Effect of the alpha 2-blocker DG-5128 on insulin and somatostatin release from the isolated perfused rat pancreas

An atlas and functional analysis of G-protein coupled receptors in human islets of Langerhans

G-protein coupled receptors (GPCRs) regulate hormone secretion from islets of Langerhans, and recently developed therapies for type-2 diabetes target islet GLP-1 receptors. However, the total number of GPCRs expressed by human islets, as well as their function and interactions with drugs, is poorly understood. In this review we have constructed an atlas of all GPCRs expressed by human islets: the 'islet GPCRome'. We have used this atlas to describe how islet GPCRs interact with their endogenous ligands, regulate islet hormone secretion, and interact with drugs known to target GPCRs, with a focus on drug/receptor interactions that may affect insulin secretion. The islet GPCRome consists of 293 GPCRs, a majority of which have unknown effects on insulin, glucagon and somatostatin secretion. The islet GPCRs are activated by 271 different endogenous ligands, at least 131 of which are present in islet cells. A large signalling redundancy was also found, with 119 ligands activating more than one islet receptor. Islet GPCRs are also the targets of a large number of clinically used drugs, and based on their coupling characteristics and effects on receptor signalling we identified 107 drugs predicted to stimulate and 184 drugs predicted to inhibit insulin secretion. The islet GPCRome highlights knowledge gaps in the current understanding of islet GPCR function, and identifies GPCR/ligand/drug interactions that might affect insulin secretion, which are important for understanding the metabolic side effects of drugs. This approach may aid in the design of new safer therapeutic agents with fewer detrimental effects on islet hormone secretion.

cAMP induces stromal interaction molecule 1 (STIM1) puncta but neither Orai1 protein clustering nor store-operated Ca2+ entry (SOCE) in islet cells

The events leading to the activation of store-operated Ca(2+) entry (SOCE) involve Ca(2+) depletion of the endoplasmic reticulum (ER) resulting in translocation of the transmembrane Ca(2+) sensor protein, stromal interaction molecule 1 (STIM1), to the junctions between ER and the plasma membrane where it binds to the Ca(2+) channel protein Orai1 to activate Ca(2+) influx. Using confocal and total internal reflection fluorescence microscopy, we studied redistribution kinetics of fluorescence-tagged STIM1 and Orai1 as well as SOCE in insulin-releasing β-cells and glucagon-secreting α-cells within intact mouse and human pancreatic islets. ER Ca(2+) depletion triggered accumulation of STIM1 puncta in the subplasmalemmal ER where they co-clustered with Orai1 in the plasma membrane and activated SOCE. Glucose, which promotes Ca(2+) store filling and inhibits SOCE, stimulated retranslocation of STIM1 to the bulk ER. This effect was evident at much lower glucose concentrations in α- than in β-cells consistent with involvement of SOCE in the regulation of glucagon secretion. Epinephrine stimulated subplasmalemmal translocation of STIM1 in α-cells and retranslocation in β-cells involving raising and lowering of cAMP, respectively. The cAMP effect was mediated both by protein kinase A and exchange protein directly activated by cAMP. However, the cAMP-induced STIM1 puncta did not co-cluster with Orai1, and there was no activation of SOCE. STIM1 translocation can consequently occur independently of Orai1 clustering and SOCE.

Chronic sympathetic innervation of islets in transgenic mice results in differential desensitization of alpha-adrenergic inhibition of insulin secretion

The effects of chronic sympathetic hyperinnervation on pancreatic beta-cell insulin secretion were investigated utilizing the in vitro perfused pancreas from transgenic mice. These mice exhibit islet hyperinnervation of sympathetic neurons resulting from overexpression of nerve growth factor in their beta-cells (1). The goal was to determine whether sympathetic hyperinnervation increased classic alpha-adrenergic inhibition of beta-cell insulin secretion or, in contrast, down-regulated beta-cell sensitivity to adrenergic input resulting in enhanced insulin secretion. Both fasting and fed blood sugars and pancreatic insulin content were normal in the transgenics. Response of the transgenic perfused pancreas to low glucose (7 mM) was primarily first phase and normal whereas high glucose (22 mM) caused enhanced, rather than reduced, insulin secretion of both first and second phases. The alpha-antagonist, phentolamine, caused a six-fold increase in glucose-stimulated insulin secretion from the control pancreas, an effect that was blunted for the transgenic pancreas. A similarly blunted response to phentolamine occurred when this agent was superimposed on a combined glucose-forskolin stimulus. (The positive effect on insulin secretion by phentolamine in normal beta-cell preparations has arguably been ascribed to non-specific ionic effects.) Therefore, as a test of possible changes in the ATP regulated K+ channel or the linked Ca++ channels, glyburide was perfused during glucose stimulation. Insulin secretion in response to glyburide was increased two fold in the control pancreas. However, with the transgenic pancreas, in contrast to the enhanced response to glucose, the effect of glyburide was almost completely inhibited. It is concluded that: 1) chronic adrenergic hyperinnervation results in enhanced glucose-stimulated insulin secretion by desensitization of a major alpha-adrenergic inhibitory site(s); and 2) adrenergic hyperinnervation acts directly or indirectly on ion flux to partially inhibit insulin release, an effect which is not desensitized. Since down-regulation of a single alpha-adrenergic receptor would be expected to desensitize both phenomena the observed differential desensitization indicates that different post receptor events or more than one adrenergic receptor are involved.

Mechanism of insulin secretion by midaglizole

Midaglizole was introduced as a hypoglycemic agent, but its insulin releasing mechanism remains unknown. In the present study, the effect of midaglizole upon the B cell function of the pancreas was investigated, using an in situ local circulation of the canine pancreas. The graded doses of midaglizole (0.2, 1.0 and 2.0 mg/kg) revealed a dose-related response of plasma insulin. The administration of yohimbine, a classical alpha 2-antagonist (1.11 mg/kg) revealed a similar increase in plasma insulin to that with midaglizole of equimolar amount. During the clonidine infusion midaglizole did not elicit any significant rise in plasma insulin, whereas yohimbine increased plasma insulin significantly. During glucagon infusion plasma insulin increased following midaglizole infusion but not by yohimbine. The simultaneous administration of diazoxide (K-channel opener) suppressed the midaglizole-induced insulin secretion. These results obtained in the present experiments revealed a different mechanism of insulin releasing action of midaglizole from that of yohimbine. Furthermore, the finding with diazoxide administration suggests that midaglizole stimulates insulin release through an interaction of K(+)-channel of the pancreatic B-cell.

Radio-immunoassay of somatostatin from isolated rat pancreatic islets

Certain aspects of radio-immunoassay of somatostatin from isolated rat pancreatic islets are described. Somatostatin-14, and not somatostatin-28, is secreted from isolated rat pancreatic islets. Less somatostatin secretion is measured per islet owing to purity of tracer in the radio-immunoassay. Theophylline apparently cross-reacts with somatostatin in the assay described, and this has to be taken into consideration when studying somatostatin release induced by theophylline in isolated islets.

Somatostatin secretion from isolated rat pancreatic islets

Certain aspects of somatostatin secretion from isolated rat pancreatic islets are described. A considerable, but falling basal secretion of somatostatin was observed in the pre-incubation periods. Both glucose and theophylline stimulation gave significant increases in somatostatin secretion, whereas carbachol inhibits the somatostatin secretion at 25 mmol l-1 glucose but not at 5 mmol l-1 glucose. The glucose effect on somatostatin secretion required a normoglycaemic pre-incubation level of 5.5 mmol l-1 glucose. Our results indicate that somatostatin secretion from isolated pancreatic islets is strongly dependent on the experimental conditions.

Effect of DG5128 on epinephrine and glucagon induced glucose output from the isolated perfused rat liver

The effect of a specific alpha 2-adrenergic antagonist 2-[2-(4,5-dihydro-1.H-imidazol-2-yl)-1-phenyl-ethyl] pyridine dihydrochloride sesquihydrate (DG5128), on the glucose output by epinephrine and/or glucagon was studied using the perfused rat liver. The administration of DG5128 alone did not affect the glucose output. However, DG5128 produced a significant inhibition of the increased glucose output when induced by 10(-6) M epinephrine alone or 10(-6) M epinephrine plus 1.4 x 10(-10) M glucagon. There were no significant changes of the glucose output by 1.4 x 10(-10) M or 7.0 x 10(-11) M glucagon alone. On the other hand, addition of 1 mU/ml insulin to the perfusate suppressed the 7.0 x 10(-11) M glucagon-induced glucose output, but failed to decrease the 1.4 x 10(-10) M glucagon effect. DG5128 suppressed further the glucagon (7.0 x 10(-11) M)-induced increase of glucose output in the presence of insulin. These results suggest that DG5128 produces a hypoglycemic effect partly through an inhibition of the increased hepatic glucose output elicited by epinephrine and glucagon.