Effects of alternations of plasma free fatty acid levels on pancreatic glucagon secretion in man

The Regulation and Secretion of Glucagon in Response to Nutrient Composition: Unraveling Their Intricate Mechanisms

Glucagon was initially regarded as a hyperglycemic substance; however, recent research has revealed its broader role in metabolism, encompassing effects on glucose, amino acids (AAs), and lipid metabolism. Notably, the interplay of glucagon with nutrient intake, particularly of AAs, and non-nutrient components is central to its secretion. Fasting and postprandial hyperglucagonemia have long been linked to the development and progression of type 2 diabetes (T2DM). However, recent studies have brought to light the positive impact of glucagon agonists on lipid metabolism and energy homeostasis. This review explores the multifaceted actions of glucagon, focusing on its regulation, signaling pathways, and effects on glucose, AAs, and lipid metabolism. The interplay between glucagon and other hormones, including insulin and incretins, is examined to provide a mechanistic understanding of its functions. Notably, the liver-α-cell axis, which involves glucagon and amino acids, emerges as a critical aspect of metabolic regulation. The dysregulation of glucagon secretion and its impact on conditions such as T2DM are discussed. The review highlights the potential therapeutic applications of targeting the glucagon pathway in the treatment of metabolic disorders.

Revisiting the role of glucagon in health, diabetes mellitus and other metabolic diseases

Insulin and glucagon exert opposing effects on glucose metabolism and, consequently, pancreatic islet β-cells and α-cells are considered functional antagonists. The intra-islet hypothesis has previously dominated the understanding of glucagon secretion, stating that insulin acts to inhibit the release of glucagon. By contrast, glucagon is a potent stimulator of insulin secretion and has been used to test β-cell function. Over the past decade, α-cells have received increasing attention due to their ability to stimulate insulin secretion from neighbouring β-cells, and α-cell-β-cell crosstalk has proven central for glucose homeostasis in vivo. Glucagon is not only the counter-regulatory hormone to insulin in glucose metabolism but also glucagon secretion is more susceptible to changes in the plasma concentration of certain amino acids than to changes in plasma concentrations of glucose. Thus, the actions of glucagon also include a central role in amino acid turnover and hepatic fat oxidation. This Review provides insights into glucagon secretion, with a focus on the local paracrine actions on glucagon and the importance of α-cell-β-cell crosstalk. We focus on dysregulated glucagon secretion in obesity, non-alcoholic fatty liver disease and type 2 diabetes mellitus. Lastly, the future potential of targeting hyperglucagonaemia and applying dual and triple receptor agonists with glucagon receptor-activating properties in combination with incretin hormone receptor agonism is discussed.

Effects of protein intake from an energy-restricted diet on the skeletal muscle composition of overweight and obese rats

Excess weight and obesity are often associated with ectopic adipose tissue accumulation in skeletal muscles. Intermuscular adipose tissue (IMAT) impairs muscle quality and reduces insulin-stimulated skeletal muscle glucose uptake. Although energy restriction and high protein intake can decrease IMAT, the effects and mechanisms of protein intake from an energy-restricted diet on protein and fat masses in skeletal muscle have received little attention. After establishing a diet-induced overweight and obese Sprague-Dawley rat model (half male and half female), rats were divided into five groups: normal control (NC; normal weight, general maintenance diet), model control (MC; overweight and obesity, high-fat diet), energy-restricted low protein (LP; overweight and obesity, 60% energy intake of NC, general maintenance diet), energy-restricted normal protein (NP; overweight and obesity, 60% energy intake of NC, high-protein diet 1), and energy-restricted high protein (HP; overweight and obesity, 60% energy intake of NC, high-protein diet 2). After 8 weeks, plasma and skeletal muscle (quadriceps femoris and gastrocnemius) samples were collected. Plasma levels of glucose, triglycerides, and hormones were analyzed, while contents of protein, fat, and factors associated with their synthesis and degradation were evaluated in skeletal muscles. Plasma concentrations of hormones contrasted protein and fat contents in skeletal muscles. Fat weights and contents of quadriceps femoris and gastrocnemius muscles in the NP group were significantly lower compared with LP and HP groups (P < 0.05). Moreover, concentrations of factors associated with the degradation of muscle fat were significantly higher in the NP group compared with LP and HP groups (P < 0.05). During energy restriction, protein intake equal to that of a normal protein diet increased lipolysis of quadriceps femoris and gastrocnemius muscles in rats of both sexes.

The past, present, and future physiology and pharmacology of glucagon

The evolution of glucagon has seen the transition from an impurity in the preparation of insulin to the development of glucagon receptor agonists for use in type 1 diabetes. In type 2 diabetes, glucagon receptor antagonists have been explored to reduce glycemia thought to be induced by hyperglucagonemia. However, the catabolic actions of glucagon are currently being leveraged to target the rise in obesity that paralleled that of diabetes, bringing the pharmacology of glucagon full circle. During this evolution, the physiological importance of glucagon advanced beyond the control of hepatic glucose production, incorporating critical roles for glucagon to regulate both lipid and amino acid metabolism. Thus, it is unsurprising that the study of glucagon has left several paradoxes that make it difficult to distill this hormone down to a simplified action. Here, we describe the history of glucagon from the past to the present and suggest some direction to the future of this field.

Human Gastrointestinal Transit and Hormonal Response to Different Meal Types: A Randomized Crossover Study

BACKGROUND

The wireless motility capsule (WMC) technique is a noninvasive and radiation-free method for measuring regional and whole gut transit in response to ingestion of a granola bar (SmartBar) or an eggbeater meal. The WMC has the potential to measure gastrointestinal transit in metabolic research as part of a standardized mixed meal tolerance test.

OBJECTIVES

To evaluate gastrointestinal transit with the WMC and postprandial plasma/serum concentrations of metabolites and gastrointestinal hormones as well as subjective appetite following ingestion of a SmartBar compared with a standardized mixed meal.

METHODS

Fourteen healthy participants [3 men, median (IQR) age 53.8 (45.8; 64.50) y, body weight 63.9 (59.9; 69.7) kg, BMI 23.1 (21.8; 23.9) kg/m2] completed a 2-d crossover study. Following ingestion of either a SmartBar (260 kcal, 7 energy percent (E%) fat, 74E% carbohydrate, and 19E% protein) or a standardized mixed meal (498 kcal, 34E% fat, 49E% carbohydrate, and 17E% protein), participants swallowed the WMC. Blood samples were drawn in the fasted state and postprandially for analyses of gastrointestinal hormones and metabolites. The primary outcome was difference in gastric emptying time between the 2 test days. Wilcoxon signed rank tests were used to test differences between test days.

RESULTS

Median (IQR) gastric emptying time was 98.0 (70.0; 113.0) min longer (P = 0.001) and incremental area under the curve of triglyceride, glucose-dependent insulinotropic polypeptide, and peptide YY were 40 mmol/L × min, 45.7%, and 63.7% greater after the standardized mixed meal compared with the SmartBar (all P < 0.001).

CONCLUSIONS

The WMC can be used in combination with a standardized mixed meal for evaluation of gastrointestinal transit in healthy men and women. Gastric emptying time was prolonged in response to the standardized mixed meal whereas transit times of the small bowel, colon, and whole gut did not differ between the test meals.

The mitochondrial β-oxidation enzyme HADHA restrains hepatic glucagon response by promoting β-hydroxybutyrate production

Disordered hepatic glucagon response contributes to hyperglycemia in diabetes. The regulators involved in glucagon response are less understood. This work aims to investigate the roles of mitochondrial β-oxidation enzyme HADHA and its downstream ketone bodies in hepatic glucagon response. Here we show that glucagon challenge impairs expression of HADHA. Liver-specific HADHA overexpression reversed hepatic gluconeogenesis in mice, while HADHA knockdown augmented glucagon response. Stable isotope tracing shows that HADHA promotes ketone body production via β-oxidation. The ketone body β-hydroxybutyrate (BHB) but not acetoacetate suppresses gluconeogenesis by selectively inhibiting HDAC7 activity via interaction with Glu543 site to facilitate FOXO1 nuclear exclusion. In HFD-fed mice, HADHA overexpression improved metabolic disorders, and these effects are abrogated by knockdown of BHB-producing enzyme. In conclusion, BHB is responsible for the inhibitory effect of HADHA on hepatic glucagon response, suggesting that HADHA activation or BHB elevation by pharmacological intervention hold promise in treating diabetes.

Impaired Suppression of Glucagon in Obese Subjects Parallels Decline in Insulin Sensitivity and Beta-Cell Function

AIM

To examine the relationship between plasma glucagon levels and insulin sensitivity and insulin secretion in obese subjects.

METHODS

Suppression of plasma glucagon was examined in 275 obese Hispanic Americans with varying glucose tolerance. All subjects received a 2-hour oral glucose tolerance test (OGTT) and a subset (n = 90) had euglycemic hyperinsulinemic clamp. During OGTT, we quantitated suppression of plasma glucagon concentration, Matsuda index of insulin sensitivity, and insulin secretion/insulin resistance (disposition) index. Plasma glucagon suppression was compared between quartiles of insulin sensitivity and beta-cell function.

RESULTS

Fasting plasma glucagon levels were similar in obese subjects with normal glucose tolerance (NGT), prediabetes, and type 2 diabetes (T2D), but the fasting glucagon/insulin ratio decreased progressively from NGT to prediabetes to T2D (9.28 ± 0.66 vs 6.84 ± 0.44 vs 5.84 ± 0.43; P < 0.001). Fasting and 2-hour plasma glucagon levels during OGTT progressively increased and correlated positively with severity of insulin resistance (both Matsuda index and euglycemic hyperinsulinemic clamp). The fasting glucagon/insulin ratio declined with worsening insulin sensitivity and beta-cell function, and correlated with whole-body insulin sensitivity (Matsuda index, r = 0.81; P < 0.001) and beta-cell function (r = 0.35; P < 0.001). The glucagon/insulin ratio also correlated and with beta-cell function during OGTT at 60 and 120 minutes (r = -0.47; P < 0.001 and r = -0.32; P < 0.001).

CONCLUSION

Insulin-mediated suppression of glucagon secretion in obese subjects is impaired with increasing severity of glucose intolerance and parallels the severity of insulin resistance and beta-cell dysfunction.

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.

Updating the Role of α-Cell Preproglucagon Products on GLP-1 Receptor-Mediated Insulin Secretion

While the field of islet biology has historically focused its attention on understanding β-cell function and the mechanisms by which these cells become dysfunctional with diabetes, there has been a scientific shift toward greater understanding of other endocrine cells of the islet and their paracrine role in regulating the β-cell. In recent years, many questions and new data have come forward regarding the paracrine role of the α-cell and specifically preproglucagon peptides in regulating insulin secretion. The role of intestinally secreted glucagon-like peptide 1 (GLP-1) in regulation of insulin secretion has been questioned, and a physiological role of pancreatic GLP-1 in regulation of insulin secretion has been proposed. In addition, in the last 2 years, a series of studies demonstrated a physiological role for glucagon, acting via the GLP-1 receptor, in paracrine regulation of insulin secretion. Altogether, this work challenges the textbook physiology of both GLP-1 and glucagon and presents a critical paradigm shift for the field. This article addresses these new findings surrounding α-cell preproglucagon products, with a particular focus on GLP-1, in the context of their roles in insulin secretion and consequently glucose metabolism.

Impact of caudal hindbrain glycogen metabolism on A2 noradrenergic neuron AMPK activation and ventromedial hypothalamic nucleus norepinephrine activity and glucoregulatory neurotransmitter marker protein expression

The brain glycogen reserve is a source of oxidizable substrate fuel. Lactoprivic-sensitive hindbrain A2 noradrenergic neurons provide crucial metabolic-sensory input to downstream hypothalamic glucose-regulatory structures. Current research examined whether hindbrain glycogen fuel supply impacts A2 energy stability and governance of ventromedial hypothalamic nucleus (VMN) metabolic transmitter signaling. Male rats were injected into the caudal fourth ventricle (CV4) with the glycogen phosphorylase inhibitor 1,4-dideoxy-1,4-imino-D-arabinitol (DAB) prior to continuous intra-CV4 infusion of L-lactate or vehicle. Lactate reversed DAB suppression of A2 neuron AMPK protein and up-regulated phosphoAMPK profiles. A2 dopamine-β-hydroxylase expression was refractory to DAB, but elevated by DAB/lactate. Lactate normalized A2 estrogen receptor-alpha and GPER proteins and up-regulated estrogen receptor-beta levels in DAB-treated rats. VMN norepinephrine content was decreased by DAB, but partially restored by lactate. DAB caused lactate-reversible or -irreversible augmentation of VMN glycogen phosphorylase-brain (GPbb) and -muscle type (GPmm) variant profiles, and correspondingly up- or down-regulated VMN protein markers of glucose-stimulatory nitrergic and glucose-inhibitory γ-aminobutyric acid transmission. DAB did not alter plasma glucose, but suppressed or elevated circulating glucagon and corticosterone in that order. Results show that diminished hindbrain glycogen breakdown is communicated to the VMN, in part by NE signaling, to up-regulate VMN glycogen breakdown and trigger neurochemical signaling of energy imbalance in that site. DAB effects on GPmm, VMN glycogen content, and counter-regulatory hormone secretion were unabated by lactate infusion, suggesting that aside from substrate fuel provision rate, additional indicators of glycogen metabolism such as turnover rate may be monitored in the hindbrain.

Integrating the inputs that shape pancreatic islet hormone release

The pancreatic islet is a complex mini organ composed of a variety of endocrine cells and their support cells, which together tightly control blood glucose homeostasis. Changes in glucose concentration are commonly regarded as the chief signal controlling insulin-secreting beta cells, glucagon-secreting alpha cells and somatostatin-secreting delta cells. However, each of these cell types is highly responsive to a multitude of endocrine, paracrine, nutritional and neural inputs, which collectively shape the final endocrine output of the islet. Here, we review the principal inputs for each islet-cell type and the physiological circumstances in which these signals arise, through the prism of the insights generated by the transcriptomes of each of the major endocrine-cell types. A comprehensive integration of the factors that influence blood glucose homeostasis is essential to successfully improve therapeutic strategies for better diabetes management.

Glucagon Receptor Signaling and Glucagon Resistance

Hundred years after the discovery of glucagon, its biology remains enigmatic. Accurate measurement of glucagon has been essential for uncovering its pathological hypersecretion that underlies various metabolic diseases including not only diabetes and liver diseases but also cancers (glucagonomas). The suggested key role of glucagon in the development of diabetes has been termed the bihormonal hypothesis. However, studying tissue-specific knockout of the glucagon receptor has revealed that the physiological role of glucagon may extend beyond blood-glucose regulation. Decades ago, animal and human studies reported an important role of glucagon in amino acid metabolism through ureagenesis. Using modern technologies such as metabolomic profiling, knowledge about the effects of glucagon on amino acid metabolism has been expanded and the mechanisms involved further delineated. Glucagon receptor antagonists have indirectly put focus on glucagon's potential role in lipid metabolism, as individuals treated with these antagonists showed dyslipidemia and increased hepatic fat. One emerging field in glucagon biology now seems to include the concept of hepatic glucagon resistance. Here, we discuss the roles of glucagon in glucose homeostasis, amino acid metabolism, and lipid metabolism and present speculations on the molecular pathways causing and associating with postulated hepatic glucagon resistance.

Effect of diet composition on insulin sensitivity in humans

Diet composition has a marked impact on the risk of developing type 2 diabetes and cardiovascular disease. Prospective studies show that dietary patterns with elevated amount of animal products and low quantity of vegetable food items raise the risk of these diseases. In healthy subjects, animal protein intake intensifies insulin resistance whereas plant-based foods enhance insulin sensitivity. Similar effects have been documented in patients with diabetes. Accordingly, pre-pregnancy intake of meat (processed and unprocessed) has been strongly associated with a higher risk of gestational diabetes whereas greater pre-pregnancy vegetable protein consumption is associated with a lower risk of gestational diabetes. Population groups that modify their traditional dietary habit increasing the amount of animal products while reducing plant-based foods experience a remarkable rise in the frequency of type 2 diabetes. The association of animal protein intake with insulin resistance is independent of body mass index. In obese individuals that consume high animal protein diets, insulin sensitivity does not improve following weight loss. Diets aimed to lose weight that encourage restriction of carbohydrates and elevated consumption of animal protein intensify insulin resistance increasing the risk of developing type 2 diabetes and cardiovascular disease. The effect of dietary components on insulin sensitivity may contribute to explain the striking impact of eating habits on the risk of type 2 diabetes and cardiovascular disease. Insulin resistance predisposes to type 2 diabetes in healthy subjects and deteriorates metabolic control in patients with diabetes. In nondiabetic and diabetic individuals, insulin resistance is a major cardiovascular risk factor.

Glucagon Receptor Signaling and Lipid Metabolism

Glucagon is secreted from the pancreatic alpha cells upon hypoglycemia and stimulates hepatic glucose production. Type 2 diabetes is associated with dysregulated glucagon secretion, and increased glucagon concentrations contribute to the diabetic hyperglycemia. Antagonists of the glucagon receptor have been considered as glucose-lowering therapy in type 2 diabetes patients, but their clinical applicability has been questioned because of reports of therapy-induced increments in liver fat content and increased plasma concentrations of low-density lipoprotein. Conversely, in animal models, increased glucagon receptor signaling has been linked to improved lipid metabolism. Glucagon acts primarily on the liver and by regulating hepatic lipid metabolism glucagon may reduce hepatic lipid accumulation and decrease hepatic lipid secretion. Regarding whole-body lipid metabolism, it is controversial to what extent glucagon influences lipolysis in adipose tissue, particularly in humans. Glucagon receptor agonists combined with glucagon-like peptide 1 receptor agonists (dual agonists) improve dyslipidemia and reduce hepatic steatosis. Collectively, emerging data support an essential role of glucagon for lipid metabolism.

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.

The New Biology and Pharmacology of Glucagon

In the last two decades we have witnessed sizable progress in defining the role of gastrointestinal signals in the control of glucose and energy homeostasis. Specifically, the molecular basis of the huge metabolic benefits in bariatric surgery is emerging while novel incretin-based medicines based on endogenous hormones such as glucagon-like peptide 1 and pancreas-derived amylin are improving diabetes management. These and related developments have fostered the discovery of novel insights into endocrine control of systemic metabolism, and in particular a deeper understanding of the importance of communication across vital organs, and specifically the gut-brain-pancreas-liver network. Paradoxically, the pancreatic peptide glucagon has reemerged in this period among a plethora of newly identified metabolic macromolecules, and new data complement and challenge its historical position as a gut hormone involved in metabolic control. The synthesis of glucagon analogs that are biophysically stable and soluble in aqueous solutions has promoted biological study that has enriched our understanding of glucagon biology and ironically recruited glucagon agonism as a central element to lower body weight in the treatment of metabolic disease. This review summarizes the extensive historical record and the more recent provocative direction that integrates the prominent role of glucagon in glucose elevation with its under-acknowledged effects on lipids, body weight, and vascular health that have implications for the pathophysiology of metabolic diseases, and the emergence of precision medicines to treat metabolic diseases.

Hindbrain A2 noradrenergic neuron adenosine 5'-monophosphate-activated protein kinase activation, upstream kinase/phosphorylase protein expression, and receptivity to hormone and fuel reporters of short-term food deprivation are regulated by estradiol

Estradiol (E) mitigates acute and postacute adverse effects of 12 hr-food deprivation (FD) on energy balance. Hindbrain 5'-monophosphate-activated protein kinase (AMPK) regulates hyperphagic and hypothalamic metabolic neuropeptide and norepinephrine responses to FD in an E-dependent manner. Energy-state information from AMPK-expressing hindbrain A2 noradrenergic neurons shapes neural responses to metabolic imbalance. Here we investigate the hypothesis that FD causes divergent changes in A2 AMPK activity in E- vs. oil (O)-implanted ovariectomized female rats, alongside dissimilar adjustments in circulating metabolic fuel (glucose, free fatty acids [FFA]) and energy deficit-sensitive hormone (corticosterone, glucagon, leptin) levels. FD decreased blood glucose in oil (O)- but not E-implanted ovariectomized female rats and elevated and reduced glucagon levels in O and E, respectively. FD decreased circulating leptin in O and E, but increased corticosterone and FFA concentrations in E only. Western blot analysis of laser-microdissected A2 neurons showed that glucocorticoid receptor type II and very-long-chain acyl-CoA synthetase 3 protein profiles were amplified in FD/E vs. FD/O. A2 total AMPK protein was elevated without change in activity in FD/O, whereas FD/E exhibited increased AMPK activation along with decreased upstream phosphatase expression. The catecholamine biosynthetic enzyme dopamine-β-hydroxylase (DβH) was increased in FD/O but not FD/E A2 cells. The data show discordance between A2 AMPK activation and glycemic responses to FD; sensor activity was refractory to glucose decrements in FD/O but augmented in FD/E despite stabilized glucose and elevated FFA levels. E-dependent amplification of AMPK activity may reflect adaptive conversion to fatty acid oxidation and/or glucocorticoid stimulation. FD augmentation of A2 DβH protein profiles in FD/O but not FD/E animals suggests that FD may correspondingly regulate NE synthesis vs. metabolism/release in the absence vs. presence of E. Mechanisms underlying translation of E-contingent A2 neuron responses to FD into regulatory signaling remain to be determined. © 2016 Wiley Periodicals, Inc.

Comparison of a carbohydrate-free diet vs. fasting on plasma glucose, insulin and glucagon in type 2 diabetes

OBJECTIVE

Hyperglycemia improves when patients with type 2 diabetes are placed on a weight-loss diet. Improvement typically occurs soon after diet implementation. This rapid response could result from low fuel supply (calories), lower carbohydrate content of the weight-loss diet, and/or weight loss per se. To differentiate these effects, glucose, insulin, C-peptide and glucagon were determined during the last 24 h of a 3-day period without food (severe calorie restriction) and a calorie-sufficient, carbohydrate-free diet.

RESEARCH DESIGN

Seven subjects with untreated type 2 diabetes were studied. A randomized-crossover design with a 4-week washout period between arms was used.

METHODS

Results from both the calorie-sufficient, carbohydrate-free diet and the 3-day fast were compared with the initial standard diet consisting of 55% carbohydrate, 15% protein and 30% fat.

RESULTS

The overnight fasting glucose concentration decreased from 196 (standard diet) to 160 (carbohydrate-free diet) to 127 mg/dl (fasting). The 24 h glucose and insulin area responses decreased by 35% and 48% on day 3 of the carbohydrate-free diet, and by 49% and 69% after fasting. Overnight basal insulin and glucagon remained unchanged.

CONCLUSIONS

Short-term fasting dramatically lowered overnight fasting and 24 h integrated glucose concentrations. Carbohydrate restriction per se could account for 71% of the reduction. Insulin could not entirely explain the glucose responses. In the absence of carbohydrate, the net insulin response was 28% of the standard diet. Glucagon did not contribute to the metabolic adaptations observed.

CD36 binds oxidized low density lipoprotein (LDL) in a mechanism dependent upon fatty acid binding

The association of unesterified fatty acid (FA) with the scavenger receptor CD36 has been actively researched, with focuses on FA and oxidized low density lipoprotein (oxLDL) uptake. CD36 has been shown to bind FA, but this interaction has been poorly characterized to date. To gain new insights into the physiological relevance of binding of FA to CD36, we characterized FA binding to the ectodomain of CD36 by the biophysical method surface plasmon resonance. Five structurally distinct FAs (saturated, monounsaturated (cis and trans), polyunsaturated, and oxidized) were pulsed across surface plasmon resonance channels, generating association and dissociation binding curves. Except for the oxidized FA HODE, all FAs bound to CD36, with rapid association and dissociation kinetics similar to HSA. Next, to elucidate the role that each FA might play in CD36-mediated oxLDL uptake, we used a fluorescent oxLDL (Dii-oxLDL) live cell assay with confocal microscopy imaging. CD36-mediated uptake in serum-free medium was very low but greatly increased when serum was present. The addition of exogenous FA in serum-free medium increased oxLDL binding and uptake to levels found with serum and affected CD36 plasma membrane distribution. Binding/uptake of oxLDL was dependent upon the FA dose, except for docosahexaenoic acid, which exhibited binding to CD36 but did not activate the uptake of oxLDL. HODE also did not affect oxLDL uptake. High affinity FA binding to CD36 and the effects of each FA on oxLDL uptake have important implications for protein conformation, binding of other ligands, functional properties of CD36, and high plasma FA levels in obesity and type 2 diabetes.

The effect of antilipolytic agents on cyclic AMP, free fatty acid and total catecholamine concentrations in plasma

Plasma concentrations of adenosine 3':5' cyclic monophosphate (cyclic AMP) and free fatty acids have been measured in 15 patients with acute myocardial infarction and in 6 dogs given infusions of isoprenaline. Plasma total catecholamines were also estimated in the patients. Inhibition of adipose tissue lipolysis with a nicotinic acid analogue did not decrease plasma cyclic AMP concentrations, either in the patients or when elevated in the dogs, thus suggesting that this tissue is not a major source of the nucleotide in plasma.

The role of nitric oxide synthase in post-operative hyperglycaemia

Post-operative hyperglycaemia is important with regard to outcomes of surgical operations. It affects post-operative morbidity, length of hospital stay, and mortality. Poor peri-operative blood glucose control leads to a higher risk of post-operative complication. Insulin resistance as a cause of post-operative hyperglycaemia has been blamed for some time. Nitric Oxide (NO) is produced by nitric oxide synthase (NOS) isoenzymes. Inducible nitric oxide synthase (iNOS) is not a normal cellular constitute. It is expressed by cytokines and non-cytokines e.g. fasting, trauma, intravenous glucose, and lipid infusion, which are encountered in surgical operations. Review of current published data on postoperative hyperglycaemia was completed. Our studies and others were explored for the possible role of NO in this scenario. Induction and expression of iNOS enzyme in pancreatic islet cells is included in the chaotic postoperative blood glucose control. The high concentrations of iNOS derived NO are toxic to pancreatic β-cells and may inhibit insulin secretion postoperatively. Hence, current peri-operative management is questionable regarding post-operative hyperglycaemia and necessitates development of a new strategy.

Surface plasmon resonance-enhanced fluorescence implementation of a single-step competition assay: demonstration of fatty acid measurement using an anti-fatty acid monoclonal antibody and a Cy5-labeled fatty acid

The development of a single-step, separation-free method for measurement of low concentrations of fatty acid using a surface plasmon resonance-enhanced fluorescence competition assay with a surface-bound antibody is described. The assay behavior was unexpectedly complex. A nonlinear coverage-dependent self-quenching of emission from surface-bound fluorescent label was deduced from the response kinetics and attributed to a surface plasmon-mediated energy transfer between adsorbed fluorophores, modified by the effects of plasmon interference. Principles of assay design to avoid complications from such effects are discussed. An anti-fatty acid mouse monoclonal antibody reacting to the alkyl chain was prepared and supported on a gold chip at a spacing appropriate for surface-plasmon field-enhanced fluorescence spectroscopy (SPEFS), by applying successively a self-assembled biotinylated monolayer, then streptavidin, then biotinylated protein A, and then the antibody, which was crosslinked to the protein A. Synthesis of a fluorescently (Cy5) tagged C-11 fatty acid is reported. SPEFS was used to follow the kinetics of the binding of the labeled fatty acid to the antibody, and to implement a competition assay with free fatty acid (undecanoic acid), sensitive at the 1 microM scale, a sensitivity limit caused by the low affinity of antibodies for free fatty acids, rather than the SPEFS technique itself. Free fatty acid concentration in human serum is in the range 0.1-1mM, suggesting that this measurement approach could be applied in a clinical diagnostic context. Finally, a predictive, theoretical model of fatty acid binding was developed that accounted for the observed "overshoot" kinetics.

Selective induction of inducible nitric oxide synthase in pancreatic islet of rat after an intravenous glucose or intralipid challenge

OBJECTIVE

Constant exposure of pancreatic islets to high levels of glucose or free fatty acids can lead to irreversible beta-cell dysfunction, a process referred to as glucotoxicity or lipotoxicity, respectively. In this context a role for nitric oxide generated by pancreatic islet has been suggested. The present investigation examined whether the route of glucose administration, i.e., given orally (OG) or infused intravenously (IVG), could have any effect on the expression and activity of inducible nitric oxide synthase (iNOS) in pancreatic islets.

METHODS

Rats were infused with glucose (50%) or Intralipid intravenously for 24 h or given glucose orally. A freely fed control group (FF) was also included. At 24 h rats were killed and blood samples were drawn for analysis of plasma insulin, glucagon, and glucose. Pancreatic islets were harvested from each animal and investigated for the occurrence of iNOS by the use of confocal microscopy, western blot, and high-performance liquid chromatographic analysis. The effect of intravenously infused glucose was then compared with the effect of an intravenous infusion of Intralipid (IL).

RESULTS

Plasma insulin levels were markedly decreased after 24 h of infusion of glucose (IVG group) or Intralipid (IL group) compared with the FF or OG group. Plasma glucagon and glucose levels were markedly increased in the IVG group, whereas both parameters were decreased in the IL group. No significant differences in plasma insulin, glucagon, or glucose were found between the OG and FF groups. Immunocytochemical (confocal microscopy), western blot, and biochemical (high-performance liquid chromatographic) analyses showed that a sustained increase in plasma level of glucose or free fatty acids by an intravenous infusion of either nutrient for 24 h resulted in a marked expression and activity of iNOS in pancreatic islets. No sign of iNOS expression could, however, be detected in the islets of FF control or OG rats.

CONCLUSION

The data suggest that impaired beta-cell function found after 24 h of an intravenous infusion of glucose or Intralipid might be mediated, at least in part, by the induction of iNOS in pancreatic islets. This may subsequently result in an exclusive production of nitric oxide, which is deleterious for beta-cells.

Reference intervals for glucose, beta-cell polypeptides, and counterregulatory factors during prolonged fasting

To establish reference intervals for the pancreatic beta-cell response and the counterregulatory hormone response to prolonged fasting, we studied 33 healthy subjects (16 males, 17 females) during a 72-h fast. Glucose, insulin, C-peptide, and proinsulin levels decreased (P < 0.001), and the levels of counterregulatory factors increased during the fast [P < 0.05; glucagon and free fatty acids (FFA) with a linear increase and epinephrine, norepinephrine, and cortisol with a clear underlying circadian rhythm]. Growth hormone secretion increased from the first to third day of fasting (P < 0.05) but actually decreased from the second to third day of fasting (P = 0.03). Males had higher glucose and glucagon levels and lower FFA levels during the fast (P < 0.05), whereas no effect of gender on beta-cell polypeptides was observed. A high body mass index resulted in higher insulin and C-peptide levels during the fast (P < 0.05). In conclusion, we have provided reference intervals for glucoregulatory factors during a 72-h fast. We observed a diminished beta-cell response concomitant with an increased secretion of counterregulatory hormones. These results should be of clinical and scientific value in the investigation of hypoglycemic disorders.

Unbound rather than total concentration and saturation rather than unsaturation determine the potency of fatty acids on insulin secretion

Isolated mouse islets were used to compare the effects of three saturated (myristate, palmitate and stearate) and three unsaturated (oleate, linoleate and linolenate) long-chain fatty acids on insulin secretion. By varying the concentrations of fatty acid (250-1250 micromol/l) and albumin simultaneously or independently, we also investigated whether the insulinotropic effect is determined by the unbound or total concentration of the fatty acids. Only palmitate and stearate slightly increased basal insulin secretion (3 mmol/l glucose). All tested fatty acids potentiated glucose-induced insulin secretion (10-15 mmol/l), and the following rank order of potency was obtained when they were compared at the same total concentrations: palmitate approximately = stearate > myristate > or = oleate > or = linoleate approximately = linolenate. The effect of a given fatty acid varied with the fatty acid to albumin molar ratio, in a way which indicated that the unbound fraction is the important one for the stimulation of beta cells. When the potentiation of insulin secretion was expressed as a function of the unbound concentrations, the following rank order emerged: palmitate > myristate > stearate approximately = oleate > linoleate approximately = linolenate. In conclusion, the acute and direct effects of long-chain fatty acids on insulin secretion are due to their unbound fraction. They are observed only at fatty acid/albumin ratios higher than those normally occurring in plasma. Saturated fatty acids are stronger insulin secretagogues than unsaturated fatty acids. Unbound palmitate is by far the most potent of the six common long-chain fatty acids.

Postprandial amplification of lipoprotein abnormalities in controlled type II diabetic subjects: relationship to postprandial lipemia and C-peptide/glucagon levels

Lipoprotein abnormalities, mainly high very-low-density lipoprotein (VLDL) and low high-density lipoprotein (HDL) levels, increase the risk of coronary heart disease (CHD) in type II diabetic patients. To investigate the relationship between these lipoprotein abnormalities and the postprandial (PP) lipid-clearing capacity, triglyceride (TG) and hormonal levels were determined hourly up to the 4th hour after a mixed meal containing 32.5 g lipids/m2 body surface in 14 treated non-obese type II diabetic patients with adequate nutritional and glycemic control (hemoglobin A1C [HbA1C] < 7%) and in 12 healthy subjects matched for age, sex, and body mass index (BMI). Mean cholesterol levels did not differ between patients and controls, with fasting TG moderately increased in diabetics (140 +/- 70 v 66 +/- 34 mg/dL, P < .01). Whereas fasting TG levels in patients showed a continuous distribution from 55 to 250 mg/dL, postprandial TG clearly identified two different subgroups. A "high-responder" or hypertriglyceridemic subgroup (HTG) showed PP TG levels significantly higher than control levels (290 +/- 62 v 106 +/- 41 mg/dL, P < .001), with higher fasting TG as well (181 +/- 52, P < .01), whereas both fasting and PP TG levels were not different from control levels in the normotriglyceridemic (NTG) diabetic subgroup. The magnitude of the PP triglyceridemic area showed a negative correlation with HDL2 cholesterol (r = .66, P < .001) and a positive correlation with PP HDL2 TG enrichment (r = .80, P < .001).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of beta-hydroxybutyrate and acetoacetate on insulin and glucagon secretion from perfused rat pancreas

To elucidate the physiological significance of ketone bodies on insulin and glucagon secretion, the direct effects of beta-hydroxybutyrate (BOHB) and acetoacetate (AcAc) infusion on insulin and glucagon release from perfused rat pancreas were investigated. The BOHB or AcAc was administered at concentrations of 10, 1, or 0.1 mM for 30 min at 4.0 ml/min. High-concentration infusions of BOHB and AcAc (10 mM) produced significant increases in insulin release in the presence of 4.4 mM glucose, but low-concentration infusions of BOHB and AcAc (1 and 0.1 mM) caused no significant changes in insulin secretion from perfused rat pancreas. BOHB (10, 1, and 0.1 mM) and AcAc (10 and 1 mM) infusion significantly inhibited glucagon secretion from perfused rat pancreas. These results suggest that physiological concentrations of ketone bodies have no direct effect on insulin release but have a direct inhibitory effect on glucagon secretion from perfused rat pancreas.

Glucose counterregulation, hypoglycemia, and intensive insulin therapy in diabetes mellitus

The prevention or correction of hypoglycemia is the result of both dissipation of insulin and activation of counterregulatory systems. In the models studied to date, glucagon and epinephrine have been shown to be the key counterregulatory factors; the potential roles of other hormones, neural factors, or substrate mechanisms in other models and during more gradual recovery from hypoglycemia remain to be defined. Deficient glucagon responses to decrements in plasma glucose, which are common in patients with IDDM and occur in some patients with NIDDM, result in altered counterregulation. But counterregulation is generally adequate, because epinephrine compensates for it. Defective glucose counterregulation due to combined deficiencies of glucagon and epinephrine secretory responses occurs in many patients, typically those with longstanding diabetes, and must be added to the list of factors known to increase the risk of hypoglycemia, at least during intensive therapy. From the material reviewed, it should be apparent that much has been learned about glucose counterregulation. It should be equally clear that much remains to be learned. Among the many possibilities, we consider four worthy of emphasis. First of all, we need to examine the physiology and pathophysiology of glucose counterregulation in additional models (e.g., during exercise) and over longer periods. Secondly, we need to determine whether central nervous system adaptation to antecedent glycemia occurs and, if so, identify its mechanisms. Thirdly, we need to develop better methods of insulin delivery or learn to correct or compensate for defective counterregulatory systems, if we are to achieve euglycemia safely in diabetic patients with defective glucose counterregulation. Finally, we need to know whether effective control of diabetes mellitus prevents development of defective glucose counterregulation.

The failure of aminophylline to modulate glucagon release in man

There are conflicting results regarding the impact of cyclic AMP on pancreatic glucagon release. The effect of aminophylline, a phosphodiesterase inhibitor, on glucagon secretion was studied in four non-obese, non-diabetic, healthy young male volunteers. The subjects received separate infusions of: 1) aminophylline; 2) aminophylline and propranolol; 3) arginine; 4) aminophylline and arginine; 5) insulin; 6) aminophylline and insulin; and 7) aminophylline and isoproterenol. Aminophylline not only failed to alter glucagon levels but also did not affect the glucagon responses observed after arginine and insulin-induced hypoglycemia. The concurrent infusion of isoproterenol and aminophylline also failed to cause a glucagon response. Although glucagon release has been evoked by cyclic AMP in some in vitro system, administration of aminophylline to human subjects does not enhance secretion. These results indirectly suggest that cyclic AMP is of little importance in the control of glucagon secretion in man, though the effects of aminophylline at the cellular level may be complex.

Evidence for a role of free fatty acids in the regulation of somatostatin secretion in normal and alloxan diabetic dogs

To investigate the effect of acute elevation of plasma free fatty acids (FFA) on the secretion of splanchnic somatostatin-like immunoreactivity (SLI), the peripheral venous, pancreatic, and gastric venous effluent levels of SLI were measured in normal and chronic alloxan diabetic dogs before and after the infusion of a fat emulsion supplemented with heparin. In normal conscious dogs heparin injected during the infusion of a fat emulsion elevated FFA levels from a mean (+/-SE) base-line level of 0.7+/-0.1 meq/liter to a peak value of 1.5+/-0.1 meq/liter (P < 0.001) and plasma SLI rose from a mean (+/-SE) base-line value of 145+/-7 pg/ml to a peak of 253+/-44 pg/ml (P < 0.05). Neither the infusion of glycerol, of fat emulsion without heparin, of heparin alone nor of saline itself had an effect on either the plasma level of FFA or SLI. In another group of anesthetized dogs with surgically implanted catheters the administration of fat emulsion plus heparin was accompanied by more than a two-fold rise in the concentration of SLI in the venous effluent of the pancreas and of the gastric fundus and antrum in association with an elevation of FFA levels. In a group of conscious diabetic dogs fat emulsion plus heparin raised FFA from a mean base-line level of 1.2+/-0.2 to 1.6+/-0.3 meq/liter (P < 0.05) and SLI rose from a mean base-line level of 185+/-9 pg/ml to a peak value of 310+/-44 pg/ml (P < 0.01). Although SLI levels were significantly greater than in normal dogs at several time points after the rise in FFA, the magnitude of the increment in diabetic dogs did not differ from normal. These results demonstrate that a rise in FFA levels is a potent stimulus for SLI secretion from the pancreas and stomach and raise the possibility that FFA is an important physiological regulator of SLI secretion.

Effect of oleic acid on arginine-induced glucagon secretion by the isolated perfused rat pancreas

The isolated perfused rat pancreas was used to investigate the effect of oleic acid on glucagon secretion in response to 10 mmol/l arginine. In the absence of oleic acid and at 2.5 mmol/l calcium, arginine induced a biphasic glucagon secretion. At lower extracellular calcium concentration (1.0 mmol/l), the second phase of glucagon release was reduced, the first phase being unchanged. In the presence of 1,500 mumol/l oleic acid, the glucagon response to arginine was also biphasic, but second phase release was markedly inhibited, the first phase glucagon release being unchanged. Such an effect was not obtained when oleic acid concentration in the medium was 750 mumol/l. These results demonstrate that high concentrations of oleic acid inhibit glucagon secretion in response to arginine from the isolated perfused rat pancreas and support the concept that circulating free fatty acid levels are involved in the control of glucagon secretion.

Effect of free fatty acids and amino acids on glucagon and insulin secretions in normal and diabetic ducks

The relationship between two metabolites free fatty acids (FFA) and amino acids (AA), and the two main pancreatic hormones, insulin and glucagon, was studied by infusing small amounts of these metabolites into normal and diabetic Peking ducks, i.e. two days after subtotal pancreatectomy. Infusion of oleic acid (0.365 g/kg/30 min as an emulsion in plasma) indicated a suppressive effect of free fatty acids on glucagon secretion, but was without effect on insulin secretion, in normal as well as in diabetic ducks, indicating that insulin might not be directly involved in the FFA-glucagon feedback in the duck. Infusions of arginine for one hour (1 g/kg/h) into normal ducks, hyperglycaemic normal birds (as a result of glucose infusion: 1 g/kg/h) and diabetic ducks, suggested the persistence of amino acid effect on glucagon secretion, and a slight reduction of the effect on insulin secretion in diabetes. This suggests that insulin may not be involved in amino acid-induced glucagon secretion in the duck.

I.V. glucose tolerance test: correlation between FFA, glucose and IRI in normal, obese and diabetic subjects

Insulin response and FFA behavior have been evaluated during an IVGTT in 63 subjects of whom 18 were normal, 31 were obese (with varying degrees of carbohydrate tolerance) and 14 were mild non insulin-dependent diabetics. The extreme reduction of insulin secretion in the early phase (delta 0-15 min) and the less severe impairment of the late phase (delta 15-60 min) have been confirmed; obese subjects showed on the average an active insulin response to venous loading; this was more marked and more consistent in the late phase. Compared to controls, FFA concentration both in basal conditions and during IVGTT was progressively higher in obese and diabetic patients. When analyzing the interplay between IRI, KG and FFA in the course of IVGTT, it was observed that: (1) a close correlation exists between IG and early insulin response (r = 0.72); (2) a correlation between delta IRI 0-15 min and percentage decrease of FFA at 45 min is found only in normal subjects; (3) a negative highly significant correlation is found between KG and mean FFA plasma level 0-60 min. This last correlation is evidence of the important role played by FFA in carbohydrate tolerance. The conflicting results reported by others have been discussed.

Influence of elevated plasma free fatty acids on the glucagon response to hypoglycemia in normal and in pregnant women

We investigated the influence of an insulin-induced hypoglycemia on plasma glucagon in nonpregnant healthy young women and in women during the last month of gestation. Both groups were tested either in the basal state or during a period where free fatty acid plasma levels were increased by infusion of a lipid emulsion supplemented with heparin. Regular insulin was injected intravenously at the dose of 0.1 U/kg body wt in controls and 0.3 U/kg in pregnant women in order to obtain a similar lowering of blood glucose in all groups. In controls, the increase in plasma glucagon was maximum 30 and 45 min after insulin injection and averaged 130 pg/ml; the infusion of triglycerides and heparin which raised plasma FFA to about 1300 muEq/liter decreased basal plasma glucagon levels and reduced, by about 70%, the glucagon response to hypoglycemia. During the last month of pregnancy, the glucagon response to insulin-induced hypoglycemia was reduced by 60% (mean maximal increase 52 pg/ml); furthermore, raising plasma FFA to about 1500 muEq/liter completely abolished the glucagon rise induced by the insulin hypoglycemia. These results support the view that the glucagon release from A-cells can be modulated by the level of circulating plasma FFA.

Exploration of the early insulin response by two small successive loads of I.V. glucose in normal and obese subjects

Two 5 g glucose loads at 1-h interval were given to healthy controls and obese subjects with slightly altered or normal OGTT in order to explore the capacity of restoration of the "rapid insulin response" to i.v. glucose. In the normal subjects, the two successive loads gave rise to identical responses as far as maximum increase (delta max), average increase at 2-5 min (delta 2-5 min), area of increase 0-15 min (delta 0-15 min) for both glucose and IRI, were concerned. Obese subjects could be divided on the basis of their insulin response to the first load into normal responders (group I) and high-responders (group II). In group I obese subjects, the responses to the second load were identical to those to the first. In group II obese subjects delta max, delta 2-5 min and delta 0-15 min of the insulin response to the second load were reduced as compared to the first.

Effects of exogenous glucagon and epinephrine in physiological amounts on the blood levels of free fatty acids and glycerol in dogs

Exogenous glucagon or epinephrine were infused into normal overnight fasted dogs to raise circulating hormone levels to concentrations within the physiologic range. Plasma levels of glycerol and free fatty acids remained unchanged during the glucagon infusion, but rose significantly during the administration of epinephrine. Plasma insulin in the systemic circulation remained unchanged during the glucagon infusion and increased slightly during the infusion of thecatecholamine. The data suggest that in normal dogs glucagon in physiological amounts has no lipolytic effect. The importance of the sympathetic nervous system in regulating lipolysis in normal mammals is stressed.

Effects of beta-hydroxybutyrate, glycerol, and free fatty acid infusions on glucagon and epinephrine secretion in dogs during acute hypoglycemia

The importance of glucagon in the regulation of carbohydrate metabolism is clearly established. However, the role played by this hormone in the regulation of the overall fuel economy is less certain, particularly with respect to such nonglucose fuels as free fatty acids, glycerol, and ketoacids. In order to elucidate glucagon's role with respect to the latter substrates, dogs were infused with solutions of these three fuels, and their A-cell responses to concomitant insulin-induced hypoglycemia were studied. In addition, epinephrine levels were also monitored. It was found that while these infusions failed to suppress glucagon release, the ketoacid infusion did significantly reduce epinephrine secretion during the insulin-induced hypoglycemic period. It was therefore concluded that glucagon secretion under these experimental conditions is not responsive to prevailing non-glucose fuel levels. Indeed, these data suggest that the sympathetic nervous system may play an important role in the regulation of the over-all fuel economy.

Effect of somatostatin on metabolic and hormonal changes induced by nicotinic acid in insulin-dependent diabetics

The study investigated the respective influences of nicotinic acid and somatostatin on plasma concentrations of blood glucose, free fatty acids, glucagon, growth hormone and cortisol in insulin-dependent diabetic subjects. After administration of nicotinic acid alone, marked depression of plasma FFA was accompanied by significant increases of plasma glucagon, growth hormone and cortisol. The glucagon and growth hormone responses to nicotinic acid were significantly reduced when plasma FFA were raised by intravenous administration of heparin and triglycerides. Somatostatin alone induced a significant decrease in blood glucose, plasma glucagon and growth hormone concentrations. Plasma FFA remained unchanged. Somatostatin did not modify the nicotinic acid-induced fall in plasma FFA, but completely blocked the corresponding increments in glucagon and growth hormone. The cortisol rise was not altered by somatostatin. Rebound of glucagon and growth hormone levels were seen upon discontinuation of the somatostatin administration. These results demonstrate that the plasma FFA concentration plays a role in the regulation of glucagon and growth hormone secretion in insulin-dependent diabetics. Furthermore, they indicate that somatostatin, previously shown to be capable of negating the stimulatory effect of various factors on glucagon and growth hormone secretion, also affects the response of these hormones to FFA depression.

Comparison of the suppressive effects of elevated plasma glucose and free fatty acid levels on glucagon secretion in normal and insulin-dependent diabetic subjects. Evidence for selective alpha-cell insensitivity to glucose in diabetes mellitus

To examine whether abnormal pancreatic alpha-cell function found in human diabetes mellitus may represent a selective insensitivity to glucose, plasma glucagon responses to hyperglycemia and elevation of plasma free fatty acid levels (both known suppressors of glucagon secretion) were compared in juvenile-onset, insulin-requiring diabetic subjects, and in normal nondiabetic subjects. In the latter, both elevation of plasma free fatty acid levels induced by heparin administration of hyperglycemia produced by intravenous infusion of glucose resulted in a comparable 30--40% suppression of circulating glucagon levels (P less than 0.01). In the diabetic subjects, glucagon suppression by hyperglycemia (less than 20%) was less than that occurring in normal subjects (P less than 0.01), even when accompanied by infusion of supraphysiologic amounts of insulin. However, suppression of glucagon levels by elevation of plasma free fatty acids in the diabetic group was similar to that found in normal subjects and of comparable magnitude to that due to hyperglycemia in the normal subjects. These results thus demonstrate a selective impairment of the diabetic alpha-cell response to glucose and provide further evidence for the presence of an abnormal alpha-cell glucoreceptor in human diabetes mellitus.

Effect of intravenous glucose and insulin on plasma tryptophan and tyrosine concentrations in normal subjects and patients with carcinoid tumors

We evaluated the effect of intravenous (i.v.) glucose on the plasma tryptophan (TRP) and tyrosine (TYR) concentration of 12 normal subjects, six patients with carcinoid tumors and the carcinoid syndrome (carcinoid syndrome), and five patients with carcinoid tumors without the carcinoid syndrome (tumor.) Following i.v. glucose administration, the plasma Trp concentration of the normal subjects and the tumor patients incresed, while the plasma Trp concentration of the carcinoid syndrome patients decreased. Following i.v. glucose administration, the plasma Tyr concentration of the normal subjects and the tumor patients decreased, while the plasma Tyr concentration of the carcinoid syndrome patients did not change. The response to i.v. insulin differed in some respects from the response to i.v. glucose: the plasma Trp of normal subjects did not change while the plasma Trp of carcinoid syndrome patients decreased; the plasma Tyr of the normal subjects increased while the plasma Tyr concentration of the carcinoid syndrome patients did not change. The carcinoid syndrome patients had high serum serotonin concentrations and impaired glucose tolerance and insulin secretion as compared to both normal subjects and tumor patients. We conclude that under appropriate experimental conditions, glucose administration can increase the plasma Trp concentration of normal human subjects.