Loss of regular oscillatory insulin secretion in islet cell antibody positive non-diabetic subjects

Polycomb Repressive Complexes: Shaping Pancreatic Beta-Cell Destiny in Development and Metabolic Disease

Pancreatic beta-cells secrete the hormone insulin, which is essential for the regulation of systemic glucose homeostasis. Insufficiency of insulin due to loss of functional beta-cells results in diabetes. Epigenetic mechanisms orchestrate the stage-specific transcriptional programs that guide the differentiation, functional maturation, growth, and adaptation of beta-cells in response to growth and metabolic signals throughout life. Primary among these mechanisms is regulation by the Polycomb Repressive Complexes (PRC) that direct gene-expression via histone modifications. PRC dependent histone modifications are pliable and provide a degree of epigenetic plasticity to cellular processes. Their modulation dictates the spatio-temporal control of gene-expression patterns underlying beta-cell homeostasis. Emerging evidence shows that dysregulation of PRC-dependent epigenetic control is also a hallmark of beta-cell failure in diabetes. This minireview focuses on the multifaceted contributions of PRC modules in the specification and maintenance of terminally differentiated beta-cell phenotype, as well as beta-cell growth and adaptation. We discuss the interaction of PRC regulation with different signaling pathways and mechanisms that control functional beta-cell mass. We also highlight recent advances in our understanding of the epigenetic regulation of beta-cell homeostasis through the lens of beta-cell pathologies, namely diabetes and insulinomas, and the translational relevance of these findings. Using high-resolution epigenetic profiling and epigenetic engineering, future work is likely to elucidate the PRC regulome in beta-cell adaptation versus failure in response to metabolic challenges and identify opportunities for therapeutic interventions.

Low Grade Islet but Marked Exocrine Pancreas Inflammation in an Adult with Autoimmune Pre-Diabetes

There is unexplained deficit in size and function of the exocrine pancreas in type 1 diabetes (T1D). We obtained pancreas from an individual with pre-T1D obtained at surgery and addressed the question, what is the relative inflammation in the exocrine and endocrine pancreas in pre-T1D in the absence of the potential confounding changes at autopsy or in brain dead organ donors. Pancreas was removed surgically from a 36 year woman for benign neuroendocrine tumors (NET). The patient had gestational diabetes at age 29 and has a 23 year old sister with T1D. Pre-operative fasting glucose of 109 mg/dl and HbA1C 5.8% revealed prediabetes with an anti-GAD 1,144 (5–250 U/ml) together with family history implying pre-T1D. There was patchy low grade immune infiltration in some, but not all, islets that met criteria for autoimmune insulitis. The exocrine pancreas showed more abundant inflammation with areas of chronic pancreatitis and acinar to ductal metaplasia, and with other areas of atrophy and fatty infiltration. In pre-T1D inflammation may be more prominent in the exocrine than the endocrine pancreas, calling into question the sequence of events and assumed islet centric basis of autoimmunity leading to T1D.

Calcium Signaling in ß-cell Physiology and Pathology: A Revisit

Pancreatic beta (β) cell dysfunction results in compromised insulin release and, thus, failed regulation of blood glucose levels. This forms the backbone of the development of diabetes mellitus (DM), a disease that affects a significant portion of the global adult population. Physiological calcium (Ca²⁺) signaling has been found to be vital for the proper insulin-releasing function of β-cells. Calcium dysregulation events can have a dramatic effect on the proper functioning of the pancreatic β-cells. The current review discusses the role of calcium signaling in health and disease in pancreatic β-cells and provides an in-depth look into the potential role of alterations in β-cell Ca²⁺ homeostasis and signaling in the development of diabetes and highlights recent work that introduced the current theories on the connection between calcium and the onset of diabetes.

Effects of Periodic Intensive Insulin Therapy: An Updated Review

Background

Traditional insulin treatment for diabetes mellitus with insulin administered subcutaneously yields nonpulsatile plasma insulin concentrations that represent a fraction of normal portal vein levels. Oral hypoglycemic medications result in the same lack of pulsatile insulin response to blood glucose levels. Intensive treatments of significant complications of diabetes are not recommended due to complicated multidrug regimens, significant weight gain, and the high risk of hypoglycemic complications. Consequently, advanced complications of diabetes do not have an effective treatment option because conventional therapy is not sufficient. Intensive insulin therapy (IIT) simulates normal pancreatic function by closely matching the periodicity and amplitude of insulin secretion in healthy subjects; however, the mechanisms involved with the observed improvement are not clearly understood.

Objective

The current review aims to analyze the pathophysiology of insulin secretion, discuss current therapies for the management of diabetes, provides an updates on the recent advancements of IIT, and proposes its mechanism of action.

Methods

A literature search on PubMed, MEDLINE, Embase, and CrossRef databases was performed on multiple key words regarding the history and current variations of pulsatile and IIT for diabetes treatment. Articles reporting the physiology of insulin secretion, advantages of pulsatile insulin delivery in patients with diabetes patients, efficacy and adverse effects of current conventional insulin therapies for the management of diabetes, benefits and shortcomings of pancreas and islet transplantation, or clinical trials on patients with diabetes treated with pulsed insulin therapy or advanced IIT were included for a qualitative analysis and categorized into the following topics: mechanism of insulin secretion in normal subjects and patients with diabetes and current therapies for the management of diabetes, including oral hypoglycemic agents, insulin therapy, pancreas and islet transplantation, pulsed insulin therapy, and advances in IIT.

Results

Our review of the literature shows that IIT improves the resolution of diabetic ulcers, neuropathy, and nephropathy, and reduces emergency room visits. The likely mechanism responsible for this improvement is increased insulin sensitivity from adipocytes, as well as increased insulin receptor expression.

Conclusions

Recent advancements show that IIT is an effective option for both type 1 diabetes mellitus and type 2 diabetes mellitus patient populations. This treatment resembles normal pancreatic function so closely that it has significantly reduced the effects of relatively common complications of diabetes in comparison to standard treatments. Thus, this new treatment is a promising advancement in the management of diabetes. (Curr Ther Res Clin Exp. 2019; 80:XXX–XXX).

Critical and Supercritical Spatiotemporal Calcium Dynamics in Beta Cells

A coordinated functioning of beta cells within pancreatic islets is mediated by oscillatory membrane depolarization and subsequent changes in cytoplasmic calcium concentration. While gap junctions allow for intraislet information exchange, beta cells within islets form complex syncytia that are intrinsically nonlinear and highly heterogeneous. To study spatiotemporal calcium dynamics within these syncytia, we make use of computational modeling and confocal high-speed functional multicellular imaging. We show that model predictions are in good agreement with experimental data, especially if a high degree of heterogeneity in the intercellular coupling term is assumed. In particular, during the first few minutes after stimulation, the probability distribution of calcium wave sizes is characterized by a power law, thus indicating critical behavior. After this period, the dynamics changes qualitatively such that the number of global intercellular calcium events increases to the point where the behavior becomes supercritical. To better mimic normal in vivo conditions, we compare the described behavior during supraphysiological non-oscillatory stimulation with the behavior during exposure to a slightly lower and oscillatory glucose challenge. In the case of this protocol, we observe only critical behavior in both experiment and model. Our results indicate that the loss of oscillatory changes, along with the rise in plasma glucose observed in diabetes, could be associated with a switch to supercritical calcium dynamics and loss of beta cell functionality.

Phase modulation of insulin pulses enhances glucose regulation and enables inter-islet synchronization

Insulin is secreted in a pulsatile manner from multiple micro-organs called the islets of Langerhans. The amplitude and phase (shape) of insulin secretion are modulated by numerous factors including glucose. The role of phase modulation in glucose homeostasis is not well understood compared to the obvious contribution of amplitude modulation. In the present study, we measured Ca²⁺ oscillations in islets as a proxy for insulin pulses, and we observed their frequency and shape changes under constant/alternating glucose stimuli. Here we asked how the phase modulation of insulin pulses contributes to glucose regulation. To directly answer this question, we developed a phenomenological oscillator model that drastically simplifies insulin secretion, but precisely incorporates the observed phase modulation of insulin pulses in response to glucose stimuli. Then, we mathematically modeled how insulin pulses regulate the glucose concentration in the body. The model of insulin oscillation and glucose regulation describes the glucose-insulin feedback loop. The data-based model demonstrates that the existence of phase modulation narrows the range within which the glucose concentration is maintained through the suppression/enhancement of insulin secretion in conjunction with the amplitude modulation of this secretion. The phase modulation is the response of islets to glucose perturbations. When multiple islets are exposed to the same glucose stimuli, they can be entrained to generate synchronous insulin pulses. Thus, we conclude that the phase modulation of insulin pulses is essential for glucose regulation and inter-islet synchronization.

Pulsatile insulin secretion, impaired glucose tolerance and type 2 diabetes

Type 2 diabetes (T2DM) results when increases in beta cell function and/or mass cannot compensate for rising insulin resistance. Numerous studies have documented the longitudinal changes in metabolism that occur during the development of glucose intolerance and lead to T2DM. However, the role of changes in insulin secretion, both amount and temporal pattern, has been understudied. Most of the insulin secreted from pancreatic beta cells of the pancreas is released in a pulsatile pattern, which is disrupted in T2DM. Here we review the evidence that changes in beta cell pulsatility occur during the progression from glucose intolerance to T2DM in humans, and contribute significantly to the etiology of the disease. We review the evidence that insulin pulsatility improves the efficacy of secreted insulin on its targets, particularly hepatic glucose production, but also examine evidence that pulsatility alters or is altered by changes in peripheral glucose uptake. Finally, we summarize our current understanding of the biophysical mechanisms responsible for oscillatory insulin secretion. Understanding how insulin pulsatility contributes to normal glucose homeostasis and is altered in metabolic disease states may help improve the treatment of T2DM.

Similar weight-adjusted insulin secretion and insulin sensitivity in short-duration late autoimmune diabetes of adulthood (LADA) and type 2 diabetes: Action LADA 9 [corrected]

AIMS

To explore insulin sensitivity and insulin secretion in people with latent autoimmune diabetes in adulthood (LADA) compared with that in people with type 2 diabetes.

METHODS

A total of 12 people with LADA, defined as glutamic acid decarboxylase (GAD) antibody positivity and > 1 year of insulin independency (group A) were age-matched pairwise to people with type 2 diabetes (group B) and to six people with type 2 diabetes of similar age and BMI (group C). β-Cell function (first-phase insulin secretion and assessment of insulin pulsatility), insulin sensitivity (hyperinsulinemic-euglycemic clamp) and metabolic response during a mixed meal were studied.

RESULTS

Both first-phase insulin secretion and insulin release during the meal were greater (P = 0.05 and P = 0.009, respectively) in type 2 diabetes as compared with LADA; these differences were lost on adjustment for BMI (group C) and could be explained by BMI alone in a multivariate analysis. Neither insulin pulsatility, incretin secretion nor insulin sensitivity differed among the groups.

CONCLUSIONS

We found no evidence that LADA and type 2 diabetes were distinct disease entities beyond the differences explained by BMI.

Phase transitions in pancreatic islet cellular networks and implications for type-1 diabetes

In many aspects the onset of a chronic disease resembles a phase transition in a complex dynamic system: Quantitative changes accumulate largely unnoticed until a critical threshold is reached, which causes abrupt qualitative changes of the system. In this study we examine a special case, the onset of type-1 diabetes (T1D), a disease that results from loss of the insulin-producing pancreatic islet β cells. Within each islet, the β cells are electrically coupled to each other via gap-junctional channels. This intercellular coupling enables the β cells to synchronize their insulin release, thereby generating the multiscale temporal rhythms in blood insulin that are critical to maintaining blood glucose homeostasis. Using percolation theory we show how normal islet function is intrinsically linked to network connectivity. In particular, the critical amount of β-cell death at which the islet cellular network loses site percolation is consistent with laboratory and clinical observations of the threshold loss of β cells that causes islet functional failure. In addition, numerical simulations confirm that the islet cellular network needs to be percolated for β cells to synchronize. Furthermore, the interplay between site percolation and bond strength predicts the existence of a transient phase of islet functional recovery after onset of T1D and introduction of treatment, potentially explaining the honeymoon phenomenon. Based on these results, we hypothesize that the onset of T1D may be the result of a phase transition of the islet β-cell network.

Pulsatile portal vein insulin delivery enhances hepatic insulin action and signaling

Insulin is secreted as discrete insulin secretory bursts at ~5-min intervals into the hepatic portal vein, these pulses being attenuated early in the development of type 1 and type 2 diabetes mellitus (T2DM). Intraportal insulin infusions (pulsatile, constant, or reproducing that in T2DM) indicated that the pattern of pulsatile insulin secretion delivered via the portal vein is important for hepatic insulin action and, therefore, presumably for hepatic insulin signaling. To test this, we examined hepatic insulin signaling in rat livers exposed to the same three patterns of portal vein insulin delivery by use of sequential liver biopsies in anesthetized rats. Intraportal delivery of insulin in a constant versus pulsatile pattern led to delayed and impaired activation of hepatic insulin receptor substrate (IRS)-1 and IRS-2 signaling, impaired activation of downstream insulin signaling effector molecules AKT and Foxo1, and decreased expression of glucokinase (Gck). We further established that hepatic Gck expression is decreased in the HIP rat model of T2DM, a defect that correlated with a progressive defect of pulsatile insulin secretion. We conclude that the physiological pulsatile pattern of insulin delivery is important in hepatic insulin signaling and glycemic control. Hepatic insulin resistance in diabetes is likely in part due to impaired pulsatile insulin secretion.

Pulsatile intravenous insulin therapy: the best practice to reverse diabetes complications?

In the basal state and after oral ingestion of carbohydrate, the normal pancreas secretes insulin into the portal vein in a pulsatile manner. The end organ of the portal vein is the liver, where approximately 80% of pancreatic insulin is extracted during first pass. In Type 1 diabetes, pancreatic insulin secretion is nearly or completely absent whilst in Type 2 diabetes the normal pattern is absent, abnormal, or blunted. Exogenous subcutaneous insulin treatment results in plasma insulin concentrations that are not pulsatile and a fraction of normal portal vein levels. Oral hypoglycemic agents also do not result in normal pulsatile response to a glucose load. Due to hypoglycemia risk, intensive treatment is not recommended after serious complications develop. Consequently, no conventional therapy has proved effective in treating advanced diabetes complications. Beta-cell replacement using whole pancreas or islet transplantation has been utilized to treat certain problems in Type 1 diabetic patients, but still unavailable for all diabetics. Pulsatile intravenous insulin therapy (PIVIT) is an insulin therapy, which mimics the periodicity and amplitude of normal pancreatic function. Numerous studies show PIVIT effective in preventing, reversing, and reducing the severity and progression of diabetes complications, however, the mechanisms involved with the improvement are not clearly understood. Here, we review the cellular basis of normal and abnormal insulin secretion, current treatments available to treat diabetes, the physiologic basis of PIVIT and possible mechanisms of action.

On high-frequency insulin oscillations

Insulin is released in a pulsatile manner, which results in oscillatory concentrations in blood. The oscillatory secretion improves release control and enhances the hormonal action. Insulin oscillates with a slow ultradian periodicity (approximately 140 min) and a high-frequency periodicity (approximately 6-10 min). Only the latter is reviewed in this article. At least 75% of the insulin secretion is released in a pulsatile manner. Individuals prone to developing diabetes or with overt type 2 diabetes are characterized by irregular oscillations of plasma insulin. Many factors have impact on insulin pulsatility such as age, insulin resistance and glycemic level. In addition, tiny glucose oscillations are capable of entraining insulin oscillations in healthy people in contrast to type 2 diabetic individuals emphasizing a profound disruption of the beta-cells in type 2 diabetes to sense or respond to physiological glucose excursions. A crucial question is how approximately 1,000,000 islets, each containing from a few to several thousand beta-cells, can be coordinated to secrete insulin in a pulsatile manner. This is blatantly a very complex operation to control involving an intra-pancreatic neural network, an intra-islet communication and metabolic oscillations in the beta-cell itself. Overnight beta-cell rest, e.g. during somatostatin administration, improves the disordered pulsatile insulin secretion in type 2 diabetes. Acute as well as long-term administration of sulphonylureas (SU) leads to substantial amplification (approximately 50%) of the pulsatile insulin secretion in type 2 diabetes. This is probably cardinal in terms of governing the hepatic glucose release in type 2 diabetes. Whether sulfonylureas also improve the ability of the beta-cells to sense glucose fluctuations remains to be explored. Thiazolidinediones reduce the pulsatile insulin secretion without affecting regularity, but appear to improve the ability of the beta-cell to be entrained by small glucose excursions. Finally, similar to SUs, the incretin hormone GLP-1 also results in an augmented pulsatile burst mass in both healthy and diabetic individuals, in the latter group, however, without influencing the disorderliness of pulses. This review will briefly describe the high-frequency insulin pulsatility during physiologic and pathophysiologic conditions as well as the influence of some hypoglycemic compounds on the insulin oscillations.

The hyperbolic effect of density and strength of inter beta-cell coupling on islet bursting: a theoretical investigation

BACKGROUND

Insulin, the principal regulating hormone of blood glucose, is released through the bursting of the pancreatic islets. Increasing evidence indicates the importance of islet morphostructure in its function, and the need of a quantitative investigation. Recently we have studied this problem from the perspective of islet bursting of insulin, utilizing a new 3D hexagonal closest packing (HCP) model of islet structure that we have developed. Quantitative non-linear dependence of islet function on its structure was found. In this study, we further investigate two key structural measures: the number of neighboring cells that each beta-cell is coupled to, nc, and the coupling strength, gc.

RESULTS

BETA-cell clusters of different sizes with number of beta-cells nbeta ranging from 1-343, nc from 0-12, and gc from 0-1000 pS, were simulated. Three functional measures of islet bursting characteristics--fraction of bursting beta-cells fb, synchronization index lambda, and bursting period Tb, were quantified. The results revealed a hyperbolic dependence on the combined effect of nc and gc. From this we propose to define a dimensionless cluster coupling index or CCI, as a composite measure for islet morphostructural integrity. We show that the robustness of islet oscillatory bursting depends on CCI, with all three functional measures fb, lambda and Tb increasing monotonically with CCI when it is small, and plateau around CCI = 1.

CONCLUSION

CCI is a good islet function predictor. It has the potential of linking islet structure and function, and providing insight to identify therapeutic targets for the preservation and restoration of islet beta-cell mass and function.

Evaluation of beta-cell mass and function in the Göttingen minipig

Increased knowledge about beta-cell mass and function is important for our understanding of the pathophysiology of type 2 diabetes (T2DM). The relationship between the two is difficult to study in humans, whereas animal models allow studies of consequences of, for example, reduction of beta-cell mass and induction of obesity and procurement of the pancreas for histological examination. An overview of results obtained in the Göttingen minipig in relation to beta-cell function, and mass is provided here. Effects of a primary reduction of beta-cell mass have indicated that not all of the defects of pulsatile insulin secretion in human T2DM can be explained by reduced beta-cell mass. Furthermore, induction of obesity has shown deterioration of beta-cell function and morphological changes in the pancreas. As in humans, obesity leads to an increased beta-cell volume in the minipig, and based on the increased number of islets, neogenesis of islets is an important factor in expansion of beta-cell mass in this species. Measurement of beta-cell function as an estimate of beta-cell mass is, at present, the only method possible in humans, and this approach has been validated using lean and obese minipigs with a range of beta-cell mass. The effects on beta-cell function and mass of obesity of longer duration and/or more pronounced hyperglycaemia remains to be determined, but the models developed so far represent a valuable tool for such investigations.

Investigating the role of islet cytoarchitecture in its oscillation using a new beta-cell cluster model

The oscillatory insulin release is fundamental to normal glycemic control. The basis of the oscillation is the intercellular coupling and bursting synchronization of β cells in each islet. The functional role of islet β cell mass organization with respect to its oscillatory bursting is not well understood. This is of special interest in view of the recent finding of islet cytoarchitectural differences between human and animal models. In this study we developed a new hexagonal closest packing (HCP) cell cluster model. The model captures more accurately the real islet cell organization than the simple cubic packing (SCP) cluster that is conventionally used. Using our new model we investigated the functional characteristics of β-cell clusters, including the fraction of cells able to burst f_b, the synchronization index λ of the bursting β cells, the bursting period T_b, the plateau fraction p_f, and the amplitude of intracellular calcium oscillation [Ca]. We determined their dependence on cluster architectural parameters including number of cells n_β, number of inter-β cell couplings of each β cell n_c, and the coupling strength g_c. We found that at low values of n_β, n_c and g_c, the oscillation regularity improves with their increasing values. This functional gain plateaus around their physiological values in real islets, at n_β∼100, n_c∼6 and g_c∼200 pS. In addition, normal β-cell clusters are robust against significant perturbation to their architecture, including the presence of non-β cells or dead β cells. In clusters with n_β>∼100, coordinated β-cell bursting can be maintained at up to 70% of β-cell loss, which is consistent with laboratory and clinical findings of islets. Our results suggest that the bursting characteristics of a β-cell cluster depend quantitatively on its architecture in a non-linear fashion. These findings are important to understand the islet bursting phenomenon and the regulation of insulin secretion, under both physiological and pathological conditions.

Loss of entrainment of high-frequency plasma insulin oscillations in type 2 diabetes is likely a glucose-specific beta-cell defect

Spontaneous high-frequency insulin oscillations are easily entrainable to exogenous glucose in vitro and in vivo, but this property is lost in type 2 diabetes (2-DM). We hypothesized that this lack of entrainment in 2-DM would be specific to glucose. This was tested in nine control and ten 2-DM subjects. Serial blood sampling at 1-min intervals was carried out for 60 min in the basal state and for 120 min while small (1-60 mg/kg) boluses of arginine were injected intravenously at exactly 29-min intervals. Samples were analyzed for insulin concentrations, and time series analysis was carried out using spectral analysis. In control subjects, the mean period of basal plasma insulin oscillations was 10.3 +/- 1.3 min and was entrained by arginine to a mean period of 14.9 +/- 0.6 min (P < 0.00001 vs. basal). Similarly, in 2-DM subjects, spontaneous insulin oscillations were entrained by arginine; mean basal insulin period was 10.0 +/- 1.0 min and 14.5 +/- 1.8 min with arginine boluses (P < 0.00001). All of the primary peaks observed in spectral analysis were statistically significant (P < 0.05). Percent total power of primary peaks ranged from 17 to 68%. Thus arginine boluses entrain spontaneous high-frequency insulin oscillations in 2-DM subjects. This represents a distinct and striking difference from the resistance of the beta-cell to glucose entrainment in 2-DM. We conclude that loss of entrainment of spontaneous high-frequency insulin oscillations in 2-DM is likely a glucose-specific manifestation of beta-cell secretory dysfunction.

Loss of beta-cell mass leads to a reduction of pulse mass with normal periodicity, regularity and entrainment of pulsatile insulin secretion in Göttingen minipigs

AIMS/HYPOTHESIS

Type 2 diabetes is associated with impaired insulin action and secretion, including disturbed pulsatile release. Impaired pulsatility has been related to impaired insulin action, thus providing a possible link between release and action of insulin. Furthermore, progressive loss of beta-cell mass has been implicated in the pathogenesis of Type 2 diabetes. The aim of this study was to evaluate a possible link between loss of beta-cell mass and impaired pulsatile insulin secretion with special focus on glucose responsiveness of insulin secretion.

METHODS

The kinetic and dynamic profiles of insulin in Göttingen minipigs are favourable for studies on pulsatility and a model of diabetes with reduced beta-cell mass has recently been established. Pigs were studied before (n=14) and after (n=10) reduction of beta-cell mass by nicotinamide (67 mg/kg) and streptozotocin (125 mg/kg) from 17.7+/-4.7 (normal animals, n=5) to 6.1+/-2.0 mg/kg. Pulsatile insulin secretion was examined during basal (n=8 normal, n=6 beta-cell reduced) and glucose entrained (n=6 normal, n=4 beta-cell reduced) conditions. Insulin concentration time series were analysed by autocorrelation and spectral analyses for periodicities and regularity, and by deconvolution for pulse frequency, mass and amplitude.

RESULTS

Reduction of beta-cell mass and secondary hyperglycaemia resulted in correspondingly (r=0.7421, p=0.0275) reduced pulse mass (42% of normal during basal and 31% during entrained conditions) with normal periodicity (6.6+/-2.2 vs 5.8+/-2.4 min, p=0.50), regularity and entrainability of insulin secretion.

CONCLUSION/INTERPRETATION

Neither beta-cell loss, nor 2 weeks of slight hyperglycaemia, as seen in the beta-cell-reduced minipig, probably accounts for the disturbed insulin pulsatility observed in human Type 2 diabetes.

Pulsatile insulin secretion: detection, regulation, and role in diabetes

Insulin concentrations oscillate at a periodicity of 5-15 min per oscillation. These oscillations are due to coordinate insulin secretory bursts, from millions of islets. The generation of common secretory bursts requires strong within-islet and within-pancreas coordination to synchronize the secretory activity from the beta-cell population. The overall contribution of this pulsatile mechanism dominates and accounts for the majority of insulin release. This review discusses the methods involved in the detection and quantification of periodicities and individual secretory bursts. The mechanism by which overall insulin secretion is regulated through changes in the pulsatile component is discussed for nerves, metabolites, hormones, and drugs. The impaired pulsatile secretion of insulin in type 2 diabetes has resulted in much focus on the impact of the insulin delivery pattern on insulin action, and improved action from oscillatory insulin exposure is demonstrated on liver, muscle, and adipose tissues. Therefore, not only is the dominant regulation of insulin through changes in secretory burst mass and amplitude, but the changes may affect insulin action. Finally, the role of impaired pulsatile release in early type 2 diabetes suggests a predictive value of studies on insulin pulsatility in the development of this disease.

Glucose-stimulated oscillations in free cytosolic ATP concentration imaged in single islet beta-cells: evidence for a Ca2+-dependent mechanism

Normal glucose-stimulated insulin secretion is pulsatile, but the molecular mechanisms underlying this pulsatility are poorly understood. Oscillations in the intracellular free [ATP]/[ADP] ratio represent one possible mechanism because they would be expected to cause fluctuations in ATP-sensitive K(+) channel activity and hence oscillatory Ca(2+) influx. After imaging recombinant firefly luciferase, expressed via an adenoviral vector in single human or mouse islet beta-cells, we report here that cytosolic free ATP concentrations oscillate and that these oscillations are affected by glucose. In human beta-cells, oscillations were observed at both 3 and 15 mmol/l glucose, but the oscillations were of a longer wavelength at the higher glucose concentration (167 vs. 66 s). Mouse beta-cells displayed oscillations in both cytosolic free [Ca(2+)] and [ATP] only at elevated glucose concentrations, both with a period of 120 s. To explore the causal relationship between [Ca(2+)] and [ATP] oscillations, the regulation of each was further investigated in populations of MIN6 beta-cells. Incubation in Ca(2+)-free medium lowered cytosolic [Ca(2+)] but increased [ATP] in MIN6 cells at both 3 and 30 mmol/l glucose. Removal of external Ca(2+) increased [ATP], possibly by decreasing ATP consumption by endoplasmic reticulum Ca(2+)-ATPases. These results allow a model to be constructed of the beta-cell metabolic oscillator that drives nutrient-induced insulin secretion.

Anesthesia rapidly suppresses insulin pulse mass but enhances the orderliness of insulin secretory process

Induction of anesthesia is accompanied by modest hyperglycemia and a decreased plasma insulin concentration. Most insulin is secreted in discrete pulses occurring at approximately 6- to 8-min intervals. We sought to test the hypothesis that anesthesia inhibits insulin release by disrupting pulsatile insulin secretion in a canine model by use of direct portal vein sampling. We report that induction of anesthesia causes an abrupt decrease in the insulin secretion rate (1.1 +/- 0.2 vs. 0.7 +/- 0.1 pmol. kg(-1). min(-1), P < 0.05) by suppressing insulin pulse mass (630 +/- 121 vs. 270 +/- 31 pmol, P < 0.01). Anesthesia also elicited an approximately 30% higher increase in insulin pulse frequency (P < 0.01) and more orderly insulin concentration profiles (P < 0.01). These effects were evoked by either sodium thiamylal or nitrous oxide and isoflurane. In conclusion, anesthesia represses insulin secretion through the mechanism of a twofold blunting of pulse mass despite an increase in orderly pulse frequency. These data thus unveil independent amplitude and frequency controls of beta-cells' secretory activity in vivo.

Insulin signalling through ultradian oscillations

Periodic oscillations appear to be a characteristic of insulin secretion at various different levels. Very rapid pulsations are seen in the isolated beta-cell and islet, while rapid (10- to 15-min) pulsations are seen both in the intact organism and in the isolated pancreas. Ultradian oscillations, particularly evident in situations of sustained exogenous glucose loading, appear to be a characteristic of intact organisms and have been hypothesized to be intrinsic to the normal glucose-insulin feedback system. Many of the features seen in experimental situations and in abnormalities of the system can be predicted by computer modelling of this system, supporting this hypothesis. A further theoretical feature of this hypothesis, borne out by experiment, is the ability to entrain insulin pulsatility by oscillations in an exogenous glucose infusion. Identification of defective ultradian oscillations and entrainment can identify subtle abnormalities of insulin sensitivity and pancreatic function, and restoration of normal function can be demonstrated after pharmaceutical intervention.

Direct measurement of pulsatile insulin secretion from the portal vein in human subjects

Insulin is secreted in a high frequency pulsatile manner. These pulses are delivered directly into the portal vein and then undergo extraction and dilution before delivery into the systemic circulation. The reported frequency of these insulin pulses estimated in peripheral blood varies from an interpulse interval of 4-20 min. We postulated that this discrepancy is due to the attenuation of the pulse signal in the systemic circulation vs. the portal circulation. In the present study we measured pulsatile insulin release directly in the portal circulation of human subjects who had indwelling transjugular intrahepatic portasystemic stent shunts (TIPSS) to decompress portal hypertension. We quantitated pulsatile insulin secretion in both the overnight fasted state (fasting) and during a hyperglycemic clamp (8 mmol/L). Direct portal vein sampling established that pulsatile insulin secretion in humans has an interval (periodicity) of approximately 5 min. The amplitude (and mass) of the insulin concentration oscillations observed in the portal vein was approximately 5-fold greater than that observed in the arterialized vein and was similar to that observed in the dog. Increased insulin release during hyperglycemia was achieved through amplification of the insulin pulse mass. In conclusion, direct portal vein sampling in humans revealed that the interpulse interval of insulin pulses in humans is about 5 min, and this frequency is also observed when sampling from the systemic circulation using a highly specific insulin assay and 1-min sampling, but is about 4-fold greater than the frequency observed at this site using single site RIAs. We confirm that enhanced insulin release in response to hyperglycemia is achieved by amplification of these high frequency pulses.

Pathophysiology of impaired pulsatile insulin release

Plasma insulin displays 5-10 min oscillations. In Type 2 diabetes the regularity of the oscillations disappears, which may lead to insulin receptor down-regulation and glucose intolerance and explain why pulsatile delivery of the hormone has a greater hypoglycemic effect than continuous delivery. The rhythm is intrinsic to the islet. Variations in metabolism, cytoplasmic Ca(2+) concentration ([Ca(2+)](i)), other hormones, neuronal signaling and possibly beta-cell insulin receptor expression have been implicated in the regulation of plasma insulin oscillations. Most of these factors are important for amplitude-regulation of the insulin pulses. Although evidence exists supporting a role of both metabolism and [Ca(2+)](i) as pacemakers of the pulses, metabolic oscillations probably have a primary role and [Ca(2+)](i) oscillations a permissive role. Results from islets from animal models of diabetes suggest that altered plasma insulin pattern could be due to lowering of pulse amplitude of insulin oscillations rather than alterations in their frequency. Supporting a role of metabolism, altered plasma insulin oscillations were found in MODY2, MIDD and glycogenosis Type VII, which are linked to alterations in glucokinase, mitochondrial tRNALeu(UUR) and phosphofructokinase. Plasma insulin oscillations require coordination of islet secretory activities in the pancreas. The intrapancreatic ganglia have been suggested as coordinators. The diabetes-associated neuropathy may contribute to the deranged pattern as indicated by glucose intolerance in chagasic patients. Continued investigation of the role and regulation of pulsatile insulin release will lead to better understanding of the pathophysiology of impaired pulsatile insulin release, which could lead to new approaches to restore normal plasma insulin oscillations in diabetes and related diseases.

INS VNTR allelic variation and dynamic insulin secretion in healthy adult non-diabetic Caucasian subjects

AIMS

To elucidate the relationship between the human insulin gene INS VNTR regulatory polymorphism and insulin secretion. The polymorphism arises from tandem repetition of 14-15 bp oligonucleotides. In Caucasians, repeat number varies from 26 to over 200, with two main and discrete allele size classes: class I (26-63 repeats) and class III (141-209 repeats). Class I allele homozygosity is associated with elevated risk of developing Type 1 diabetes, while the class III allele has been associated with increased risk of Type 2 diabetes, polycystic ovary syndrome (PCOS) and with larger size at birth, which may influence development of adult disease.

METHODS

Thirty-one healthy adult subjects with normal glucose tolerance, underwent an intravenous glucose tolerance test with one minute sampling. Seventeen subjects were homozygous for class I alleles (14 excluding individuals carrying alleles associated with parent-of-origin effects and heterogeneity in allele transmission) and 14 homozygous for class III alleles. The groups were well matched.

RESULTS

No significant differences in amount or rate of insulin secretion, or beta cell function were detected between the two groups. There was a difference in pattern of pulsatile insulin secretion with more 9-minute oscillations in class I homozygotes (P<0.026). The after-load glucose concentration was also higher in subjects with class I alleles (P<0.03).

CONCLUSIONS

These results warrant further analysis of possible association between allelic variation of the INS VNTR and the pulsatility of insulin secretion.

Preserved pulsatile insulin release from prediabetic mouse islets

During the development of type I diabetes, the plasma insulin pattern changes. Because the islet secretory pattern has been implicated in this phenomenon, insulin release was measured from female nonobese diabetic (NOD) mouse islets isolated at different ages. Islets from 5-week-old mice were used as controls because they had no infiltrating mononuclear cells and insulin release rose almost 9-fold with maintained oscillatory frequency when the glucose concentration was raised from 3 to 11 mM. Islets isolated from 13- and 25-week-old mice were infiltrated with mononuclear cells. In these islets, increase in the glucose concentration from 3 to 11 mM only doubled insulin release. However, despite the cellular infiltration, insulin release was pulsatile. Islets from 13-week-old mice had reduced glucose oxidation rate. Culture of such islets for 7 days at 11.1 mM glucose causes a decrease in the number of mononuclear cells infiltrating the islets, which in the present study was accompanied by a normalization of both glucose oxidation and glucose-induced insulin release. In the presence of the mitochondrial substrate alpha-keto-isocaproate (5 mM) both control and infiltrated islets responded with pronounced insulin pulses with similar amplitudes. The results suggest that the deranged plasma insulin pattern observed during the development of type I diabetes may be related to decrease in the insulin pulse amplitude rather than loss of the pulsatile release from the islets.

The role of plasma membrane K+ and Ca2+ permeabilities for glucose induction of slow Ca2+ oscillations in pancreatic beta-cells

In individual pancreatic beta-cells the rise of the cytoplasmic Ca2+ concentration ([Ca2+]i), induced by 11 mM glucose, is manifested either as oscillations (0.2-0.5 min-1) or as a sustained elevation. The significance of the plasma membrane permeability of Ca2+ and K- for the establishment of these slow oscillations was investigated by dual wavelength microfluorometric measurements of [Ca2+]i in individual ob/ob mouse beta-cells loaded with fura-2. Increasing the extracellular Ca2+ to 10 mM or the addition of Ca2+ channel agonist BAY K 8644 (1 microM) or K+ channel blocker tetraethylammonium+ (TEA: 10-20 mM) caused steeper rises and higher peaks of the glucose-induced oscillations. However, when extracellular Ca2+ was lowered to 0.5 mM the oscillations were transformed into a sustained suprabasal level. When the beta-cells exhibited glucose-stimulated sustained elevation of [Ca2+]i in the presence of a physiological Ca2+ concentration (1.3 mM), it was possible to induce slow oscillations by promoting the entry of Ca2+ either by raising the extracellular Ca2+ concentration to 10 mM or adding TEA or BAY K 8644. The results indicate that glucose-induced slow oscillations of [Ca2+]i depend on the closure of ATP-regulated K+ channels and require that the rate of Ca2+ influx exceeds a critical level. Apart from an inherent periodicity in ATP production it is proposed that Ca(2+)-induced ATP consumption in the submembrane space contributes to the cyclic changes of the membrane potential determining periodic entry of Ca2+.

Oscillatory patterns of electrical activity in mouse pancreatic islets of Langerhans recorded in vivo

Pancreatic beta-cells secrete insulin as a function of blood glucose concentration. One of the key steps in stimulus-secretion coupling is the depolarisation of the membrane and the appearance of bursts of calcium action potentials. Recently, the characteristics and glucose dependence of the oscillations in electrical activity in vivo have been described. The experiments described here were designed to determine the temporal evolution of such electrical activity when no experimental changes in the glycaemia are imposed. The absolute duration of the active and silent phases has been analysed and compared with the values obtained in vitro. We have found that in vivo, at glycaemia ranging from 6.0 to 7.5 mM, the electrical activity of the islets of Langerhans is permanently oscillatory, the mean duration of the depolarisation phase being 28 s. In general, the oscillatory pattern remains very constant for relatively long (up to 60 min) periods of time. In some experiments, slow or transitory changes in the degree of beta-cell activation could be observed, as well as the existence, in a very few cases, of oscillatory non-periodic patterns. Key words beta-cells middle dot Pancreas middle dot Electrophysiology middle dot Oscillations

Differential beta-cell response to glucose, glucagon, and arginine during progression to type I (insulin-dependent) diabetes mellitus

Acute insulin responses to glucose (AIRG), glucagon (AIRGln), and arginine (AIRArg) were evaluated prospectively in nine subjects positive for islet-cell antibodies (ICAs) who later progressed to type I diabetes or impaired glucose tolerance (IGT) (progressors), 64 ICA-positive subjects at risk who did not develop type I diabetes, 365 ICA-negative relatives of diabetic patients who also remained free of the disease, and 89 control subjects. Seven progressors already had a low AIRG at entry into the study, and the other two became low responders 3 to 9 months before diabetes or IGT, with a progressive decline of AIRG over serial intravenous (IV) glucose tolerance tests. At entry into the study, the group of progressors displayed lower AIRG, AIRGln, and AIRArg than the other three groups (P<.001). However, AIRArg was less altered than AIRG. During the course of the prediabetic phase, there was a progressive decline in AIRG and AIRGln analyzed as a function either of time (P<.006) or of basal glycemia (P<.05), ie, two different ways of estimating worsening of the disease process. Conversely, there was no significant decrease in AIRArg with time or with increasing basal glycemia, so that AIRArg was not totally blunted in these prediabetic subjects even a few months before the onset of diabetes. The persistence of a substantial response to arginine, ie, higher than the fifth control percentile, even at a late stage, was confirmed in five of nine diabetic patients tested either at onset of the disease or during non-insulin-receiving remission. Whereas AIRG deteriorates during prediabetes, AIRArg appears to be less altered with time and increased basal glycemia, remaining substantial even at the onset of the disease. This reinforces the supposition that the prediabetic state may be associated with a glucose-specific beta-cell functional abnormality in addition to a decreasing beta-cell mass.

Impact of sampling technique on appraisal of pulsatile insulin secretion by deconvolution and cluster analysis

Little is known about the optimal experimental conditions for assessing pulsatile insulin secretion in vivo. To address this, we employed a recently validated canine model (n = 12) to determine the consequences of 1) sampling from the systemic circulation (SC) vs. the portal vein (PV), 2) sampling intensity and duration, and 3) deconvolution vs. cluster analysis on assessing pulsatile insulin secretion. PV vs. SC sampling resulted in a approximately 40% higher pulse frequency by deconvolution (9.0 +/- 0.5 vs. 6.6 +/- 0.9 pulses/h, P < 0.02) and cluster analysis (7.5 +/- 0.3 vs. 5.6 +/- 0.6 pulses/h, P < 0.01) due to a higher signal-to-noise ratio (19 +/- 4.8 PV vs. 12 +/- 1.8 SC). PV sampling also disclosed a higher calculated contribution of the pulsatile vs. nonpulsatile mode of delivery to total insulin secretion (57 +/- 4 vs. 28 +/- 5%, P < 0.001). Analysis of the relevance of sampling intensity revealed that 1-min data yielded a markedly higher estimate of pulse frequency with PV sampling than 2-min data (9.0 +/- 0.5 vs. 5.4 +/- 0.5, P < 0.02, deconvolution; 7.5 +/- 0.3 vs. 4.3 +/- 0.6 pulses/h, P < 0.001, cluster). Optimal sampling duration was shown to be 40 min or more. We conclude that the resolving power of the analytical tool, the anatomic site of blood withdrawal, the frequency of blood sampling, and the duration of the total observation interval all significantly influence estimated insulin secretory pulse frequency and the fraction of insulin secreted in pulses. With the assumption that PV 1-min insulin data constitute the "gold standard," our in vivo inferences of 7.5-9.0 insulin pulses/h closely recapitulate in vitro islet secretory activity.

Impaired Ca2+ response to glucose in mouse beta-cells infected with coxsackie B or Echo virus

Five strains of Coxsackie B4 virus and one of Echo 11 virus were tested with regard to their ability to replicate in pancreatic mouse beta-cells and interfere with the alterations of the cytoplasmic Ca2+ concentration ([Ca2+]i) induced by glucose. All strains except one both multiplied and caused cytopathic effect. In a control group 68% of the beta-cells responded to 11 mM glucose with large amplitude oscillations of [Ca2+]i. After inoculation with the infectious strains these oscillations appeared in only 5% of the beta-cells, whereas the non-infectious strain did not modify the glucose effect on [Ca2+]i. Despite the virus interference with the glucose response, [Ca2+]i was increased after depolarization with excessive extracellular K+ and the oscillations were induced in most beta-cells when glucose was combined with the insulin-releasing sulfonylurea tolbutamide.

Glucose induces oscillatory Ca2+ signalling and insulin release in human pancreatic beta cells

Mechanisms of pulsatile insulin release in man were explored by studying the induction of oscillatory Ca2+ signals in individual beta cells and islets isolated from the human pancreas. Evidence was provided for a glucose-induced closure of ATP-regulated K+ channels, resulting in voltage-dependent entry of Ca2+. The observation of step-wise increases of capacitance in response to depolarizing pulses suggests that an enhanced influx of Ca2+ is an effective means of stimulating the secretory activity of the isolated human beta cell. Activation of muscarinic receptors (1-10 mumol/l carbachol) and of purinergic P2 receptors (0.01-1 mumol/l ATP) resulted in repetitive transients followed by sustained elevation of the cytoplasmic Ca2+ concentration ([Ca2+]i). Periodic mobilisation of intracellular calcium was seen also when injecting 100 mumol/l GTP-gamma-S into beta cells hyperpolarized to -70 mV. Individual beta cells responded to glucose and tolbutamide with increases of [Ca2+]i, manifested either as large amplitude oscillations (frequency 0.1-0.5/min) or as a sustained elevation. Glucose regulation was based on sudden transitions between the basal and the two alternative states of raised [Ca2+]i at threshold concentrations of the sugar characteristic for the individual beta cells. The oscillatory characteristics of coupled cells were determined collectively rather than by particular pacemaker cells. In intact pancreatic islets the glucose induction of well-synchronized [Ca2+]i oscillations had its counterpart in 2-5 min pulses of insulin. Each of these pulses could be resolved into regularly occurring short insulin transients. It is concluded that glucose stimulation of insulin release in man is determined by the number of beta cells entering into a state with Ca(2+)-induced secretory pulses.

Alterations in pulsatile insulin secretion in the Zucker diabetic fatty rat

Insulin secretion from the isolated perfused pancreas is characterized by pulses occurring every 5-15 min. The present experiments were performed to explore the role of glucose in regulating these pulses. The pancreata from 12 Wistar (W), 12 Zucker diabetic fatty (ZDF), and 6 nondiabetic lean Zucker control (ZC) male rats were isolated and perfused at 37 degrees C with an oxygenated Krebs Ringer solution containing bovine serum albumin and glucose. In W and ZDF, insulin secretion was pulsatile during constant glucose, as assessed by pulse analysis (ULTRA). The pulse period in W was significantly shorter than in ZDF (7.1 +/- 0.6 vs. 14.7 +/- 1.0 min; P < 0.001), whereas the median relative pulse amplitude was not different. When glucose was administered as a series of 10-min sine waves, spectral analysis showed that the normalized spectral power at 10 min was greater in W and ZC compared with ZDF (34.2 +/- 5.9 and 32.9 +/- 2.9 vs. 3.2 +/- 0.9; P < 0.0001), demonstrating entrainment of the insulin pulses to the exogenous glucose oscillations in W and ZC but not in ZDF. Furthermore, in ZDF, the insulin secretory rates were not higher when 28 mM rather than 7 mM glucose were used. In additional studies, islets of Langerhans from one W, three ZDF, and three ZC rats were isolated and perifused using an oscillatory glucose concentration. Single and groups of islets were studied. Islets from diabetic rats demonstrated the same lack of entrainment by glucose seen in the perfused pancreas, suggesting that the defect is at the cellular level.(ABSTRACT TRUNCATED AT 250 WORDS)

Analytical problems in detecting rapid insulin secretory pulses in normal humans

The present study was undertaken to define the optimal experimental and analytical conditions necessary to reproducibly detect, in the systemic blood, small-amplitude rapid oscillations (period 8-15 min) of insulin and C-peptide. Samples for insulin, C-peptide, and glucose were drawn at 2-min intervals for 2 h from six normal subjects during constant glucose infusion and from five of the same subjects under basal conditions. To reduce measurement error, insulin and C-peptide levels were measured 16 times at each time point. Three algorithms for pulse analysis (ULTRA, Cluster, PulseFit) were used to identify significant pulses, whereas autocorrelation and spectral analysis were used to identify potential regular periodic components in the data. In three of the five subjects studied under basal conditions, regular rapid oscillations could be consistently detected by autocorrelation when the analysis was based on eight replicates but not on duplicate series. In the remaining two basal studies and in all studies during glucose infusion, the majority of profiles did not have a significant periodic component. However, formal pulse analysis demonstrated that the number of pulses was similar during glucose infusion and basal conditions. Reproducibility was enhanced by increasing the number of replicates used in the analysis. We conclude that in the analysis of small-amplitude rapid insulin and C-peptide oscillations, the sensitivity and specificity of the analysis is likely to be enhanced by performing multiple estimations at each time point and by using a minimum of two contrasting analytical approaches for pulse detection, which incorporate a method evaluating periodicity in conjunction with a pulse detection program designed to evaluate each individual oscillation separately.