Sulphonylurea receptor-1, sulphonylureas and amplification of insulin secretion by Epac activation in ß cells.

Amplification of insulin secretion by cyclic AMP involves activation of protein kinase A (PKA) and Epac2 in pancreatic ß cells. Recent hypotheses suggest that sulphonylurea receptor-1 (SUR1), the regulatory subunit of ATP-sensitive potassium channels, is implicated in Epac2 effects and that direct activation of Epac2 by hypoglycaemic sulphonylureas contributes to the stimulation of insulin secretion by these drugs. In the present experiments, using islets from Sur1KO mice, we show that dibutyryl-cAMP and membrane-permeant selective activators of Epac or PKA normally amplify insulin secretion in ß cells lacking SUR1. In contrast to Epac activator, sulphonylureas (glibenclamide and tolbutamide) did not increase insulin secretion in Sur1KO islets, as would be expected if they were activating Epac2 directly. Furthermore, glibenclamide and tolbutamide did not augment the amplification of insulin secretion produced by Epac activator or dibutyryl-cAMP. Collectively, the results show that SUR1 is dispensable for amplification of insulin secretion by Epac2 activation and that direct activation of Epac2 is unimportant for the action of therapeutic concentrations of sulphonylureas in ß cells.

Pancreatic alpha cell mass in European subjects with type 2 diabetes.

AIMS/HYPOTHESIS: Type 2 diabetes is a bi-hormonal disease characterised by relative hypoinsulinaemia and hyperglucagonaemia with elevated blood glucose levels. Besides pancreatic beta cell defects, a low number of beta cells (low beta cell mass) may contribute to the insufficient secretion of insulin. In this study our aim was to determine whether the alpha cell mass is also altered. METHODS: Using a point counting method, we measured the ratio of alpha to beta cell areas in pancreas samples obtained at autopsy from 50 type 2 diabetic subjects, whose beta cell mass had previously been found to be 36% lower than that of 52 non-diabetic subjects. RESULTS: The topography of alpha and beta cells was similar in both groups: many alpha cells were localised in the centre of the islets and the ratio of alpha/beta cell areas increased with islet size. The average ratio was significantly higher in type 2 diabetic subjects (0.72) than in non-diabetic subjects (0.42), with, however, a large overlap between the two groups. In contrast, the alpha cell mass was virtually identical in type 2 diabetic subjects (366 mg) and non-diabetic subjects (342 mg), and was not influenced by sex, BMI or type of diabetes treatment. CONCLUSIONS: The higher proportion of alpha to beta cells in the islets of some type 2 diabetic subjects is due to a decrease in beta cell number rather than an increase in alpha cell number. This imbalance may contribute to alterations in the normal inhibitory influence exerted by beta cells on alpha cells, and lead to the relative hyperglucagonaemia observed in type 2 diabetes.

Subplasmalemmal Ca(2+) measurements in mouse pancreatic beta cells support the existence of an amplifying effect of glucose on insulin secretion.

AIMS/HYPOTHESIS: Glucose-induced insulin secretion is attributed to a rise of beta cell cytosolic free [Ca(2+)] ([Ca(2+)](c)) (triggering pathway) and amplification of the action of Ca(2+). This concept of amplification rests on observations that glucose can increase Ca(2+)-induced insulin secretion without further elevating an imposed already high [Ca(2+)](c). However, it remains possible that this amplification results from an increase in [Ca(2+)] just under the plasma membrane ([Ca(2+)](SM)), which escaped detection by previous measurements of global [Ca(2+)](c). This was the hypothesis that we tested here by measuring [Ca(2+)](SM). METHODS: The genetically encoded Ca(2+) indicators D3-cpv (untargeted) and LynD3-cpv (targeted to plasma membrane) were expressed in clusters of mouse beta cells. LynD3-cpv was also expressed in beta cells within intact islets. [Ca(2+)](SM) changes were monitored using total internal reflection fluorescence microscopy. Insulin secretion was measured in parallel. RESULTS: Beta cells expressing D3cpv or LynD3cpv displayed normal [Ca(2+)] changes and insulin secretion in response to glucose. Distinct [Ca(2+)](SM) fluctuations were detected during repetitive variations of KCl between 30 and 32-35 mmol/l, attesting to the adequate sensitivity of our system. When the amplifying pathway was evaluated (high KCl + diazoxide), increasing glucose from 3 to 15 mmol/l consistently lowered [Ca(2+)](SM) while stimulating insulin secretion approximately two fold. Blocking Ca(2+) uptake by the endoplasmic reticulum largely attenuated the [Ca(2+)](SM) decrease produced by high glucose but did not unmask localised [Ca(2+)](SM) increases. CONCLUSIONS/INTERPRETATION: Glucose can increase Ca(2+)-induced insulin secretion without causing further elevation of beta cell [Ca(2+)](SM). The phenomenon is therefore a true amplification of the triggering action of Ca(2+).

Pharmacological stimulation and inhibition of insulin secretion in mouse islets lacking ATP-sensitive K+ channels.

BACKGROUND AND PURPOSE: ATP-sensitive potassium channels (K(ATP) channels) in beta cells are a major target for insulinotropic drugs. Here, we studied the effects of selected stimulatory and inhibitory pharmacological agents in islets lacking K(ATP) channels. EXPERIMENTAL APPROACH: We compared insulin secretion (IS) and cytosolic calcium ([Ca(2+)](c)) changes in islets isolated from control mice and mice lacking sulphonylurea receptor1 (SUR1), and thus K(ATP) channels in their beta cells (Sur1KO). KEY RESULTS: While similarly increasing [Ca(2+)](c) and IS in controls, agents binding to site A (tolbutamide) or site B (meglitinide) of SUR1 were ineffective in Sur1KO islets. Of two non-selective blockers of potassium channels, quinine was inactive, whereas tetraethylammonium was more active in Sur1KO compared with control islets. Phentolamine, efaroxan and alinidine, three imidazolines binding to K(IR)6.2 (pore of K(ATP) channels), stimulated control islets, but only phentolamine retained weaker stimulatory effects on [Ca(2+)](c) and IS in Sur1KO islets. Neither K(ATP) channel opener (diazoxide, pinacidil) inhibited Sur1KO islets. Calcium channel blockers (nimodipine, verapamil) or diphenylhydantoin decreased [Ca(2+)](c) and IS in both types of islets, verapamil and diphenylhydantoin being more efficient in Sur1KO islets. Activation of alpha(2)-adrenoceptors or dopamine receptors strongly inhibited IS while partially (clonidine > dopamine) lowering [Ca(2+)](c) (control > Sur1KO islets). CONCLUSIONS AND IMPLICATIONS: Those drugs retaining effects on IS in islets lacking K(ATP) channels, also affected [Ca(2+)](c), indicating actions on other ionic channels. The greater effects of some inhibitors in Sur1KO than in control islets might be relevant to medical treatment of congenital hyperinsulinism caused by inactivating mutations of K(ATP) channels.

Shortcomings of current models of glucose-induced insulin secretion.

Glucose-induced insulin secretion by pancreatic beta-cells is generally schematized by a 'consensus model' that involves the following sequence of events: acceleration of glucose metabolism, closure of ATP-sensitive potassium channels (K(ATP) channels) in the plasma membrane, depolarization, influx of Ca(2+) through voltage-dependent calcium channels and a rise in cytosolic-free Ca(2+) concentration that induces exocytosis of insulin-containing granules. This model adequately depicts the essential triggering pathway but is incomplete. In this article, we first make a case for a model of dual regulation in which a metabolic amplifying pathway is also activated by glucose and augments the secretory response to the triggering Ca(2+) signal under physiological conditions. We next discuss experimental evidence, largely but not exclusively obtained from beta-cells lacking K(ATP) channels, which indicates that these channels are not the only possible transducers of glucose effects on the triggering Ca(2+)signal. We finally address the identity of the widely neglected background inward current (Cl(-) efflux vs. Na(+) or Ca(2+) influx through voltage-independent channels) that is necessary to cause beta-cell depolarization when glucose closes K(ATP) channels. More attention should be paid to the possibility that some components of this background current are influenced by glucose metabolism and have their place in a model of glucose-induced insulin secretion.

Regulation of insulin secretion: a matter of phase control and amplitude modulation.

The consensus model of stimulus-secretion coupling in beta cells attributes glucose-induced insulin secretion to a sequence of events involving acceleration of metabolism, closure of ATP-sensitive K(+) channels, depolarisation, influx of Ca(2+) and a rise in cytosolic free Ca(2+) concentration ([Ca(2+)](c)). This triggering pathway is essential, but would not be very efficient if glucose did not also activate a metabolic amplifying pathway that does not raise [Ca(2+)](c) further but augments the action of triggering Ca(2+) on exocytosis. This review discusses how both pathways interact to achieve temporal control and amplitude modulation of biphasic insulin secretion. First-phase insulin secretion is triggered by the rise in [Ca(2+)](c) that occurs synchronously in all beta cells of every islet in response to a sudden increase in the glucose concentration. Its time course and duration are shaped by those of the Ca(2+) signal, and its amplitude is modulated by the magnitude of the [Ca(2+)](c) rise and, substantially, by amplifying mechanisms. During the second phase, synchronous [Ca(2+)](c) oscillations in all beta cells of an individual islet induce pulsatile insulin secretion, but these features of the signal and response are dampened in groups of intrinsically asynchronous islets. Glucose has hardly any influence on the amplitude of [Ca(2+)](c) oscillations and mainly controls the time course of triggering signal. Amplitude modulation of insulin secretion pulses largely depends on the amplifying pathway. There are more similarities than differences between the two phases of glucose-induced insulin secretion. Both are subject to the same dual, hierarchical control over time and amplitude by triggering and amplifying pathways, suggesting that the second phase is a sequence of iterations of the first phase.

Pancreatic beta-cell mass in European subjects with type 2 diabetes.

Decreases in both beta-cell function and number can contribute to insulin deficiency in type 2 diabetes. Here, we quantified the beta-cell mass in pancreas obtained at autopsy of 57 type 2 diabetic (T2D) and 52 non-diabetic subjects of European origin. Sections from the body and tail were immunostained for insulin. The beta-cell mass was calculated from the volume density of beta-cells (measured by point-counting methods) and the weight of the pancreas. The pancreatic insulin concentration was measured in some of the subjects. beta-cell mass increased only slightly with body mass index (BMI). After matching for BMI, the beta-cell mass was 41% (BMI < 25) and 38% (BMI 26-40) lower in T2D compared with non-diabetic subjects, and neither gender nor type of treatment influenced these differences. beta-cell mass did not correlate with age at diagnosis but decreased with duration of clinical diabetes (24 and 54% lower than controls in subjects with <5 and >15 years of overt diabetes respectively). Pancreatic insulin concentration was 30% lower in patients. In conclusion, the average beta-cell mass is about 39% lower in T2D subjects compared with matched controls. Its decrease with duration of the disease could be a consequence of diabetes that, with further impairment of insulin secretion, contributes to the progressive deterioration of glucose homeostasis. We do not believe that the small difference in beta-cell mass observed within 5 years of onset could cause diabetes in the absence of beta-cell dysfunction.

Nutrient control of insulin secretion in perifused adult pig islets.

OBJECTIVES: Xenotransplantation of pig islets is a potential solution to the shortage of human islets, but our knowledge of how these islets secrete insulin in response to nutrients is still fragmentary. This was the question addressed in the present study. METHODS: After 24 h culture adult pig islets were perifused to characterize the dynamics of insulin secretion. Some responses were compared to those in human islets. RESULTS: Increasing glucose from 1 to 15 mM weakly (approximately 2x) stimulated insulin secretion, which was potentiated (approximately 12x) by the cAMP-producing agent, forskolin. The effect of glucose was concentration-dependent (threshold at 3-5 mM and maximum at approximately 10 mM). The pattern of secretion was biphasic with a small first phase and an ascending second phase, and a paradoxical increase when the glucose concentration was abruptly lowered. Diazoxide abolished glucose-induced insulin secretion and tolbutamide reversed the inhibition. Glucose also increased secretion when islets were depolarized with tolbutamide or KCl. Insulin secretion was increased by leucine+glutamine, arginine, alanine or a mixture of amino acids, but their effect was significant only in the presence of forskolin. Upon stimulation by glucose alone, human islets secreted approximately 10x more insulin than pig islets, and the kinetics was characterized by a large first phase, a flat second phase, and rapid reversibility. CONCLUSIONS: Compared with human islets, in vitro insulin secretion by adult pig islets is characterized by a different kinetics and a major quantitative deficiency that can be corrected by cAMP.

Morphological localisation of sulfonylurea receptor 1 in endocrine cells of human, mouse and rat pancreas.

AIMS/HYPOTHESIS: Sulfonylurea receptor 1 (SUR1) is the regulatory subunit of ATP-sensitive K channels in beta cells. Morphological methods (immunohistochemistry and sulfonylurea binding) were used to establish the cellular and subcellular location of SUR1 in human and rodent islets. RESULTS: In the human, mouse and rat pancreas, all endocrine cells of the islets were immunolabelled with an anti-SUR1 antibody, whereas tissues containing SUR2 were consistently negative, as were those from Sur1 (also known as Abcc8)(-/-) mice. In beta cells of the three species, the plasma membrane was distinctly stained, but SUR1 was mainly present over the cytoplasm, with an intensity that varied between cells. Electron microscopy showed that SUR1 was immunolocalised in insulin, glucagon and somatostatin granules. In rat beta cells degranulated by in vivo treatment with glibenclamide (known as glyburide in the USA and Canada), the insulin and SUR1 staining intensity was similarly decreased by approximately 45%, whereas SUR1 staining was not changed in non-beta cells. In all islet cells, binding of glibenclamide labelled with fluorescent dipyrromethane boron difluoride (BODIPY-FL) was punctate over the cytoplasm, compatible with the labelling of endocrine granules. A faint labelling persisted in Sur1 (-/-) mice, but it was not different from that obtained with BODIPY-FL alone used as negative control. CONCLUSIONS/INTERPRETATION: Our study immunolocalised SUR1 in alpha, beta and delta cells of human, mouse and rat islets, and for the first time visualised it in the plasma membrane. We also show that SUR1 is abundant in endocrine granules, where its function remains to be established. No specific sulfonylurea-binding sites other than SUR1 are identified in islet cells by the glibenclamide-BODIPY-FL technique.

Antioxidants N-acetyl-L-cysteine and manganese(III)tetrakis (4-benzoic acid)porphyrin do not prevent beta-cell dysfunction in rat islets cultured in high glucose for 1 wk.

We previously showed that the stimulation of heme oxygenase-1 expression by high glucose and hydrogen peroxide (H(2)O(2)) in cultured rat islets is prevented by antioxidants and suggested that this effect of high glucose results from an oxidative stress. However, the role of oxidative stress in high-glucose-induced beta-cell dysfunction is unclear. We therefore compared the preventative effects of N-acetyl-l-cysteine (NAC), a free radical scavenger, and manganese(III)tetrakis (4-benzoic acid)porphyrin (MnTBAP), a superoxide dismutase/catalase mimetic agent, on the alteration of stimulus-secretion coupling induced in rat islets by overnight exposure to hydrogen peroxide (H(2)O(2)-treated islets) or 1-wk culture in 30 vs. 10 mmol/l glucose (High-glucose vs. Control islets). The features of beta-cell dysfunction differed between the two groups: reduced glucose-induced insulin secretion without changes in glucose sensitivity in H(2)O(2)-treated islets; increased sensitivity to glucose with parallel reductions in insulin content and maximal rate of glucose-induced insulin secretion in High-glucose islets. The latter alterations were accompanied by a decrease in preproinsulin without changes in pancreatic and duodenal homeobox gene 1 mRNA levels. The functional alterations induced by H(2)O(2) were significantly prevented by addition of NAC or MnTBAP in the culture medium. In contrast, neither NAC nor MnTBAP affected the functional alterations induced by high glucose. These results suggest that beta-cell dysfunction induced by 1-wk culture in high glucose does not result from an increase in oxidative stress.

Congenital hyperinsulinism and mosaic abnormalities of the ploidy.

BACKGROUND: Congenital hyperinsulinism and Beckwith-Wiedemann syndrome both lead to beta islet hyperplasia and neonatal hypoglycaemia. They may be related to complex genetic/epigenetic abnormalities of the imprinted 11p15 region. The possibility of common pathophysiological determinants has not been thoroughly investigated. OBJECTIVE: To report abnormalities of the ploidy in two unrelated patients with congenital hyperinsulinism. METHODS: Two patients with severe congenital hyperinsulinism, one overlapping with Beckwith-Wiedemann syndrome, had pancreatic histology, ex vivo potassium channel electrophysiological studies, and mutation detection of the encoding genes. The parental genetic contribution was explored using genome-wide polymorphism, fluorescent in situ hybridisation (FISH), and blood group typing studies. RESULTS: Histological findings diverged from those described in focal congenital hyperinsulinism or Beckwith-Wiedemann syndrome. No potassium channel dysfunction and no mutation of its encoding genes (SUR1, KIR6.2) were detected. In patient 1 with congenital hyperinsulinism and Beckwith-Wiedemann syndrome, paternal isodisomy for the whole haploid set was homogeneous in the pancreatic lesion, and mosaic in the leucocytes and skin fibroblasts (hemihypertrophic segment). Blood group typing confirmed the presence of two erythroid populations (bi-parental v paternal only contribution). Patient 2 had two pancreatic lesions, both revealing triploidy with paternal heterodisomy. Karyotype and FISH analyses done on the fibroblasts and leucocytes of both patients were unremarkable (diploidy). CONCLUSIONS: Diploid (biparental/paternal-only) mosaicism and diploid/triploid mosaicism were present in two distinct patients with congenital hyperinsulinism. These chromosomal abnormalities led to paternal disomy for the whole haploid set in pancreatic lesions (with isodisomy or heterodisomy), thereby extending the range and complexity of the mechanisms underlying congenital hyperinsulinism, associated or not with Beckwith-Wiedemann syndrome.

High glucose and hydrogen peroxide increase c-Myc and haeme-oxygenase 1 mRNA levels in rat pancreatic islets without activating NFkappaB.

AIMS/HYPOTHESIS: Hyperglycaemia and the pro-inflammatory cytokine IL-1beta induce similar alterations of beta cell gene expression, including up-regulation of c-Myc and haeme-oxygenase 1. These effects of hyperglycaemia may result from nuclear factor-kappa B (NFkappaB) activation by oxidative stress. To test this hypothesis, we compared the effects of IL-1beta, high glucose, and hydrogen peroxide, on NFkappaB DNA binding activity and target gene mRNA levels in cultured rat islets. METHODS: Rat islets were pre-cultured for 1 week in serum-free RPMI medium containing 10 mmol/l glucose, and further cultured in glucose concentrations of 5-30 mmol/l plus various test substances. Islet NFkappaB activity was measured by ELISA and gene mRNA expression was measured by RT-PCR. RESULTS: IL-1beta consistently increased islet NFkappaB activity and c-Myc, haeme-oxygenase 1, inducible nitric oxide synthase (iNOS), Fas, and inhibitor of NFkappaB alpha (IkappaBalpha) mRNA levels. In comparison, 1- to 7-day culture in 30 mmol/l instead of 10 mmol/l glucose stimulated islet c-Myc and haeme-oxygenase 1 expression without affecting NFkappaB activity or iNOS and IkappaBalpha mRNA levels. Fas mRNA levels only increased after 1 week in 30 mmol/l glucose. Overnight exposure to hydrogen peroxide mimicked the effects of 30 mmol/l glucose on haeme-oxygenase 1 and c-Myc mRNA levels without activating NFkappaB. On the other hand, the antioxidant N-acetyl-L-cysteine inhibited the stimulation of haeme-oxygenase 1 and c-Myc expression by 30 mmol/l glucose and/or hydrogen peroxide. CONCLUSIONS/INTERPRETATION: In contrast to IL-1beta, high glucose and hydrogen peroxide do not activate NFkappaB in cultured rat islets. It is suggested that the stimulation of islet c-Myc and haeme-oxygenase 1 expression by 30 mmol/l glucose results from activation of a distinct, probably oxidative-stress-dependent signalling pathway.

Increased glucose sensitivity of both triggering and amplifying pathways of insulin secretion in rat islets cultured for 1 wk in high glucose.

Chronic hyperglycemia has been shown to induce either a lack of response or an increased sensitivity to glucose in pancreatic beta-cells. We reinvestigated this controversial issue in a single experimental model by culturing rat islets for 1 wk in 10 or 30 mmol/l glucose (G10, Controls; or G30, High-glucose islets) before testing the effect of stepwise glucose stimulation from G0.5 to G20 on key beta-cell stimulus-secretion coupling events. Compared with Controls, the glucose sensitivity of High-glucose islets was markedly increased, leading to maximal stimulation of oxidative metabolism and both triggering and amplifying pathways of insulin secretion in G6 rather than G20, hence to loss of glucose effect above G6. This enhanced glucose sensitivity occurred despite an approximately twofold increase in islet uncoupling protein 2 mRNA expression. Besides this increased glucose sensitivity, the maximal glucose stimulation of insulin secretion in High-glucose islets was reduced by approximately 50%, proportionally to the reduction of insulin content. In High-glucose islets, changes in (45)Ca(2+) influx induced by glucose and diazoxide were qualitatively similar but quantitatively smaller than in Control islets and, paradoxically, did not lead to detectable changes in the intracellular Ca(2+) concentration measured by microspectrofluorimetry (fura PE 3). In conclusion, after 1 wk of culture in G30, the loss of glucose stimulation of insulin secretion in the physiological range of glucose concentrations (G5-G10) results from the combination of an increased sensitivity to glucose of both triggering and amplifying pathways of insulin secretion and an approximately 50% reduction in the maximal glucose stimulation of insulin secretion.

Haeme-oxygenase 1 expression in rat pancreatic beta cells is stimulated by supraphysiological glucose concentrations and by cyclic AMP.

AIM/HYPOTHESIS: Increased expression of haeme-oxygenase 1 (HO1) and other antioxidant enzymes could improve pancreatic beta-cell survival under stressful conditions, including hyperglycaemia. However, how hyperglycaemia increases islet HO1 expression is not known. METHODS: Rat islets were pre-cultured for 1 week in RPMI medium containing 10 mmol x l(-1) glucose (G10), and further cultured overnight in G5-G30 plus various test substances. Islet HO1 mRNA and protein expression was measured by semiquantitative RT-PCR, western blot, and immunohistochemistry. RESULTS: Islet HO1 mRNA expression was minimal after overnight culture in G10, slightly increased in G5, and increased by five- to ten-fold in G30 in parallel with a heterogeneous increase in beta-cell HO1 protein expression. The effect of G30 was fully inhibited by agents decreasing cytosolic Ca2+ (diazoxide, nimodipine), but was only slightly reproduced by agents raising Ca2+ (tolbutamide, 30 mmol x l(-1) potassium). It was also suppressed by the alpha2-adrenoceptor agonist clonidine, whereas dibutyryl-cyclic-AMP largely increased beta-cell HO1 expression. The induction of HO1 mRNA expression by G30 was independent from changes in medium insulin concentration, but was completely inhibited by a cocktail of antioxidants. In contrast to HO1, islet mRNA expression of glutathione peroxidase and constitutive haeme-oxygenase 2 were not affected by G30, nor by dibutyryl-cyclic-AMP. CONCLUSION/INTERPRETATION: High glucose and dibutyryl-cyclic-AMP stimulate expression of HO1 in rat pancreatic beta cells. The inhibition of HO1 expression in G30 by nimodipine, clonidine, and antioxidants, suggests that Ca2+ influx and cyclic-AMP are necessary for the generation of oxidative stress by G30, or for the stimulation of beta-cell HO1 expression by increased oxidative stress.

Disorganization of cytoplasmic Ca(2+) oscillations and pulsatile insulin secretion in islets from ob/ obmice.

AIMS/HYPOTHESIS: In normal mouse islets, glucose induces synchronous cytoplasmic [Ca(2+)](i) oscillations in beta cells and pulses of insulin secretion. We investigated whether this fine regulation of islet function is preserved in hyperglycaemic and hyperinsulinaemic ob/ obmice. METHODS: Intact islets from ob/ ob mice and their lean littermates were used after overnight culture for measurement of [Ca(2+)](i) and insulin secretion. RESULTS: We observed three types of [Ca(2+)](i) responses during stimulation by 9 to 12 mmol/l of glucose: sustained increase, rapid oscillations and slow (or mixed) oscillations. They occurred in 8, 18 and 74% of lean islets and 9, 0 and 91% of ob/ ob islets, respectively. Subtle desynchronisation of [Ca(2+)](i) oscillations between regions occurred in 11% of lean islets. In ob/ ob islets, desynchronisation was frequent (66-82% depending on conditions) and prominent: oscillations were out of phase in different regions because of distinct periods and shapes. Only small ob/ ob islets were well synchronised, but sizes of synchronised lean and desynchronised ob/ ob islets were markedly overlapped. The occurrence of desynchronisation in clusters of 5 to 50 islet cells from ob/ obmice and not from lean mice further indicates that islet hypertrophy is not the only causal factor. In both types of islets, synchronous [Ca(2+)](i) oscillations were accompanied by oscillations of insulin secretion. In poorly synchronised ob/ ob islets, secretion was irregular but followed the pattern of the global [Ca(2+)](i) changes. CONCLUSIONS/INTERPRETATION: The regularity of glucose-induced [Ca(2+)](i) oscillations is disrupted in islets from ob/ ob mice and this desynchronisation perturbs the pulsatility of insulin secretion. A similar mechanism could contribute to the irregularity of insulin oscillations in Type II (non-insulin-dependent) diabetes mellitus.