] i ) is a key signal in the initiation of insulin secretion from the pancreatic ␤-cell. This increase principally results from calcium influx through plasma membrane (PM) Ca 2ϩ channels, which open in response to secretagogues, primarily glucose. The metabolism of glucose through glycolysis and the tricarboxylic acid cycle leads to an increase in the cytoplasmic ATP-to-ADP (ATP/ADP) ratio. This causes closure of ATP-sensitive K ϩ (K ATP ) channels followed by depolarization of the ␤-cell membrane to the threshold potential where Ca 2ϩ channels open, initiating Ca 2ϩ influx (4). These events underlie glucose-induced electrical activity that, in pancreatic islets, consists of Ca 2ϩ -dependent action potentials.There is extensive literature describing ␤-cell electrical activity and its relationship to [Ca 2ϩ ] i in intact islets of Langerhans, isolated islet cells, and insulinoma cell lines. Most of the work has been carried out using mouse islets, with some studies using islets from rat, hamster, human, and other species.Mouse pancreatic ␤-cells exhibit complex and cyclic spike-burst activity in response to a rise in extracellular glucose concentration. The bursts consist of a depolarized phase of Ca 2ϩ -carrying action potentials alternating with a silent phase of repolarization, resulting in oscillations in intracellular Ca 2ϩ , which can drive pulses of insulin secretion (28, 37).The only stimulus required for a complex cyclic spike-burst activity and corresponding [Ca 2ϩ ] i oscillations in islets and ␤-cell clusters is elevation of glucose to levels above 5 and less than ϳ20 mM. Intermediate glucose concentrations induce two main types of oscillations in mouse pancreatic islets: fast, where the period ranges from 10 to 30 s, and slow, with periods of several minutes (37,54,83). Single mouse ␤-cells can also respond to glucose stimulation with regular oscillations (37).We have previously studied slow and fast [Ca 2ϩ ] i oscillations in islets in response to a variety of conditions (70, 73; unpublished observations). We have also previously reported that a stable, transgenically derived murine insulinoma cell line (␤TC3-neo) responds to glucose with slow, large amplitude [Ca 2ϩ ] i oscillations but only in the presence of 10-20 mM tetraethylammonium (TEA), a blocker of K ϩ channels (74). We have utilized this cell line to characterize glucose-stimulated oscillatory activity (74).However, the precise interpretation of previous results is limited due to the numerous channels and pumps in ␤-cells that work concurrently, and identification of physiologically slow variables that drive oscillations remains unclear. To clarify these complex experimental results, we used a mathematical modeling approach. Our goals, then, are twofold: to develop a model for ␤-cell ion homeostasis, including the bursts and [Ca 2ϩ ] i oscillations that can simulate cellular behavior, and to explain on this basis the experimental data for single cells and islets.Several mathematical approaches in the literature have provid...

Glucose-dependent insulin secretion (GDIS), reactive oxygen species (ROS) production, and oxidative stress in pancreatic ␤-cells may be tightly linked processes. Here we suggest that the same pathways used in the activation of GDIS (increased glycolytic flux, ATP-to-ADP ratio, and intracellular Ca 2؉ concentration) can dramatically enhance ROS production and manifestations of oxidative stress and, possibly, apoptosis. The increase in ROS production and oxidative stress produced by GDIS activation itself suggests a dual role for metabolic insulin secretagogues, as an initial sharp increase in insulin secretion rate can be accompanied by progressive ␤-cell injury. We propose that therapeutic strategies targeting enhancement of GDIS should be carefully considered in light of possible loss of ␤-cell function and mass.

The early stages of type 2 diabetes mellitus are characterized by the development of insulin resistance (IRe) in muscle cells and adipocytes with the concomitant loss of beta-cell compensation. We have extensively reviewed the literature related to metabolic and signalling pathways of reactive oxygen species (ROS) in regard to the coordinated development of oxidative stress and IRe. We considered the hypothesis that oxidative stress leads to IRe in muscle cells and adipocytes, but found that the data are more consistent with the hypothesis that the cellular mechanisms that protect against oxidative stress per se are capable of creating an ROS-dependent insulin-resistant state. Furthermore, ROS-induced mitochondrial dysfunction can lead to disruptions of lipid metabolism, increasing the intracellular lipid content, and, in addition, contribute to lipid-dependent IRe in myocytes. Together, these two ROS-activated pathways to IRe can contribute to a global state of profound resistance to insulin action. Therapeutic strategies should, therefore, be directed towards reducing insulin resistance without an increase in ROS production or concentration. Pharmacological or other approaches to IRe that result in the activation of mitochondrial biogenesis in particular could be highly beneficial in the prevention or treatment of both insulin resistance and type 2 diabetes.

Fridlyand LE, Tamarina N, Philipson LH. Bursting and calcium oscillations in pancreatic ␤-cells: specific pacemakers for specific mechanisms. Am J Physiol Endocrinol Metab 299: E517-E532, 2010. First published July 13, 2010; doi:10.1152/ajpendo.00177.2010.-Oscillatory phenomenon in electrical activity and cytoplasmic calcium concentration in response to glucose are intimately connected to multiple key aspects of pancreatic ␤-cell physiology. However, there is no single model for oscillatory mechanisms in these cells. We set out to identify possible pacemaker candidates for burst activity and cytoplasmic Ca 2ϩ oscillations in these cells by analyzing published hypotheses, their corresponding mathematical models, and relevant experimental data. We found that although no single pacemaker can account for the variety of oscillatory phenomena in ␤-cells, at least several separate mechanisms can underlie specific kinds of oscillations. According to our analysis, slowly activating Ca 2ϩ -sensitive K ϩ channels can be responsible for very fast Ca 2ϩ oscillations; changes in the ATP/ADP ratio and in the endoplasmic reticulum calcium concentration can be pacemakers for both fast bursts and cytoplasmic calcium oscillations, and cyclical cytoplasmic Na ϩ changes may underlie patterning of slow calcium oscillations. However, these mechanisms still lack direct confirmation, and their potential interactions raises new issues. Further studies supported by improved mathematical models are necessary to understand oscillatory phenomena in ␤-cell physiology. endoplasmic reticulum; channels; diabetes; mathematical model; metabolism IN PANCREATIC ␤-CELLS, glucose-stimulated insulin secretion is mediated by an elevated cytosolic free calcium concentration ([Ca 2ϩ ] c ). This results from calcium influx through voltagedependent Ca 2ϩ channels (VDCCs) located in the plasma membrane (PM), which open in response to secretagogues, primarily glucose. Stimulation-secretion coupling in ␤-cells is different from most other cell types, because instead of being mediated by receptor binding, glucose must be transported into the cytoplasm and metabolized.Glucose initiates changes in the PM potential via an increase in the cytoplasmic [ATP]/[ADP] ratio derived from glycolysis and oxidative phosphorylation. This results in closure of ATPsensitive K ϩ (K ATP ) channels. Closure of these channels leads to PM depolarization up to a threshold potential, causing the cell to move from quiescence to initiate electrical activity and VDCC opening. Ca 2ϩ influx through VDCCs leads to increased [Ca 2ϩ ] c , which is a key signal in the initiation of insulin secretion from the pancreatic ␤-cells. Consensus mechanisms of glucose-induced PM, cytosolic, and mitochondrial processes are summarized in Fig. 1 (for recent review, see Refs. 71,74,76,and 140).The ␤-cell membrane is hyperpolarized to a resting potential of about Ϫ60 mV at low glucose levels (ϳ3-6 mM). Dean and Matthews (32,33) showed that when glucose rises the PM depolarizes and then generates an ...

BackgroundPancreatic beta-cells respond to rising blood glucose by increasing oxidative metabolism, leading to an increased ATP/ADP ratio in the cytoplasm. This leads to a closure of KATP channels, depolarization of the plasma membrane, influx of calcium and the eventual secretion of insulin. Such mechanism suggests that beta-cell metabolism should have a functional regulation specific to secretion, as opposed to coupling to contraction. The goal of this work is to uncover contributions of the cytoplasmic and mitochondrial processes in this secretory coupling mechanism using mathematical modeling in a systems biology approach.MethodsWe describe a mathematical model of beta-cell sensitivity to glucose. The cytoplasmic part of the model includes equations describing glucokinase, glycolysis, pyruvate reduction, NADH and ATP production and consumption. The mitochondrial part begins with production of NADH, which is regulated by pyruvate dehydrogenase. NADH is used in the electron transport chain to establish a proton motive force, driving the F1F0 ATPase. Redox shuttles and mitochondrial Ca2+ handling were also modeled.ResultsThe model correctly predicts changes in the ATP/ADP ratio, Ca2+ and other metabolic parameters in response to changes in substrate delivery at steady-state and during cytoplasmic Ca2+ oscillations. Our analysis of the model simulations suggests that the mitochondrial membrane potential should be relatively lower in beta cells compared with other cell types to permit precise mitochondrial regulation of the cytoplasmic ATP/ADP ratio. This key difference may follow from a relative reduction in respiratory activity. The model demonstrates how activity of lactate dehydrogenase, uncoupling proteins and the redox shuttles can regulate beta-cell function in concert; that independent oscillations of cytoplasmic Ca2+ can lead to slow coupled metabolic oscillations; and that the relatively low production rate of reactive oxygen species in beta-cells under physiological conditions is a consequence of the relatively decreased mitochondrial membrane potential.ConclusionThis comprehensive model predicts a special role for mitochondrial control mechanisms in insulin secretion and ROS generation in the beta cell. The model can be used for testing and generating control hypotheses and will help to provide a more complete understanding of beta-cell glucose-sensing central to the physiology and pathology of pancreatic β-cells.