NIDDM is associated with loss of pancreatic beta-cell L-type Ca2+ channel activity

Endoplasmic Reticulum (ER) Stress and Its Role in Pancreatic β-Cell Dysfunction and Senescence in Type 2 Diabetes

An increased life span and accompanying nutritional affluency have led to a rapid increase in diseases associated with aging, such as obesity and type 2 diabetes, imposing a tremendous economic and health burden on society. Pancreatic β-cells are crucial for controlling glucose homeostasis by properly producing and secreting the glucose-lowering hormone insulin, and the dysfunction of β-cells determines the outcomes for both type 1 and type 2 diabetes. As the native structure of insulin is formed within the endoplasmic reticulum (ER), ER homeostasis should be appropriately maintained to allow for the proper metabolic homeostasis and functioning of β-cells. Recent studies have found that cellular senescence is critically linked with cellular stresses, including ER stress, oxidative stress, and mitochondrial stress. These studies implied that β-cell senescence is caused by ER stress and other cellular stresses and contributes to β-cells’ dysfunction and the impairment of glucose homeostasis. This review documents and discusses the current understanding of cellular senescence, β-cell function, ER stress, its associated signaling mechanism (unfolded protein response), and the effect of ER stress on β-cell senescence and dysfunction.

Exposure to maternal obesity programs sex differences in pancreatic islets of the offspring in mice

AIMS/HYPOTHESIS

Obesity during pregnancy increases offspring type 2 diabetes risk. Given that nearly half of women of child-bearing age in many populations are currently overweight/obese, it is key that we improve our understanding of the impact of the in utero/early life environment on offspring islet function. Whilst a number of experimental studies have examined the effect of maternal obesity on offspring islet architecture and/or function, it has not previously been delineated whether these changes are independent of other confounding risk factors such as obesity, postnatal high-fat-feeding and ageing. Thus, we aimed to study the impact of exposure to maternal obesity on offspring islets in young, glucose-tolerant male and female offspring.

METHODS

Female C57BL/6J mice were fed ad libitum either chow or obesogenic diet prior to and throughout pregnancy and lactation. Offspring were weaned onto a chow diet and remained on this diet until the end of the study. An IPGTT was performed on male and female offspring at 7 weeks of age. At 8 weeks of age, pancreatic islets were isolated from offspring for measurement of insulin secretion and content, mitochondrial respiration, ATP content, reactive oxygen species levels, beta and alpha cell mass, granule and mitochondrial density (by transmission electron microscopy), and mRNA and protein expression by real-time RT-PCR and Western blotting, respectively.

RESULTS

Glucose tolerance was similar irrespective of maternal diet and offspring sex. However, blood glucose was lower (p < 0.001) and plasma insulin higher (p < 0.05) in female offspring of obese dams 15 min after glucose administration. This was associated with higher glucose- (p < 0.01) and leucine/glutamine-stimulated (p < 0.05) insulin secretion in these offspring. Furthermore, there was increased mitochondrial respiration (p < 0.01) and density (p < 0.05) in female offspring of obese dams compared with same-sex controls. Expression of mitochondrial and nuclear-encoded components of the electron transport chain, L-type Ca²⁺ channel subtypes that play a key role in stimulus-secretion coupling [Cacna1d (p < 0.05)], and oestrogen receptor α (p < 0.05) was also increased in islets from these female offspring of obese dams. Moreover, cleaved caspase-3 expression and BAX:Bcl-2 were decreased (p < 0.05) reflecting reduced susceptibility to apoptosis. In contrast, in male offspring, glucose and leucine/glutamine-stimulated insulin secretion was comparable between treatment groups. There was, however, compromised mitochondrial respiration characterised by decreased ATP synthesis-driven respiration (p < 0.05) and increased uncoupled respiration (p < 0.01), reduced docked insulin granules (p < 0.001), decreased Cacna1c (p < 0.001) and Cacna1d (p < 0.001) and increased cleaved caspase-3 expression (p < 0.05).

CONCLUSIONS/INTERPRETATION

Maternal obesity programs sex differences in offspring islet function. Islets of female but not male offspring appear to be primed to cope with a nutritionally-rich postnatal environment, which may reflect differences in future type 2 diabetes risk.

Regulation of L-type Ca2+ Channel Activity and Insulin Secretion by Huntingtin-associated Protein 1

Huntingtin-associated protein 1 (Hap1) was originally identified as a protein that binds to the Huntington disease protein, huntingtin. Growing evidence has shown that Hap1 participates in intracellular trafficking via its association with various microtubule-dependent transporters and organelles. Recent studies also revealed that Hap1 is involved in exocytosis such as insulin release from pancreatic β-cells. However, the mechanism underlying the action of Hap1 on insulin release remains to be investigated. We found that Hap1 knock-out mice had a lower plasma basal insulin level than control mice. Using cultured pancreatic β-cell lines, INS-1 cells, we confirmed that decreasing Hap1 reduces the number of secreted vesicles and inhibits vesicle exocytosis. Electrophysiology and imaging of intracellular Ca²⁺ measurements demonstrated that Hap1 depletion significantly reduces the influx of Ca²⁺ mediated by L-type Ca²⁺ channels (Cav). This decrease is not due to reduced expression of Cav1.2 channel mRNA but results from the decreased distribution of Cav1.2 on the plasma membrane of INS-1 cells. Fluorescence recovery after photobleaching showed a defective movement of Cav1.2 in Hap1 silencing INS-1 cells. Our findings suggest that Hap1 is important for insulin secretion of pancreatic β-cells via regulating the intracellular trafficking and plasma membrane localization of Cav1.2, providing new insight into the mechanisms that regulate insulin release from pancreatic β-cells.

CaV1.2 and CaV1.3 channel hyperactivation in mouse islet β cells exposed to type 1 diabetic serum

The voltage-gated Ca(2+) (CaV) channel acts as a key player in β cell physiology and pathophysiology. β cell CaV channels undergo hyperactivation subsequent to exposure to type 1 diabetic (T1D) serum resulting in increased cytosolic free Ca(2+) concentration and thereby Ca(2+)-triggered β cell apoptosis. The present study was aimed at revealing the subtypes of CaV1 channels hyperactivated by T1D serum as well as the biophysical mechanisms responsible for T1D serum-induced hyperactivation of β cell CaV1 channels. Patch-clamp recordings and single-cell RT-PCR analysis were performed in pancreatic β cells from CaV1 channel knockout and corresponding control mice. We now show that functional CaV1.3 channels are expressed in a subgroup of islet β cells from CaV1.2 knockout mice (CaV1.2(-/-)). T1D serum enhanced whole-cell CaV currents in islet β cells from CaV1.3 knockout mice (CaV1.3(-/-)). T1D serum increased the open probability and number of functional unitary CaV1 channels in CaV1.2(-/-) and CaV1.3(-/-) β cells. These data demonstrate that T1D serum hyperactivates both CaV1.2 and CaV1.3 channels by increasing their conductivity and number. These findings suggest CaV1.2 and CaV1.3 channels as potential targets for anti-diabetes therapy.

Alterations of the Ca²⁺ signaling pathway in pancreatic beta-cells isolated from db/db mice

Upon glucose elevation, pancreatic beta-cells secrete insulin in a Ca²⁺-dependent manner. In diabetic animal models, different aspects of the calcium signaling pathway in beta-cells are altered, but there is no consensus regarding their relative contributions to the development of beta-cell dysfunction. In this study, we compared the increase in cytosolic Ca²⁺ ([Ca²⁺]_i) via Ca²⁺ influx, Ca²⁺ mobilization from endoplasmic reticulum (ER) calcium stores, and the removal of Ca²⁺ via multiple mechanisms in beta-cells from both diabetic db/db mice and non-diabetic C57BL/6J mice. We refined our previous quantitative model to describe the slow [Ca²⁺]_i recovery after depolarization in beta-cells from db/db mice. According to the model, the activity levels of the two subtypes of the sarco-endoplasmic reticulum Ca²⁺-ATPase (SERCA) pump, SERCA2 and SERCA3, were severely down-regulated in diabetic cells to 65% and 0% of the levels in normal cells. This down-regulation may lead to a reduction in the Ca²⁺ concentration in the ER, a compensatory up-regulation of the plasma membrane Na⁺/Ca²⁺ exchanger (NCX) and a reduction in depolarization-evoked Ca²⁺ influx. As a result, the patterns of glucose-stimulated calcium oscillations were significantly different in db/db diabetic beta-cells compared with normal cells. Overall, quantifying the changes in the calcium signaling pathway in db/db diabetic beta-cells will aid in the development of a disease model that could provide insight into the adaptive transformations of beta-cell function during diabetes development.

Transplantation of Encapsulated Pancreatic Islets as a Treatment for Patients with Type 1 Diabetes Mellitus

Encapsulation of pancreatic islets has been proposed and investigated for over three decades to improve islet transplantation outcomes and to eliminate the side effects of immunosuppressive medications. Of the numerous encapsulation systems developed in the past, microencapsulation have been studied most extensively so far. A wide variety of materials has been tested for microencapsulation in various animal models (including nonhuman primates or NHPs) and some materials were shown to induce immunoprotection to islet grafts without the need for chronic immunosuppression. Despite the initial success of microcapsules in NHP models, the combined use of islet transplantation (allograft) and microencapsulation has not yet been successful in clinical trials. This review consists of three sections: introduction to islet transplantation, transplantation of encapsulated pancreatic islets as a treatment for patients with type 1 diabetes mellitus (T1DM), and present challenges and future perspectives.

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

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

Islet preconditioning via multimodal microfluidic modulation of intermittent hypoxia

Simultaneous stimulation of ex vivo pancreatic islets with dynamic oxygen and glucose is a critical technique for studying how hypoxia alters glucose-stimulated response, especially in transplant environments. Standard techniques using a hypoxic chamber cannot provide both oxygen and glucose modulations, while monitoring stimulus-secretion coupling factors in real-time. Using novel microfluidic device with integrated glucose and oxygen modulations, we quantified hypoxic impairment of islet response by calcium influx, mitochondrial potentials, and insulin secretion. Glucose-induced calcium response magnitude and phase were suppressed by hypoxia, while mitochondrial hyperpolarization and insulin secretion decreased in coordination. More importantly, hypoxic response was improved by preconditioning islets to intermittent hypoxia (IH, 1 min/1 min 5-21% cycling for 1 h), translating to improved insulin secretion. Moreover, blocking mitochondrial K(ATP) channels removed preconditioning benefits of IH, similar to mechanisms in preconditioned cardiomyocytes. Additionally, the multimodal device can be applied to a variety of dynamic oxygen-metabolic studies in other ex vivo tissues.

Encapsulation of human islets in novel inhomogeneous alginate-ca2+/ba2+ microbeads: in vitro and in vivo function

Microencapsulation may allow for immunosuppression-free islet transplantation. Herein we investigated whether human islets can be shipped safely to a remote encapsulation core facility and maintain in vitro and in vivo functionality. In non-encapsulated islets before and encapsulated islets after shipment, viability was 88.3+/-2.5 and 87.5+/-2.7% (n=6, p=0.30). Stimulation index after static glucose incubation was 5.4+/-0.5 and 6.3+/-0.4 (n=6, p=0.18), respectively. After intraperitoneal transplantation, long-term normoglycemia was consistently achieved with 3,000, 5,000, and 10,000 IEQ encapsulated human islets. When transplanting 1,000 IEQ, mice returned to hyperglycemia after 30-55 (n=4/7) and 160 days (n=3/7). Transplanted mice showed human oral glucose tolerance with lower glucose levels than non-diabetic control mice. Capsules retrieved after transplantation were intact, with only minimal overgrowth. This study shows that human islets maintained the viability and in vitro function after encapsulation and the inhomogeneous alginate-Ca(2+)/Ba(2+) microbeads allow for long-term in vivo human islet graft function, despite long-distance shipment.

Growth factor control of pancreatic islet regeneration and function

Type 1 and type 2 diabetes mellitus together are predicted to affect over 300 million people worldwide by the year 2020. A relative or absolute paucity of functional β-cells is a central feature of both types of disease, and identifying the pathways that mediate the embryonic origin of new β-cells and mechanisms that underlie the proliferation of existing β-cells are major efforts in the fields of developmental and islet biology. A poor secretory response of existing β-cells to nutrients and hormones and the defects in hormone processing also contribute to the hyperglycemia observed in type 2 diabetes and has prompted studies aimed at enhancing β-cell function. The factors that contribute to a greater susceptibility in aging individuals to develop diabetes is currently unclear and may be linked to a poor turnover of β-cells and/or enhanced susceptibility of β-cells to apoptosis. This review is an update on the recent work in the areas of islet/β-cell regeneration and hormone processing that are relevant to the pathophysiology of the endocrine pancreas in type 1, type 2 and obesity-associated diabetes.

Glucotoxicity inhibits late steps of insulin exocytosis

Prolonged exposure of beta-cells to high glucose (glucotoxicity) diminishes insulin secretion in response to glucose and has been linked to altered generation of metabolism-secretion coupling factors. We have investigated whether glucotoxicity may also alter calcium handling and late steps in secretion such as exocytosis. Clonal INS-1E beta-cells cultured at high glucose (20 or 30 mM vs. 5.5 mM) for 72 h exhibited elevated basal intracellular calcium ([Ca2+]i), which was KATP-channel dependent and due to long-term activation of protein kinase A. An increased amplitude and shortened duration of depolarization-evoked rises in [Ca2+]i were apparent. These changes were probably linked to the observed increased filling of intracellular stores and to short-term activation of protein kinase A. Insulin secretion was reduced not only by acute stimulation with either glucose or KCl but more importantly by direct calcium stimulation of permeabilized cells. These findings indicate a defect in the final steps of exocytosis. To confirm this, we measured expression levels of some 30 proteins implicated in trafficking/exocytosis of post-Golgi vesicles. Several proteins required for calcium-induced exocytosis of secretory granules were down-regulated, such as the soluble N-ethylmaleimide-sensitive factor-sensitive factor attachment receptor (SNARE) proteins VAMP-2 [vesicle (v)-SNARE, vesicle-associated membrane protein 2] and syntaxin 1 as well as complexin. VAMP-2 was also reduced in human islets. In contrast, cell immunostaining and expression levels of several fluorescent proteins suggested that other post-trans-Golgi trafficking steps and compartments are preserved and that cells were not degranulated. Thus, these studies indicate that, in addition to known metabolic changes, glucotoxicity impedes generation of signals for secretion and diminishes the efficiency of late steps in exocytosis.

Limited role for SREBP-1c in defective glucose-induced insulin secretion from Zucker diabetic fatty rat islets: a functional and gene profiling analysis

Accumulation of intracellular lipid may contribute to defective insulin secretion in type 2 diabetes. Although Zucker diabetic fatty (ZDF; fa/fa) rat islets are fat-laden and overexpress the lipogenic master gene, sterol regulatory element binding protein 1c (SREBP-1c), the contribution of SREBP-1c to the secretory defects observed in this model remains unclear. Here we compare the gene expression profile of lean control (fa/+) and ZDF rat islets in the absence or presence of dominant-negative SREBP-1c (SREBP-1c DN). ZDF islets displayed elevated basal insulin secretion at 3 mmol/l glucose but a severely depressed response to 17 mmol/l glucose. While SREBP-1c DN reduced basal insulin secretion from ZDF islets, glucose-stimulated insulin secretion was not improved. Of 57 genes differentially regulated in ZDF islets and implicated in glucose metabolism, vesicle trafficking, ion fluxes, and/or exocytosis, 21 were upregulated and 5 were suppressed by SREBP-1c DN. Genes underrepresented in ZDF islets were either unaffected (Glut-2, Kir6.2, Rab3), stimulated (voltage-dependent Ca(2+) channel subunit alpha1D, CPT2, SUR2, rab9, syt13), or inhibited (syntaxin 7, secretogranin-2) by SREBP-1c inhibition. Correspondingly, SREBP-1c DN largely corrected decreases in the expression of the transcription factors Pdx-1 and MafA but did not affect the abnormalities in Pax6, Arx, hepatic nuclear factor-1alpha (HNF1alpha), HNF3beta/Forkhead box-a2 (Foxa2), inducible cyclic AMP early repressor (ICER), or transcription factor 7-like 2 (TCF7L2) expression observed in ZDF islets. We conclude that upregulation of SREBP-1c and mild increases in triglyceride content do not explain defective glucose-stimulated insulin secretion from ZDF rats. However, overexpression of SREBP-1c may contribute to enhanced basal insulin secretion in this model.

Regulation of two insulin granule populations within the reserve pool by distinct calcium sources

Insulin granule trafficking is a key step of glucose-stimulated insulin secretion from pancreatic beta cells. Using quantitative live cell imaging, we examined insulin granule movements within the reserve pool upon secretory stimulation in betaTC3 cells. For this study, we developed a custom image analysis program that permitted automatic tracking of the individual motions of over 20,000 granules. This analysis of a large sample size enabled us to study micro-populations of granules that were not quantifiable in previous studies. While over 90% of the granules depend on Ca2+ efflux from the endoplasmic reticulum for their mobilization, a small and fast-moving population of granules responds to extracellular Ca2+ influx after depolarization of the plasma membrane. We show that this differential regulation of the two granule populations is consistent with localized Ca2+ signals, and that the cytoskeletal network is involved in both types of granule movement. The fast-moving granules are correlated temporally and spatially to the replacement of the secreted insulin granules, which supports the hypothesis that these granules are responsible for replenishing the readily releasable pool. Our study provides a model by which glucose and other secretory stimuli can regulate the readily releasable pool through the same mechanisms that regulate insulin secretion.

Role of high-voltage-activated calcium channels in glucose-regulated beta-cell calcium homeostasis and insulin release

High-voltage-activated (HVA) calcium channels are known to be the primary source of calcium for glucose-stimulated insulin secretion. However, few studies have investigated how these channels can be regulated by chronically elevated levels of glucose. In the present study, we determined the level of expression of the four major HVA calcium channels (N-type, P/Q-type, L(C)-type, and L(D)-type) in rat pancreatic beta-cells. Using quantitative real-time PCR (QRT-PCR), we found the expression of all four HVA genes in rat insulinoma cells (INS-1) and in primary isolated rat islet cells. We then determined the role of each channel in insulin secretion by using channel-selective antagonists. Insulin secretion analysis revealed that N- and L-type channels are both involved in immediate glucose-induced insulin secretion. However, L-type was preferentially coupled to secretion at later time points. P/Q-type channels were not found to play a role in insulin secretion at any stage. It was also found that long-term exposure to elevated glucose increases basal calcium in these cells. Interestingly, chronically elevated glucose decreased the mRNA expression of the channels involved with insulin secretion and diminished the level of stimulated calcium influx in these cells. Using whole cell patch clamp, we found that N- and L-type channel currents increase gradually subsequent to lower intracellular calcium perfusion, suggesting that these channels may be regulated by glucose-induced changes in calcium.

Islet secretory defect in insulin receptor substrate 1 null mice is linked with reduced calcium signaling and expression of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA)-2b and -3

Mice with deletion of insulin receptor substrate (IRS)-1 (IRS-1 knockout [KO] mice) show mild insulin resistance and defective glucose-stimulated insulin secretion and reduced insulin synthesis. To further define the role of IRS-1 in islet function, we examined the insulin secretory defect in the knockouts using freshly isolated islets and primary beta-cells. IRS-1 KO beta-cells exhibited a significantly shorter increase in intracellular free Ca(2+) concentration ([Ca(2+)](i)) than controls when briefly stimulated with glucose or glyceraldehyde and when l-arginine was used to potentiate the stimulatory effect of glucose. These changes were paralleled by a lower number of exocytotic events in the KO beta-cells in response to the same secretagogues, indicating reduced insulin secretion. Furthermore, the normal oscillations in intracellular Ca(2+) and O(2) consumption after glucose stimulation were dampened in freshly isolated KO islets. Semiquantitative RT-PCR showed a dramatically reduced islet expression of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA)-2b and -3 in the mutants. These data provide evidence that IRS-1 modulation of insulin secretion is associated with Ca(2+) signaling and expression of SERCA-2b and -3 genes in pancreatic islets and provides a direct link between insulin resistance and defective insulin secretion.

[Ca2+]iregulates trafficking of Cav1.3 (α1DCa2+channel) in insulin-secreting cells

Chronic exposure of pancreatic β-cells to high concentrations of glucose impairs the insulin secretory response to further glucose stimulation. This phenomenon is referred to as glucose desensitization. It has been shown that glucose desensitization is associated with abnormal elevation of β-cell basal intracellular free Ca2+concentration ([Ca2+]i). We have investigated the relationship between the basal intracellular free Ca2+and the L-type (Cav1.3) Ca2+channel translocation in insulin-secreting cells. Glucose stimulation or membrane depolarization induced a nifedipine-sensitive Ca2+influx, which was attenuated when the basal [Ca2+]iwas elevated. Using voltage-clamp techniques, we found that changing [Ca2+]icould regulate the amplitude of the Ca2+current. This effect was attenuated by drugs that interfere with the cytoskeleton. Immunofluorescent labeling of Cav1.3 showed an increase in the cytoplasmic distribution of the channels under high [Ca2+]iconditions by deconvolution microscopy. The [Ca2+]i-dependent translocation of Cav1.3 channel was also demonstrated by Western blot analysis of biotinylation/NeutrAvidin-bead-eluted surface proteins in cells preincubated at various [Ca2+]i. These results suggest that Cav1.3 channel trafficking is involved in glucose desensitization of pancreatic β-cells.

Localized calcium influx in pancreatic beta-cells: its significance for Ca2+-dependent insulin secretion from the islets of Langerhans

Ca2+ influx through voltage-dependent Ca2+ channels plays a crucial role in stimulus-secretion coupling in pancreatic islet beta-cells. Molecular and physiologic studies have identified multiple Ca2+ channel subtypes in rodent islets and insulin-secreting cell lines. The differential targeting of Ca2+ channel subtypes to the vicinity of the insulin secretory apparatus is likely to account for their selective coupling to glucose-dependent insulin secretion. In this article, I review these studies. In addition, I discuss temporal and spatial aspects of Ca2+ signaling in beta-cells, the former involving the oscillatory activation of Ca2+ channels during glucose-induced electrical bursting, and the latter involving [Ca2+]i elevation in restricted microscopic "domains," as well as direct interactions between Ca2+ channels and secretory SNARE proteins. Finally, I review the evidence supporting a possible role for Ca2+ release from the endoplasmic reticulum in glucose-dependent insulin secretion, and evidence to support the existence of novel Ca2+ entry pathways. I also show that the beta-cell has an elaborate and complex set of [Ca2+]i signaling mechanisms that are capable of generating diverse and extremely precise [Ca2+]i patterns. These signals, in turn, are exquisitely coupled in space and time to the beta-cell secretory machinery to produce the precise minute-to-minute control of insulin secretion necessary for body energy homeostasis.

Hyperosmolality-induced abnormal patterns of calcium mobilization in smooth muscle cells from non-diabetic and diabetic rats

Hyperglycemia and/or hyperosmolality may disturb calcium homeostasis in vascular smooth muscle cells (SMCs), leading to altered vascular contractility in diabetes. To test this hypothesis, the KCl-induced increases in [Ca2+]i in primarily cultured vascular SMCs exposed to different concentrations of glucose were examined. With glucose concentration in solutions kept at 5.5 mM, KCl induced a fast increase in [Ca2+]i which then slowly declined (type 1 response) in 83% of SMCs from non-diabetic rats. In 9% of non-diabetic SMCs KCl induced a slow increase in [Ca2+]i (type 2 response). Interestingly, under the same culture conditions KCl induced type 1 and type 2 responses in 47 and 35% of SMCs from diabetic rats. When SMCs from non-diabetic or diabetic rats were cultured in 36 mM glucose, KCl induced a fast increase in [Ca2+]i which, however, maintained at a high level (type 3 response). The sustained level of [Ca2+]i in the presence of KCl was significantly higher in cells cultured with 36 mM glucose than that in non-diabetic cells cultured with 5.5 mM glucose. Furthermore, the hyperglycemia-induced alterations in calcium mobilization were similarly observed in cells cultured in high concentration of mannitol (30.5 mM) or L-glucose, indicating that hyperosmolality was mainly responsible for the abnormal calcium mobilization in diabetic SMCs.

Isoforms of endoplasmic reticulum Ca(2+)-ATPase are differentially expressed in normal and diabetic islets of Langerhans

Glucose-dependent sequestration of Ca2+ into endoplasmic reticulum and its subsequent release play an important role in the control of intracellular Ca2+ concentration, which regulates insulin secretion in pancreatic beta-cells. The active uptake of cytosolic Ca2+ into endoplasmic reticulum is mediated by sarco-(endo)plasmic reticulum Ca(2+)-ATPases (SERCAs). We found, using RT-PCR with isoform-specific primers, that SERCA 2 and SERCA 3 mRNAs are co-expressed in human and rat islets of Langerhans and in the RINm5F beta-cell line. Immunochemical analysis also revealed the existence of two SERCA proteins with molecular masses of 110 and 115 kDa in beta-cell membranes. The 115 kDa protein was identified as SERCA 2b by its reaction with an isoform-specific antibody and the 110 kDa protein most probably corresponds to SERCA 3. The presence of two functionally different SERCA isoforms raises the possibility that they are located in distinct Ca2+ stores. There is evidence that altered Ca2+ handling in the beta-cell may contribute to the decreased insulin secretion seen in non-insulin dependent diabetes mellitus (NIDDM). We therefore investigated SERCA 2 and SERCA 3 mRNA expression by quantitative RT-PCR in islets prepared from Goto-Kakizaki (GK) rats, a non-obese spontaneous model of NIDDM. We found a significant reduction (about 68%) in SERCA 3 isoform expression. Since SERCA 2 expression was not significantly reduced, these genes are independently regulated and probably play distinct roles in islets of Langerhans. The marked decrease of SERCA 3 expression may constitute a defect in Ca2+ signalling in GK rat islets which could be a component of NIDDM.