measuring free plasma amino acid concentration before and during pancreatic stimulation with secretin (1 CU/kg body wt.h) and cerulein (50 ng/kg body wt.h). Pancreatic stimulation with these peptides caused a significant decrease in plasma amino acid concentrations. Somatostatin, given at the dosages of 0.15 and 1.35 micrograms/kg body wt.h, significantly diminished this decrease. The effect of the higher dose of somatostatin was more marked than that produced by the lower dose, compatible with dose dependence. The results suggest that somatostatin is able to inhibit the amino acid uptake by the pancreatic acinar cells. This inhibitory effect could be an important mechanism by which the peptide decreases pancreatic enzyme secretion.

Hyperglucagonemia and insulin-mediated glucose metabolism

The effect of chronic physiologic hyperglucagonemia on basal and insulin-mediated glucose metabolism was evaluated in normal subjects, using the euglycemic insulin clamp technique (+50, +100, and +500 microU/ml). After glucagon infusion fasting glucose increased from 76 +/- 4 to 93 +/- 2 mg/dl and hepatic glucose production (HGP) rose from 1.96 +/- 0.08 to 2.25 +/- 0.08 mg/kg X min (P less than 0.001). Basal glucose oxidation after glucagon increased (P less than 0.05) and correlated inversely with decreased free fatty acid concentrations (r = -0.94; P less than 0.01) and decreased lipid oxidation (r = -0.75; P less than 0.01). Suppression of HGP and stimulation of total glucose disposal were impaired at each insulin step after glucagon (P less than 0.05-0.01). The reduction in insulin-mediated glucose uptake was entirely due to diminished non-oxidative glucose utilization. Glucagon infusion also caused a decrease in basal lipid oxidation and an enhanced ability of insulin to inhibit lipid oxidation and augment lipid synthesis. These results suggest that hyperglucagonemia may contribute to the disturbances in glucose and lipid metabolism in some diabetic patients.

The effect of vasopressin infusion on glucose metabolism in man

Studies on intact animals and isolated rat hepatocytes have shown that arginine vasopression (AVP) stimulates glycogen phosphorylase to break down glycogen and raise plasma glucose concentrations. Since no similar work has been performed on healthy human adults, the effect of moderate (25 pmol/min) and high (75 pmol/min) dose AVP infusion on plasma glucose, intermediary metabolites, glucose kinetics, and circulating glucagon and insulin concentrations was investigated. After AVP infusion, plasma glucose rose from 4.9 +/- 0.1 to a peak of 5.7 +/- 0.2 mmol/l (P less than 0.001), but no changes in blood lactate, pyruvate, alanine, glycerol or 3-hydroxybutyrate concentrations were observed. The glucose rise was accounted for entirely by an increase in the rate of appearance of glucose from 11.19 +/- 0.43 to 13.38 +/- 0.63 mu mol/kg/min (P less than 0.001). Infusion of AVP also increased plasma glucagon concentrations from 38 +/- 8 to 79 +/- 20 pg/l (P less than 0.01). The hyperglycaemic effect of AVP may be mediated solely by stimulation of glucagon release, but we cannot exclude direct stimulation of glycogen phosphorylase activity.

Modulation of hepatic protein kinase activity by indomethacin

We previously demonstrated that treatment with indomethacin in vivo significantly blunted the glucagon-induced glycemic response in the rat. This prostaglandin synthetase (cyclo-oxygenase) inhibitor also accentuated the evanescent effect of glucagon on hepatic glucose output in the intact, anesthetized rat. In this report, we present evidence that impairment of glucagon action in the rat liver by indomethacin is mediated through its inhibitory effect on both cAMP-dependent and cAMP-independent hepatic protein kinase. Indomethacin treatment did not have a measurable effect on any of the other components of the glucagon transducer system. Furthermore, infusion with glucagon for two hours that maintained plasma glucagon values at high physiological levels significantly reduced hepatic cAMP-dependent protein kinase activity without altering its Km. Glucagon infusion also down-regulated its own hepatic receptors and glucagon-stimulated cAMP production; prostaglandin E1-stimulated cAMP production was not affected. We concluded that prostaglandins may play a role in the regulation of hepatic protein kinases involved in the glucagon-stimulated glycogenolytic response and that glucagon-induced down-regulation extends at least to the hepatic protein kinases. However, a direct effect of indomethacin or protein kinase and the adenylate cyclase complex cannot be ruled out.

Glucose counterregulation during prolonged hypoglycemia in normal humans

To study glucose counterregulation under conditions approximating those of clinical disorders in which hypoglycemia develops gradually and is reversed over a prolonged period, we injected regular insulin subcutaneously, in a dose (0.15 U/kg) selected to produce two- to threefold increases in plasma insulin, in 11 normal human volunteers and measured plasma glucose, insulin, C-peptide, and counterregulatory hormone concentrations as well as rates of glucose production, glucose utilization, and insulin secretion over 12 h. The data suggest that the mechanisms of gradual recovery from prolonged hypoglycemia may differ from those of rapid recovery from short-term hypoglycemia produced by intravenous injection of insulin in that 1) both stimulation of glucose production and limitation of glucose utilization contribute to recovery from prolonged hypoglycemia; 2) increases in glucagon, epinephrine, growth hormone, and cortisol secretion as well as a decrease in insulin secretion may all participate in glucose counterregulation during prolonged hypoglycemia; 3) epinephrine may play a more important role than glucagon during prolonged hypoglycemia. The latter two conclusions are based primarily on the temporal relationships between changes in the rates of glucose turnover and changes in plasma hormone concentrations and should not be considered proved. However, they provide the basis for testable hypotheses concerning the physiology of gradual recovery from prolonged hypoglycemia that can be expected to be relevant to the pathophysiology of clinical hypoglycemia.

Regulatory effect of glucagon on its own receptor concentrations and target-cell sensitivity in the rat

To evaluate the role of glucagon on its hepatocyte receptor concentrations, groups of rats were injected with a long-acting glucagon preparation (20 [G-20], 40 [G-40] or 60 [G-60] micrograms/100 g body weight) every 8 h for 4 days. Glucagon receptors in liver plasma membranes of treated animals were decreased in number (control = 1.66 +/- 0.20 ng/0.5 mg protein versus G-20 = 1.24 +/- 0.26, G-40 = 1.03 +/- 0.26, G-60 = 0.70 +/- 0.03 ng/0.5 mg protein; p less than 0.05, less than 0.001, less than 0.001, respectively), but they were indistinguishable from receptors of control rats by other criteria including affinity and kinetics of association. Degradation of both glucagon and receptor sites did not account for differences observed in binding. Similar results were obtained with isolated hepatocytes. In relation to controls, isolated hepatocytes of treated rats had a reduced number of receptors (control = 0.70 +/- 0.05 versus G-40 = 0.47 +/- 0.04 ng/10(6) cells; p less than 0.02) proportionate to the decreased glucagon-stimulated production of cyclic AMP and glucose. Four to eight hours exposure of cultured hepatocytes of nontreated rats to 4 x 10(-8) mol/l glucagon produced a decreased binding of 125I-glucagon to its receptor (p less than 0.05). In contrast, hormone exposure for shorter periods of time (0-2 h) was without effect. These results suggest (1) an inverse relationship between circulating glucagon levels and hepatocyte glucagon receptor concentration, and (2) a direct relation between receptor number and target-cell response.

Metabolic interactions of glucagon and cortisol in man--studies with somatostatin

The metabolic response to pathophysiologic concentrations of glucagon, induced by glucagon infusion, has been examined in normal man before and after 36-60 hr hypercortisolaemia, induced by administration of tetracosactrin-depot. Glucagon alone increased serum insulin levels twofold but blood glucose was unaltered. Plasma NEFA and blood ketone body concentrations were decreased by glucagon infusion. Tetracosactrin produced a threefold rise in serum cortisol levels and caused mild fasting hyperglycemia and hyperinsulinaemia. Subsequent glucagon infusion had no effect on circulating insulin, glucose, NEFA or ketone body concentrations. Simultaneous infusion of somatostatin, to produce partial insulin-deficiency, unmasked a hyperglycemic action of glucagon (+ 3.8 +/- 0.2 mmol/l at 90 min, p less than 0.02). This glucagon-induced rise in blood glucose was diminished by prior tetracosactrin administration. Tetracosactrin revealed a mild lipolytic action of glucagon in partial insulin deficiency, not apparent in the euadrenal state. Glucagon was equally hyperketonemic during somatostatin infusion before and after tetracosactrin. Thus the hyperglycemic and hyperketonemic actions of glucagon at pathophysiologic levels are restricted to insulin deficiency. Hypercortisolaemia reveals a lipolytic action of glucagon in insulin-deficient man but does not potentiate the hyperglycemic or hyperketonemic effects.

Theophylline potentiates glucagon-induced hepatic glucose production in man but does not prevent hepatic downregulation to glucagon

Prolonged hyperglucagonemia causes only a transient increase in hepatic glucose production. To determine whether activation of hepatic phosphodiesterase by glucagon is responsible for the transient nature of this response, the effect of infusion of the phosphodiesterase inhibitor theophylline alone, glucagon alone, and glucagon plus theophylline on isotopically determined glucose production was examined in normal human subjects. Infusion of theophylline alone did not alter rates of glucose production or utilization. Infusion of glucagon alone increased glucose production transiently from a basal rate of 1.9 +/- 0.1 mg/kg/min to a maximum at min 30 of 2.8 +/- 0.3 mg/kg/min followed by a return to rates no different from basal by min 60; plasma glucose increased from 89 +/- 3 mg/dl to a maximum of 114 +/- 5 mg/dl. Infusion of glucagon in the presence of theophylline resulted in greater increases in both plasma glucose (maximum at min 60 of 134 +/- 9 mg/dl) and glucose production (maximum at min 30 of 3.5 +/- 0.3 mg/kg/min) than had occurred during infusion of glucagon alone; the increase in glucose production, however, was not sustained. Thus theophylline potentiated glucagon-induced stimulation of hepatic glucose production, but it did not prevent the evanescent hepatic response to sustained hyperglucagonemia. Therefore, the present studies indices that glucagon activation of hepatic phosphodiesterase does not appear to be responsible for the transient nature of the increase in hepatic glucose production observed during prolonged hyperglucagonemia.

Hyperglycemia of diabetic rats decreased by a glucagon receptor antagonist

The glucagon analog [l-N alpha-trinitrophenylhistidine, 12-homoarginine]-glucagon (THG) was examined for its ability to lower blood glucose concentrations in rats made diabetic with streptozotocin. In vitro, THG is a potent antagonist of glucagon activation of the hepatic adenylate cyclase assay system. Intravenous bolus injections of THG caused rapid decreases (20 to 35 percent) of short duration in blood glucose. Continuous infusion of low concentrations of the inhibitor led to larger sustained decreases in blood glucose (30 to 65 percent). These studies demonstrate that a glucagon receptor antagonist can substantially reduce blood glucose levels in diabetic animals without addition of exogenous insulin.