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Hypoglycaemia (low plasma glucose) is a serious and potentially fatal complication of insulin-treated diabetes. In healthy individuals, hypoglycaemia triggers glucagon secretion, which restores normal plasma glucose levels by stimulation of hepatic glucose production. This counterregulatory mechanism is impaired in diabetes. Here we show in mice that therapeutic concentrations of insulin inhibit glucagon secretion by an indirect (paracrine) mechanism mediated by stimulation of intra-islet somatostatin release. Insulin’s capacity to inhibit glucagon secretion is lost following genetic ablation of insulin receptors in the somatostatin-secreting δ-cells, when insulin-induced somatostatin secretion is suppressed by dapagliflozin (an inhibitor of sodium-glucose co-tranporter-2; SGLT2) or when the action of secreted somatostatin is prevented by somatostatin receptor (SSTR) antagonists. Administration of these compounds in vivo antagonises insulin’s hypoglycaemic effect. We extend these data to isolated human islets. We propose that SSTR or SGLT2 antagonists should be considered as adjuncts to insulin in diabetes therapy. Impaired glucagon secretion in patients with diabetes causes hypoglycemia. Here the authors show that therapeutic concentrations of insulin inhibit alpha-cell glucagon secretion by stimulating delta-cell insulin receptor and the release of somatostatin. Blocking somatostatin secretion or action ameliorates this effect.

Metabolic surgery leads to rapid and effective diabetes reversal in humans, by weight-independent mechanisms. The crucial improvement in pancreatic islet function observed after surgery is induced by alteration in several factors, including gut hormones. In addition to glucagon-like peptide 1 (GLP-1), increasing lines of evidence show that peptide tyrosine tyrosine (PYY) plays a key role in the metabolic benefits associated with the surgery, ranging from appetite regulation to amelioration of islet secretory properties and survival. Here, we summarize the current knowledge and the latest advancements in the field, which pitch a strong case for the development of novel PYY-based therapy for the treatment of diabetes.

Strategies aimed at mimicking or enhancing the action of the incretin hormone glucagon-like peptide 1 (GLP-1) therapeutically improve glucose-stimulated insulin secretion (GSIS); however, it is not clear whether GLP-1 directly drives insulin secretion in pancreatic islets. Here, we examined the mechanisms by which GLP-1 stimulates insulin secretion in mouse and human islets. We found that GLP-1 enhances GSIS at a half-maximal effective concentration of 0.4 pM. Moreover, we determined that GLP-1 activates PLC, which increases submembrane diacylglycerol and thereby activates PKC, resulting in membrane depolarization and increased action potential firing and subsequent stimulation of insulin secretion. The depolarizing effect of GLP-1 on electrical activity was mimicked by the PKC activator PMA, occurred without activation of PKA, and persisted in the presence of PKA inhibitors, the KATP channel blocker tolbutamide, and the L-type Ca(2+) channel blocker isradipine; however, depolarization was abolished by lowering extracellular Na(+). The PKC-dependent effect of GLP-1 on membrane potential and electrical activity was mediated by activation of Na(+)-permeable TRPM4 and TRPM5 channels by mobilization of intracellular Ca(2+) from thapsigargin-sensitive Ca(2+) stores. Concordantly, GLP-1 effects were negligible in Trpm4 or Trpm5 KO islets. These data provide important insight into the therapeutic action of GLP-1 and suggest that circulating levels of this hormone directly stimulate insulin secretion by β cells.

Voltage-Gated Ion Channels in Human Pancreatic β-Cells: Electrophysiological Characterization and Role in Insulin Secretion Matthias Braun 1 , Reshma Ramracheya 1 , Martin Bengtsson 1 , Quan Zhang 1 , Jovita Karanauskaite 1 , Chris Partridge 1 , Paul R. Johnson 2 and Patrik Rorsman 1 1 Oxford Centre for Diabetes Endocrinology and Metabolism, University of Oxford, Churchill Hospital, Oxford, U.K 2 Nuffield Department of Surgery, John Radcliffe Hospital, Oxford, U.K Corresponding author: Matthias Braun, Oxford Centre for Diabetes, Endocrinology, and Metabolism, Churchill Hospital, Old Road, Oxford OX37 LJ, U.K. E-mail: matthias.braun{at}drl.ox.ac.uk Abstract OBJECTIVE— To characterize the voltage-gated ion channels in human β-cells from nondiabetic donors and their role in glucose-stimulated insulin release. RESEARCH DESIGN AND METHODS— Insulin release was measured from intact islets. Whole-cell patch-clamp experiments and measurements of cell capacitance were performed on isolated β-cells. The ion channel complement was determined by quantitative PCR. RESULTS— Human β-cells express two types of voltage-gated K + currents that flow through delayed rectifying (K V 2.1/2.2) and large-conductance Ca 2+ -activated K + (BK) channels. Blockade of BK channels (using iberiotoxin) increased action potential amplitude and enhanced insulin secretion by 70%, whereas inhibition of K V 2.1/2.2 (with stromatoxin) was without stimulatory effect on electrical activity and secretion. Voltage-gated tetrodotoxin (TTX)-sensitive Na + currents (Na V 1.6/1.7) contribute to the upstroke of action potentials. Inhibition of Na + currents with TTX reduced glucose-stimulated (6–20 mmol/l) insulin secretion by 55–70%. Human β-cells are equipped with L- (Ca V 1.3), P/Q- (Ca V 2.1), and T- (Ca V 3.2), but not N- or R-type Ca 2+ channels. Blockade of L-type channels abolished glucose-stimulated insulin release, while inhibition of T- and P/Q-type Ca 2+ channels reduced glucose-induced (6 mmol/l) secretion by 60–70%. Membrane potential recordings suggest that L- and T-type Ca 2+ channels participate in action potential generation. Blockade of P/Q-type Ca 2+ channels suppressed exocytosis (measured as an increase in cell capacitance) by >80%, whereas inhibition of L-type Ca 2+ channels only had a minor effect. CONCLUSIONS— Voltage-gated T-type and L-type Ca 2+ channels as well as Na + channels participate in glucose-stimulated electrical activity and insulin secretion. Ca 2+ -activated BK channels are required for rapid membrane repolarization. Exocytosis of insulin-containing granules is principally triggered by Ca 2+ influx through P/Q-type Ca 2+ channels. BK, Ca2+-activated K+ IbTX, iberiotoxin I-V, current-voltage relationship KATP channel, ATP-sensitive K+ channel TEA, tetraethylammonium TTX, tetrodotoxin Footnotes Published ahead of print at http://diabetes.diabetesjournals.org on 17 March 2008. DOI: 10.2337/db07-0991. M.B. and R.R. contributed equally to this article. Additional information for this article can be found in an online appendix at http://dx.doi.org/10.2337/db07-0991 . The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. Accepted March 13, 2008. Received July 17, 2007. DIABETES

Diabetes is characterized by hyperglycaemia due to impaired insulin secretion and aberrant glucagon secretion resulting from changes in pancreatic islet cell function and/or mass. The extent to which hyperglycaemia per se underlies these alterations remains poorly understood. Here we show that β-cell-specific expression of a human activating K ATP channel mutation in adult mice leads to rapid diabetes and marked alterations in islet morphology, ultrastructure and gene expression. Chronic hyperglycaemia is associated with a dramatic reduction in insulin-positive cells and an increase in glucagon-positive cells in islets, without alterations in cell turnover. Furthermore, some β-cells begin expressing glucagon, whilst retaining many β-cell characteristics. Hyperglycaemia, rather than K ATP channel activation, underlies these changes, as they are prevented by insulin therapy and fully reversed by sulphonylureas. Our data suggest that many changes in islet structure and function associated with diabetes are attributable to hyperglycaemia alone and are reversed when blood glucose is normalized. In patients with diabetes, insulin release from pancreatic β-cells is reduced due to altered islet structure and function. Here, Brereton et al . show that elevated blood glucose underlies these changes and is sufficient to reversibly alter β-cell identity in a mouse model of β-cell dysfunction.

Glucagon, secreted from pancreatic islet alpha cells, stimulates gluconeogenesis and liver glycogen breakdown. The mechanism regulating glucagon release is debated, and variously attributed to neuronal control, paracrine control by neighbouring beta cells, or to an intrinsic glucose sensing by the alpha cells themselves. We examined hormone secretion and Ca(2+) responses of alpha and beta cells within intact rodent and human islets. Glucose-dependent suppression of glucagon release persisted when paracrine GABA or Zn(2+) signalling was blocked, but was reversed by low concentrations (1-20 muM) of the ATP-sensitive K(+) (KATP) channel opener diazoxide, which had no effect on insulin release or beta cell responses. This effect was prevented by the KATP channel blocker tolbutamide (100 muM). Higher diazoxide concentrations (>/=30 muM) decreased glucagon and insulin secretion, and alpha- and beta-cell Ca(2+) responses, in parallel. In the absence of glucose, tolbutamide at low concentrations (<1 muM) stimulated glucagon secretion, whereas high concentrations (>10 muM) were inhibitory. In the presence of a maximally inhibitory concentration of tolbutamide (0.5 mM), glucose had no additional suppressive effect. Downstream of the KATP channel, inhibition of voltage-gated Na(+) (TTX) and N-type Ca(2+) channels (omega-conotoxin), but not L-type Ca(2+) channels (nifedipine), prevented glucagon secretion. Both the N-type Ca(2+) channels and alpha-cell exocytosis were inactivated at depolarised membrane potentials. Rodent and human glucagon secretion is regulated by an alpha-cell KATP channel-dependent mechanism. We propose that elevated glucose reduces electrical activity and exocytosis via depolarisation-induced inactivation of ion channels involved in action potential firing and secretion.

Pancreatic islets are complex micro-organs consisting of at least three different cell types: glucagon-secreting alpha, insulin-producing beta and somatostatin-releasing delta cells 1 . Somatostatin is a powerful paracrine inhibitor of insulin and glucagon secretion 2 . In diabetes, increased somatostatinergic signalling leads to defective counter-regulatory glucagon secretion 3 . This increases the risk of severe hypoglycaemia, a dangerous complication of insulin therapy 4 . The regulation of somatostatin secretion involves both intrinsic and paracrine mechanisms 5 but their relative contributions and whether they interact remain unclear. Here we show that dapagliflozin-sensitive glucose- and insulin-dependent sodium uptake stimulates somatostatin secretion by elevating the cytoplasmic Na + concentration (intracellular [Na + ]; [Na + ] i ) and promoting intracellular Ca 2+ -induced Ca 2+ release. This mechanism also becomes activated when [Na + ] i is elevated following the inhibition of the plasmalemmal Na + -K + pump by reductions of the extracellular K + concentration emulating those produced by exogenous insulin in vivo 6 . Islets from some donors with type-2 diabetes hypersecrete somatostatin, leading to suppression of glucagon secretion that can be alleviated by a somatostatin receptor antagonist. Our data highlight the role of Na + as an intracellular second messenger, illustrate the significance of the intra-islet paracrine network and provide a mechanistic framework for pharmacological correction of the hormone secretion defects associated with diabetes that selectively target the delta cells. Somatostatin is secreted by delta cells and inhibits insulin and glucagon secretion. Here Vergari et al. demonstrate a mechanism for somatostatin secretion that is dependent on decreased extracellular potassium and increased intracellular sodium levels, and is altered in islets from patients with diabetes.

The aim of the study was to elucidate the cellular mechanism underlying the suppression of glucose-induced insulin secretion in mice fed a high-fat diet (HFD) for 15 weeks. C57BL6J mice were fed a HFD or a normal diet (ND) for 3 or 15 weeks. Plasma insulin and glucose levels in vivo were assessed by intraperitoneal glucose tolerance test. Insulin secretion in vitro was studied using static incubations and a perfused pancreas preparation. Membrane currents, electrical activity, and exocytosis were examined by patch-clamp technique measurements. Intracellular calcium concentration ([Ca(2+)](i)) was measured by microfluorimetry. Total internal reflection fluorescence microscope (TIRFM) was used for optical imaging of exocytosis and submembrane depolarization-evoked [Ca(2+)](i). The functional data were complemented by analyses of histology and gene transcription. After 15 weeks, but not 3 weeks, mice on HFD exhibited hyperglycemia and hypoinsulinemia. Pancreatic islet content and beta-cell area increased 2- and 1.5-fold, respectively. These changes correlated with a 20-50% reduction of glucose-induced insulin secretion (normalized to insulin content). The latter effect was not associated with impaired electrical activity or [Ca(2+)](i) signaling. Single-cell capacitance and TIRFM measurements of exocytosis revealed a selective suppression (>70%) of exocytosis elicited by short (50 ms) depolarization, whereas the responses to longer depolarizations were (500 ms) less affected. The loss of rapid exocytosis correlated with dispersion of Ca(2+) entry in HFD beta-cells. No changes in gene transcription of key exocytotic protein were observed. HFD results in reduced insulin secretion by causing the functional dissociation of voltage-gated Ca(2+) entry from exocytosis. These observations suggest a novel explanation to the well-established link between obesity and diabetes.

Closure of ATP-regulated K + channels (K ATP channels) plays a central role in glucose-stimulated insulin secretion in beta cells. K ATP channels are also highly expressed in glucagon-producing alpha cells, where their function remains unresolved. Under hypoglycaemic conditions, K ATP channels are open in alpha cells but their activity is low and only ~1% of that in beta cells. Like beta cells, alpha cells respond to hyperglycaemia with K ATP channel closure, membrane depolarisation and stimulation of action potential firing. Yet, hyperglycaemia reciprocally regulates glucagon (inhibition) and insulin secretion (stimulation). Here we discuss how this conundrum can be resolved and how reduced K ATP channel activity, via membrane depolarisation, paradoxically reduces alpha cell Ca 2+ entry and glucagon exocytosis. Finally, we consider whether the glucagon secretory defects associated with diabetes can be attributed to impaired K ATP channel regulation and discuss the potential for remedial pharmacological intervention using sulfonylureas.