Glucagon Secretion In Response Research Articles

α- or β-Adrenergic blockade does not affect transplanted islet cell responses to hypoglycemia in type 1 diabetes.

Type 1 diabetes recipients of intrahepatic islet transplantation exhibit glucose-dependent suppression of insulin and activation of glucagon secretion in response to insulin-induced hypoglycemia associated with clinical protection from hypoglycemia. Whether sympathetic activation of adrenergic receptors on transplanted islets is required for these responses in defense against hypoglycemia is not known. To evaluate the adrenergic contribution to post-transplant glucose counterregulation, we performed a randomized, double-blind crossover study of responses during a hyperinsulinemic euglycemic-hypoglycemic clamp under phentolamine (α-adrenergic blockage), propranolol (β-adrenergic blockage), or placebo infusion. Participants (5 female/4 male) were median (range) age 53 (34-63) years, diabetes duration 29 (18-56) years, post-transplant 7.0 (1.9-8.4) years, HbA1c 5.8 (4.5-6.8)%, insulin in-/dependent 5/4, all on tacrolimus-based immunosuppression. During the clamp, blood pressure was lower with phentolamine and heart rate lower with propranolol vs. placebo (P <0.05). There was no difference in suppression of endogenous insulin secretion (derived from C-peptide measurements) during the euglycemic or hypoglycemic phases, and while levels of glucagon were similar with phentolamine or propranolol vs. placebo, the increase in glucagon from eu- to hypoglycemia was greater with propranolol vs. placebo (P < 0.05). Pancreatic polypeptide was greater with phentolamine vs. placebo during the euglycemic phase (P < 0.05), and free fatty acids were lower and the glucose infusion rate higher with propranolol vs. placebo during the hypoglycemic phase (P < 0.05). These results indicate that neither physiologic α- nor β-adrenergic blockade attenuates transplanted islet responses to hypoglycemia, suggesting sympathetic re-innervation of the islet graft is not necessary for post-transplant glucose counterregulation.

Phenotypic Characterization of Congenital Hyperinsulinism Due to Novel Activating Glucokinase Mutations.

Our evaluation of six novel and two previously published activating GCK mutations revealed that the detrimental effects of these mutations on glucose homeostasis can be attributed not only to a lowering of the glucose threshold of insulin secretion but also to a decreased counterregulatory glucagon secretory response. These studies provide insights into the pathophysiology of GCK-hyperinsulinism and the dual role of glucokinase in β-cells and α-cells to regulate glucose homeostasis.

α-cell electrophysiology and the regulation of glucagon secretion.

Glucagon is the principal glucose-elevating hormone that forms the first-line defence against hypoglycaemia. Along with insulin, glucagon also plays a key role in maintaining systemic glucose homeostasis. The cells that secrete glucagon, pancreatic α-cells, are electrically excitable cells and use electrical activity to couple its hormone secretion to changes in ambient glucose levels. Exactly how glucose regulates α-cells has been a topic of debate for decades but it is clear that electrical signals generated by the cells play an important role in glucagon secretory response. Decades of studies have already revealed the key players involved in the generation of these electrical signals and possible mechanisms controlling them to tune glucagon release. This has offered the opportunity to fully understand the enigmatic α-cell physiology. In this review, we describe the current knowledge on cellular electrophysiology and factors regulating excitability, glucose sensing, and glucagon secretion. We also discuss α-cell pathophysiology and the perspective of addressing glucagon secretory defects in diabetes for developing better diabetes treatment, which bears the hope of eliminating hypoglycaemia as a clinical problem in diabetes care.

Intercellular contacts affect secretion and biosynthesis of pancreatic islet cells.

Cell protein biosynthesis is regulated by different factors, but implication of intercellular contacts on alpha and beta cell protein biosyntheses activity has not been yet investigated. Islet cell biosynthetic activity is essential in regulating not only the hormonal reserve within cells but also in renewing all the proteins involved in the control of secretion. Here we aimed to assess whether intercellular interactions affected similarly secretion and protein biosynthesis of rat alpha and beta cells. Insulin and glucagon secretion were analyzed by ELISA or reverse hemolytic plaque assay, and protein biosynthesis evaluated at single cell level using bioorthogonal noncanonical amino acid tagging. Regarding beta cells, we showed a positive correlation between insulin secretion and protein biosynthesis. We also observed that homologous contacts increased both activities at low or moderate glucose concentrations. By contrast, at high glucose concentration, homologous contacts increased insulin secretion and not protein biosynthesis. In addition, heterogeneous contacts between beta and alpha cells had no impact on insulin secretion and protein biosynthesis. Regarding alpha cells, we showed that when they were in contact with beta cells, they increased their glucagon secretion in response to a drop of glucose concentration, but, on the other hand, they decreased their protein biosynthesis under any glucose concentrations. Altogether, these results emphasize the role of intercellular contacts on the function of islet cells, showing that intercellular contacts increased protein biosynthesis in beta cells, except at high glucose, and decreased protein biosynthesis in alpha cells even when glucagon secretion is stimulated.

LBSUN331 Rescuing Alpha Cell Responses To Hypoglycemia In Type 1 Diabetes

Abstract Increased glucagon secretion from the pancreatic alpha cell is the first and most important defense against hypoglycemia. In type 1 diabetes this defensive mechanism is lost, increasing the mortality risk. In addition to the reduced glucagon response to hypoglycemia in T1D, basal glucagon secretion is elevated further stimulating hyperglycemia and desensitizing autocrine signaling pathways. Furthermore, alpha cells lose their ability to respond appropriately to rising glucose levels. In healthy individuals, rising glucose levels lead to increased insulin secretion, resulting in inhibition of glucagon secretion. In T1D however, the loss of beta cells results in the absence of these paracrine signals that mediate glucagon suppression under hyperglycemic conditions. We tested the hypothesis that reactivating paracrine and autocrine signaling pathways pharmacologically reverses alpha cell glucose blindness. We used living pancreas slices, which allow functional assessments of damaged and infiltrated islets within their native environment. We studied tissues from non-diabetic donors and donors with type 1 diabetes to determine alpha cell responses to (a) changes in glycemia, (b) agonists, antagonists, and positive allosteric modulators of autocrine and paracrine signaling pathways, and (c) reference stimuli such as adrenaline and KCl depolarization using functional recordings. We further performed in vivo studies using mouse models with defective glucose counterregulation to determine whether alpha cell responses to hypoglycemia could be restored. We found that human alpha cells in slices from type 1 diabetic donors had normal glucagon content and responded to KCl depolarization but failed to respond to decreases in glucose concentration. Furthermore, we found severely diminished Ca2+ responses to both lowering in glucose concentration and glutamate receptor stimulation. Our in vitro results not only reproduced important features seen in vivo but also revealed mechanistic defects related to autocrine and paracrine inputs. Importantly, reestablishing these inputs pharmacologically rescued glucagon secretion in response to a lowering in glucose concentration in human tissue slices/islets from donors with T1D as well as to hypoglycemia in the nonobese diabetic (NOD) mouse model. Several of these signaling molecules are used in other medical indications and could be repurposed to develop therapies aimed at preventing hypoglycemia in T1D. Presentation: Sunday, June 12, 2022 12:30 p.m. - 2:30 p.m.

255-LB: Defining Functional Phenotype in Human Islets across Endocrine Cell Composition, Donor Age, Sex, and Ancestry

The Integrated Islet Distribution Program (IIDP) is currently the largest source of human islets for research in the U.S. The IIDP Human Islet Phenotyping Program (HIPP) provides standardized islet assessment and makes these data available to researchers. Since 2016, HIPP has phenotyped 337 islet samples from nondiabetic organ donors (40%F / 60%M, 45±12 years, BMI 29.0±5.4; 52% European, 26% Latino, 8% African, 3% Asian, and 11% other ancestry) . To define the islet structure-function relationships, we integrated physiological profiling of hormone secretion with islet cell composition measured by immunohistochemistry­. We found that % β cells varied significantly (range 34 - 92%; CV 21%) and had a strong negative correlation with % α (range 3 - 59%; CV 33%; r -0.95; p&amp;lt;1.78E-163) and % δ cells (range 1 - 19%; CV 44%; r -0.32, p=2.69E-09) . This variation did not impact basal insulin secretion at 5.6 mM glucose, but insulin secretory responses stimulated by high 16.7 mM glucose, cAMP-evoked potentiation, and KCl-mediated depolarization, were all negatively correlated with % α and δ cells. By contrast, glucagon response to basal 5.6 mM glucose had the strongest negative correlation with % β cells (r -0.31, p=4.25E-09) , but not % δ cells (r 0.03, p=0.63) . Furthermore, the β cell composition did not correlate with age or BMI, but was higher in males vs. females (59.1±12.2% vs. 55.7±11.0%, p=0.0107) and in islets from donors of Asian ancestry (71.2±14.7%, p&amp;lt;0.01) . Finally, we noted that insulin response to KCl-mediated depolarization was significantly lower in islets from donors of African vs. European or Asian ancestry (8.3±4.0 vs. 16.5±8.8 and 13.0±7.0ng/IEQ; p&amp;lt;0.01) . Thus, these data highlight the importance of endocrine cell composition in islet hormone secretion possibly influencing β-α cell paracrine interactions and setpoint of glucagon secretion in response to low glucose. In addition, they suggest that β cell composition and insulin secretion are influenced by sex and ancestry. Disclosure M. Brissova: None. G. Poffenberger: None. D. C. Saunders: None. A. C. Powers: None. J. C. Niland: None. C. Evans-molina: Advisory Panel; Avotres Inc., DiogenX, Isla Technologies, MaiCell Therapeutics, Provention Bio, Inc., Other Relationship; Astellas Pharma Inc., Dompé, Lilly, Research Support; BMS, Nimbus Therapeutics. H. H. Durai: None. S. Mei: None. A. Coldren: None. C. Davis: None. C. Reihsmann: None. A. L. Hopkirk: None. D. Gibson: None. R. Aramandla: None.

149-OR: Genetic Modulation of HNF1A Activity via SGLT2 Deficiency Leads to Transient Intermittent Hyperglycemia: Consequences for HNF1A-MODY

Hepatocyte Nuclear Factor-1A (HNF1A) is a master regulator of key glucose-responsive genes in metabolic organs such as the intestine, liver, kidney, and pancreas. Heterozygous mutations in HNF1A cause Maturity-Onset-Diabetes-of-the-Young (HNF1A-MODY) . Mutant carriers are normoglycemic in childhood, but with age, they develop hyperglycemia. Although insulin secretory defects have been extensively studied, it is unclear whether HNF1A deficiency contributes to a progressive decline in alpha cell function. HNF1A-MODY patients display glycosuria and endogenous glucose production linked to reduced renal SGLT2 expression. Since SGLT2 is also expressed in alpha cells and is required for glucagon regulation, we hypothesized that HNF1A coordinates renal and alpha cell SGLT2 activity to maintain glucose homeostasis. Here, we studied Hnf1a regulation of renal Sglt2, Sglt1, and Glut2 in Hnf1a-/- mutant mice, glucagon expression and secretion in siHNF1A transfected human islets and we developed a novel Hnf1a mutant mouse model (Hnf1a+/Δe4-10) . Sglt2 protein was significantly reduced in the kidney of Hnf1a-/- mice, with no changes in Sglt1 or Glut2 protein expression. We observed that HNF1A is heterogeneously expressed in alpha, beta, and delta cells. siHNF1A islets displayed reduced insulin secretion and content, which was rescued by GLP1 treatment. HNF1A deficiency reduced SGLT2 levels, correlating with increased glucagon content and secretion. The Hnf1a+/Δe4-mice were born normoglycemic but became glucose intolerant at 12 weeks of age, mimicking the clinical characteristics of HNF1A-MODY patients. Collectively, these findings indicate that HNF1A deficiency downregulates renal and alpha cell SGLT2 expression, which is associated with an alteration in glucagon secretion in response to glucose stimulation. The new Hnf1a+/Δe4-mouse model may be a useful tool to study the progression of MODY with and without chronic drug treatment. Disclosure A.Acosta-montalvo: None. V.Gmyr: None. M.Cnop: None. J.A.Kerr-conte: None. F.Pattou: None. M.Pontoglio: None. A.Liston: None. C.Bonner: None. C.Saponaro: None. J.Thevenet: None. M.Chiral: None. A.Piron: None. N.Delalleau: None. G.Pasquetti: None. A.Coddeville: None. M.Moreno: None. Funding EFSD/Lilly (2017) awarded to Caroline Bonner, I-SITE ULNE awarded to Caroline Bonner, the European Genomic Institute for Diabetes (ANR-10-LABX-46) , the European Consortium for Islet Transplantation funded by the Juvenile Diabetes Research Foundation. TIGER was developed within the European Union’s Horizon 2020 research and innovation program project T2DSystems, under grant agreement 667191 (to Miriam Cnop) .

Hypoglycemia: Control of counter‐regulatory hormone secretion by the brain

Hypoglycaemia is a common side‐effect of the treatment of type 1 diabetes with insulin. The Diabetes Control and Complications Trial indicated that, while intensive insulin therapy reduced the incidence and severity of diabetic complications such as retinopathy, neuropathy and nephropathy, the incidence of severe hypoglycaemia was increased 2‐3 fold. In non‐diabetic individuals, hypoglycaemia triggers the glucose counter‐regulatory response. This response consists of inhibition of insulin secretion and activation of glucagon secretion. Prolonged hypoglycaemia also evokes adrenaline secretion and less importantly, growth hormone and cortisol secretion. However, in individuals that have type 1 diabetes, the glucagon response to hypoglycaemia is lost. In these patients, the pancreatic α‐cells continue to synthesise glucagon since glucagon secretagogues such as arginine produce a robust glucagon secretory response. Our research has examined the hypothalamic and medullary neurocircuits that mediate adrenaline and glucagon secretion under conditions of neuroglucoprivation or hypoglycaemia. These studies have shown that the response to neuroglucoprivation involves specialised sympathetic premotor neurons in the rostral ventrolateral medulla (RVLM) that mediate adrenaline secretion. These neurons are readily distinguishable from sympathetic vasomotorneurons also located in the RVLM. In addition, we have described the properties of perifornical hypothalamic (PeH) orexinergic (Ox) neurons that are activated by insulin‐induced hypoglycaemia (IHH) or neuroglucoprivation. PeH neurons project to many different targets including the RVLM and the dorsal motor nucleus of the vagus (DMV). Ox2‐R in the RVLM mediate the adrenaline secretory response. PeH neurons also mediate the glucagon response to IHH since the release of glucagon in response to IHH is diminished in Ataxin3 rats which have an age‐related loss of PeH orexin neurons.

E4orf1-induced reduction in endogenous insulin level is independent of pancreas endocrine function.

Obesity is often associated with hyperinsulinemia due to insulin resistance. In mice models of hyperinsulinemia, adenovirus-derived E4orf1 protein promotes glucose disposal via insulin-independent pathway, and reduces insulin response to glucose load, described as its "Insulin Sparing Action". This is likely because less insulin is needed for disposing glucose in presence of E4orf1, however, there are other potential possibilities. This study determined if E4orf1 reduces insulin response to glucose load because it a) suppresses the ability of pancreatic β-cells to secret insulin, or b) upregulates glucagon production by the pancreas. C57BL/6J wild type (control) and transgenic C57BL/6J (E4orf1) mice that express E4orf1 protein in adipose tissue upon doxycycline feeding, were used. Post-doxycycline feeding, insulin and glucagon secretion in response to glibenclamide or phenylephrine were compared between the two groups. The pancreases were examined for histological changes. In response to glibenclamide, E4orf1 mice secreted more insulin and exhibited lower blood glucose compared to control (47.4 ± 4.4 vs 27.4 ± 3.7 mg/dl, p < 0.003), but showed no difference in glucagon secretion. Post-phenylephrine injection, no differences were observed between the two groups for glucagon or insulin, except E4orf1 mice had a lower blood glucose rise after 10-min of injection compared to the control (39.7 ± 4.7 vs. 58.3 ± 7.5 mg/dl, p < 0.05). E4orf1 mice had significantly larger pancreatic islets and higher number of islets per mm2 tissue area. Neither the size nor the number of islets met the criteria of hypertrophy or hyperplasia. E4orf1 retains and may enhance the ability of the pancreases to secret insulin in response to insulin secretagogue. Glucagon does not seem to play a role in the Insulin Sparing Action of E4orf1. Overall, the histology studies support better pancreatic islet health in presence of E4orf1, compared to that in control mice. The "insulin-independent" role of E4orf1 has potential therapeutic implications in addressing hyperinsulinemia in obesity.

Glucose metabolism in amyotrophic lateral sclerosis: it is bitter-sweet.

Glucose metabolism in amyotrophic lateral sclerosis: it is bitter-sweet.

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.

GRK2 contributes to glucose mediated calcium responses and insulin secretion in pancreatic islet cells

Diabetes is a metabolic syndrome rooted in impaired insulin and/or glucagon secretory responses within the pancreatic islets of Langerhans (islets). Insulin secretion is primarily regulated by two key factors: glucose-mediated ATP production and G-protein coupled receptors (GPCRs) signaling. GPCR kinase 2 (GRK2), a key regulator of GPCRs, is reported to be downregulated in the pancreas of spontaneously obesogenic and diabetogenic mice (ob/ob). Moreover, recent studies have shown that GRK2 non-canonically localizes to the cardiac mitochondrion, where it can contribute to glucose metabolism. Thus, islet GRK2 may impact insulin secretion through either mechanism. Utilizing Min6 cells, a pancreatic ß-cell model, we knocked down GRK2 and measured glucose-mediated intracellular calcium responses and insulin secretion. Silencing of GRK2 attenuated calcium responses, which were rescued by pertussis toxin pre-treatment, suggesting a Gαi/o-dependent mechanism. Pancreatic deletion of GRK2 in mice resulted in glucose intolerance with diminished insulin secretion. These differences were due to diminished insulin release rather than decreased insulin content or gross differences in islet architecture. Furthermore, a high fat diet feeding regimen exacerbated the metabolic phenotype in this model. These results suggest a new role for pancreatic islet GRK2 in glucose-mediated insulin responses that is relevant to type 2 diabetes disease progression.

Functional Heterogeneity Among Pancreatic Alpha Cells

Historically, endocrine cells in the pancreatic islets have been assumed to function as relatively homogenous populations largely because we lacked the ability to measure individual cell activity with sufficient throughput to reliably detect heterogeneity within each population. The glucagon-secreting alpha cells play a vital role in regulating glycemia, but the mechanisms that control alpha cell activity and whether the alpha cells behave as a single unit or heterogeneously remain incompletely understood. To overcome the limitations in throughput that have to date prevented the study of alpha cells at the population level, we used genetically-encoded fluorescent indicators selectively expressed in alpha cells. Imaging intact mouse islets with these indicators in 3D responding to treatments in real time yields hundreds of individual alpha cell recordings per experiment. Calcium imaging showed reproducible heterogeneous responses to a panel of known physiological potentiators of glucagon secretion such as arginine vasopressin, epinephrine, and amino acids. Separate dose response experiments revealed that the proportion of alpha cells responding to each signal plateaus at different proportions of alpha cells. The calcium data correlate both with direct glucagon secretion levels as well as cAMP measurement. Our findings highlight previously unappreciated levels of functional heterogeneity among alpha cells and demonstrate that alpha cells are not a single uniform unit. Our observations suggest that dose-dependent increases in glucagon secretion in response to different physiological cues may be the result of mobilizing progressively larger proportions of the total alpha cell mass. We hypothesize that this functional heterogeneity is a built-in mechanism through which different physiological cues elicit graded glucagon responses from the alpha cells.

Evidence for Glucagon Secretion and Function Within the Human Gut.

Glucagon is secreted by pancreatic α cells in response to hypoglycemia and increases hepatic glucose output through hepatic glucagon receptors (GCGRs). There is evidence supporting the notion of extrapancreatic glucagon but its source and physiological functions remain elusive. Intestinal tissue samples were obtained from patients undergoing surgical resection of cancer. Mass spectrometry analysis was used to detect glucagon from mucosal lysate. Static incubations of mucosal tissue were performed to assess glucagon secretory response. Glucagon concentration was quantitated using a highly specific sandwich enzyme-linked immunosorbent assay. A cholesterol uptake assay and an isolated murine colonic motility assay were used to assess the physiological functions of intestinal GCGRs. Fully processed glucagon was detected by mass spectrometry in human intestinal mucosal lysate. High glucose evoked significant glucagon secretion from human ileal tissue independent of sodium glucose cotransporter and KATP channels, contrasting glucose-induced glucagon-like peptide 1 (GLP-1) secretion. The GLP-1 receptor agonist Exendin-4 attenuated glucose-induced glucagon secretion from the human ileum. GCGR blockade significantly increased cholesterol uptake in human ileal crypt culture and markedly slowed ex vivo colonic motility. Our findings describe the human gut as a potential source of extrapancreatic glucagon and demonstrate a novel enteric glucagon/GCGR circuit with important physiological functions beyond glycemic regulation.

Pancreatic Islet Composition Affects Hormone Secretion in Isolated Alpha and Beta cells

Background/Objective: &#x0D; The Integrated Islet Distribution Program (IIDP) distributes islets from five isolation Centers and serves as the main source of human islets for research in the U.S. In 2016, the IIDP initiated the Human Islet Phenotyping Program (HIPP), which provides standardized post-shipment assessment of islet hormone secretion and endocrine cell composition for each IIDP-supported islet isolation. To date, islets from 276 non-diabetic donors have been analyzed. We hypothesized that analysis of this unique resource will provide novel insights into how demographic and clinical features impact islet health. &#x0D; &#x0D; Methods: &#x0D; Relationships between insulin and glucagon secretion assessed by perifusion and islet composition (% of β- and α-cells) were analyzed using SAS Version 9.4. For each analysis, the isolation center was used as a covariate. &#x0D; &#x0D; Results: &#x0D; Of the 276 donors, 60% were male; 59% of donors were Caucasian, 28% were Hispanic, 9% were African-American; 4% were Asian; and 0.36% were American Indian. The % of β-cells was moderately correlated with insulin responses to 16.7 mM glucose (r=0.2785; p&lt;0.0001) and 20 mM KCL (r=0.3109, p&lt;0.0001). Similarly, the α-cell% was moderately correlated with total glucagon content (r=0.3362, p&lt;0.0001) and glucagon responses to 1.7 mM glucose + 1mM epinephrine (r=0.2015, p=0.0001). The % of β -cells was negatively correlated with glucagon total content (r=-.243,p=.0001), while the α-cell% was negatively correlated with insulin stimulation index to KCL (r=-0.2573, p&lt;0.0001). Notably, Asian donors exhibited a significantly higher β-cell% compared to other groups (p&lt;0.05). Consistent with this, glucose-stimulated insulin secretion was higher in Asian donors compared to responses observed in islets from African American donors (p&lt;0.01). &#x0D; &#x0D; Conclusion: &#x0D; These data indicate that islet cell composition influences insulin and glucagon secretory responses and suggests that race may impact islet composition and hormone secretion. Continued analysis of the HIPP dataset may aid in our understanding of risk factors for the development islet dysfunction in diabetes.

359-OR: a- or ß-Adrenergic Blockade Does Not Affect Transplanted Islet Responses to Hypoglycemia in Type 1 Diabetes (T1D)

Recipients of intrahepatic islet transplantation for T1D exhibit appropriate suppression of endogenous insulin and activation of glucagon secretion in response to insulin-induced hypoglycemia with restored glucose counterregulation and protection against clinically significant hypoglycemia. Whether sympathetic activation of adrenergic receptors on transplanted islets is required for these responses is not known. To evaluate the adrenergic contribution to post-transplant glucose counterregulation, we performed a randomized, double-blind crossover study of responses during a hyperinsulinemic (1 mU·kg-1·min-1) euglycemic (EU, ∼90 mg/dL)-hypoglycemic (HYPO, ∼50 mg/dL) clamp conducted under phentolamine (0.95 µg·kg-1·min-1; α-adrenergic blockage), propranolol (0.48 µg·kg-1·min-1; β-adrenergic blockage), or placebo infusion. Subjects were 5F/4M with median (range) age 53 (34 - 63) years, T1D duration 29 (18 - 56) years, post-transplant 7 (2 - 8) years, HbA1c 5.8 (4.5 - 6.8) %, insulin in-/dependent 5/4, on tacrolimus-based immunosuppression, and spent 97 (76 - 99) % time in range 70 - 180 mg/dL and 1 (0 - 3) % time with hypoglycemia &amp;lt; 70 mg/dL by CGM. During the EU-HYPO clamp, blood pressure was lower with phentolamine and heart rate lower with propranolol vs. placebo (P &amp;lt; 0.05). There was no difference in suppression of C-peptide or activation of glucagon with phentolamine or propranolol vs. placebo. Pancreatic polypeptide was greater with phentolamine vs. placebo during EU (P &amp;lt; 0.05), and free fatty acids were lower and the glucose infusion rate higher with propranolol vs. placebo during HYPO (P &amp;lt; 0.05) with no differences seen for endogenous glucose production. These results indicate physiologic α- and β-adrenergic blockage had no effect on transplanted islet responses to hypoglycemia. Sympathetic re-innervation, as occurs with re-vascularization of intrahepatic islets, may not be necessary for post-transplant recovery of glucose counterregulation in T1D. Disclosure M.R. Rickels: Consultant; Self; Semma Therapeutics, Inc. Research Support; Self; Xeris Pharmaceuticals, Inc. M. Bellin: Research Support; Self; Dexcom, Inc., Viacyte, Inc. Other Relationship; Self; Insulet Corporation. D. Stefanovski: None. A.J. Peleckis: None. C.V. Dalton-Bakes: None. E. Markmann: None. H.T. Nguyen: None. A. Naji: None. Funding National Institutes of Health (R01DK091331, UL1TR001878, P30DK19525)

γ-Hydroxybutyrate does not mediate glucose inhibition of glucagon secretion

Hypersecretion of glucagon from pancreatic α-cells strongly contributes to diabetic hyperglycemia. Moreover, failure of α-cells to increase glucagon secretion in response to falling blood glucose concentrations compromises the defense against hypoglycemia, a common complication in diabetes therapy. However, the mechanisms underlying glucose regulation of glucagon secretion are poorly understood and likely involve both α-cell–intrinsic and intraislet paracrine signaling. Among paracrine factors, glucose-stimulated release of the GABA metabolite γ-hydroxybutyric acid (GHB) from pancreatic β-cells might mediate glucose suppression of glucagon release via GHB receptors on α-cells. However, the direct effects of GHB on α-cell signaling and glucagon release have not been investigated. Here, we found that GHB (4–10 μm) lacked effects on the cytoplasmic concentrations of the secretion-regulating messengers Ca2+ and cAMP in mouse α-cells. Glucagon secretion from perifused mouse islets was also unaffected by GHB at both 1 and 7 mm glucose. The GHB receptor agonist 3-chloropropanoic acid and the antagonist NCS-382 had no effects on glucagon secretion and did not affect stimulation of secretion induced by a drop in glucose from 7 to 1 mm. Inhibition of endogenous GHB formation with the GABA transaminase inhibitor vigabatrin also failed to influence glucagon secretion at 1 mm glucose and did not prevent the suppressive effect of 7 mm glucose. In human islets, GHB tended to stimulate glucagon secretion at 1 mm glucose, an effect mimicked by 3-chloropropanoic acid. We conclude that GHB does not mediate the inhibitory effect of glucose on glucagon secretion.

Interindividual Heterogeneity of SGLT2 Expression and Function in Human Pancreatic Islets.

Studies implicating sodium-glucose cotransporter 2 (SGLT2) inhibitors in glucagon secretion by pancreatic α-cells reported controversial results. We hypothesized that interindividual heterogeneity in SGLT2 expression and regulation may affect glucagon secretion by human α-cells in response to SGLT2 inhibitors. An unbiased RNA-sequencing analysis of 207 donors revealed an unprecedented level of heterogeneity of SLC5A2 expression. To determine heterogeneity of SGLT2 expression at the protein level, the anti-SGLT2 antibody was first rigorously evaluated for specificity, followed by Western blot and immunofluorescence analysis on islets from 10 and 12 donors, respectively. The results revealed a high interdonor variability of SGLT2 protein expression. Quantitative analysis of 665 human islets showed a significant SGLT2 protein colocalization with glucagon but not with insulin or somatostatin. Moreover, glucagon secretion by islets from 31 donors at low glucose (1 mmol/L) was also heterogeneous and correlated with dapagliflozin-induced glucagon secretion at 6 mmol/L glucose. Intriguingly, islets from three donors did not secrete glucagon in response to either 1 mmol/L glucose or dapagliflozin, indicating a functional impairment of the islets of these donors to glucose sensing and SGLT2 inhibition. Collectively, these data suggest that heterogeneous expression of SGLT2 protein and variability in glucagon secretory responses contribute to interindividual differences in response to SGLT2 inhibitors.

Somatostatin analogues for the treatment of hyperinsulinaemic hypoglycaemia.

Hyperinsulinaemic hypoglycaemia (HH) is a biochemical finding of low blood glucose levels due to the dysregulation of insulin secretion from pancreatic β-cells. Under normal physiological conditions, glucose metabolism is coupled to β-cell insulin secretion so that blood glucose levels are maintained within the physiological range of 3.5–5.5 mmol/L. However, in HH this coupling of glucose metabolism to insulin secretion is perturbed so that insulin secretion becomes unregulated. HH typically occurs in the neonatal, infancy and childhood periods and can be due to many different causes. Adults can also present with HH but the causes in adults tend to be different. Somatostatin (SST) is a peptide hormone that is released by the delta cells (δ-cells) in the pancreas. It binds to G protein-coupled SST receptors to regulate a variety of location-specific and selective functions such as hormone inhibition, neurotransmission and cell proliferation. SST plays a potent role in the regulation of both insulin and glucagon secretion in response to changes in glucose levels by negative feedback mechanism. The half-life of SST is only 1–3 min due to quick degradation by peptidases in plasma and tissues. Thus, a direct continuous intravenous or subcutaneous infusion is required to achieve the therapeutic effect. These limitations prompted the discovery of SST analogues such as octreotide and lanreotide, which have longer half-lives and therefore can be administered as injections. SST analogues are used to treat different forms of HH in children and adults and therapeutic effect is achieved by suppressing insulin secretion from pancreatic β-cells by complex mechanisms. These treatments are associated with several side effects, especially in the newborn period, with necrotizing enterocolitis being the most serious side effect and hence SS analogues should be used with extreme caution in this age group.

Impaired glucose tolerance, glucagon, and insulin responses in mice lacking the loop diuretic-sensitive Nkcc2a transporter.

The Na+K+2Cl- cotransporter-2 (Nkcc2, Slc12a1) is abundantly expressed in the kidney and its inhibition with the loop-diuretics bumetanide and furosemide has been linked to transient or permanent hyperglycemia in mice and humans. Notably, Slc12a1 is expressed at low levels in hypothalamic neurons and in insulin-secreting β-cells of the endocrine pancreas. The present study was designed to determine if global elimination of one of the Slc12a1 products, i.e., Nkcc2 variant a (Nkcc2a), the main splice version of Nkcc2 found in insulin-secreting β-cells, has an impact on the insulin and glucagon secretory responses and fuel homeostasis in vivo. We have used dynamic tests of glucose homeostasis in wild-type mice and mice lacking both alleles of Nkcc2a (Nkcc2aKO) and assessed their islet secretory responses in vitro. Under basal conditions, Nkcc2aKO mice have impaired glucose homeostasis characterized by increased blood glucose, intolerance to the sugar, delayed/blunted in vivo insulin and glucagon responses to glucose, and increased glycemic responses to the gluconeogenic substrate alanine. Further, we provide evidence of conserved quantitative secretory responses of Nkcc2aKO islets within a context of increased islet size related to hyperplastic/hypertrophic glucagon- and insulin-positive cells (α-cells and β-cells, respectively), normal total islet Cl- content, and reduced β-cell expression of the Cl- extruder Kcc2.