New mechanisms for sulfonylurea control of insulin secretion

Observing Islet Function and Islet-Immune Cell Interactions in Live Pancreatic Tissue Slices

Live pancreatic tissue slices allow for the study of islet physiology and function in the context of an intact islet microenvironment. Slices are prepared from live human and mouse pancreatic tissue embedded in agarose and cut using a vibratome. This method allows for the tissue to maintain viability and function in addition to preserving underlying pathologies such as type 1 (T1D) and type 2 diabetes (T2D). The slice method enables new directions in the study of the pancreas through the maintenance of the complex structures and various intercellular interactions that comprise the endocrine and exocrine tissues of the pancreas. This protocol demonstrates how to perform staining and time-lapse microscopy of live endogenous immune cells within pancreatic slices along with assessments of islet physiology. Further, this approach can be refined to discern immune cell populations specific for islet cell antigens using major histocompatibility complex-multimer reagents.

Anti-incretin, Anti-proliferative Action of Dopamine on β-Cells

Human islet β-cells exploit an autocrine dopamine (DA)-mediated inhibitory circuit to regulate insulin secretion. β-Cells also express the DA active transporter and the large neutral amino acid transporter heterodimer enabling them to import circulating DA or its biosynthetic precursor, L-3,4-dihydroxyphenylalanine (L-DOPA). The capacity to import DA or L-DOPA from the extracellular space possibly indicates that DA may be an endocrine signal as well. In humans, a mixed meal stimulus is accompanied by contemporary serum excursions of incretins, DA and L-DOPA, suggesting that DA may act as an anti-incretin as postulated by the foregut hypothesis proposed to explain the early effects of bariatric surgery on type 2 diabetes. In this report, we take a translational step backwards and characterize the kinetics of plasma DA and incretin production after a mixed meal challenge in a rat model and study the integration of incretin and DA signaling at the biochemical level in a rodent β-cell line and islets. We found that there are similar excursions of incretins and DA in rats, as those reported in humans, after a mixed meal challenge and that DA counters incretin enhanced glucose-stimulated insulin secretion and intracellular signaling at multiple points from dampening calcium fluxes to inhibiting proliferation as well as apoptosis. Our data suggest that DA is an important regulator of insulin secretion and may represent 1 axis of a gut level circuit of glucose and β-cell mass homeostasis.

Triphenyltin impairs insulin secretion by decreasing glucose-induced NADP(H) and ATP production in hamster pancreatic β-cells

Oral administration of triphenyltin chloride (TPT) (6 mg/100g body weight) inhibits insulin secretion by decreasing glucose-induced cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) in pancreatic β-cells of the hamster. To test the possibility that the abnormal level of the [Ca(2+)](i) induced by TPT administration could be due to a defect in the metabolic signal of glucose in the β-cells, we tested the effects of TPT administration on the glucose-induced NAD(P)H and ATP production, and on the changes of membrane potential and [Ca(2+)](i) by glucose and high K(+) in the β-cells. The [Ca(2+)](i) was measured in islet cells loaded with fura-2. TPT administration significantly reduced the NAD(P)H and ATP production, the depolarization of plasma membrane, and insulin secretion by 15 mM glucose in islet cells. TPT administration also reduced the insulin secretion by 10mM dihydroxyacetone and glyceraldehyde. However, TPT administration did not affect the increase of [Ca(2+)](i) and the insulin secretion by 30 mMK(+) or 100 μM tolbutamide, and the membrane potential by 30 mMK(+), and the insulin secretion by 10mM α-ketoisocaproic acid and 0.5mM formycin A, an analog of ATP in the presence of 15 mM glucose. These results suggested that the pathogenesis of TPT-induced hyperglycemia in hamster involves the reduction of [Ca(2+)](i) and insulin secretion in response to K(ATP) channel-dependent depolarization, which is related to the decrease of NAD(P)H and ATP production in pancreatic islet cells after glucose metabolism.

Antidiabetic activity and toxicity of Zizyphus spina-christi leaves

The effect of the butanol extract of Zizyphus spina-christi (L.), Willd (Rhamnaceae) leaves and its major saponin glycoside, christinin-A, on the serum glucose and insulin levels was studied in non-diabetic control, type-I (insulin-dependent) and type-II (non-insulin-dependent) diabetic rats. Pretreatment either with 100 mg/kg butanol extract or christinin-A potentiated glucose-induced insulin release in non-diabetic control rats. In type-II but not in type-I diabetic rats pretreatment with the butanol extract or christinin-A improved the oral glucose tolerance and potentiated glucose-induced insulin release. Treatment either with 100 mg/kg butanol extract or christinin-A reduced the serum glucose level and increased the serum insulin level of non-diabetic control and type-II diabetic rats but not of type-I diabetic rats. Effects of the butanol extract and christinin-A were similar. Pretreatment of non-diabetic control and type-II diabetic rats either with 100 mg/kg butanol extract or christinin-A enhanced the glucose lowering and insulinotropic effects of 5 g/kg glibenclamide. The hyperglycemic and hypoinsulinemic effects of 30 mg/kg diazoxide in non-diabetic control and type-II diabetic rats were inhibited and antagonized, respectively by pretreatment with the butanol extract or christinin-A. The relaxant effects of different concentrations of diazoxide on the isolated norepinephrine-contracted aortic strips were inhibited by 100 micromol/l christinin-A or 10 micromol/l glibenclamide. The combination of glibenclamide and christinin-A led to complete inhibition of the relaxant effects of different concentrations of diazoxide. At a dose level much higher than that required to produce satisfactory insulinotropic and hypoglycemic effects, the butanol extract of Zizyphus spina-christi leaves produced a depressant effect on the central nervous system in rats. Treatment of rats with 100mg/kg butanol extract for 3 months produced no functional or structural disturbances in liver and kidney and no haematological changes. In addition, the oral LD50 of the butanol extract in mice was 3820 mg/kg, while that of glibenclamide was 3160 mg/kg. Thus, Zizyphusspina-christi leaves appears to be a safe alternative to lower blood glucose. The safe insulinotropic and subsequent hypoglycemic effects of Zizyphus spina-christi leaves may be due to a sulfonylurea-like activity.

The potential antidiabetic activity of some alpha-2 adrenoceptor antagonists

The effect of alpha-2 adrenoceptor antagonists, yohimbine and efaroxan, on the plasma glucose and insulin levels was studied in non-diabetic control, type-I (insulin-dependent) and type-II (non-insulin-dependent) diabetic rats. Pretreatment with either yohimbine or efaroxan potentiated glucose-induced insulin release in non-diabetic control rats and produced an improvement of the oral glucose tolerance and potentiated glucose-induced insulin release in type-II but not in type-I diabetic rats. Treatment with either yohimbine or efaroxan reduced the plasma glucose level and increased the plasma insulin level of non-diabetic control and type-II diabetic rats but not of type-I diabetic rats. Effects of efaroxan were more marked. Pretreatment of non-diabetic control and type-II diabetic rats with either yohimbine or efaroxan inhibited clonidine-induced hyperglycaemia and suppressed or reversed clonidine-induced hypoinsulinaemia. Also, pretreatment of these animals with either yohimbine or efaroxan enhanced the hypoglycaemic and insulinotropic effects of glibenclamide. The combination of glibenclamide and efaroxan led to a synergistic increase in insulin secretion, while that of glibenclamide and yohimbine led to an additive increase. The hyperglycaemic effect of diazoxide in non-diabetic control and type-II diabetic rats was inhibited by pretreatment with either yohimbine or efaroxan. The hypoinsulinaemic effect of diazoxide in these animals was antagonized and reversed by pretreatment with yohimbine and efaroxan, respectively. In type-I diabetic rats, there was no change in the plasma glucose and insulin levels induced by the treatment of animals with each of clonidine or diazoxide alone or in combination with either yohimbine or efaroxan. Glibenclamide produced a slight decrease in the plasma glucose level of type-I diabetic rats, at the end of the 120 min period of investigation but there was no change in the plasma insulin level. Pretreatment of these animals with either yohimbine or efaroxan produced no change in glibenclamide effects. Additionally, bath application of efaroxan or glibenclamide inhibited the relaxant effects of different concentrations of diazoxide on the isolated norepinephrine-contracted aortic strips, while the application of yohimbine produced insignificant changes. The combination of glibenclamide and efaroxan led to complete inhibition of the relaxant effects of different concentrations of diazoxide, while that of glibenclamide and yohimbine did not produce such an effect. It is concluded that yohimbine, via blockade of postsynaptic alpha-2 adrenoceptors, and efaroxan, via blockade of postsynaptic alpha-2 adrenoceptors and adenosine triphosphate-sensitive potassium channels in the pancreatic beta-cell membrane, produce insulinotropic and subsequent hypoglycaemic effects.

Tolbutamide stimulation of pancreatic beta-cells involves both cell recruitment and increase in the individual Ca(2+) response

Individual pancreatic beta-cells are functionally heterogeneous. Their sensitivity to glucose is variable, so that the proportion of active cells increases with the glucose concentration (recruitment). We have investigated whether sulphonylureas also recruit beta-cells, by measuring cytoplasmic Ca(2+) ([Ca(2+)](i)) - the triggering signal of insulin secretion - in single cells and clusters of cells prepared from mouse islets. In 4 mM glucose, the threshold concentration of tolbutamide inducing a [Ca(2+)](i) rise was variable (5 - 50 microM). The proportion of responsive cells and clusters therefore increased with the tolbutamide concentration, to reach a maximum of 90% of the cells and 100% of the clusters. This recruitment occurred faster when the glucose concentration was increased from 4 to 5 mM (EC(50) of approximately 14 and approximately 4 microM tolbutamide respectively). Within responsive clusters little recruitment was observed; when a cluster was active, all or nearly all cells were active probably because of cell coupling. Thus, tolbutamide-induced [Ca(2+)](i) oscillations were synchronous in all cells of each cluster, whereas there was no synchrony between clusters or individual cells. Independently of cell recruitment, tolbutamide gradually augmented the magnitude of the [Ca(2+)](i) rise in single cells and clusters. This increase occurred over a broader range of concentrations than did recruitment (EC(50) of approximately 50 and 25 microM tolbutamide at 4 and 5 mM glucose respectively). Tolbutamide (10 microM) accelerated the recruitment of single cells and clusters brought about by increasing glucose concentrations (range of 3 - 7 mM instead of 4 - 10 mM glucose), and potentiated the amplification of the individual responses that glucose also produced. In conclusion, both metabolic (glucose) and pharmacologic (sulphonylurea) inhibition of K(+)-ATP channels recruits beta-cells to generate a [Ca(2+)](i) response. However, the response is not of an all-or-none type; it increases in amplitude with the concentration of either glucose or tolbutamide.

Chloride channels regulate HIT cell volume but cannot fully account for swelling-induced insulin secretion

Insulin-secreting pancreatic islet beta-cells possess anion-permeable Cl- channels (I(Cl,islet)) that are swelling-activated, but the role of these channels in the cells is unclear. The Cl- channel blockers 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) and niflumic acid were evaluated for their ability to inhibit I(Cl,islet) in clonal beta-cells (HIT cells). Both drugs blocked the channel, but the blockade due to niflumic acid was less voltage-dependent than the blockade due to DIDS. HIT cell volume initially increased in hypotonic solution and was followed by a regulatory volume decrease (RVD). The addition of niflumic acid and, to a lesser extent, DIDS to the hypotonic solution potentiated swelling and blocked the RVD. In isotonic solution, niflumic acid produced swelling, suggesting that islet Cl- channels are activated under basal conditions. The channel blockers glyburide, gadolinium, or tetraethylammonium-Cl did not alter hypotonic-induced swelling or volume regulation. The Na/K/2Cl transport blocker furosemide produced cell shrinkage in isotonic solution and blocked cell swelling normally induced by hypotonic solution. Perifused HIT cells secreted insulin when challenged with hypotonic solutions. However, this could not be completely attributed to I(Cl,islet)-mediated depolarization, because secretion persisted even when Cl- channels were fully blocked. To test whether blocker-resistant secretion occurred via a distal pathway, distal secretion was isolated using 50 mmol/l potassium and diazoxide. Under these conditions, glucose-dependent secretion was blunted, but hypotonically induced secretion persisted, even with Cl- channel blockers present. These results suggest that beta-cell swelling stimulates insulin secretion primarily via a distal I(Cl,islet)-independent mechanism, as has been proposed for K(ATP)-independent glucose- and sulfonylurea-stimulated insulin secretion. Reverse transcriptase-polymerase chain reaction of HIT cell mRNA identified a CLC-3 transcript in HIT cells. In other systems, CLC-3 is believed to mediate swelling-induced outwardly rectifying Cl- channels. This suggests that the proximal effects of swelling to regulate cell volume may be mediated by CLC-3 or a closely related Cl- channel.

Alterations of insulin secretion from mouse islets treated with sulphonylureas: perturbations of Ca2+ regulation prevail over changes in insulin content

1. To determine how pretreatment with sulphonylureas alters the beta cell function, mouse islets were cultured (18 - 20 h) without (controls) or with (test) 0.01 microM glibenclamide. Acute responses to glucose were then determined in the absence of glibenclamide. 2. Test islets were insensitive to drugs (sulphonylureas and diazoxide) acting on K+-ATP channels, and their [Ca2+]i was already elevated in the absence of stimulation. 3. Insulin secretion was increased in the absence of glucose, and mainly stimulated between 0 - 10 instead of 7 - 20 mM glucose in controls. The maximum response was halved, but this difference disappeared after correction for the 45% decrease in the islet insulin content. 4. The first phase of glucose-induced insulin secretion was abrogated because of a paradoxical decrease of the high basal [Ca2+]i in beta cells. The second phase was preserved but occurred with little rise of [Ca2+]i. These abnormalities did not result from alterations of glucose metabolism (NADPH fluorescence). 5. In islets cultured with 50 microM tolbutamide, glucose induced biphasic increases in [Ca2+]i and insulin secretion. The decrease in the secretory response was matched by the decrease in insulin content (45%) except at maximal glucose concentrations. Islets pretreated with tolbutamide, however, behaved like those cultured with glibenclamide if tolbutamide was also present during the acute functional tests. 6. In conclusion, treatment with a low glibenclamide concentration causes long-lasting blockade of K+-ATP channels and rise of [Ca2+]i in beta cells. Glucose-induced insulin secretion occurs at lower concentrations, is delayed and is largely mediated by a modulation of Ca2+ action on exocytosis. It is suggested that glucose regulation of insulin secretion mainly depends on a K+-ATP channel-independent pathway during in vivo sulphonylurea treatment.

Potassium Channels, Sulphonylurea Receptors and Control of Insulin Release

Clinical profiles of the glucose regulation disorders persistent hyperinsulinaemic hypoglycaemia of infancy (PHHI) and diabetes mellitus are diametrically opposed: unregulated insulin secretion versus insulin insufficiency. Yet, despite this, recent studies of PHHI and other rare neonatal conditions have revealed common pathways of cellular dysfunction relevant to our understanding of diabetes. Such work has been based upon integration of the genetics of these diseases with the cellular and molecular biology of a potassium channel known to play a major role in the 'glucose-sensing apparatus' of the pancreatic beta cell - the ATP-sensitive K+ (KATP) channel. The structure of this protein complex is unique among ion channel families, because it is composed partly of a K+ channel and partly of an ATP-binding cassette protein that has an extraordinarily high affinity for sulphonylurea compounds. Here, we describe how defects in KATP channel genes give rise to insulin hypersecretion, and may also predispose to the onset of Type 2 diabetes, and how acquired losses of function of these channels have been implicated in maturity onset diabetes of the young and reactive hyperinsulinaemia-induced hypoglycaemia.

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.

Modulation of the bursting properties of single mouse pancreatic beta-cells by artificial conductances

Glucose triggers bursting activity in pancreatic islets, which mediates the Ca2+ uptake that triggers insulin secretion. Aside from the channel mechanism responsible for bursting, which remains unsettled, it is not clear whether bursting is an endogenous property of individual beta-cells or requires an electrically coupled islet. While many workers report stochastic firing or quasibursting in single cells, a few reports describe single-cell bursts much longer (minutes) than those of islets (15-60 s). We studied the behavior of single cells systematically to help resolve this issue. Perforated patch recordings were made from single mouse beta-cells or hamster insulinoma tumor cells in current clamp at 30-35 degrees C, using standard K+-rich pipette solution and external solutions containing 11.1 mM glucose. Dynamic clamp was used to apply artificial KATP and Ca2+ channel conductances to cells in current clamp to assess the role of Ca2+ and KATP channels in single cell firing. The electrical activity we observed in mouse beta-cells was heterogeneous, with three basic patterns encountered: 1) repetitive fast spiking; 2) fast spikes superimposed on brief (<5 s) plateaus; or 3) periodic plateaus of longer duration (10-20 s) with small spikes. Pattern 2 was most similar to islet bursting but was significantly faster. Burst plateaus lasting on the order of minutes were only observed when recordings were made from cell clusters. Adding gCa to cells increased the depolarizing drive of bursting and lengthened the plateaus, whereas adding gKATP hyperpolarized the cells and lengthened the silent phases. Adding gCa and gKATP together did not cancel out their individual effects but could induce robust bursts that resembled those of islets, and with increased period. These added currents had no slow components, indicating that the mechanisms of physiological bursting are likely to be endogenous to single beta-cells. It is unlikely that the fast bursting (class 2) was due to oscillations in gKATP because it persisted in 100 microM tolbutamide. The ability of small exogenous currents to modify beta-cell firing patterns supports the hypothesis that single cells contain the necessary mechanisms for bursting but often fail to exhibit this behavior because of heterogeneity of cell parameters.

Neurotransmitters and their receptors in the islets of Langerhans of the pancreas: what messages do acetylcholine, glutamate, and GABA transmit?

Although neurotransmitters are present in pancreatic islets of Langerhans and can be shown to alter hormone secretion, their precise physiological roles in islet function and their cellular mechanisms of action are unclear. Recent research has identified specific neurotransmitter receptor isoforms in islets that may be important physiologically, because selective receptor agonists activate islet ion channels, modify intracellular [Ca2+], and affect secretion. This article focuses on the putative roles of acetylcholine, glutamate, and GABA in islet function. It has been hypothesized that acetylcholine potentiates insulin secretion by either promoting Ca release from cellular stores, activating a store depletion-activated channel, or activating a novel Na channel. GABA and glutamate, in contrast, have been proposed to mediate a novel paracrine signaling pathway whereby alpha- and beta-cells communicate within the islet. The evidence supporting these hypotheses will be critically evaluated.

Interactions between imidazoline compounds and sulphonylureas in the regulation of insulin secretion

1. Imidazoline alpha 2-antagonist drugs such as efaroxan have been shown to increase the insulin secretory response to sulphonylureas from rat pancreatic B-cells. We have investigated whether this reflects binding to an islet imidazoline receptor or whether alpha 2-adrenoceptor antagonism is involved. 2. Administration of (+/-)-efaroxan or glibenclamide to Wistar rats was associated with a transient increase in plasma insulin. When both drugs were administered together, the resultant increase in insulin levels was much greater than that obtained with either drug alone. 3. Use of the resolved enantiomers of efaroxan revealed that the ability of the compound to enhance the insulin secretory response to glibenclamide resided only in the alpha 2-selective-(+)-enantiomer; the imidazoline receptor-selective-(-)-enantiomer was ineffective. 4. In vitro, (+)-efaroxan increased the insulin secretory response to glibenclamide in rat freshly isolated and cultured islets of Langerhans, whereas (-)-efaroxan was inactive. By contrast, (+)-efaroxan did not potentiate glucose-induced insulin secretion but (-)-efaroxan induced a marked increase in insulin secretion from islets incubated in the presence of 6 mM glucose. 5. Incubation of rat islets under conditions designed to minimize the extent of alpha 2-adrenoceptor signalling (by receptor blockade with phenoxybenzamine; receptor down-regulation or treatment with pertussis toxin) abolished the capacity of (+)- and (+/-)-efaroxan to enhance the insulin secretory response to glibenclamide. However, these manoeuvres did not alter the ability of (+/-)-efaroxan to potentiate glucose-induced insulin secretion. 6. The results indicate that the enantiomers of efaroxan exert differential effects on insulin secretion which may result from binding to effector sites having opposite stereoselectivity. Binding of (-)-efaroxan (presumably to imidazoline receptors) results in potentiation of glucose-induced insulin secretion, whereas interaction of (+)-efaroxan with a second site leads to selective enhancement of sulphonylurea-induced insulin release.