Sulphonylureas do not increase insulin secretion by a mechanism other than a rise in cytoplasmic Ca2+ in pancreatic B-cells

Harnessing Human Pluripotent Stem Cell-Derived Pancreatic In Vitro Models for High-Throughput Toxicity Testing and Diabetes Drug Discovery

The long-standing goals in diabetes research are to improve β-cell survival, functionality and increase β-cell mass. Current strategies to manage diabetes progression are still not ideal for sustained maintenance of normoglycemia, thereby increasing demand for the development of novel drugs. Available pancreatic cell lines, cadaveric islets, and their culture methods and formats, either 2D or 3D, allow for multiple avenues of experimental design to address diverse aims in the research setting. More specifically, these pancreatic cells have been employed in toxicity testing, diabetes drug screens, and with careful curation, can be optimized for use in efficient high-throughput screenings (HTS). This has since spearheaded the understanding of disease progression and related mechanisms, as well as the discovery of potential drug candidates which could be the cornerstone for diabetes treatment. This book chapter will touch on the pros and cons of the most widely used pancreatic cells, including the more recent human pluripotent stem cell-derived pancreatic cells, and HTS strategies (cell models, design, readouts) that can be used for the purpose of toxicity testing and diabetes drug discovery.

Assessment of re-aggregated human pancreatic islets for secondary drug screening

BACKGROUND AND PURPOSE

Insulin secretion from isolated pancreatic islets is a pivotal assay in developing novel insulin secretagogues, given its good correlation with in vivo efficacy. Because the supply of human islets is limited, this assay is typically run with rodent islets, which do not address species differences and are low-throughput, because of the size matching or volume normalization required. Here we have evaluated the suitability of human re-aggregated islets for this assay.

EXPERIMENTAL APPROACH

We generated re-aggregated human islets of a consistent size, using micromolds and compared their responses with those of native human and rat islets, to known secretagogues and inhibitors of insulin release.

KEY RESULTS

Insulin secretion from rat islets, human islets and human re-aggregated cell clusters was concentration-dependently increased by glucose. The calcium channel agonist, Bay K 8644, stimulated insulin secretion in native rat islets and human re-aggregated islets, but not native human islets. Glibenclamide and tolbutamide were more effective and potent in re-aggregated human clusters compared with the other two preparations. Rat islets outperformed both human preparations of islets in response to caffeine, carbachol and glucagon-like peptide-1. Re-aggregated human islet clusters were more sensitive to somatostatin, diazoxide and sodium azide, but rodent islets were more sensitive to nifedipine.

CONCLUSIONS AND IMPLICATIONS

Human re-aggregated clusters of islet cells, of a constant size were more responsive to all compounds tested than native human islets. Importantly, the assay variability was less in the re-aggregated cluster preparations, which suggests that such re-aggregated cells could be useful for drug development.

Activators of PKA and Epac distinctly influence insulin secretion and cytosolic Ca2+ in female mouse islets stimulated by glucose and tolbutamide

Amplification of insulin secretion by cAMP is mediated by protein kinase A (PKA) and exchange protein directly activated by cAMP (Epac). Using selective activators, we determined how each effector influences the cytosolic free Ca(2+) concentration ([Ca(2+)]c) and insulin secretion in mouse islets. Alone PKA activator amplified glucose- and tolbutamide-induced insulin secretion, with a greater impact on second than first phase. Epac activator strongly amplified both phases in response to either secretagogue. Amplification was even greater when activators were combined. Although both activators similarly amplified glucose-induced insulin secretion, Epac activator was particularly efficient on tolbutamide-induced insulin secretion. That greater efficacy is attributed to higher [Ca(2+)]c rather than interaction of tolbutamide with Epac, because it was also observed during KCl stimulation. Moreover, in contrast to Epac activator, tolbutamide was inactive when insulin secretion was increased by gliclazide, and its effect on glucose-induced insulin secretion was unaffected by an inhibitor of Epac2. PKA activator increased [Ca(2+)]c during acute or steady-state glucose stimulation, whereas Epac activator had no effect alone or in combination. Neither activator affected [Ca(2+)]c response to tolbutamide or KCl. Metabolic (glucose-mediated) amplification of insulin secretion was unaffected by PKA activator. It was attenuated when insulin secretion was augmented by Epac activator but insensitive to Epac2 inhibitor, which suggests distinct although somewhat overlapping mechanisms. In conclusion, activators of PKA and Epac amplify insulin secretion by augmenting the action of Ca(2+) on exocytosis and, for PKA only, slightly increasing glucose-induced [Ca(2+)]c rise. The influence of Epac seems more important than that of PKA during first phase.

Impaired Ca(2+) signaling in β-cells lacking leptin receptors by Cre-loxP recombination

Obesity is a major risk factor for diabetes and is typically associated with hyperleptinemia and a state of leptin resistance. The impact of chronically elevated leptin levels on the function of insulin-secreting β-cells has not been elucidated. We previously generated mice lacking leptin signaling in β-cells by using the Cre-loxP strategy and showed that these animals develop increased body weight and adiposity, hyperinsulinemia, impaired glucose-stimulated insulin secretion and insulin resistance. Here, we performed several in vitro studies and observed that β-cells lacking leptin signaling in this model are capable of properly metabolizing glucose, but show impaired intracellular Ca²⁺ oscillations and lack of synchrony within the islets in response to glucose, display reduced response to tolbutamide and exhibit morphological abnormalities including increased autophagy. Defects in intracellular Ca²⁺ signaling were observed even in neonatal islets, ruling out the possible contribution of obesity to the β-cell irregularities observed in adults. In parallel, we also detected a disrupted intracellular Ca²⁺ pattern in response to glucose and tolbutamide in control islets from adult transgenic mice expressing Cre recombinase under the rat insulin promoter, despite these animals being glucose tolerant and secreting normal levels of insulin in response to glucose. This unexpected observation impeded us from discerning the consequences of impaired leptin signaling as opposed to long-term Cre expression in the function of insulin-secreting cells. These findings highlight the need to generate improved Cre-driver mouse models or new tools to induce Cre recombination in β-cells.

The molecular mechanisms of pancreatic β-cell glucotoxicity: recent findings and future research directions

It is well established that regular physiological stimulation by glucose plays a crucial role in the maintenance of the β-cell differentiated phenotype. In contrast, prolonged or repeated exposure to elevated glucose concentrations both in vitro and in vivo exerts deleterious or toxic effects on the β-cell phenotype, a concept termed as glucotoxicity. Evidence indicates that the latter may greatly contribute to the pathogenesis of type 2 diabetes. Through the activation of several mechanisms and signaling pathways, high glucose levels exert deleterious effects on β-cell function and survival and thereby, lead to the worsening of the disease over time. While the role of high glucose-induced β-cell overstimulation, oxidative stress, excessive Unfolded Protein Response (UPR) activation, and loss of differentiation in the alteration of the β-cell phenotype is well ascertained, at least in vitro and in animal models of type 2 diabetes, the role of other mechanisms such as inflammation, O-GlcNacylation, PKC activation, and amyloidogenesis requires further confirmation. On the other hand, protein glycation is an emerging mechanism that may play an important role in the glucotoxic deterioration of the β-cell phenotype. Finally, our recent evidence suggests that hypoxia may also be a new mechanism of β-cell glucotoxicity. Deciphering these molecular mechanisms of β-cell glucotoxicity is a mandatory first step toward the development of therapeutic strategies to protect β-cells and improve the functional β-cell mass in type 2 diabetes.

Both triggering and amplifying pathways contribute to fuel-induced insulin secretion in the absence of sulfonylurea receptor-1 in pancreatic beta-cells

In normal beta-cells glucose induces insulin secretion by activating both a triggering pathway (closure of K(ATP) channels, depolarization, and rise in cytosolic [Ca(2+)](i)) and an amplifying pathway (augmentation of Ca(2+) efficacy on exocytosis). It is unclear if and how nutrients can regulate insulin secretion by beta-cells lacking K(ATP) channels (Sur1 knockout mice). We compared glucose- and amino acid-induced insulin secretion and [Ca(2+)](i) changes in control and Sur1KO islets. In 1 mm glucose (non-stimulatory for controls), the triggering signal [Ca(2+)](i) was high (loss of regulation) and insulin secretion was stimulated in Sur1KO islets. This "basal" secretion was decreased or increased by imposed changes in [Ca(2+)](i) and was dependent on ATP production, indicating that both triggering and amplifying signals are involved. High glucose stimulated insulin secretion in Sur1KO islets, by an unsuspected, transient increase in [Ca(2+)](i) and a sustained activation of the amplifying pathway. Unlike controls, Sur1KO islets were insensitive to diazoxide and tolbutamide, which rules out effects of either drug at sites other than K(ATP) channels. Amino acids potently increased insulin secretion by Sur1KO islets through both a further electrogenic rise in [Ca(2+)](i) and a metabolism-dependent activation of the amplifying pathway. After sulfonylurea blockade of their K(ATP) channels, control islets qualitatively behaved like Sur1KO islets, but their insulin secretion rate was consistently lower for a similar or even higher [Ca(2+)](i). In conclusion, fuel secretagogues can control insulin secretion in beta-cells without K(ATP) channels, partly by an unsuspected influence on the triggering [Ca(2+)](i) signal and mainly by the modulation of a very effective amplifying pathway.

Capacitance measurements of exocytosis in mouse pancreatic alpha-, beta- and delta-cells within intact islets of Langerhans

Capacitance measurements of exocytosis were applied to functionally identified alpha-, beta- and delta-cells in intact mouse pancreatic islets. The maximum rate of capacitance increase in beta-cells during a depolarization to 0 mV was equivalent to 14 granules s(-1), <5% of that observed in isolated beta-cells. Beta-cell secretion exhibited bell-shaped voltage dependence and peaked at +20 mV. At physiological membrane potentials (up to approximately -20 mV) the maximum rate of release was approximately 4 granules s(-1). Both exocytosis (measured by capacitance measurements) and insulin release (detected by radioimmunoassay) were strongly inhibited by the L-type Ca(2+) channel blocker nifedipine (25 microm) but only marginally (<20%) affected by the R-type Ca(2+) channel blocker SNX482 (100 nm). Exocytosis in the glucagon-producing alpha-cells peaked at +20 mV. The capacitance increases elicited by pulses to 0 mV exhibited biphasic kinetics and consisted of an initial transient (150 granules s(-1)) and a sustained late component (30 granules s(-1)). Whereas addition of the N-type Ca(2+) channel blocker omega-conotoxin GVIA (0.1 microm) inhibited glucagon secretion measured in the presence of 1 mm glucose to the same extent as an elevation of glucose to 20 mm, the L-type Ca(2+) channel blocker nifedipine (25 microm) had no effect. Thus, glucagon release during hyperglycaemic conditions depends principally on Ca(2+)-influx through N-type rather than L-type Ca(2+) channels. Exocytosis in the somatostatin-secreting delta-cells likewise exhibited two kinetically separable phases of capacitance increase and consisted of an early rapid (600 granules s(-1)) component followed by a sustained slower (60 granules s(-1)) component. We conclude that (1) capacitance measurements in intact pancreatic islets are feasible; (2) exocytosis measured in beta-cells in situ is significantly slower than that of isolated cells; and (3) the different types of islet cells exhibit distinct exocytotic features.

Desensitization of insulin secretion

Desensitization of insulin secretion describes a reversible state of decreased secretory responsiveness of the pancreatic beta-cell, induced by a prolonged exposure to a multitude of stimuli. These include the main physiological stimulator, glucose, but also other nutrients like free fatty acids and practically all pharmacological stimulators acting by depolarization and Ca2+ influx into the beta-cell. Desensitization of insulin secretion appears to be an important step in the manifestation of type 2 diabetes and in the secondary failure of oral antidiabetic treatment. In this commentary, the basic concepts and the controversial issues in the field will be outlined. With regard to glucose-induced desensitization, two fundamentally opposing concepts have emerged. The first is that desensitization is the consequence of functional changes in the beta-cell that impair glucose-recognition. The second is that long-term increased secretory activity leads to a depletion of releasable insulin, often in spite of increased insulin synthesis. The latter concept is more appropriately termed beta-cell exhaustion. The same dichotomy applies to the desensitization evoked by pharmacological stimuli: again the relative contributions of a decreased insulin content versus alterations in signal transduction are in dispute. The action of tolbutamide on beta-cells may be an example of desensitization caused by a lack of releasable insulin since the signaling mechanisms are nearly unchanged, whereas the action of phentolamine, an imidazoline, induces a strong desensitization without reducing insulin content or secretory granules, apparently by abolishing Ca2+ influx. With pharmacological agents it seems that both, alterations in signal transduction and decreased availability of releasable insulin, can contribute to the desensitized state of the beta-cell, the relative contribution being variable depending upon the exact nature of the secretory stimulus.

Relationships between the Na(+)/K(+) pump and ATP and ADP content in mouse pancreatic islets: effects of meglitinide and glibenclamide

We have previously demonstrated that both D-glucose and glibenclamide stimulate the Na(+)/K(+) pump and suggested that this may be part of the membrane repolarization process, following the primary depolarization by these agents. The aim of this study was to investigate whether the non-sulphonylurea meglitinide (HB 699) exerts similar effects as glibenclamide or glucose on the islet Na(+)/K(+) pump and if effects of meglitinide or glibenclamide on this pump activity is paralleled by changes in islet ATP content and/or ATP/ADP ratio. The acyl-amino-alkyl benzoic acid derivative, meglitinide, stimulated the islet ouabain-sensitive portion of (86)Rb(+) influx (Na(+)/K(+) pump) by 53%, while the ouabain-resistant portion was inhibited by 70%. The stimulatory effect was not additive to that caused by D-glucose, suggesting that both agents may activate the Na(+)/K(+) pump via the same mechanism. Glibenclamide (10 microM) decreased the islet ATP and ADP content as well as the ATP/ADP ratio at 0 mM glucose. These effects were no longer observed at 10 mM glucose. Meglitinide (10 or 50 microM) lowered the islet ATP and ADP content at 0 mM glucose without affecting the ATP/ADP ratio. At 10 mM glucose, however, 10 microM of the drug reduced the islet ATP content but not the ATP/ADP ratio, while 50 microM of the drug, besides lowering the ATP content, also reduced the ATP/ADP ratio. Diazoxide (0.5 mM) increased the islet ATP content in the absence of glucose, an effect not seen in the presence of 10 mM glucose. The rate of glucose oxidation at 1, 10 or 20 mM of the sugar was not affected by glibenclamide (0.1 - 10 microM) and at 10 or 20 mM of the sugar not affected by meglitinide (1 - 100 microM). These results suggest that glibenclamide and meglitinide lower the islet ATP level by indirectly activating the beta-cell Na(+)/K(+) pump, which is a major consumer of ATP in the islets, while diazoxide increases the ATP level due to inhibition of the pump.

Calcium-stimulated insulin secretion in diffuse and focal forms of congenital hyperinsulinism

OBJECTIVES

To identify infants with hyperinsulinism caused by defects of the beta-cell adenosine triphosphate-dependent potassium channel complex and to distinguish focal and diffuse forms of hyperinsulinism caused by these mutations.

STUDY DESIGN

The acute insulin response to intravenous calcium stimulation (CaAIR) was determined in 9 patients <20 years with diffuse hyperinsulinism caused by defective beta-cell sulfonylurea receptor (SUR1(-/-)), 3 patients with focal congenital hyperinsulinism (6 weeks to 18 months), a 10-year-old with insulinoma, 5 with hyperinsulinism/hyperammonemia syndrome caused by defective glutamate dehydrogenase (6 months to 28 years), 4 SUR1(+/-) heterozygotes with no symptoms, and 9 normal adults. Three infants with congenital focal disease, 1 with diffuse hyperinsulinism, and the child with insulinoma underwent selective pancreatic intra-arterial calcium stimulation with hepatic venous sampling.

RESULTS

Children with diffuse SUR1(-/-) disease and infants with congenital focal hyperinsulinism responded to CaAIR, whereas the normal control group, patients with hyperinsulinism/hyperammonemia syndrome, and SUR1(+/-) carriers did not. Selective arterial calcium stimulation of the pancreas with hepatic venous sampling revealed selective, significant step-ups in insulin secretion that correlated anatomically with the location of solitary lesions confirmed surgically in 2 of 3 infants with congenital focal disease and in the child with insulinoma. Selective arterial calcium stimulation of the pancreas with hepatic venous sampling demonstrated markedly elevated baseline insulin levels throughout the pancreas of the infant with diffuse hyperinsulinism.

CONCLUSIONS

The intravenous CaAIR is a safe and simple test for identifying infants with diffuse SUR1(-/-) hyperinsulinism or with focal congenital hyperinsulinism. Preoperative selective arterial calcium stimulation of the pancreas with hepatic venous sampling can localize focal lesions causing hyperinsulinism in children. The combination of these calcium stimulation tests may help distinguish focal lesions suitable for cure by local surgical resection.

Potentiation of quantal catecholamine secretion by glibenclamide: evidence for a novel role of sulphonylurea receptors in regulating the Ca(2+) sensitivity of exocytosis

Electrochemical detection of quantal catecholamine release from PC-12 cells revealed that glibenclamide, an inhibitor of ATP-sensitive K(+) channels, potentiated Ca(2+)-dependent exocytosis evoked by raised extracellular [K(+)] and by exposure of cells to caffeine. Glibenclamide was without effect on voltage-gated Ca(2+) currents, membrane potential, or rises of [Ca(2+)](i) evoked by either raised extracellular [K(+)] or caffeine. The dependence of K(+)-evoked secretion on extracellular Ca(2+) was shifted leftward in the presence of glibenclamide, with a small increase in the plateau level of release, suggesting that glibenclamide primarily increased the Ca(2+) sensitivity of the exocytotic apparatus. Enhancement of secretion by glibenclamide was reversed by pinacidil and cromakalim, indicating that the effects of glibenclamide were mediated via an action on a sulfonylurea receptor. These results demonstrate that sulfonylurea receptors can modulate Ca(2+)-dependent exocytosis via a mechanism downstream of Ca(2+) influx or mobilization.

Imidazolines and pancreatic hormone secretion

A range of imidazoline derivatives are known to be effective stimulators of insulin secretion, and this response correlates with closure of ATP-sensitive potassium channels in the pancreatic beta-cell. However, mounting evidence indicates that potassium channel blockade may form only part of the mechanism by which imidazolines exert their effects on insulin secretion. Additionally, it remains unclear whether members of this class of drugs can bind directly to potassium channel components and whether occupation of a single binding site accounts for their functional activity. This review considers recent developments in the field and highlights evidence that does not fit readily with the concept that a single mechanism of action is sufficient to mediate the effects of imidazolines on pancreatic hormone secretion.

Imidazolines and the pancreatic B-cell. Actions and binding sites

Stimulation of insulin secretion by imidazoline compounds displays variable characteristics. Phentolamine (10-100 microM) increased secretion of perifused mouse islets at nonstimulatory glucose concentrations (5 mM) and even in the absence of glucose. Idazoxan (20-100 microM) elicited a moderate increase in insulin secretion, which required the presence of a stimulatory glucose concentration (10 mM). Phentolamine is therefore a stimulator of secretion in its own right, whereas idazoxan may be termed an enhancer of secretion. Both compounds inhibited the activity of ATP-dependent K+ channels in inside-out patches from B-cells; however, idazoxan achieved only an incomplete block. Both compounds depolarized the B-cell plasma membrane to an extent that permitted the opening of voltage-dependent Ca2+ channels (-40 to -30 mV). An increase in cytoplasmic Ca2+ concentration was induced by phentolamine and much less so by idazoxan. Activation of protein kinase C, a possible mechanism to amplify Ca(2+)-induced secretion, could not be verified for phentolamine. It thus appears that stimulation of insulin secretion by phentolamine is due to its blocking effect on KATP channels, which may be the correlate of non-adrenergic imidazoline binding sites which were characterized in insulin-secreting HIT cells. Whether incomplete closure of KATP channels by idazoxan or additional effects are responsible for the requirement of high glucose to stimulate secretion remains to be clarified.

The stimulatory action of tolbutamide on Ca2+-dependent exocytosis in pancreatic beta cells is mediated by a 65-kDa mdr-like P-glycoprotein

Intracellular application of the sulfonylurea tolbutamide during whole-cell patch-clamp recordings stimulated exocytosis >5-fold when applied at a cytoplasmic Ca2+ concentration of 0.17 microM. This effect was not detectable in the complete absence of cytoplasmic Ca2+ and when exocytosis was elicited by guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS). The stimulatory action could be antagonized by the sulfonamide diazoxide, by the Cl--channel blocker 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), by intracellular application of the antibody JSB1 [originally raised against a 170-kDa multidrug resistance (mdr) protein], and by tamoxifen (an inhibitor of the mdr- and volume-regulated Cl- channels). Immunocytochemistry and Western blot analyses revealed that JSB1 recognizes a 65-kDa protein in the secretory granules. This protein exhibited no detectable binding of sulfonylureas and is distinct from the 140-kDa sulfonylurea high-affinity sulfonylurea receptors also present in the granules. We conclude that (i) tolbutamide stimulates Ca2+-dependent exocytosis secondary to its binding to a 140-kDa high-affinity sulfonylurea receptor in the secretory granules; and (ii) a granular 65-kDa mdr-like protein mediates the action. The processes thus initiated culminate in the activation of a granular Cl- conductance. We speculate that the activation of granular Cl- fluxes promotes exocytosis (possibly by providing the energy required for membrane fusion) by inducing water uptake and an increased intragranular hydrostatic pressure.

Glucose regulation of insulin secretion independent of the opening or closure of adenosine triphosphate-sensitive K+ channels in beta cells

Two major pathways are implicated in the stimulation of insulin secretion by glucose. The K+-ATP channel-dependent pathway involves closure of these channels, depolarization of the beta-cell membrane, acceleration of Ca2+ influx, and a rise in cytosolic free Ca2+ ([Ca2+]i). The K+-ATP channel-independent pathway potentiates the stimulation of exocytosis by high [Ca2+]i. To determine whether this second pathway is influenced by the configuration of the channel, we compared the effects of glucose on [Ca2+]i and insulin secretion in mouse islets under three conditions. First, in the presence of 20, 25, and 30 mM K+, i.e. without pharmacological action on K+-ATP channels, [Ca2+]i and insulin secretion were already elevated at 3 mM glucose. High glucose (20 mM) caused a transient decrease in [Ca2+]i followed by an ascent to slightly above control levels, and rapidly stimulated insulin secretion. Second, opening of K+-ATP channels with diazoxide did not influence [Ca2+]i and insulin secretion at 3 mM glucose and high K+. However, high glucose now caused a sustained lowering of [Ca2+]i accompanied by a slow increase in secretion that augmented with the K+ concentration. Third, when K+-ATP channels were blocked and beta-cells depolarized by high concentrations of tolbutamide or glibenclamide, [Ca2+]i and insulin secretion were elevated even in low glucose. High glucose transiently lowered [Ca2+]i, which then increased to or slightly above control levels, while insulin secretion was rapidly stimulated. Under all conditions the correlation between [Ca2+]i and insulin secretion was excellent at low and high glucose levels, and high glucose increased release at all [Ca2+]i. The potentiation of Ca2+-induced exocytosis by glucose is thus independent of the closed or open state of K+-ATP channels. It is only when the channels are opened by diazoxide that the increase in release is a strict amplification of the action of Ca2+. When the channels are closed (sulfonylureas) or still closable (high K+ alone), the effect of glucose on secretion also comprises a slight increase in [Ca2+]i and, in the latter case, is not strictly K+-ATP channel independent.

Stimulation of bile duct epithelial secretion by glybenclamide in normal and cholestatic rat liver

Cholestasis is a cardinal complication of liver disease, but most treatments are merely supportive. Here we report that the sulfonylurea glybenclamide potently stimulates bile flow and bicarbonate excretion in the isolated perfused rat liver. Video-microscopic studies of isolated hepatocyte couplets and isolated bile duct segments show that this stimulatory effect occurs at the level of the bile duct epithelium, rather than through hepatocytes. Measurements of cAMP, cytosolic pH, and Ca2+ in isolated bile duct cells suggest that glybenclamide directly activates Na+-K+-2Cl- cotransport, rather than other transporters or conventional second-messenger systems that link to secretory pathways in these cells. Finally, studies in livers from rats with endotoxin- or estrogen-induced cholestasis show that glybenclamide retains its stimulatory effects on bile flow and bicarbonate excretion even under these conditions. These findings suggest that bile duct epithelia may represent an important new therapeutic target for treatment of cholestatic disorders.

Signaling and sites of interaction for RX-871024 and sulfonylurea in the stimulation of insulin release

The objective of this study was to compare effects of RX-871024, a compound with imidazoline structure, and the sulfonylurea glibenclamide, representatives of two groups of ATP-dependent potassium channel (KATP) blockers, on insulin secretion and cytoplasmic free calcium concentration ([Ca2+]i). Furthermore, we studied the interaction of the compounds on these two parameters. The experiments were performed in the perfused rat pancreas, isolated rat pancreatic islets, and dispersed beta-cells. At maximal effective concentrations of the compounds, RX-871024 had a more pronounced insulinotropic effect than glibenclamide, but the increase in [Ca2+]i was similar. Glibenclamide enhanced the insulinotropic effect of suboptimal concentrations of RX-871024 at 3.3 and 16.7 mM glucose. Notably, glibenclamide and RX-871024 also stimulated insulin secretion under Ca(2+)-clamped conditions, i.e., during plasma membrane depolarization with KCl and glucose or in permeabilized islets. The magnitudes of insulin stimulation under the latter types of conditions were similar for both compounds. It is concluded that RX-871024 and the sulfonylurea glibenclamide promote insulin secretion by two mechanisms, namely closure of KATP channels and a direct stimulation of exocytosis. At a similar increase in [Ca2+]i, the maximal stimulatory effect of RX-871024 on insulin secretion was stronger than that of glibenclamide, implying that RX-871024 also affects insulin secretion by a signal transduction pathway that is not activated by glibenclamide.

Regulation by tolbutamide and diazoxide of the electrical activity in mouse pancreatic beta-cells recorded in vivo

1. The glucose-dependence of beta-cell electrical activity and the effects of tolbutamide and diazoxide were studied in anaesthetized mice. 2. In untreated animals there was a direct relationship between glycaemia and the burst pattern of electrical activity. Animals with high glucose concentration showed continuous electrical activity. The application of insulin led to a steady decrease in blood glucose concentration and a transition from continuous to oscillatory activity at 7.7+/-0.1 mM glucose (mean+/-s.d.) and a subsequent transition from oscillatory to silent at 4.7+/-0.6 mM glucose. 3. At physiological blood glucose concentrations the electrical activity was oscillatory. The injection of tolbutamide (1800 mg kg[-1]) transformed this oscillatory pattern into one of continuous electrical activity. The increased electrical activity was associated with a decrease in blood glucose concentration from 7.1+/-0.9 (control) to 5.5+/-1.0 mM (10 min after tolbutamide injection). The effects of tolbutamide are consistent with a direct blocking effect on the K(ATP) channel that leads to membrane depolarization. 4. The injection of diazoxide (6000 mg kg[-1]) hyperpolarized the cells and transformed the oscillatory pattern into a silent one. This is consistent with a direct stimulant effect by diazoxide on the K(ATP) channel. The use of tolbutamide or diazoxide correspondingly led to the lengthening or shortening of the active phase of electrical activity, respectively. This indicates that in vivo, such activity can be modulated by the relative degree of activation or inhibition of the K(ATP) channel. 5. These results indicate that under physiological conditions, tolbutamide and diazoxide have direct and opposite effects on the electrical activity of pancreatic beta-cells, most likely through their action on K(ATP) channels. This is consistent with previous work carried out on in vitro models and explains the drugs hypo- and hyperglycaemic effects.

Adrenaline inhibits depolarization-induced increases in capacitance the presence of elevated [Ca2+]i in insulin secreting cells

Cell capacitance (Cm), cell conductance (Gm), access conductance (Ga) and membrane voltage (Vm) were measured simultaneously in insulin secreting cells using the dual frequency method. Depolarization and stimulation of the cells with secretagogues increased Cm. EGTA abolished the increase in [Ca2+]i and prevented the rise of Cm. Adrenaline inhibited the augmentation of Cm without lowering [Ca2+]i. In pertussis toxin pretreated cells adrenaline had no effect. Thus, stimulation of insulin secretion is accompanied by an increase in Cm. Inhibition of exocytosis by adrenaline occurs even in the presence of elevated [Ca2+]i, i.e. at a more distal step of exocytosis.

Oscillatory Ca2+ signaling in somatostatin-producing cells from the human pancreas

Oscillatory Ca2+ signaling was studied in human somatostatin-releasing pancreatic delta cells identified by immunostaining. A ratiometric fura-2 technique was used for measuring cytoplasmic concentrations of Ca2+ and Sr2+ in delta cells exposed to the respective cation. Rhythmic activity in terms of slow (frequency, 0.1 to 0.4 per minute) oscillations from close to the basal level was seen in the presence of 3 to 20 mmol/L glucose during superfusion with medium containing 2.6 to 5 mmol/L Ca2+ or 5 mmol/L Sr2. These oscillations could be transformed into a sustained increase by decreasing extracellular Ca2+ or adding 1 mmol/L tolbutamide or 20 nmol/L glucagon. Addition of glucagon to a medium containing 20 mmol/L glucose resulted in the generation of short (< 30 seconds) transients, which disappeared upon exposure to 100 nmol/L of the intracellular Ca(2+)-adenosine triphosphatase (ATPase) inhibitor thapsigargin. When analyzing small aggregates of islet cells, it became evident that oscillatory activity in delta cells can be synchronous with that in adjacent non-delta cells. It is concluded that secretion of pancreatic somatostatin in man involves Ca2+ signaling similar to that regulating the pulsatile release of insulin.