Insulin-induced hypoglycemia increases hepatic sensitivity to glucagon in dogs

Hepatic glucagon action: beyond glucose mobilization

Glucagon's ability to promote hepatic glucose production has been known for over a century, with initial observations touting this hormone as a diabetogenic agent. However, glucagon receptor agonism [when balanced with an incretin, including glucagon-like peptide 1 (GLP-1) to dampen glucose excursions] is now being developed as a promising therapeutic target in the treatment of metabolic diseases, like metabolic dysfunction-associated steatotic disease/metabolic dysfunction-associated steatohepatitis (MASLD/MASH), and may also have benefit for obesity and chronic kidney disease. Conventionally regarded as the opposing tag-team partner of the anabolic mediator insulin, glucagon is gradually emerging as more than just a "catabolic hormone." Glucagon action on glucose homeostasis within the liver has been well characterized. However, growing evidence, in part thanks to new and sensitive "omics" technologies, has implicated glucagon as more than just a "glucose liberator." Elucidation of glucagon's capacity to increase fatty acid oxidation while attenuating endogenous lipid synthesis speaks to the dichotomous nature of the hormone. Furthermore, glucagon action is not limited to just glucose homeostasis and lipid metabolism, as traditionally reported. Glucagon plays key regulatory roles in hepatic amino acid and ketone body metabolism, as well as mitochondrial turnover and function, indicating broader glucagon signaling consequences for metabolic homeostasis mediated by the liver. Here we examine the broadening role of glucagon signaling within the hepatocyte and question the current dogma, to appreciate glucagon as more than just that "catabolic hormone."

Ultra-stable insulin-glucagon fusion protein exploits an endogenous hepatic switch to mitigate hypoglycemic risk

The risk of hypoglycemia and its serious medical sequelae restrict insulin replacement therapy for diabetes mellitus. Such adverse clinical impact has motivated development of diverse glucose-responsive technologies, including algorithm-controlled insulin pumps linked to continuous glucose monitors ("closed-loop systems") and glucose-sensing ("smart") insulins. These technologies seek to optimize glycemic control while minimizing hypoglycemic risk. Here, we describe an alternative approach that exploits an endogenous glucose-dependent switch in hepatic physiology: preferential insulin signaling (under hyperglycemic conditions) versus preferential counter-regulatory glucagon signaling (during hypoglycemia). Motivated by prior reports of glucagon-insulin co-infusion, we designed and tested an ultra-stable glucagon-insulin fusion protein whose relative hormonal activities were calibrated by respective modifications; physical stability was concurrently augmented to facilitate formulation, enhance shelf life and expand access. An N-terminal glucagon moiety was stabilized by an α-helix-compatible Lys 13 -Glu 17 lactam bridge; A C-terminal insulin moiety was stabilized as a single chain with foreshortened C domain. Studies in vitro demonstrated (a) resistance to fibrillation on prolonged agitation at 37 °C and (b) dual hormonal signaling activities with appropriate balance. Glucodynamic responses were monitored in rats relative to control fusion proteins lacking one or the other hormonal activity, and continuous intravenous infusion emulated basal subcutaneous therapy. Whereas efficacy in mitigating hyperglycemia was unaffected by the glucagon moiety, the fusion protein enhanced endogenous glucose production under hypoglycemic conditions. Together, these findings provide proof of principle toward a basal glucose-responsive insulin biotechnology of striking simplicity. The fusion protein's augmented stability promises to circumvent the costly cold chain presently constraining global insulin access.

Significance Statement

The therapeutic goal of insulin replacement therapy in diabetes is normalization of blood-glucose concentration, which prevents or delays long-term complications. A critical barrier is posed by recurrent hypoglycemic events that results in short- and long-term morbidities. An innovative approach envisions co-injection of glucagon (a counter-regulatory hormone) to exploit a glycemia-dependent hepatic switch in relative hormone responsiveness. To provide an enabling technology, we describe an ultra-stable fusion protein containing insulin- and glucagon moieties. Proof of principle was obtained in rats. A single-chain insulin moiety provides glycemic control whereas a lactam-stabilized glucagon extension mitigates hypoglycemia. This dual-hormone fusion protein promises to provide a basal formulation with reduced risk of hypoglycemia. Resistance to fibrillation may circumvent the cold chain required for global access.

Integration of metabolic flux with hepatic glucagon signaling and gene expression profiles in the conscious dog

Glucagon rapidly and profoundly stimulates hepatic glucose production (HGP), but for reasons that are unclear, this effect normally wanes after a few hours, despite sustained plasma glucagon levels. This study characterized the time course of glucagon-mediated molecular events and their relevance to metabolic flux in the livers of conscious dogs. Glucagon was either infused into the hepato-portal vein at a sixfold basal rate in the presence of somatostatin and basal insulin, or it was maintained at a basal level in control studies. In one control group, glucose remained at basal, whereas in the other, glucose was infused to match the hyperglycemia that occurred in the hyperglucagonemic group. Elevated glucagon caused a rapid (30 min) and largely sustained increase in hepatic cAMP over 4 h, a continued elevation in glucose-6-phosphate (G6P), and activation and deactivation of glycogen phosphorylase and synthase activities, respectively. Net hepatic glycogenolysis increased rapidly, peaking at 15 min due to activation of the cAMP/PKA pathway, then slowly returned to baseline over the next 3 h in line with allosteric inhibition by glucose and G6P. Glucagon's stimulatory effect on HGP was sustained relative to the hyperglycemic control group due to continued PKA activation. Hepatic gluconeogenic flux did not increase due to the lack of glucagon's effect on substrate supply to the liver. Global gene expression profiling highlighted glucagon-regulated activation of genes involved in cellular respiration, metabolic processes, and signaling, as well as downregulation of genes involved in extracellular matrix assembly and development.NEW & NOTEWORTHY Glucagon rapidly stimulates hepatic glucose production, but these effects are transient. This study links the molecular and metabolic flux changes that occur in the liver over time in response to a rise in glucagon, demonstrating the strength of the dog as a translational model to couple findings in small animals and humans. In addition, this study clarifies why the rapid effects of glucagon on liver glycogen metabolism are not sustained.

Evaluating the effectiveness of a novel somatostatin receptor 2 antagonist, ZT-01, for hypoglycemia prevention in a rodent model of type 2 diabetes

Background: Elevated levels of somatostatin blunt glucagon counterregulation during hypoglycemia in type 1 diabetes (T1D) and this can be improved using somatostatin receptor 2 (SSTR2) antagonists. Hypoglycemia also occurs in late-stage type 2 diabetes (T2D), particularly when insulin therapy is initiated, but the utility of SSTR2 antagonists in ameliorating hypoglycemia in this disease state is unknown. We examined the efficacy of a single-dose of SSTR2 antagonists in a rodent model of T2D. Methods: High-fat fed (HFF), low dose streptozotocin (STZ, 35 mg/kg)-induced T2D and HFF only, nondiabetic (controls-no STZ) rats were treated with the SSTR2 antagonists ZT-01/PRL-2903 or vehicle (n = 9-11/group) 60 min before an insulin tolerance test (ITT; 2-12 U/kg insulin aspart) or an oral glucose tolerance test (OGTT; 2 g/kg glucose via oral gavage) on separate days. Results: This rodent model of T2D is characterized by higher baseline glucose and HbA1c levels relative to HFF controls. T2D rats also had lower c-peptide levels at baseline and a blunted glucagon counterregulatory response to hypoglycemia when subjected to the ITT. SSTR2 antagonists increased the glucagon response and reduced incidence of hypoglycemia, which was more pronounced with ZT-01 than PRL-2903. ZT-01 treatment in the T2D rats increased glucagon levels above the control response within 60 min of dosing, and values remained elevated during the ITT (glucagon Cmax: 156 ± 50 vs. 77 ± 46 pg/mL, p < 0.01). Hypoglycemia incidence was attenuated with ZT-01 vs. controls (63% vs. 100%) and average time to hypoglycemia onset was also delayed (103.1 ± 24.6 vs. 66.1 ± 23.6 min, p < 0.05). ZT-01 administration at the OGTT onset increased the glucagon response without exacerbating hyperglycemia (2877 ± 806 vs. 2982 ± 781), potentially due to the corresponding increase in c-peptide levels (6251 ± 5463 vs. 14008 ± 5495, p = 0.013). Conclusion: Treatment with SSTR2 antagonists increases glucagon responses in a rat model of T2D and results in less hypoglycemia exposure. Future studies are required to determine the best dosing periods for chronic SSTR2 antagonism treatment in T2D.

Biochemical Toxicological Study of Insulin Overdose in Rats: A Forensic Perspective

Due to nonspecific pathological changes and the rapid degradation of insulin in postmortem blood samples, the identification of the cause of death during insulin overdose has always been a difficulty in forensic medicine. At present, there is a lack of studies on the toxicological changes and related mechanisms of an insulin overdose, and the specific molecular markers of insulin overdose are still unclear. In this study, an animal model of insulin overdose was established, and 24 SD rats were randomly divided into a control group, insulin overdose group, and a recovery group (n = 8). We detected the biochemical changes and analyzed the toxicological mechanism of an insulin overdose. The results showed that after insulin overdose, the rats developed irregular convulsions, Eclampsia, Opisthotonos, and other symptoms. The levels of glucose, glycogen, and C-peptide in the body decreased significantly, while the levels of lactate, insulin, and glucagon increased significantly. The decrease in plasma K+ was accompanied by the increase in skeletal muscle K+. The PI3K-AKT signaling pathway was significantly activated in skeletal muscle, and the translocation of GLUT4/Na+-K+-ATPase to sarcolemma was significantly increased. Rare glycogenic hepatopathy occurred in the recovery group after insulin overdose. Our study showed that insulin overdose also plays a role in skeletal muscle cells, mainly through the PI3K-Akt signaling pathway. Therefore, the detection of signaling pathway proteins of the skeletal muscle cell membrane GLUT4 and Na+-K+-ATPase has a certain auxiliary diagnostic value for forensic insulin overdose identification. Glycogen detection in the liver and skeletal muscle is important for the diagnosis of insulin overdose, but it still needs to be differentiated from other causes of death. Skeletal muscle has great potential for insulin detection, and the ratio of insulin to the C-peptide (I:C) can determine whether an exogenous insulin overdose is present.

Integration of metabolic flux with hepatic glucagon signaling and gene expression profiles in the conscious dog

Glucagon rapidly and profoundly simulates hepatic glucose production (HGP), but for reasons which are unclear, this effect normally wanes after a few hours, despite sustained plasma glucagon levels. This study characterized the time course and relevance (to metabolic flux) of glucagon mediated molecular events in the livers of conscious dogs. Glucagon was either infused into the hepato-portal vein at a 6-fold basal rate in the presence of somatostatin and basal insulin, or it was maintained at a basal level in control studies. In one control group glucose remained at basal while in the other glucose was infused to match the hyperglycemia that occurred in the hyperglucagonemic group. Elevated glucagon caused a rapid (30 min) but only partially sustained increase in hepatic cAMP over 4h, a continued elevation in G6P, and activation and deactivation of glycogen phosphorylase and synthase activities, respectively. Net hepatic glycogenolysis and HGP increased rapidly, peaking at 30 min, then returned to baseline over the next 3h (although glucagons stimulatory effect on HGP was sustained relative to the hyperglycemic control group). Hepatic gluconeogenic flux did not increase due to lack of glucagon effect on substrate supply to the liver. Global gene expression profiling highlighted glucagon-regulated activation of genes involved in cellular respiration, metabolic processes, and signaling, and downregulation of genes involved in extracellular matrix assembly and development.

Short-term fasting lowers glucagon levels under euglycemic and hypoglycemic conditions in healthy humans

Fasting is associated with increased susceptibility to hypoglycemia in people with type 1 diabetes, thereby making it a significant health risk. To date, the relationship between fasting and insulin-induced hypoglycemia has not been well characterized, so our objective was to determine whether insulin-independent factors, such as counterregulatory hormone responses, are adversely impacted by fasting in healthy control individuals. Counterregulatory responses to insulin-induced hypoglycemia were measured in 12 healthy people during 2 metabolic studies. During one study, participants ate breakfast and lunch, after which they underwent a 2-hour bout of insulin-induced hypoglycemia (FED). During the other study, participants remained fasted prior to hypoglycemia (FAST). As expected, hepatic glycogen concentrations were lower in FAST, and associated with diminished peak glucagon levels and reduced endogenous glucose production (EGP) during hypoglycemia. Accompanying lower EGP in FAST was a reduction in peripheral glucose utilization, and a resultant reduction in the amount of exogenous glucose required to maintain glycemia. These data suggest that whereas a fasting-induced lowering of glucose utilization could potentially delay the onset of insulin-induced hypoglycemia, subsequent reductions in glucagon levels and EGP are likely to encumber recovery from it. As a result of this diminished metabolic flexibility in response to fasting, susceptibility to hypoglycemia could be enhanced in patients with type 1 diabetes under similar conditions.

A reappraisal of the role of cyclic AMP in the physiological action of glucagon

Effects of the metabolic hormone glucagon can be physiological or supraphysiological, based on agonist concentration and the mediating cellular signal. The threshold concentration (TC) for activating the AC/cAMP signal pathway in liver is ≥ 100 pM. By contrast, mean plasma concentrations are around 20-45 pM, depending on the vascular bed. Accordingly, effects produced at TCs below 100 pM are physiological and mediated by cellular signal pathways other than AC/cAMP. Effects generated at concentrations above 100 pM are supraphysiological, often mediated by simultaneous activation of cAMP-independent and -dependent pathways. Physiological responses, and their established or implicated signal pathways, include stimulation of: glucose mobilization, fatty acid oxidation, and urea synthesis in liver (PLC/IP3/Ca²⁺/CaM); lipolysis in white and brown adipose tissue and oxygen consumption in brown adipose of the rat but not in humans (PLC/IP3/Ca²⁺/CaM); renal potassium and phosphate excretion in rodents and GFR in humans (signal undetermined); and glucose utilization in rat heart (PI3K/akt). Supraphysiological responses involve the AC/cAMP pathway and include: enhanced stimulation of glucose mobilization and stimulation of urea synthesis in liver; further stimulation of white and brown adipose lipolysis and thermogenesis in brown adipose tissue; stimulation of renal Cl^- transport; and increased rat heart contractility. The AC/cAMP pathway is likely recruited when plasma glucagon rises above 100 pM during periods of elevated metabolic stress and systemic glucose demand, such as in the early neonate or strenuously exercising adult. The current cAMP-centered model should therefore be reconsidered and replaced with one that places more emphasis on cAMP-independent pathways.

In vitro generation of transplantable insulin-producing cells from canine adipose-derived mesenchymal stem cells

Canine mesenchymal stem cells (cMSCs) have potential applications for regenerative therapy, including the generation of insulin-producing cells (IPCs) for studying and treating diabetes. In this study, we established a useful protocol for generating IPCs from canine adipose mesenchymal stem cells (cAD-MSCs). Subsequently, in vitro preservation of pluronic F127-coated alginate (ALGPA)-encapsulated cAD-MSC-derived IPCs was performed to verify ready-to-use IPCs. IPCs were induced from cAD-MSCs with the modulated three-stepwise protocol. The first step of definitive endoderm (DE) induction showed that the cooperation of Chir99021 and Activin A created the effective production of Sox17-expressed DE cells. The second step for pancreatic endocrine (PE) progenitor induction from DE indicated that the treatment with taurine, retinoic acid, FGF2, EGF, TGFβ inhibitor, dorsomorphin, nicotinamide, and DAPT showed the significant upregulation of the pancreatic endocrine precursor markers Pdx1 and Ngn3. The last step of IPC production, the combination of taurine, nicotinamide, Glp-1, forskolin, PI3K inhibitor, and TGFβ inhibitor, yielded efficiently functional IPCs from PE precursors. Afterward, the maintenance of ALGPA-encapsulated cAD-MSC-derived IPCs with VSCBIC-1, a specialized medium, enhanced IPC properties. Conclusion, the modulated three-stepwise protocol generates the functional IPCs. Together, the encapsulation of cAD-MSC-derived IPCs and the cultivation with VSCBIC-1 enrich the maturation of generated IPCs.

Prandial hepatic glucose production during hypoglycemia is altered after gastric bypass surgery and sleeve gastrectomy

AIMS/HYPOTHESIS

Roux-en Y gastric bypass surgery (GB) and sleeve gastrectomy (SG) alter prandial glucose metabolism, producing lower nadir glucose values and predisposing susceptible individuals to prandial hypoglycemia. The glycemic phenotype of GB or SG is associated with prandial hyperinsulinemia and hyperglucagonemia along with an increased influx of ingested glucose. Following insulin-induced hypoglycemia, glucagon is the most important stimulus for hepatic glucose production (HGP). It is unclear whether prandial hyperglucagonemia after GB or SG changes HGP under hyperinsulinemic hypoglycemia conditions. This study examined the hypothesis that prandial glucose production is reduced after GB and SG during hypoglycemia.

METHODS

Glucose kinetics and islet-cell and gut hormone secretion during hyperinsulinemic (120 mU.m^-2.min^-1) hypoglycemic clamp (~3.2 mM) were measured before and after mixed meal ingestion in 9 non-diabetic subjects with GB, 7 with SG, and 5 matched non-operated controls (CN).

RESULTS

Systemic appearance of ingested glucose was faster in GB compared to SG, and in SG compared to CN (p < 0.05). Subjects with GB and SG had greater plasma glucagon levels after eating (AUC_Glucagon) compared to CN (p < 0.05). But prandial HGP response during insulin-induced hypoglycemia (AUC_HGP) was smaller and shorter in duration in surgical groups (p < 0.05). In the absence of meal stimuli, however, glucose counterregulatory response to hypoglycemia was comparable among the 3 groups during hyperinsulinemic clamp.

CONCLUSION

After bariatric surgery, prandial glucose counterregulatory response to hypoglycemia is impaired. Considering post-meal hyperglucagonemia after GB or SG the blunted HGP response suggests a lower sensitivity of liver to glucagon that can predispose to hypoglycemia in this population.

Glucagon, cyclic AMP, and hepatic glucose mobilization: A half‐century of uncertainty

For at least 50 years, the prevailing view has been that the adenylate cyclase (AC)/cyclic AMP (cAMP)/protein kinase A pathway is the predominant signal mediating the hepatic glucose‐mobilizing actions of glucagon. A wealth of evidence, however, supports the alternative, that the operative signal most of the time is the phospholipase C (PLC)/inositol‐phosphate (IP3)/calcium/calmodulin pathway. The evidence can be summarized as follows: (1) The consensus threshold glucagon concentration for activating AC ex vivo is 100 pM, but the statistical hepatic portal plasma glucagon concentration range, measured by RIA, is between 28 and 60 pM; (2) Within that physiological concentration range, glucagon stimulates the PLC/IP3 pathway and robustly increases glucose output without affecting the AC/cAMP pathway; (3) Activation of a latent, amplified AC/cAMP pathway at concentrations below 60 pM is very unlikely; and (4) Activation of the PLC/IP3 pathway at physiological concentrations produces intracellular effects that are similar to those produced by activation of the AC/cAMP pathway at concentrations above 100 pM, including elevated intracellular calcium and altered activities and expressions of key enzymes involved in glycogenolysis, gluconeogenesis, and glycogen synthesis. Under metabolically stressful conditions, as in the early neonate or exercising adult, plasma glucagon concentrations often exceed 100 pM, recruiting the AC/cAMP pathway and enhancing the activation of PLC/IP3 pathway to boost glucose output, adaptively meeting the elevated systemic glucose demand. Whether the AC/cAMP pathway is consistently activated in starvation or diabetes is not clear. Because the importance of glucagon in the pathogenesis of diabetes is becoming increasingly evident, it is even more urgent now to resolve lingering uncertainties and definitively establish glucagon’s true mechanism of glycemia regulation in health and disease.

New Horizons: Next-Generation Insulin Analogues: Structural Principles and Clinical Goals

Design of "first-generation" insulin analogues over the past 3 decades has provided pharmaceutical formulations with tailored pharmacokinetic (PK) and pharmacodynamic (PD) properties. Application of a molecular tool kit-integrating protein sequence, chemical modification, and formulation-has thus led to improved prandial and basal formulations for the treatment of diabetes mellitus. Although PK/PD changes were modest in relation to prior formulations of human and animal insulins, significant clinical advantages in efficacy (mean glycemia) and safety (rates of hypoglycemia) were obtained. Continuing innovation is providing further improvements to achieve ultrarapid and ultrabasal analogue formulations in an effort to reduce glycemic variability and optimize time in range. Beyond such PK/PD metrics, next-generation insulin analogues seek to exploit therapeutic mechanisms: glucose-responsive ("smart") analogues, pathway-specific ("biased") analogues, and organ-targeted analogues. Smart insulin analogues and delivery systems promise to mitigate hypoglycemic risk, a critical barrier to glycemic control, whereas biased and organ-targeted insulin analogues may better recapitulate physiologic hormonal regulation. In each therapeutic class considerations of cost and stability will affect use and global distribution. This review highlights structural principles underlying next-generation design efforts, their respective biological rationale, and potential clinical applications.

Structural principles of insulin formulation and analog design: A century of innovation

BACKGROUND

The discovery of insulin in 1921 and its near-immediate clinical use initiated a century of innovation. Advances extended across a broad front, from the stabilization of animal insulin formulations to the frontiers of synthetic peptide chemistry, and in turn, from the advent of recombinant DNA manufacturing to structure-based protein analog design. In each case, a creative interplay was observed between pharmaceutical applications and then-emerging principles of protein science; indeed, translational objectives contributed to a growing molecular understanding of protein structure, aggregation and misfolding.

SCOPE OF REVIEW

Pioneering crystallographic analyses-beginning with Hodgkin's solving of the 2-Zn insulin hexamer-elucidated general features of protein self-assembly, including zinc coordination and the allosteric transmission of conformational change. Crystallization of insulin was exploited both as a step in manufacturing and as a means of obtaining protracted action. Forty years ago, the confluence of recombinant human insulin with techniques for site-directed mutagenesis initiated the present era of insulin analogs. Variant or modified insulins were developed that exhibit improved prandial or basal pharmacokinetic (PK) properties. Encouraged by clinical trials demonstrating the long-term importance of glycemic control, regimens based on such analogs sought to resemble daily patterns of endogenous β-cell secretion more closely, ideally with reduced risk of hypoglycemia.

MAJOR CONCLUSIONS

Next-generation insulin analog design seeks to explore new frontiers, including glucose-responsive insulins, organ-selective analogs and biased agonists tailored to address yet-unmet clinical needs. In the coming decade, we envision ever more powerful scientific synergies at the interface of structural biology, molecular physiology and therapeutics.

C-peptide enhances glucagon secretion in response to hyperinsulinemia under euglycemic and hypoglycemic conditions

Several studies have associated the presence of residual insulin secretion capability (also referred to as being C-peptide positive) with lower risk of insulin-induced hypoglycemia in patients with type 1 diabetes (T1D), although the reason is unclear. We tested the hypothesis that C-peptide infusion would enhance glucagon secretion in response to hyperinsulinemia during euglycemic and hypoglycemic conditions in dogs (5 male/4 female). After a 2-hour basal period, an intravenous (IV) infusion of insulin was started, and dextrose was infused to maintain euglycemia for 2 hours. At the same time, an IV infusion of either saline (SAL) or C-peptide (CPEP) was started. After this euglycemic period, the insulin and SAL/CPEP infusions were continued for another 2 hours, but the glucose was allowed to fall to approximately 50 mg/dL. In response to euglycemic-hyperinsulinemia, glucagon secretion decreased in SAL but remained unchanged from the basal period in CPEP condition. During hypoglycemia, glucagon secretion in CPEP was 2 times higher than SAL, and this increased net hepatic glucose output and reduced the amount of exogenous glucose required to maintain glycemia. These data suggest that the presence of C-peptide during IV insulin infusion can preserve glucagon secretion during euglycemia and enhance it during hypoglycemia, which could explain why T1D patients with residual insulin secretion are less susceptible to hypoglycemia.

Liver glycogen-induced enhancements in hypoglycemic counterregulation require neuroglucopenia

Iatrogenic hypoglycemia is a prominent barrier to achieving optimal glycemic control in patients with diabetes, in part due to dampened counterregulatory hormone responses. It has been demonstrated that elevated liver glycogen content can enhance these hormonal responses through signaling to the brain via afferent nerves, but the role that hypoglycemia in the brain plays in this liver glycogen effect remains unclear. During the first 4 h of each study, the liver glycogen content of dogs was increased by using an intraportal infusion of fructose to stimulate hepatic glucose uptake (HG; n = 13), or glycogen was maintained near fasting levels with a saline infusion (NG; n = 6). After a 2-h control period, during which the fructose/saline infusion was discontinued, insulin was infused intravenously for an additional 2 h to bring about systemic hypoglycemia in all animals, whereas brain euglycemia was maintained in a subset of the HG group by infusing glucose bilaterally into the carotid and vertebral arteries (HG-HeadEu; n = 7). Liver glycogen content was markedly elevated in the two HG groups (43 ± 4, 73 ± 3, and 75 ± 7 mg/g in NG, HG, and HG-HeadEu, respectively). During the hypoglycemic period, arterial plasma glucose levels were indistinguishable between groups (53 ± 2, 52 ± 1, and 51 ± 1 mg/dL, respectively), but jugular vein glucose levels were kept euglycemic (88 ± 5 mg/dL) only in the HG-HeadEu group. Glucagon and epinephrine responses to hypoglycemia were higher in HG compared with NG, whereas despite the increase in liver glycogen, neither increased above basal in HG-HeadEu. These data demonstrate that the enhanced counterregulatory hormone secretion that accompanies increased liver glycogen content requires hypoglycemia in the brain.NEW & NOTEWORTHY It is well known that iatrogenic hypoglycemia is a barrier to optimal glycemic regulation in patients with diabetes. Our data confirm that increasing liver glycogen content 75% above fasting levels enhances hormonal responses to insulin-induced hypoglycemia and demonstrate that this enhanced hormonal response does not occur in the absence of hypoglycemia in the brain. These data demonstrate that information from the liver regarding glycogen availability is integrated in the brain to optimize the counterregulatory response.

Serum spexin concentration, body condition score and markers of obesity in dogs

Background

Spexin (SPX) is a peptide hormone that regulates body weight, adipose tissue metabolism, and food intake.

Hypothesis

Serum SPX concentration correlates with body condition score (BCS) and markers of obesity in dogs.

Animals

Fifty‐seven dogs of varying body condition assessed using a 5‐point BCS.

Methods

Prospective, nonblinded, observational cohort study. Serum SPX concentration was measured using commercially available radioimmunoassay (RIA) in dogs with varying BCS. Spexin mRNA and protein expression were detected using real‐time quantitative polymerase chain reaction and immunofluorescence staining.

Results

Serum SPX concentration was lower in dogs with BCS4 (8.56 +/‐ 2.86) and BCS5 (6.7 +/‐ 2.12) compared to BCS2 (11.96 +/‐ 2.23) and BCS3 (10.51 +/‐ 2.19; BCS2 vs BCS5, P < .001 and BCS2 vs BCS4, P = .005; BCS3 vs BCS5, P = .002). Spexin mRNA was detected in adipose tissue, liver and pancreas. Spexin protein was expressed in adipose tissue and liver but not in pancreas. There were negative correlations between SPX and serum concentration of insulin (P < .05); leptin (P < .01), triglycerides (P < .01), total cholesterol (P < .01), nonesterified fatty acids (P < .01), and fructosamine (P < .01). There was a positive correlation between SPX and serum concentration of adiponectin (P < .01).

Conclusions and Clinical Importance

Spexin could be involved in pathogenesis of obesity in dogs, and might be considered as a potential marker for obesity.

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.

iNOS as a metabolic enzyme under stress conditions

Nitric oxide (NO) is a free radical acting as a cellular signaling molecule in many different biochemical processes. NO is synthesized from l-arginine through the action of the nitric oxide synthase (NOS) family of enzymes, which includes three isoforms: endothelial NOS (eNOS), neuronal NOS (nNOS) and inducible NOS (iNOS). iNOS-derived NO has been associated with the pathogenesis and progression of several diseases, including liver diseases, insulin resistance, obesity and diseases of the cardiovascular system. However, transient NO production can modulate metabolism to survive and cope with stress conditions. Accumulating evidence strongly imply that iNOS-derived NO plays a central role in the regulation of several biochemical pathways and energy metabolism including glucose and lipid metabolism during inflammatory conditions. This review summarizes current evidence for the regulation of glucose and lipid metabolism by iNOS during inflammation, and argues for the role of iNOS as a metabolic enzyme in immune and non-immune cells.

Dual treatment with a fixed ratio of glucagon and insulin increases the therapeutic window of insulin in diabetic rats

The current available insulin therapies decrease blood glucose but are associated with the risk of developing hypoglycemia. Glucagon is a counter regulatory hormone and we hypothesize that a fixed ratio of insulin and a long‐acting glucagon‐analogue can reduce the risk of hypoglycemia. To define an appropriate ratio we tested two fixed glucagon doses (3.5 and 10 nmol/kg) in combination with increasing doses of insulin in diabetic rats. We observed a plateau in blood glucose at 15.2 mmol/L with 10 nmol/kg of the glucagon‐analogue. The mechanism behind this plateau, protecting against hypoglycemia, was investigated by measuring the glucose output, cAMP production, and hormone binding in primary rat hepatocytes. While glucose output could contribute to the observed plateau in blood glucose, cAMP response or hormone binding did not explain the observation. Though such plateau indicated decreased risk of hypoglycemia a full normalization of blood glucose was still needed. Based on the data obtained with 3.5 nmol/kg of the glucagon‐analogue, a 1:23 (glucagon‐analogue:insulin) ratio was chosen and a dose‐response was performed in diabetic rats. At low doses (≤20 nmol/kg), insulin and the 1:23 ratio showed similar efficacy of lowering blood glucose. Interestingly, the insulin‐dose resulting in hypoglycemia was increased from 40 nmol/kg insulin alone to 160 nmol/kg insulin in the 1:23 ratio. Analysis of the liver glycogen content at the end of the experiment showed that the highest dose in the 1:23 ratio almost emptied the liver from glycogen. Thus, liver glycogen is essential for the protective effect of glucagon in hypoglycemia.

Intraperitoneal, subcutaneous and intravenous glucagon delivery and subsequent glucose response in rats: a randomized controlled crossover trial

OBJECTIVE

Hypoglycemia is a frequent and potentially dangerous event among patients with diabetes mellitus type 1. Subcutaneous glucagon is an emergency treatment to counteract severe hypoglycemia. The effect of intraperitoneal glucagon delivery is sparsely studied. We performed a direct comparison of the blood glucose response following intraperitoneally, subcutaneously and intravenously administered glucagon.

RESEARCH DESIGN AND METHODS

This is a prospective, randomized, controlled, open-label, crossover trial in 20 octreotide-treated rats. Three interventions, 1  week apart, in a randomized order, were done in each rat. All 20 rats were given intraperitoneal and subcutaneous glucagon injections, from which 5 rats were given intravenous glucagon injections and 15 rats received placebo (intraperitoneal isotonic saline) injection. The dose of glucagon was 5 µg/kg body weight for all routes of administration. Blood glucose levels were measured before and until 60 min after the glucagon/placebo injections.

RESULTS

Compared with placebo-treated rats, a significant increase in blood glucose was observed 4 min after intraperitoneal glucagon administration (p=0.009), whereas after subcutaneous and intravenous glucagon administration significant increases were seen after 8 min (p=0.002  and p<0.001, respectively). In intraperitoneally treated compared with subcutaneously treated rats, the increase in blood glucose was higher after 4 min (p=0.019) and lower after 40 min (p=0.005) and 50 min (p=0.011). The maximum glucose response occurred earlier after intraperitoneal compared with subcutaneous glucagon injection (25 min vs 35 min; p=0.003).

CONCLUSIONS

Glucagon administered intraperitoneally gives a faster glucose response compared with subcutaneously administered glucagon in rats. If repeatable in humans, the more rapid glucose response may be of importance in a dual-hormone artificial pancreas using the intraperitoneal route for administration of insulin and glucagon.

Glucose autoregulation is the dominant component of the hormone-independent counterregulatory response to hypoglycemia in the conscious dog

The contribution of hormone-independent counterregulatory signals in defense of insulin-induced hypoglycemia was determined in adrenalectomized, overnight-fasted conscious dogs receiving hepatic portal vein insulin infusions at a rate 20-fold basal. Either euglycemia was maintained (group 1) or hypoglycemia (≈45 mg/dl) was allowed to occur. There were three hypoglycemic groups: one in which hepatic autoregulation against hypoglycemia occurred in the absence of sympathetic nervous system input (group 2), one in which autoregulation occurred in the presence of norepinephrine (NE) signaling to fat and muscle (group 3), and one in which autoregulation occurred in the presence of NE signaling to fat, muscle, and liver (group 4). Average net hepatic glucose balance (NHGB) during the last hour for groups 1-4 was -0.7 ± 0.1, 0.3 ± 0.1 (P < 0.01 vs. group 1), 0.7 ± 0.1 (P = 0.01 vs. group 2), and 0.8 ± 0.1 (P = 0.7 vs. group 3) mg·kg^-1·min^-1, respectively. Hypoglycemia per se (group 2) increased NHGB by causing an inhibition of net hepatic glycogen synthesis. NE signaling to fat and muscle (group 3) increased NHGB further by mobilizing gluconeogenic precursors resulting in a rise in gluconeogenesis. Lowering glucose per se decreased nonhepatic glucose uptake by 8.9 mg·kg^-1·min^-1, and the addition of increased neural efferent signaling to muscle and fat blocked glucose uptake further by 3.2 mg·kg^-1·min^-1 The addition of increased neural efferent input to liver did not affect NHGB or nonhepatic glucose uptake significantly. In conclusion, even in the absence of increases in counterregulatory hormones, the body can defend itself against hypoglycemia using glucose autoregulation and increased neural efferent signaling, both of which stimulate hepatic glucose production and limit glucose utilization.

The biology of glucagon and the consequences of hyperglucagonemia

The proglucagon-derived peptide hormone, glucagon, comprises 29 amino acids. Its secretion from the pancreatic α cells is regulated by several factors. Glucagon increases blood glucose levels through gluconeogenesis and glycogenolysis. Elevated plasma concentrations of glucagon, hyperglucagonemia, may contribute to diabetes. However, hyperglucagonemia is also observed in other clinical conditions than diabetes, including nonalcoholic fatty liver disease, glucagon-producing tumors and after gastric bypass surgery. Here, we review the current literature on hyperglucagonemia in disease with a particular focus on diabetes, and finally speculate that the primary physiological importance of glucagon may not reside in glucose homeostasis but in regulation of amino acid metabolism exerted via a hitherto unrecognized hepato-pancreatic feedback loop.

Comparative Pharmacokinetic/Pharmacodynamic Study of Liquid Stable Glucagon Versus Lyophilized Glucagon in Type 1 Diabetes Subjects

BACKGROUND

There is currently no stable liquid form of glucagon commercially available. The aim of this study is to assess the speed of absorption and onset of action of G-Pump™ glucagon at 3 doses as compared to GlucaGen®, all delivered subcutaneously via an OmniPod®.

METHODS

Nineteen adult subjects with type 1 diabetes participated in this Phase 2, randomized, double-blind, cross-over, pharmacokinetic/pharmacodynamic study. Subjects were given 0.3, 1.2, and 2.0 µg/kg each of G-Pump glucagon and GlucaGen via an OmniPod.

RESULTS

G-Pump glucagon effectively increased blood glucose levels in a dose-dependent fashion with a glucose Cmax of 183, 200, and 210 mg/dL at doses of 0.3, 1.2, and 2.0 µg/kg, respectively (P = ns vs GlucaGen). Mean increases in blood glucose from baseline were 29.2, 52.9, and 77.7 mg/dL for G-Pump doses of 0.3, 1.2, and 2.0 µg/kg, respectively. There were no statistically significant differences between treatments in the glucose T50%-early or glucagon T50%-early with one exception. The glucagon T50%-early was greater following G-Pump treatment at the 2.0 μg/kg dose (13.9 ± 4.7 min) compared with GlucaGen treatment at the 2.0 μg/kg dose (11.0 ± 3.1 min, P = .018). There was more pain and erythema at the infusion site with G-Pump as compared to GlucaGen. No serious adverse events were reported, and no unexpected safety issues were observed.

CONCLUSIONS

G-Pump glucagon is a novel, stable glucagon formulation with similar PK/PD properties as GlucaGen, but was associated with more pain and infusion site reactions as the dose increased, as compared to GlucaGen.

Hepatic glycogen can regulate hypoglycemic counterregulation via a liver-brain axis

Liver glycogen is important for the counterregulation of hypoglycemia and is reduced in individuals with type 1 diabetes (T1D). Here, we examined the effect of varying hepatic glycogen content on the counterregulatory response to low blood sugar in dogs. During the first 4 hours of each study, hepatic glycogen was increased by augmenting hepatic glucose uptake using hyperglycemia and a low-dose intraportal fructose infusion. After hepatic glycogen levels were increased, animals underwent a 2-hour control period with no fructose infusion followed by a 2-hour hyperinsulinemic/hypoglycemic clamp. Compared with control treatment, fructose infusion caused a large increase in liver glycogen that markedly elevated the response of epinephrine and glucagon to a given hypoglycemia and increased net hepatic glucose output (NHGO). Moreover, prior denervation of the liver abolished the improved counterregulatory responses that resulted from increased liver glycogen content. When hepatic glycogen content was lowered, glucagon and NHGO responses to insulin-induced hypoglycemia were reduced. We conclude that there is a liver-brain counterregulatory axis that is responsive to liver glycogen content. It remains to be determined whether the risk of iatrogenic hypoglycemia in T1D humans could be lessened by targeting metabolic pathway(s) associated with hepatic glycogen repletion.

Weight-loss diets and 2-y changes in circulating amino acids in 2 randomized intervention trials

BACKGROUND

Circulating amino acids, such as branched-chain amino acids (BCAAs) and aromatic amino acids (AAAs), have been associated with diabetes risk; however, little is known about how a long-term dietary intervention for weight loss affects circulating amino acids.

OBJECTIVES

We examined the effects of weight-loss diets on long-term changes in plasma amino acids and the associations of these changes with weight loss and the improvement of insulin resistance.

DESIGN

We repeatedly measured plasma amino acid profiles over 2 y in overweight or obese participants from 2 randomized, dietary intervention, weight-loss trials [774 subjects from the POUNDS LOST (Preventing Overweight Using Novel Dietary Strategies Trial) and 318 subjects from the DIRECT (Dietary Intervention Randomized Controlled Trial)].

RESULTS

Intervention diets consistently lowered most of the amino acid concentrations, including BCAAs and AAAs, in both trials. In the POUNDS LOST, average-protein diets (15% of daily energy) showed stronger effects than did high-protein diets (25% of daily energy) on reducing concentrations of the diabetes-associated BCAA valine at 6 mo independent of the weight change. In both trials, weight loss was directly related to the concurrent reduction of the BCAAs leucine and isoleucine, the AAAs tyrosine and phenylalanine, and 4 other amino acids. For example, per kilogram of weight loss, there was a 0.04-SD decrease in log tyrosine (∼0.6 μmol/L) in both trials. In addition, we showed that reductions in alanine and the AAA tyrosine were significantly related to improved insulin resistance (measured with the use of the homeostasis model assessment of insulin resistance), independent of weight loss, in both trials (both P < 0.05). For example, per 1-SD decrease in log tyrosine (∼17 μmol/L), there was a 0.04-SD (∼3%) improvement in insulin resistance in the POUNDS LOST and a 0.13-SD (∼8%) improvement in insulin resistance in the DIRECT.

CONCLUSION

Our findings underscore the potential importance of dietary interventions in improving amino acid profiles (i.e., reducing diabetes risk-enhancing amino acid concentrations) along with and beyond weight loss. The POUNDS LOST and the DIRECT were registered at clinicaltrials.gov as NCT00072995 and NCT00160108, respectively.

Glucagon sensitivity and clearance in type 1 diabetes: insights from in vivo and in silico experiments

Glucagon use in artificial pancreas for type 1 diabetes (T1D) is being explored for prevention and rescue from hypoglycemia. However, the relationship between glucagon stimulation of endogenous glucose production (EGP) viz., hepatic glucagon sensitivity, and prevailing glucose concentrations has not been examined. To test the hypothesis that glucagon sensitivity is increased at hypoglycemia vs. euglycemia, we studied 29 subjects with T1D randomized to a hypoglycemia or euglycemia clamp. Each subject was studied at three glucagon doses at euglycemia or hypoglycemia, with EGP measured by isotope dilution technique. The peak EGP increments and the integrated EGP response increased with increasing glucagon dose during euglycemia and hypoglycemia. However, the difference in dose response based on glycemia was not significant despite higher catecholamine concentrations in the hypoglycemia group. Knowledge of glucagon's effects on EGP was used to develop an in silico glucagon action model. The model-derived output fitted the obtained data at both euglycemia and hypoglycemia for all glucagon doses tested. Glucagon clearance did not differ between glucagon doses studied in both groups. Therefore, the glucagon controller of a dual hormone control system may not need to adjust glucagon sensitivity, and hence glucagon dosing, based on glucose concentrations during euglycemia and hypoglycemia.

Novel PEGylated basal insulin LY2605541 has a preferential hepatic effect on glucose metabolism

The impact of the novel basal insulin LY2605541 (LY) on hepatic and nonhepatic glucose uptake (non-HGU) was evaluated. Conscious dogs underwent euglycemic clamps with tracer and hepatic balance measurements. Clamp period infusions were peripheral venous regular insulin (0.1 nmol ⋅ kg(-1) ⋅ h(-1) [control], n = 6) or LY (bolus [nmol/kg], continuous [nmol ⋅ kg(-1) ⋅ h(-1)]: 0.5, 0.5 [n = 6]; 0.375, 0.375 [n = 5]; 0.25, 0.25 [n = 4]), somatostatin, and glucose, as well as intraportal glucagon (basal). During the clamp, the dogs switched from net hepatic glucose output to uptake (rates reached 2.1 ± 1.2, 0.9 ± 2.1, 8.6 ± 2.3, and 6.0 ± 1.1 µmol ⋅ kg(-1) ⋅ min(-1) within 5 h in control, LY0.25, LY0.375, and LY0.5, respectively). Non-HGU in LY increased less than in control; the ratio of change from basal in non-HGU to change in net hepatic glucose balance, calculated when glucose infusion rates (GIRs) were ~20 µmol ⋅ kg(-1) ⋅ min(-1) in all groups, was higher in control (1.17 ± 0.38) versus LY0.25 (0.39 ± 0.33), LY0.375 (-0.01 ± 0.13), and LY0.5 (-0.09 ± 0.07). Likewise, the change from baseline in glucose Rd-to-Ra ratio was greatest in control (1.4 ± 0.3 vs. 0.6 ± 0.4, 0.5 ± 0.2, and 0.6 ± 0.2 in LY0.25, LY0.375, and LY0.5, respectively). In contrast to exogenously administered human insulin, LY demonstrated preferential hepatic effects, similar to endogenously secreted insulin. Therefore, the analog might reduce complications associated with current insulin therapy.

A novel antihypoglycemic role of inducible nitric oxide synthase in liver inflammatory response induced by dietary cholesterol and endotoxemia

AIMS

The current study aim was to elucidate the antihypoglycemic role and mechanism of inducible nitric oxide synthase (iNOS) under inflammatory stress.

METHODS

Liver inflammatory stress was induced in wild-type (WT) and iNOS-knockout (iNOS(-/-)) mice by lipopolysaccharide (LPS) (5 mg/kg) with and without the background of nonalcoholic steatohepatitis (NASH)-Induced by high cholesterol diet (HCD, 6 weeks).

RESULTS

HCD led to steatohepatitis in WT and iNOS(-/-) mice. LPS administration caused marked liver inflammatory damage only in cholesterol-fed mice, which was further exacerbated in the absence of iNOS. Glucose homeostasis was significantly impaired and included fatal hypoglycemia and inhibition of glycogen decomposition. In iNOS(-/-) hypoxia-inducible factor-1 (HIF1), signaling was impaired compared to control WT. Using hydrodynamic gene transfer method HIF1α was expressed in the livers of iNOS(-/-) mice, and significantly ameliorated cholesterol and LPS-induced liver damage. WT mice overexpressing HIF1α exhibited higher blood glucose levels and lower glycogen contents after LPS injection. Conversely, induction of HIF1α was not effective in preventing LPS-induced glucose lowering effect in iNOS(-/-) mice. The critical role of NO signaling in hepatocytes glucose output mediated by HIF1 pathway was also confirmed in vitro. Results also demonstrated increased oxidative stress and reduced heme oxygenase-1 mRNA in the livers of iNOS(-/-) mice. Furthermore, the amounts of plasma tumor necrosis factor-α (TNFα) and intrahepatic TNFα mRNA were significantly elevated in the absence of iNOS.

INNOVATION AND CONCLUSION

These data highlight the essential role of iNOS in the glycemic response to LPS in NASH conditions and argues for the beneficial effects of iNOS.

Increased plasma β-hydroxybutyrate levels during the fasting test in patients with endogenous hyperinsulinaemic hypoglycaemia

OBJECTIVE

The objective of the present study was to determine whether a plasma β-hydroxybutyrate (BOHB) level >2700 μmol/l during the 72-h fasting test is sufficient to rule out the diagnosis of endogenous hyperinsulinaemic hypoglycaemia (EHH).

RESEARCH DESIGN AND METHODS

We retrospectively studied BOHB levels in 39 patients with EHH who had undergone a 72-H fasting test to make the diagnosis of EHH, and we compared EHH patients with BOHB levels 2700 MOL/L (group 1), EHH PATIENTS with BOHB levels 2700 MOL/L (group 2) and 59 controls (median glycaemia: 3.2  mmol/l and median BOHB: 6095 μmol/l).

RESULTS

During a 72-h fasting test, nine patients (group 1) had BOHB levels >2700  μmol/l (median 6140 and range 2957-7824) and 30 patients (group 2) had BOHB levels <2700 μmol/l (median 542 and range 0-2607). In group 1, four patients had undergone partial pancreatectomy previously and were evaluated for the recurrence of hypoglycaemia, whereas none of the group 2 patients had been operated. The duration of the fasting test was longer in group 1 than in group 2 (P<0.0001), and at the end of the fasting test, plasma glucose concentrations were not significantly different (P=0.0617), but insulin (P=0.004), C-peptide (P=0.0015) and proinsulin (P=0.0038) levels were significantly lower in group 1 patients than in group 2 patients, suggesting lower insulin secretion and/or impaired glycaemic counter-regulation.

CONCLUSION

During a fasting test, a BOHB level >2700 μmol/l is observed in some EHH patients, suggesting that BOHB levels cannot rule out the recurrence of EHH, in particular, after partial pancreatectomy.

Emerging role of AMP-activated protein kinase in endocrine control of metabolism in the liver

This review summarizes the emerging role of AMP-activated protein kinase (AMPK) in mediating endocrine regulation of metabolic fluxes in the liver. There are a number of hormones which, when acting on the liver, alter AMPK activation. Here we describe those hormones associated with activation and de-activation of AMPK and the potential mechanisms for changes in AMPK activation state. The actions of these hormones, in many cases, are consistent with downstream effects of AMPK signaling thus strengthening the circumstantial case for AMPK-mediated hormone action. In recent years, genetic mouse models have also been used in an attempt to establish the role of AMPK in hormone-stimulated metabolism in the liver. Few experiments have, however, firmly established a causal relationship between hormone action at the liver and AMPK signaling.

Ectopic expression of glucagon receptor in skeletal muscles improves glucose homeostasis in a mouse model of diabetes

AIMS/HYPOTHESIS

Excessive secretion of glucagon partially contributes to the development of diabetic hyperglycaemia. However, complete blocking of glucagon action will lead to adverse effects, since glucagon exerts certain beneficial effects via its receptor in many organs. We aimed to study the effects of a 'decoy receptor' for circulating glucagon on modulating beta cell function and glucose homeostasis in mice by over-producing the glucagon receptor (GCGR) in skeletal muscles.

METHODS

We generated transgenic mice in which the expression of Gcgr is driven by the muscle specific creatine kinase (Mck) promoter, and assessed the effects of glucagon on the modulation of glucose homeostasis under conditions of extremes of glucose influx or efflux.

RESULTS

Mck/Gcgr mice showed increased circulating levels of glucagon and insulin, resulting in an unchanged ratio of glucagon-to-insulin. The levels of hepatic glucose-6-phosphatase (G6PC) and fructose-1,6-bisphosphatase (F1,6P2ase) were significantly decreased, whereas the phosphorylation level of pancreatic cAMP-response-element-binding-protein (CREB) was significantly increased in these transgenic mice. Under basal conditions, the mice displayed normal blood glucose levels and unchanged glucose tolerance and insulin sensitivity when compared with their age-matched wild-type (WT) littermates. However, following multiple low-dose streptozotocin injections, Mck/Gcgr mice exhibited a delay in the onset of hyperglycaemia compared with the WT controls. This was associated with preserved beta cell mass and beta cell secretory capacity in response to glucose challenge.

CONCLUSIONS/INTERPRETATION

We suggest that mild and chronic hyperglucagonaemia, through a strategy involving neutralising peripheral glucagon action, provides beneficial effects on beta cell function and glucose homeostasis. Mck/Gcgr mice thus represent a novel mouse model for studying the physiological effects of glucagon.

Safe glycemic management during closed-loop treatment of type 1 diabetes: the role of glucagon, use of multiple sensors, and compensation for stress hyperglycemia

Patients with type 1 diabetes mellitus (T1DM) must make frequent decisions and lifestyle adjustments in order to manage their disorder. Automated treatment would reduce the need for these self-management decisions and reduce the risk for long-term complications. Investigators in the field of closed-loop glycemic control systems are now moving from inpatient to outpatient testing of such systems. As outpatient systems are developed, the element of safety increases in importance. One such concern is the risk for hypoglycemia, due in part to the delayed onset and prolonged action duration of currently available subcutaneous insulin preparations. We found that, as compared to an insulin-only closed-loop system, a system that also delivers glucagon when needed led to substantially less hypoglycemia. Though the capability of glucagon delivery would mandate the need for a second hormone chamber, glucagon in small doses is tolerated very well. People with T1DM often develop hyperglycemia from emotional stress or medical stress. Automated closed-loop systems should be able to detect such changes in insulin sensitivity and adapt insulin delivery accordingly. We recently verified the adaptability of a model-based closed-loop system in which the gain factors that govern a proportional-integral-derivative-like system are adjusted according to frequently measured insulin sensitivity. Automated systems can be tested by physical exercise to increase glucose uptake and insulin sensitivity or by administering corticosteroids to reduce insulin sensitivity. Another source of risk in closed-loop systems is suboptimal performance of amperometric glucose sensors. Inaccuracy can result from calibration error, biofouling, and current drift. We found that concurrent use of more than one sensor typically leads to better sensor accuracy than use of a single sensor. For example, using the average of two sensors substantially reduces the proportion of large sensor errors. The use of more than two allows the use of voting algorithms, which can temporarily exclude a sensor whose signal is outlying. Elements such as the use of glucagon to minimize hypoglycemia, adaptation to changes in insulin sensitivity, and sensor redundancy will likely increase safety during outpatient use of closed-loop glycemic control systems.

Physiologic action of glucagon on liver glucose metabolism

Glucagon is a primary regulator of hepatic glucose production (HGP) in vivo during fasting, exercise and hypoglycaemia. Glucagon also plays a role in limiting hepatic glucose uptake and producing the hyperglycaemic phenotype associated with insulin deficiency and insulin resistance. In response to a physiological rise in glucagon, HGP is rapidly stimulated. This increase in HGP is entirely attributable to an enhancement of glycogenolysis, with little to no acute effect on gluconeogenesis. This dramatic rise in glycogenolysis in response to hyperglucagonemia wanes with time. A component of this waning effect is known to be independent of hyperglycemia, though the molecular basis for this tachyphylaxis is not fully understood. In the overnight fasted state, the presence of basal glucagon secretion is essential in countering the suppressive effects of basal insulin, resulting in the maintenance of appropriate levels of glycogenolysis, fasting HGP and blood glucose. The enhancement of glycogenolysis in response to elevated glucagon is critical in the life-preserving counterregulatory response to hypoglycaemia, as well as a key factor in providing adequate circulating glucose for working muscle during exercise. Finally, glucagon has a key role in promoting the catabolic consequences associated with states of deficient insulin action, which supports the therapeutic potential in developing glucagon receptor antagonists or inhibitors of glucagon secretion.