Dependency of cyclic AMP-induced insulin release on intra- and extracellular calcium in rat islets of Langerhans

Proteomic Analysis of Rap1A GTPase Signaling-Deficient C57BL/6 Mouse Pancreas and Functional Studies Identify an Essential Role of Rap1A in Pancreas Physiology

Ras-related Rap1A GTPase is implicated in pancreas β-cell insulin secretion and is stimulated by the cAMP sensor Epac2, a guanine exchange factor and activator of Rap1 GTPase. In this study, we examined the differential proteomic profiles of pancreata from C57BL/6 Rap1A-deficient (Null) and control wild-type (WT) mice with nanoLC-ESI-MS/MS to assess targets of Rap1A potentially involved in insulin regulation. We identified 77 overlapping identifier proteins in both groups, with 8 distinct identifier proteins in Null versus 56 distinct identifier proteins in WT mice pancreata. Functional enrichment analysis showed four of the eight Null unique proteins, ERO1-like protein β (Ero1lβ), triosephosphate isomerase (TP1), 14-3-3 protein γ, and kallikrein-1, were exclusively involved in insulin biogenesis, with roles in insulin metabolism. Specifically, the mRNA expression of Ero1lβ and TP1 was significantly (p < 0.05) increased in Null versus WT pancreata. Rap1A deficiency significantly affected glucose tolerance during the first 15-30 min of glucose challenge but showed no impact on insulin sensitivity. Ex vivo glucose-stimulated insulin secretion (GSIS) studies on isolated Null islets showed significantly impaired GSIS. Furthermore, in GSIS-impaired islets, the cAMP-Epac2-Rap1A pathway was significantly compromised compared to the WT. Altogether, these studies underscore an essential role of Rap1A GTPase in pancreas physiological function.

Islet Gene View-a tool to facilitate islet research

Characterization of gene expression in pancreatic islets and its alteration in type 2 diabetes (T2D) are vital in understanding islet function and T2D pathogenesis. We leveraged RNA sequencing and genome-wide genotyping in islets from 188 donors to create the Islet Gene View (IGW) platform to make this information easily accessible to the scientific community. Expression data were related to islet phenotypes, diabetes status, other islet-expressed genes, islet hormone-encoding genes and for expression in insulin target tissues. The IGW web application produces output graphs for a particular gene of interest. In IGW, 284 differentially expressed genes (DEGs) were identified in T2D donor islets compared with controls. Forty percent of DEGs showed cell-type enrichment and a large proportion significantly co-expressed with islet hormone-encoding genes; glucagon (<i>GCG</i>, 56%), amylin (<i>IAPP</i>, 52%), insulin (<i>INS</i>, 44%), and somatostatin (<i>SST</i>, 24%). Inhibition of two DEGs, <i>UNC5D</i> and <i>SERPINE2</i>, impaired glucose-stimulated insulin secretion and impacted cell survival in a human β-cell model. The exploratory use of IGW could help designing more comprehensive functional follow-up studies and serve to identify therapeutic targets in T2D.

Biosynthetic Activity Differs Between Islet Cell Types and in Beta Cells Is Modulated by Glucose and Not by Secretion

A correct biosynthetic activity is thought to be essential for the long-term function and survival of islet cells in culture and possibly also after islet transplantation. Compared to the secretory activity, biosynthetic activity has been poorly studied in pancreatic islet cells. Here we aimed to assess biosynthetic activity at the single cell level to investigate if protein synthesis is dependent on secretagogues and increased as a consequence of hormonal secretion. Biosynthetic activity in rat islet cells was studied at the single cell level using O-propargyl-puromycin (OPP) that incorporates into newly translated proteins and chemically ligates to a fluorescent dye by "click" reaction. Heterogeneous biosynthetic activity was observed between the four islet cell types, with delta cells showing the higher relative protein biosynthesis. Beta cells protein biosynthesis was increased in response to glucose while 3-isobutyl-1-methylxanthine and phorbol-12-myristate-13-acetate, 2 drugs known to stimulate insulin secretion, had no similar effect on protein biosynthesis. However, after several hours of secretion, protein biosynthesis remained high even when cells were challenged to basal conditions. These results suggest that mechanisms regulating secretion and biosynthesis in islet cells are different, with glucose directly triggering beta cells protein biosynthesis, independently of insulin secretion. Furthermore, this OPP labeling approach is a promising method to identify newly synthesized proteins under various physiological and pathological conditions.

Functionalization of Alginate with Extracellular Matrix Peptides Enhances Viability and Function of Encapsulated Porcine Islets

Translation of transplanted alginate-encapsulated pancreatic islets to treat type 1 diabetes has been hindered by inconsistent long-term efficacy. This loss of graft function can be partially attributed to islet dysfunction associated with the destruction of extracellular matrix (ECM) interactions during the islet isolation process as well as immunosuppression-associated side effects. This study aims at recapitulating islet-ECM interactions by the direct functionalization of alginate with the ECM-derived peptides RGD, LRE, YIGSR, PDGEA, and PDSGR. Peptide functionalization is controlled in a concentration-dependent manner and its presentation is found to be homogeneous across the microcapsule environment. Preweaned porcine islets are encapsulated in peptide-functionalized alginate microcapsules, and those encapsulated in RGD-functionalized alginate displays enhanced viability and glucose-stimulated insulin release. Effects are RGD-specific and not observed with its scrambled control RDG nor with LRE, YIGSR, PDGEA, and PDSGR. This study supports the sustained presentation of ECM-derived peptides in helping to maintain health of encapsulated pancreatic islets and may aid in prolonging longevity of encapsulated islet grafts.

Ficus deltoidea: A Potential Alternative Medicine for Diabetes Mellitus

Ficus deltoidea from the Moraceae family has been scientifically proven to reduce hyperglycemia at different prandial states. In this study, we evaluate the mechanisms that underlie antihyperglycemic action of Ficus deltoidea. The results had shown that hot aqueous extract of Ficus deltoidea stimulated insulin secretion significantly with the highest magnitude of stimulation was 7.31-fold (P < 0.001). The insulin secretory actions of the hot aqueous extract involved K⁺  _ATP channel-dependent and K⁺  _ATP-channel-independent pathway. The extract also has the ability to induce the usage of intracellular Ca²⁺ to trigger insulin release. The ethanolic and methanolic extracts enhanced basal and insulin-mediated glucose uptake into adipocytes cells. The extracts possess either insulin-mimetic or insulin-sensitizing property or combination of both properties during enhancing glucose uptake into such cells. Meanwhile, the hot aqueous and methanolic extracts augmented basal and insulin-stimulated adiponectin secretion from adipocytes cells. From this study, it is suggested that Ficus deltoidea has the potential to be developed as future oral antidiabetic agent.

Adenylyl cyclase 8 is central to glucagon-like peptide 1 signalling and effects of chronically elevated glucose in rat and human pancreatic beta cells

AIMS/HYPOTHESIS

Glucose and incretins regulate beta cell function, gene expression and insulin exocytosis via calcium and cAMP. Prolonged exposure to elevated glucose (also termed glucotoxicity) disturbs calcium homeostasis, but little is known about cAMP signalling. We therefore investigated long-term effects of glucose on this pathway with special regard to the incretin glucagon-like peptide 1 (GLP-1).

METHODS

We exposed INS-1E cells and rat or human islets to different levels of glucose for 3 days and determined functional responses in terms of second messengers (cAMP, Ca(2+)), transcription profiles, activation of cAMP-responsive element (CRE) and secretion by measuring membrane capacitance. Moreover, we modulated directly the abundance of a calcium-sensitive adenylyl cyclase (ADCY8) and GLP-1 receptor (GLP1R).

RESULTS

GLP-1- or forskolin-mediated increases in cytosolic calcium, cAMP-levels or insulin secretion were largely reduced in INS-1E cells cultured at elevated glucose (>5.5 mmol/l). Statistical analysis of transcription profiles identified cAMP pathways as major targets regulated by glucose. Quantitative PCR confirmed these findings and unravelled marked downregulation of the calcium-sensitive adenylyl cyclase ADCY8 also in rat and in human islets. Re-expression of ADCY8, but not of the GLP1R, recovered GLP-1 signalling in glucotoxicity in INS-1E cells and in rat islets. Moreover, knockdown of this adenylyl cyclase showed that GLP-1-induced cAMP generation, calcium signalling, activation of the downstream target CRE and direct amplification of exocytosis by cAMP-raising agents (evaluated by capacitance measurement) proceeds via ADCY8.

CONCLUSIONS/INTERPRETATION

cAMP-mediated pathways are modelled by glucose, and downregulation of the calcium-sensitive ADCY8 plays a central role herein, including signalling via the GLP1R.

Regulation of pancreatic beta-cell insulin secretion by actin cytoskeleton remodelling: role of gelsolin and cooperation with the MAPK signalling pathway

We have previously isolated two MIN6 beta-cell sublines, B1, highly responsive to glucose-stimulated insulin secretion, and C3, markedly refractory (Lilla, V., Webb, G., Rickenbach, K., Maturana, A., Steiner, D. F., Halban, P. A. and Irminger, J. C. (2003) Endocrinology 144, 1368-1379). We now demonstrate that C3 cells have substantially increased amounts of F-actin stress fibres whereas B1 cells have shorter cortical F-actin. Consistent with these data, B1 cells display glucose-dependent actin remodelling whereas, in C3 cells, F-actin is refractory to this secretagogue. Furthermore, F-actin depolymerisation with latrunculin B restores glucose-stimulated insulin secretion in C3 cells. In parallel, glucose-stimulated ERK1/2 activation is greater in B1 than in C3 cells, and is potentiated in both sublines following F-actin depolymerisation. Glucose-activated phosphoERK1/2 accumulates at actin filament tips adjacent to the plasma membrane, indicating that these are the main sites of action for this kinase during insulin secretion. In addition, B1 cell expression of the calcium-dependent F-actin severing protein gelsolin is >100-fold higher than that of C3 cells. Knock-down of gelsolin reduced glucose-stimulated insulin secretion, whereas gelsolin over-expression potentiated secretion from B1 cells. Gelsolin localised along depolymerised actin fibres after glucose stimulation. Taken together, these data demonstrate that F-actin reorganization prior to insulin secretion requires gelsolin and plays a role in the glucose-dependent MAPK signal transduction that regulates beta-cell insulin secretion.

Dopaminergic regulation of glucose-induced insulin secretion through dopamine D2 receptors in the pancreatic islets in vitro

The stimulatory effect of dopamine through dopamine D2 receptor on glucose-induced insulin secretion was studied in the pancreatic islets in vitro. Dopamine significantly stimulated insulin secretion at a concentration of 10-8 M in the presence of high glucose (20 mM). The higher concentrations of dopamine (10(-7)-10(-4)) inhibited glucose-induced insulin secretion in the presence of both 4 mM and 20 mM glucose. Stimulatory and inhibitory effect of dopamine on glucose-induced insulin secretion was reverted by the addition of dopamine D2 receptor antagonists such as butaclamol and sulpiride. Norepinephrine (NE) at 10(-4) M concentration inhibited the dopamine uptake as well as its stimulatory effect at 10(-8) M concentration on glucose induced insulin secretion. Our results suggest that dopamine exerts a differential effect on glucose-induced insulin secretion through dopamine D2 receptor and it is essential for the regulation of glucose-induced insulin secretion by pancreatic islets.

Beta-cell-targeted overexpression of phosphodiesterase 3B in mice causes impaired insulin secretion, glucose intolerance, and deranged islet morphology

The second messenger cAMP mediates potentiation of glucose-stimulated insulin release. Use of inhibitors of cAMP-hydrolyzing phosphodiesterase (PDE) 3 and overexpression of PDE3B in vitro have demonstrated a regulatory role for this enzyme in insulin secretion. In this work, the physiological significance of PDE3B-mediated degradation of cAMP for the regulation of insulin secretion in vivo and glucose homeostasis was investigated in transgenic mice overexpressing PDE3B in pancreatic beta-cells. A 2-fold overexpression of PDE3B protein and activity blunted the insulin response to intravenous glucose, resulting in reduced glucose disposal. The effects were "dose"-dependent because mice overexpressing PDE3B 7-fold failed to increase insulin in response to glucose and hence exhibited pronounced glucose intolerance. Also, the insulin secretory response to intravenous glucagon-like peptide 1 was reduced in vivo. Similarly, islets stimulated in vitro exhibited reduced insulin secretory capacity in response to glucose and glucagon-like peptide 1. Perifusion experiments revealed that the reduction specifically affected the first phase of glucose-stimulated insulin secretion. Furthermore, morphological examinations demonstrated deranged islet cytoarchitecture. In conclusion, these results are consistent with an essential role for PDE3B in cAMP-mediated regulation of insulin release and glucose homeostasis.

Piccolo, a Ca2+ sensor in pancreatic beta-cells. Involvement of cAMP-GEFII.Rim2. Piccolo complex in cAMP-dependent exocytosis

We have previously shown that cAMP-binding protein cAMP-guanidine nucleotide exchange factor II (GEFII) (or Epac2) interacting with Rim2 is involved in cAMP-dependent, protein kinase A-independent exocytosis in pancreatic beta-cells. The action of the cAMP-GEFII.Rim2 complex requires both intracellular cAMP and Ca(2+). Although Rim2 has C(2) domains, its role as a Ca(2+) sensor has remained unclear. In the present investigation, we have discovered that Piccolo, a CAZ (cytoskeletal matrix associated with the active zone) protein in neurons that is structurally related to Rim2, also binds to cAMP-GEFII and that it forms both homodimer and heterodimer with Rim2 in a Ca(2+)-dependent manner, whereas Rim2 alone does not form the homodimer. The association of Piccolo.Rim2 heterodimerization is stronger than Piccolo. Piccolo homodimerization. Treatment of pancreatic islets with antisense oligodeoxynucleotides against Piccolo inhibits insulin secretion induced by cAMP analog 8-bromo-cyclic AMP plus high glucose stimulation. These results suggest that Piccolo serves as a Ca(2+) sensor in exocytosis in pancreatic beta-cells and that the formation of a cAMP-GEFII.Rim2. Piccolo complex is important in cAMP-induced insulin secretion. In addition, this study suggests that CAZ proteins similar to those in neurons are also function in pancreatic beta-cells.

The ryanodine receptor calcium channel of beta-cells: molecular regulation and physiological significance

The list of Ca(2+) channels involved in stimulus-secretion coupling in beta-cells is increasing. In this respect the roles of the voltage-gated Ca(2+) channels and IP(3) receptors are well accepted. There is a lack of consensus about the significance of a third group of Ca(2+) channels called ryanodine (RY) receptors. These are large conduits located on Ca(2+) storage organelle. Ca(2+) gates these channels in a concentration- and time-dependent manner. Activation of these channels by Ca(2+) leads to fast release of Ca(2+) from the stores, a process called Ca(2+)-induced Ca(2+) release (CICR). A substantial body of evidence confirms that beta-cells have RY receptors. CICR by RY receptors amplifies Ca(2+) signals. Some properties of RY receptors ensure that this amplification process is engaged in a context-dependent manner. Several endogenous molecules and processes that modulate RY receptors determine the appropriate context. Among these are several glycolytic intermediates, long-chain acyl CoA, ATP, cAMP, cADPR, NO, and high luminal Ca(2+) concentration, and all of these have been shown to sensitize RY receptors to the trigger action of Ca(2+). RY receptors, thus, detect co-incident signals and integrate them. These Ca(2+) channels are targets for the action of cAMP-linked incretin hormones that stimulate glucose-dependent insulin secretion. In beta-cells some RY receptors are located on the secretory vesicles. Thus, despite their low abundance, RY receptors are emerging as distinct players in beta-cell function by virtue of their large conductance, strategic locations, and their ability to amplify Ca(2+) signals in a context-dependent manner.

The insulinotropic mechanism of the novel hypoglycaemic agent JTT-608: direct enhancement of Ca(2+) efficacy and increase of Ca(2+) influx by phosphodiesterase inhibition

We examined the effects of the novel hypoglycaemic agent JTT-608 [trans-4-(4-methylcyclohexyl)-4-oxobutyric acid] on insulin secretion using rat pancreatic islets, and analysed the mechanism of its effect. JTT-608 augmented 8.3 mM glucose-induced insulin secretion dose-dependently, and there was a stimulatory effect of 100 microM JTT-608 at both moderate and high concentrations (8.3, 11. 1 and 16.7 mM) of glucose, but not at low concentrations (3.3 and 5. 5 mM). In perifusion experiments, both phases of insulin release were enhanced, and the effect was eliminated 10 min after withdrawal of the agent. In the presence of 200 microM diazoxide and a depolarizing concentration (30 mM) of K(+), there was an augmentation of insulin secretion by 100 microM JTT-608, not only under high levels of glucose but also under low levels, and the effects were abolished by 10 microM nitrendipine. JTT-608 augmented insulin secretion from electrically permeabilized islets in the presence of stimulatory concentrations (0.3 and 1.0 microM) of Ca(2+), and the intracellular Ca(2+) concentration ([Ca(2+)](i)) response under 16.7 mM glucose, 200 microM diazoxide, and 30 mM K(+) was also increased. The cyclic AMP content in the islets was increased by 100 microM JTT-608, and an additive effect to 1 microM forskolin was observed, but not to 50 microM 3-isobutyl-1-methylxanthine (IBMX). JTT-608 inhibited phosphodiesterase (PDE) activity dose-dependently. We conclude that JTT-608 augments insulin secretion by enhancing Ca(2+) efficacy and by increasing Ca(2+) influx. This appears to be a result of the increased intracellular cyclic AMP concentration due to PDE inhibition.

Glucose-induced insulin secretion in uremia: effects of aminophylline infusion and glucose loads

To explain mechanisms responsible for derangement of insulin release in uremia, we investigated glucose metabolism through three different tests in 14 patients with end-stage chronic renal failure. These tests were: intravenous glucose tolerance test with 0.33 g/kg of glucose solution (IVGTT); IVGTT with 0.5 g/kg of glucose solution (IVGTT2); IVGTT during aminophylline infusion (IVGTT + A). Twelve of the patients had IVGTT repeated after two to four months of thrice-weekly regular hemodialysis (IVGTT3). In each test we measured plasma glucose (G), immunoreactive insulin (IRI) and C-peptide. We also calculated glucose constant decay (K), insulin production (IRI area), insulinogenic index (IGI), and insulin resistance index (RI). Twenty-nine healthy volunteers formed the normal controls for IVGTT. As compared to controls, during IVGTT uremic patients showed significantly lower values in K, IRI area and IGI, and showed a significant RI value increase. During IVGTT2, IRI are values were higher than during IVGTT but IGI and K values were unchanged. During IVGTT + A both IRI area and IGI values were higher than during IVGTT. After hemodialysis treatment (IVGTT3) K, IRI areas and IGI increased significantly as compared to the predialysis period. K increase after hemodialysis correlated directly to IGI increase and inversely to RI changes. IGI increase during IVGTT3 was directly correlated to IGI rise during IVGTT + A. From these data we infer that defective insulin release in uremia is due to a decrease of beta-cell glucose sensitivity rather than to their functional exhaustion. An impaired adenyl cyclase-cAMP system may have an important role in the pathogenesis of this abnormality.

Maternal diet and insulin secretion in islets of lactating, nonlactating, and young rats

The effects of dietary phosphorus (P) on insulin secretion by pancreatic islets of rats at peak lactation (14 days after parturition) and by islets of nonlactating previously pregnant rats have been determined. In addition, the effects of changed dietary P in the maternal diet on insulin secretion by islets of the 1- and 14-day-old offspring were studied. Lactating rats had increased food intake relative to nonlactating rats and correspondingly increased P intake on the 0.4 and 0.7% P diets. Serum P levels were elevated in lactating rats consuming the 0.7% P feed relative to those on the 0.4% P feed and also relative to the nonlactating rats on the 0.7% P feed. Lactating rats had decreased bone calcium compared with the nonlactating rats. Islets from lactating rats on the 0.7% P feed secreted more insulin in response to glucose or to 3-isobutyl-1-methylxanthine (IBMX) than lactating rats on the 0.4% P feed. Additionally, lactating rats on the 0.7% P feed secreted more insulin in response to glucose than nonlactating rats on the same 0.7% P feed. The maternal diet had no influence on the endocrine pancreatic responses of the 1- and 14-day-old offspring in response to glucose or IBMX.

The mechanisms by which vasoactive intestinal peptide (VIP) and thyrotropin releasing hormone (TRH) stimulate prolactin release from pituitary cells

The effect of vasoactive intestinal peptide (VIP) on prolactin (PRL) secretion from pituitary cells is reviewed and compared to the effect of thyrotropin releasing hormone (TRH). These two peptides induced different secretion profiles from parafused lactotrophs in culture. TRH was found to increase PRL secretion within 4 s and induced a biphasic secretion pattern, while VIP induced a monophasic secretion pattern after a lag time of 45-60 s. The secretion profiles are compared to changes in adenylate cyclase activity, production of inositol polyphosphates, changes in intracellular calcium concentrations and changes in electrophysiological properties of the cell membrane.

Effect of low concentrations of glucagon on insulin release and cyclic AMP content in isolated rat islets

Studies on hormone action in isolated islets have generally been carried out using concentrations far above physiologic levels. This study investigates whether glucagon at concentrations close to the physiologic level is insulinotropic in isolated islets and how this relates to islet cyclic AMP levels. Collagenase isolated rat islets were tested directly after isolation or after a 24-hour tissue culture. Insulin release and islet cyclic AMP content were determined during a 30-minute incubation by radioimmunoassay. After maintenance in tissue culture glucose-induced (16.7 mmol/L) insulin release was clearly enhanced by glucagon concentrations between 2 and 1,000 ng/mL in a dose-related manner. Islet cyclic AMP was increased only by glucagon 1 mumol/L (3.8 micrograms/mL). When phosphodiesterases were inhibited (0.1 mmol/L 3-isobutyl-1-methylxanthine) insulin release was stimulated by 1 ng/mL and cyclic AMP by 100 ng/mL. By contrast, in freshly isolated islets, glucagon concentrations in the range of 1 to 100 micrograms/mL were needed to augment glucose-induced insulin release. These findings demonstrate that the hormone sensitivity of collagenase isolated islets is markedly improved by short-term maintenance in tissue culture. The threshold level for a detectable effect on islet cyclic AMP is considerably higher than for glucose-induced insulin release. Since in hepatocytes two signal transduction systems for glucagon have recently been demonstrated, the results could mean that at low concentrations glucagon effects may not be mediated via cyclic AMP.

Abnormal regulation of insulin secretion in the genetically obese (ob/ob) mouse

To determine whether the abnormal insulin-secretory activity encountered in obese mice is due to an anomaly in the production of cyclic AMP, islets of lean and obese mice were incubated with forskolin under various conditions. Our data show that, in addition to the well-known quantitative differences in insulin-secretory activity between islets of lean and obese mice, there are important qualitative differences. The islets of obese mice accumulated less cyclic AMP than did those of lean mice in response to given doses of forskolin, yet their insulin secretion was enhanced to much higher values. In the islets of obese mice, but not in those of lean mice, the stimulatory effect of forskolin on insulin secretion was evident even at non-stimulatory concentrations of glucose or in Ca2+-deprived incubation media, showing that the islet of the obese mouse is less dependent on a primary stimulus (glucose) and on the provision of normal concentrations of Ca2+ in the bathing medium than is the islet of the lean mouse.

Stimulation of insulin release in isolated rat islets by GIP in physiological concentrations and its relation to islet cyclic AMP content

Several insulinotropic hormones have been shown to increase the level of cyclic AMP in isolated islets. This study was performed to investigate whether gastric inhibitory polypeptide (glucose-dependent insulin-releasing polypeptide) has a similar effect, in particular at concentrations close to the physiological level in blood. Collagenase isolated rat islets were maintained for 24 h in tissue culture (medium 199) and then incubated for 30 min for measurement of insulin release and cyclic AMP content. Glucose-induced (16.7 mmol/l) insulin release was enhanced by gastric inhibitory polypeptide 1-100 ng/ml (0.196-19.6 nmol/l) in a dose-related fashion. The cyclic AMP content was enhanced only by 100 ng/ml. However, when 0.1 mmol/l of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine was present, even 1 ng/ml of gastric inhibitory polypeptide increased both cyclic AMP content and insulin release. Such a concentration of the hormone can be measured in human blood after a meal. In contrast, in freshly isolated islets no effect of the hormone on glucose-induced insulin release or cyclic AMP content could be detected for concentrations ranging from 1 to 100 ng/ml. These findings demonstrate that the hormone sensitivity of isolated islets is markedly enhanced by short-term maintenance in tissue culture. The results suggest that an increase in cyclic AMP is seen in response to gastric inhibitory polypeptide and may be causally related to the insulinotropic effect of the hormone.

Effects of alpha- and beta-adrenoceptor stimulation on 45Ca2+ efflux and insulin secretion from perfused rat islets

Exposure to the beta 2-adrenoceptor agonist terbutaline resulted in a transient stimulation of 45Ca2+ efflux from 45Ca2+ preloaded rat islets perfused in 2 mM Ca2+ and 8.3 mM glucose. Concomitantly, an increase in insulin secretion occurred. Under the same experimental conditions, the alpha-adrenoceptor agonist noradrenaline promptly inhibited insulin release without any apparent influence on 45Ca2+ efflux. In contrast, in a medium containing 2 mM Ca2+ and a low glucose concentration (2.8 mM), terbutaline stimulated insulin secretion without any apparent effects on 45Ca2+ efflux. Noradrenaline had no effect on insulin secretion or 45Ca2+ efflux in this medium. When islets were perfused with 8.3 mM glucose in a Ca2+ deficient medium, with or without addition of the chelating agent EGTA, terbutaline induced a marginal stimulation of insulin secretion and a negligible stimulation of 45Ca2+ efflux. On the contrary, noradrenaline stimulated to an immediate and notable 45Ca2+ efflux in these Ca2+ deficient media. Noradrenaline also clearly inhibited insulin secretion, though less markedly and with a slower onset than in islets perfused in 2 mM Ca2+. When the islets were perfused in a Ca2+ deficient medium with 2.8 mM glucose, terbutaline had a slight insulin releasing effect, but stimulated 45Ca2+ efflux potently. Noradrenaline had no influence on insulin secretion but a weak stimulatory effect on 45Ca2+ efflux. The data suggest that the beta 2-adrenoceptor agonist terbutaline has the ability to stimulate insulin secretion in perfused rat islets, requiring extracellular Ca2+ for the full expression of its effects. These effects may be exerted through a Ca2+-Ca2+ exchange over the cell membrane and/or through cAMP and intracellular Ca2+ perturbations.(ABSTRACT TRUNCATED AT 250 WORDS)

Early phase of pancreatic polypeptide release is augmented by calcium infusion

This study evaluated the effect on an intravenous infusion of calcium chloride (4 and 8 mg/kg/hr) on both fasting and meal-stimulated pancreatic polypeptide (PP) release in an awake canine model. Intravenous calcium had no effect on fasting levels of PP. A protein-rich meal was a potent stimulus for PP release with a peak response of 1474 +/- 266 pg/ml occurring at 45 min after the meal. Intravenous calcium at a dose of 8 mg/kg/hr augmented meal-stimulated PP release from 30 to 180 min after the meal compared to control values. Significant increases in serum calcium occurred at 60 to 180 min during infusion of calcium at 8 mg/kg/hr. This study demonstrates that the early cephalic (vagal) phase of pancreatic polypeptide release appears to be sensitive to subthreshold increases in extracellular calcium, suggesting that calcium is a factor in the stimulus-secretion coupling for the endogenous release of pancreatic polypeptide.

Mechanisms of synergism between glucose and cAMP on stimulation of insulin release

The mechanism of synergism between glucose and adenosine 3',5'-cyclic monophosphate (cAMP) on insulin release has been studied. Synergism may result from 1) inhibition of Na+-Ca2+ exchange by glucose and 2) a cAMP-induced sensitization of the release machinery to Ca2+. To distinguish between these two possibilities, isolated rat pancreatic islets were perifused with agents that raise intracellular levels of cAMP [3-isobutyl-1-methylxanthine (IBMX) and forskolin] and others that increase intracellular concentrations of Ca2+ either by blocking Na2+-Ca2+ exchange (ouabain and choline-Ringer solution) or by causing increased Ca2+ influx (KCl, carbachol, and 10 mM Ca2+). The results indicate that both the combination of cAMP and increased Ca2+ influx or blocked Na2-Ca2+ exchange and increased Ca2+ influx potentiated insulin release. When the relative potentiating abilities of cAMP and blocked Na2+-Ca2+ exchange were compared by determining the individual effects of IBMX and 1 mM ouabain (a concentration that causes similar inhibition of 45C2+ efflux as 16.7 mM glucose) in the presence of carbachol, cAMP was only 1.4 times more potent as a potentiating agent than blocked Na+-Ca2+ exchange. The greatest potentiation of insulin release was observed when Na+-Ca2+ exchange was blocked in the presence of increased levels of intracellular cAMP.

Glucagon and forskolin have dual effects upon islet cell electrical activity

We have investigated the effects of glucagon and forskolin upon pancreatic islet cell electrical activity using intracellular recordings from single mouse islets. Glucagon (0.1-2.0 microM) and forskolin (0.5-5.0 microM), both adenylate cyclase activators, potentiated glucose (200 mg/dl)-induced electrical activity. In the steady-state, islet cells have cyclic electrical activity with periodically recurring "plateau" depolarizations (with superimposed Ca++ action potentials) separated by silent hyperpolarizations. Both glucagon and forskolin mimicked glucose stimulation by increasing the fraction of each cycle spent in the plateau phase (the "plateau fraction"). Unlike glucose, however, glucagon and forskolin increased, rather than decreased, the overall frequency of plateaus, suggesting that plateau frequency is not tightly linked to changes of plateau fraction. This dissociation was also apparent during the onset of drug action. Plateau fraction increased immediately (within one minute), fell to a nadir and then rose to a new steady state level. Plateau frequency, however, rose slowly and monotonically to a new level. Following drug withdrawal plateau fraction returned to control levels several minutes before plateau fraction. From these results it was concluded that cAMP has two effects upon islet cell electrical activity: one is to increase plateau fraction possibly by stimulating glucose-dependent process, which results in increasing in Ca++ influx, and the other to increase plateau frequency possibly by reducing intracellular Ca++ buffering.

Effects of theophylline and dibutyryl cyclic adenosine monophosphate on the membrane potential of mouse pancreatic beta-cells

The effects of theophylline and dibutyryl cyclic AMP on the membrane potential of mouse beta-cells were studied with micro-electrodes. They were compared to their effects on insulin release by perifused mouse islets. In 3 mM-glucose, theophylline (10 mM) depolarized the beta-cell membrane and stimulated insulin release, but did not induce electrical activity. Dibutyryl cyclic AMP (1 mM) was without effect. In 7 mM-glucose, theophylline (0.5-2 mM) and dibutyryl cyclic AMP (1 mM) slightly depolarized the beta-cell membrane, induced electrical activity in otherwise silent cells and increased insulin release. A higher concentration of theophylline (10 mM) hyperpolarized the beta-cell membrane, did not induce electrical activity, but also stimulated insulin release. In 10 mM-glucose, the membrane potential of beta-cells exhibited repetitive slow waves with bursts of spikes on the plateau. Under steady state, these slow waves were differently affected by low or high concentrations of theophylline. At 0.5-2 mM, theophylline shortened the intervals, lengthened the slow waves and slightly increased their frequency. On the other hand, 10 mM-theophylline markedly decreased the duration of both intervals and slow waves, and increased their frequency. The effects of 1 mM-dibutyryl cyclic AMP were similar to those of 2 mM-theophylline. With 2-10 mM-theophylline, two other effects were also observed: a transient hyperpolarization with suppression of electrical activity immediately after addition of the methylxanthine and an increase in electrical activity upon its withdrawal. Theophylline and dibutyryl cyclic AMP markedly potentiated insulin release induced by 10 mM-glucose. The magnitude of these changes did not correlate well with the importance of the changes in electrical activity. However, with 2-10 mM-theophylline the increase in release was also preceded by an initial transient inhibition, whereas withdrawal of the methylxanthine was accompanied by a further increase. When Ca influx was inhibited by D600, the slow waves were suppressed, the membrane was depolarized to the plateau level and only few spikes were present. Although theophylline markedly increased insulin release under these conditions, it did not affect the membrane potential. Several conclusions can be drawn from this study. Insulin release and electrical activity in beta-cells can be dissociated when intracellular Ca is used to trigger exocytosis. High concentrations of theophylline produce effects unrelated to cyclic AMP.(ABSTRACT TRUNCATED AT 400 WORDS)

Cobalt inhibition of insulin release: evidence for an action not related to Ca2+ uptake

Insulin release, Ca2+ fluxes, and adenosine 3',5'-cyclic monophosphate (cAMP) levels were measured under various conditions to test the selectivity of cobalt (Co2+) actions in pancreatic islets. Insulin release was measured simultaneously either with 45Ca2+ uptake or 45Ca2+ efflux from rat islets maintained 2 days in tissue culture. Glucose (16.7 mM)-stimulated insulin release was inhibited by Co2+ (0.25, 0.5 mM) without concomitant inhibition of glucose-stimulated 45Ca2+ uptake. At higher concentrations of Co2+ (1.0, 2.5 mM) glucose- as well as K+-stimulated insulin release were inhibited in parallel with 45Ca2+ uptake. In the presence or the absence of external Ca2+, Co2+ (0.5 mM) also decreased 3-isobutyl-1-methylxanthine-stimulated insulin release, which does not depend on the stimulation of Ca2+ uptake. Co2+ did not alter islet cAMP levels. Co2+ also did not affect the unidirectional basal 45Ca2+ efflux measured in the absence of Ca2+ but completely overcame the stimulated 45Ca2+ efflux evoked by the removal of external Mg2+. Under the latter conditions, 50, but not 5 microM verapamil, decreased the stimulated 45Ca2+ efflux. It is concluded that Co2+ is able to inhibit insulin release and lower Ca2+ efflux without apparent interference with Ca2+ uptake. This suggests that Co2+, in addition to its well-known inhibition of Ca2+ uptake, may exert its action at a distal step in the insulin release process.

Role of Ca2+ in secretagogue-stimulated breakdown of phosphatidylinositol in rat pancreatic islets

Breakdown of phosphatidylinositol (PI) has been shown to be increased during Ca2+-mediated stimulation of cellular responses in many systems and has been proposed to be involved in stimulus-secretion coupling. The effects on PI breakdown of insulin secretagogues that alter cellular Ca2+ or cyclic (c)AMP levels were investigated in perifused rat islets of Langerhans. Isolated islets were labeled with myo-[2-3H(N)]inositol and the efflux of 3H-labeled metabolites was monitored. Glucose (16.7 mM) greatly increased 3H release in a manner that paralleled the second phase of the insulin secretory response; by 60 min, the amount of [3H]PI in the islet decreased by 50%. Removal of Ca2+ from the perifusate or blockade of Ca2+ entry through the voltage-dependent channels by D600 (20 microM) abolished the glucose-induced increase in 3H efflux. Depolarization with 47 mM K+, which increases Ca2+ entry, stimulated protracted 3H and insulin release. Glucose-stimulated output of 3H was not prevented by epinephrine (1 microM) even though the insulin response was abolished. In contrast, 3H output was not affected by isobutylmethylxanthine (1 mM), known to raise cellular levels of cAMP, although insulin release was stimulated. These findings indicate that PI breakdown is not related to the exocytotic process since stimulation of insulin release and PI breakdown could be uncoupled, and that it is not associated with cAMP-mediated regulation of insulin release. PI breakdown in islets differs from the immediate, transient phenomenon reported in other systems in both its timing and requirement for Ca2+. It appears to result from the entry of Ca2+ and not to be the mechanism by which glucose initiates Ca2+ influx.

Vitamin D Deficiency, hypocalcemia, and increased skeletal muscle degradation in rats

The myopathy associated with vitamin D deficiency was examined in vitamin D-deficient and vitamin D-supplemented rats. When compared with either vitamin D-supplemented ad lib. or pair-fed rats, weight gain and muscle mass were decreased in vitamin D-deficient hypocalcemic animals. With the exception of a modest decrease in muscle creatine phosphate levels, muscle composition was unchanged by vitamin D deficiency. Muscle protein turnover rates were determined in both in vivo and in vitro studies and demonstrated that myofibrillar protein degradation was increased in vitamin D deficiency. Normal growth rates could be maintained be feeding the rats vitamin D-deficient diets containing 1.6% calcium, which maintained plasma calcium within the normal range. In addition to its role in maintaining plasma calcium, vitamin D-supplemented rats had significantly higher levels of the anabolic hormone insulin. Vitamin D supplementation may affect muscle protein turnover by preventing hypocalcemia, as well as directly stimulating insulin secretion, rather than by a direct effect within skeletal muscle.

Modulation of active Ca2+ uptake by the islet-cell endoplasmic reticulum

The possible effects of calmodulin and cyclic AMP on active Ca2+ uptake by the islet-cell endoplasmic reticulum were investigated. Neither calmodulin nor cyclic AMP affected the rate of active Ca2+ uptake, or the steady-state filling capacity of the endoplasmic reticulum when measured in the absence of oxalate. Consistent with these results, calmodulin did not activate the Ca2+-stimulated ATPase activity associated with this cell fraction. During the course of these experiments., it was unexpectedly discovered that the rate of Ca2+ uptake, as well as the steady-state Ca2+ filling capacity of the endoplasmic reticulum, were markedly increased by unidentified factor(s) in the cytosol. This effect could be demonstrated by reconstitution of the membranes in cytosol, or by direct addition of fresh or dialysed cytosol to the Ca2+ uptake assays. The degree of activation by the cytosol indicates that the endoplasmic reticulum may play a prominent role in controlling beta-cell Ca2+ concentrations and that the unidentified activator(s) present in the cytosol may be involved in regulation of this function.

Selective inhibition of glucose-stimulated insulin release by dantrolene

Dantrolene sodium, which interferes with excitation-contraction coupling by inhibiting the Ca2+ release from sarcoplasmic reticulum in muscle, was used to investigate the role of stored calcium in the stimulation of insulin release by various secretagogues. Insulin release was measured simultaneously with 45Ca2+ uptake or 45Ca2+ efflux from isolated rat pancreatic islets. Glucose-stimulated insulin release was inhibited by dantrolene (10-100 microM) as was glyceraldehyde- or mannose-stimulated release. In contrast, dantrolene failed to inhibit insulin release stimulated by leucine, arginine, ouabain, potassium, or 3-isobutyl-1-methylxanthine. Although dantrolene lowered glucose-stimulated 45Ca2+ uptake, nonspecific blockade of voltage-dependent Ca2+ channels may not be a primary action of dantrolene because K+-stimulated 45Ca2+ uptake was not inhibited. Glucose utilization (3H2O formation) was unaffected by dantrolene, whereas glucose oxidation (14CO2 production) was decreased. In the absence of Ca2+, the glucose-inhibited 45Ca2+ efflux was unchanged. At normal Ca2+, dantrolene inhibited glucose-stimulated 45Ca2+ efflux and veratridine induced insulin release. This suggests an interference with mobilization of beta-cell calcium stores. The selective action of dantrolene on insulin release makes it an interesting tool for further studies on stimulus-secretion coupling.

Effects of hypophysectomy and growth hormone on cultured islets of Langerhans of the rat

The effects of islet function of addition to the culture medium of rat growth hormone was studied in 4-day cultured islets of Langerhans from normal and hypophysectomised rats. In islets from hypophysectomised rats, rates of insulin release were 34% lower than in control rat islets; rates of insulin plus proinsulin and total protein biosynthesis were also lower by 48% and 16% respectively. The rates of glucose oxidation and the islet content of cyclic AMP were unchanged in islets from hypophysectomised rats but the islet content of calmodulin was decreased by 68%. The presence of rat growth hormone during the culture period restored the secretory response of hypophysectomised rat islets to that seen in control islets cultured without growth hormone but had only a marginal effect on the rate of insulin plus proinsulin biosynthesis, and no significant effect on islet calmodulin content. Glucose oxidation was increased by the presence of growth hormone during the culture period in both control (73% increase) and hypophysectomized (38% increase) rat islets. Addition of growth hormone to the culture medium also enhanced rates of insulin release and biosynthesis in control islets by 116% and 20% respectively. It is suggested that these changes arise primarily from modification of the synthesis of specific islet proteins.

Glucagon and tolbutamide as insulin secretagogues in the pig (Sus domesticus)

1. Glucagon tolbutamide, either alone or in combination, were injected i.v. into pigs and the effect upon plasma glucose and insulin concentrations measured. 2. Glucagon gave similar insulin responses to those seen in humans, but insulin responses to tolbutamide were less than in humans. 3. Combined doses of glucagon and tolbutamide gave similar, though reduced, responses to those seen in humans. At the highest combined doses applied, glucose concentration remained reduced for up to 6 hr. The insulin responses were approximately equal to the sum of the responses to each substance given alone.

Insulin secretion induced by glucose and by stimulation of beta 2 -adrenoceptors in the rat. Different sensitivity to somatostatin

The effects of somatostatin on insulin secretion stimulated by glucose or by the selective beta 2-adrenoceptor agonist terbutaline were studied in vivo in the anaesthetized rat. Infusion of low doses of glucose (5 mg/min) or terbutaline (2 microgram/min) caused slight stimulation of insulin secretion, whereas infusion of higher doses of glucose (12.5 mg/min) or terbutaline (200 microgram/min) yielded higher rates of insulin release. In both instances plasma insulin concentrations were of comparable magnitudes immediately prior to somatostatin infusion. Somatostatin (0.1 microgram/min) inhibited the insulin response to glucose and terbutaline, both at the low and high secretory rates. However, the inhibitory effect of somatostatin was much more pronounced on insulin release during glucose infusion than during infusion of terbutaline. Thus, at the high rate of insulin secretion somatostatin depressed plasma insulin by 46% during glucose and by 22% during terbutaline infusion. The results at the low rate insulin secretion were 66% and 48%, respectively. Both at high and low secretory rates somatostatin depressed plasma insulin levels more potently during glucose infusion than during terbutaline infusion (p less than 0.01 and P less than 0.05, respectively). Furthermore, the plasma insulin levels during inhibition by somatostatin following terbutaline stimulation stabilized after approximately 10 min of somatostatin infusion, whereas following glucose stimulation the insulin levels continued to decline. The results suggest that somatostatin inhibits insulin secretion via mechanisms that are more closely related to the insulin secretory pathway induced by glucose than to that induced by beta-adrenoceptor agonists.

Evidence for the involvement of Na/Ca exchange in glucose-induced insulin release from rat pancreatic islets

Glucose-induced inhibition of Ca(++) extrusion from the beta-cell may contribute to the rise in cytosol Ca(++) that leads to insulin release. To study whether interference with Na/Ca exchange is involved in this inhibition the effects of glucose were compared to those of ouabain. This substance inhibits Na/K ATPase, decreases the transmembrane Na(+) gradient in islets, and thus interferes with Na/Ca exchange. Collagenase isolated rat islets were maintained for 2 d in tissue culture with a trace amount of (45)Ca(++). Insulin release and (45)Ca(++) efflux were then measured during perifusion. In Ca(++)-deprived medium (to avoid changes in tissue specific radioactivity) 16.7 mM glucose inhibited (45)Ca(++) efflux. Initially 1 mM ouabain inhibited (45)Ca(++) efflux in a similar fashion, the onset being even faster than that of glucose. The effects of 16.7 mM glucose and ouabain were not additive, indicating that both substances may interfere with Na/Ca exchange. In the presence of Ca(++), 16.7 mM glucose induced biphasic insulin release. Ouabain alone caused a gradual increase of insulin release. Again, the effects of ouabain and 16.7 mM glucose were not additive. In contrast, at a submaximal glucose concentration (7 mM) ouabain enhanced both phases of release. An important role for Na/Ca exchange is suggested from experiments in which Ca(++) was removed at the time of glucose-stimulation (16.7 mM). The resulting marked inhibition of insulin release was completely overcome during first phase by ouabain added at the time of Ca(++) removal; second phase was restored to 60%. This could be due to the rapid inhibitory action of ouabain on Ca(++) efflux thereby preventing loss of cellular calcium critical for glucose to induce insulin release. It appears, therefore, that interference with Na/Ca exchange is an important event in the stimulation of insulin release by glucose.

Possible role for calmodulin in insulin release. Studies with trifluoperazine in rat pancreatic islets

The role of calmodulin in insulin secretion from rat pancreatic islets has been examined by the use of trifluoperazine, an inhibitor of calmodulin-Ca++-directed functions. It was found that 30 microM trifluoperazine caused 50% inhibition, and 100 microM, up to 73% inhibition of 16.7 mM glucose-stimulated insulin release. 100 microM trifluoperazine caused a similar inhibition of 10 mM glyceraldehyde-stimulated release. Therefore, the site of action of trifluoperazine in glucose stimulus-secretion coupling appears to be after the trioses. As trifluoperazine had no effect upon insulin release stimulated by 1 mM 3-isobutyl-1-methylxanthine, the inhibitory effect of trifluoperazine appears to be rather specific. Further, the process of exocytosis per se is not affected. It was also found that although trifluoperazine inhibited the effect of glucose to stimulate insulin release, it did not affect the synergism between glucose and 3-isobutyl-1-methylxanthine to potentiate insulin release. It may be concluded that trifluoperazine selectively inhibits one part of the mechanism by which glucose stimulates insulin release. Calmodulin plays a role in the stimulation of insulin release by glucose at a site between metabolism of trioses and elevation of cytosol Ca++, but is not involved in the final process of exocytosis.

Role of Ca2+ in impaired insulin release from islets of diabetic (C57BL/KsJ-db/db) mice

The involvement of Ca2+ in the impaired insulin release of diabetic C57BL/KsJ-db/db mice was studied. Twenty-week-old severely hyperglycemic mice were compared to nondiabetic C57BL/KsJ mice as controls. Collagenase-isolated islets were maintained for 46 h in tissue culture allowing for equilibration at the same glucose concentration (8.3) mM). The insulin content of both types of islets was similar. In control islets preloaded during culture with 45Ca2+ glucose-induced insulin release was associated with increased 45Ca2+ effux. Islets from diabetic mice showed markedly reduced insulin response to glucose and a smaller increase in 45Ca2+ efflux. Because insulin release was strikingly potentiated by 3-isobutyl-1-methylxanthine (IBMX), even more than in control islets, there was no generalized release defect. In both types of islets, IBMX potentiation was accompanied by a further enhanced 45Ca2+ efflux, possibly suggesting that cAMP effects are associated with increased cytosol Ca2+% concentrations. As Ca2+ uptake was stimulated by glucose in both types of islets, a defect may lie in the mechanism by which glucose uses cellulr calcium to raise cytosol Ca2+ in the beta-cell of these diabetic mice.