Insulin Secretion and Ca2+ Dynamics in β-Cells Are Regulated by PERK (EIF2AK3) in Concert with Calcineurin

Protein kinase R (PKR)-like endoplasmic reticulum kinase (PERK) (EIF2AK3) is essential for normal development and function of the insulin-secreting β-cell. Although genetic ablation of PERK in β-cells results in permanent neonatal diabetes in humans and mice, the underlying mechanisms remain unclear. Here, we used a newly developed and highly specific inhibitor of PERK to determine the immediate effects of acute ablation of PERK activity. We found that inhibition of PERK in human and rodent β-cells causes a rapid inhibition of secretagogue-stimulated subcellular Ca²⁺ signaling and insulin secretion. These dysfunctions stem from alterations in store-operated Ca²⁺ entry and sarcoplasmic endoplasmic reticulum Ca²⁺-ATPase activity. We also found that PERK regulates calcineurin, and pharmacological inhibition of calcineurin results in similar defects on stimulus-secretion coupling. Our findings suggest that interplay between calcineurin and PERK regulates β-cell Ca²⁺ signaling and insulin secretion, and that loss of this interaction may have profound implications in insulin secretion defects associated with diabetes.     

Ca2+ Influx through Store-operated Ca2+ Channels Reduces Alzheimer Disease β-Amyloid Peptide Secretion

Alzheimer disease (AD), the leading cause of dementia, is characterized by the accumulation of β-amyloid peptides (Aβ) in senile plaques in the brains of affected patients. Many cellular mechanisms are thought to play important roles in the development and progression of AD. Several lines of evidence point to the dysregulation of Ca²⁺ homeostasis as underlying aspects of AD pathogenesis. Moreover, direct roles in the regulation of Ca²⁺ homeostasis have been demonstrated for proteins encoded by familial AD-linked genes such as PSEN1, PSEN2, and APP, as well as Aβ peptides. Whereas these studies support the hypothesis that disruption of Ca²⁺ homeostasis contributes to AD, it is difficult to disentangle the effects of familial AD-linked genes on Aβ production from their effects on Ca²⁺ homeostasis. Here, we developed a system in which cellular Ca²⁺ homeostasis could be directly manipulated to study the effects on amyloid precursor protein metabolism and Aβ production. We overexpressed stromal interaction molecule 1 (STIM1) and Orai1, the components of the store-operated Ca²⁺ entry pathway, to generate cells with constitutive and store depletion-induced Ca²⁺ entry. We found striking effects of Ca²⁺ entry induced by overexpression of the constitutively active STIM1_D76A mutant on amyloid precursor protein metabolism. Specifically, constitutive activation of Ca²⁺ entry by expression of STIM1_D76A significantly reduced Aβ secretion. Our results suggest that disruptions in Ca²⁺ homeostasis may influence AD pathogenesis directly through the modulation of Aβ production.     

Store-operated Ca2+ entry and Ca2+ responses to hypothalamic releasing hormones in anterior pituitary cells from Orai1−/− and heptaTRPC knockout mice

Store operated Ca²⁺ entry (SOCE) is the most important Ca²⁺ entry pathway in non-excitable cells. However, SOCE can also play a pivotal role in excitable cells such as anterior pituitary (AP) cells. The AP gland contains five different cell types that release six major AP hormones controlling most of the entire endocrine system. AP hormone release is modulated by Ca²⁺ signals induced by different hypothalamic releasing hormones (HRHs) acting on specific receptors in AP cells. TRH and LHRH both induce Ca²⁺ release and Ca²⁺ entry in responsive cells while GHRH and CRH only induce Ca²⁺ entry. SOCE has been shown to contribute to Ca²⁺ responses induced by TRH and LHRH but no molecular evidence has been provided. Accordingly, we used AP cells isolated from mice devoid of Orai1 channels (noted as Orai1−/− or Orai1 KO mice) and mice lacking expression of all seven canonical TRP channels (TRPC) from TRPC1 to TRPC7 (noted as heptaTRPC KO mice) to investigate contribution of these putative channel proteins to SOCE and intracellular Ca²⁺ responses induced by HRHs. We found that thapsigargin-evoked SOCE is lost in AP cells from Orai1−/− mice but unaffected in cells from heptaTRPC KO mice. Conversely, while spontaneous intracellular Ca²⁺-oscillations related to electrical activity were not affected in the Orai1−/− mice, these responses were significantly reduced in heptaTRPC KO mice. We also found that Ca²⁺ entry induced by TRH and LHRH is decreased in AP cells isolated from Orai1−/−. In addition, Ca²⁺ responses to several HRHs, particularly TRH and GHRH, are decreased in the heptaTRPC KO mice. These results indicate that expression of Orai1, and not TRPC channel proteins, is necessary for thapsigargin-evoked SOCE and is required to support Ca²⁺ entry induced by TRH and LHRH in mouse AP cells. In contrast, TRPC channel proteins appear to contribute to spontaneous Ca²⁺-oscillations and Ca²⁺ responses induced by TRH and GHRH. We conclude that expression of Orai1 and TRPC channels proteins may play differential and significant roles in AP physiology and endocrine control.     

Intracellular Serotonin Modulates Insulin Secretion from Pancreatic β-Cells by Protein Serotonylation

While serotonin (5-HT) co-localization with insulin in granules of pancreatic β-cells was demonstrated more than three decades ago, its physiological role in the etiology of diabetes is still unclear. We combined biochemical and electrophysiological analyses of mice selectively deficient in peripheral tryptophan hydroxylase (Tph1−/−) and 5-HT to show that intracellular 5-HT regulates insulin secretion. We found that these mice are diabetic and have an impaired insulin secretion due to the lack of 5-HT in the pancreas. The pharmacological restoration of peripheral 5-HT levels rescued the impaired insulin secretion in vivo. These findings were further evidenced by patch clamp experiments with isolated Tph1−/− β-cells, which clearly showed that the secretory defect is downstream of Ca²⁺-signaling and can be rescued by direct intracellular application of 5-HT via the clamp pipette. In elucidating the underlying mechanism further, we demonstrate the covalent coupling of 5-HT by transglutaminases during insulin exocytosis to two key players in insulin secretion, the small GTPases Rab3a and Rab27a. This renders them constitutively active in a receptor-independent signaling mechanism we have recently termed serotonylation. Concordantly, an inhibition of such activating serotonylation in β-cells abates insulin secretion. We also observed inactivation of serotonylated Rab3a by enhanced proteasomal degradation, which is in line with the inactivation of other serotonylated GTPases. Our results demonstrate that 5-HT regulates insulin secretion by serotonylation of GTPases within pancreatic β-cells and suggest that intracellular 5-HT functions in various microenvironments via this mechanism in concert with the known receptor-mediated signaling.     

Impaired Ca2+ 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.     

Chloroquine Stimulates Cl? Secretion by Ca2+ Activated Cl? Channels in Rat Ileum

Chloroquine (CQ), a bitter tasting drug widely used in treatment of malaria, is associated gastrointestinal side effects including nausea or diarrhea. In the present study, we investigated the effect of CQ on electrolyte transport in rat ileum using the Ussing chamber technique. The results showed that CQ evoked an increase in short circuit current (I_SC) in rat ileum at lower concentration (≤5×10⁻⁴ M ) but induced a decrease at higher concentrations (≥10⁻³ M). These responses were not affected by tetrodotoxin (TTX). Other bitter compounds, such as denatoniumbenzoate and quinine, exhibited similar effects. CQ-evoked increase in I_SC was partly reduced by amiloride(10⁻⁴ M), a blocker of epithelial Na⁺ channels. Furosemide (10⁻⁴ M), an inhibitor of Na⁺-K⁺ -2Cl⁻ co-transporter, also inhibited the increased I_SC response to CQ, whereas another Cl⁻ channel inhibitor, CFTR(inh)-172(10⁻⁵M), had no effect. Intriguingly, CQ-evoked increases were almost completely abolished by niflumic acid (10⁻⁴M), a relatively specific Ca²⁺-activated Cl⁻ channel (CaCC) inhibitor. Furthermore, other CaCC inhibitors, such as DIDS and NPPB, also exhibited similar effects. CQ-induced increases in I_SC were also abolished by thapsigargin(10⁻⁶M), a Ca²⁺ pump inhibitor and in the absence of either Cl⁻ or Ca²⁺ from bathing solutions. Further studies demonstrated that T2R and CaCC-TMEM16A were colocalized in small intestinal epithelial cells and the T2R agonist CQ evoked an increase of intracelluar Ca²⁺ in small intestinal epithelial cells. Taken together, these results demonstrate that CQ induces Cl⁻ secretion in rat ileum through CaCC at low concentrations, suggesting a novel explanation for CQ-associated gastrointestinal side-effects during the treatment of malaria.     

Chronic Exposure to Excess Nutrients Left-shifts the Concentration Dependence of Glucose-stimulated Insulin Secretion in Pancreatic β-Cells

Hyperinsulinemia (HI) is elevated plasma insulin at basal glucose. Impaired glucose tolerance is associated with HI, although the exact cause and effect relationship remains poorly defined. We tested the hypothesis that HI can result from an intrinsic response of the β-cell to chronic exposure to excess nutrients, involving a shift in the concentration dependence of glucose-stimulated insulin secretion. INS-1 (832/13) cells were cultured in either a physiological (4 mm) or high (11 mm) glucose concentration with or without concomitant exposure to oleate. Isolated rat islets were also cultured with or without oleate. A clear hypersensitivity to submaximal glucose concentrations was evident in INS-1 cells cultured in excess nutrients such that the 25% of maximal (S_0.25) glucose-stimulated insulin secretion was significantly reduced in cells cultured in 11 mm glucose (S_0.25 = 3.5 mm) and 4 mm glucose with oleate (S_0.25 = 4.5 mm) compared with 4 mm glucose alone (S_0.25 = 5.7 mm). The magnitude of the left shift was linearly correlated with intracellular lipid stores in INS-1 cells (r² = 0.97). We observed no significant differences in the dose responses for glucose stimulation of respiration, NAD(P)H autofluorescence, or Ca²⁺ responses between left- and right-shifted β-cells. However, a left shift in the sensitivity of exocytosis to Ca²⁺ was documented in permeabilized INS-1 cells cultured in 11 versus 4 mm glucose (S_0.25 = 1.1 and 1.7 μm, respectively). Our results suggest that the sensitivity of exocytosis to triggering is modulated by a lipid component, the levels of which are influenced by the culture nutrient environment.     

Brain-selective Kinase 2 (BRSK2) Phosphorylation on PCTAIRE1 Negatively Regulates Glucose-stimulated Insulin Secretion in Pancreatic β-Cells

Brain-selective kinase 2 (BRSK2) has been shown to play an essential role in neuronal polarization. In the present study, we show that BRSK2 is also abundantly expressed in pancreatic islets and MIN6 β-cell line. Yeast two-hybrid screening, GST fusion protein pull-down, and co-immunoprecipitation assays reveal that BRSK2 interacts with CDK-related protein kinase PCTAIRE1, a kinase involved in neurite outgrowth and neurotransmitter release. In MIN6 cells, BRSK2 co-localizes with PCTAIRE1 in the cytoplasm and phosphorylates one of its serine residues, Ser-12. Phosphorylation of PCTAIRE1 by BRSK2 reduces glucose-stimulated insulin secretion (GSIS) in MIN6 cells. Conversely, knockdown of BRSK2 by siRNA increases serum insulin levels in mice. Our results reveal a novel function of BRSK2 in the regulation of GSIS in β-cells via a PCTAIRE1-dependent mechanism and suggest that BRSK2 is an attractive target for developing novel diabetic drugs.     

Glucagon-Like Peptide-1 Induced Signaling and Insulin Secretion Do Not Drive Fuel and Energy Metabolism in Primary Rodent Pancreatic β-Cells

Background

Glucagon like peptide-1 (GLP-1) and its analogue exendin-4 (Ex-4) enhance glucose stimulated insulin secretion (GSIS) and activate various signaling pathways in pancreatic β-cells, in particular cAMP, Ca²⁺ and protein kinase-B (PKB/Akt). In many cells these signals activate intermediary metabolism. However, it is not clear whether the acute amplification of GSIS by GLP-1 involves in part metabolic alterations and the production of metabolic coupling factors.

Methodology/Prinicipal Findings

GLP-1 or Ex-4 at high glucose caused release (∼20%) of the total rat islet insulin content over 1 h. While both GLP-1 and Ex-4 markedly potentiated GSIS in isolated rat and mouse islets, neither had an effect on β-cell fuel and energy metabolism over a 5 min to 3 h time period. GLP-1 activated PKB without changing glucose usage and oxidation, fatty acid oxidation, lipolysis or esterification into various lipids in rat islets. Ex-4 caused a rise in [Ca²⁺]_i and cAMP but did not enhance energy utilization, as neither oxygen consumption nor mitochondrial ATP levels were altered.

Conclusions/Significance

The results indicate that GLP-1 barely affects β-cell intermediary metabolism and that metabolic signaling does not significantly contribute to GLP-1 potentiation of GSIS. The data also indicate that insulin secretion is a minor energy consuming process in the β-cell, and that the β-cell is different from most cell types in that its metabolic activation appears to be primarily governed by a “push” (fuel substrate driven) process, rather than a “pull” mechanism secondary to enhanced insulin release as well as to Ca²⁺, cAMP and PKB signaling.     

Glucose Evokes Rapid Ca2+ and Cyclic AMP Signals by Activating the Cell-Surface Glucose-Sensing Receptor in Pancreatic β-Cells

Glucose is a primary stimulator of insulin secretion in pancreatic β-cells. High concentration of glucose has been thought to exert its action solely through its metabolism. In this regard, we have recently reported that glucose also activates a cell-surface glucose-sensing receptor and facilitates its own metabolism. In the present study, we investigated whether glucose activates the glucose-sensing receptor and elicits receptor-mediated rapid actions. In MIN6 cells and isolated mouse β-cells, glucose induced triphasic changes in cytoplasmic Ca²⁺ concentration ([Ca²⁺]_c); glucose evoked an immediate elevation of [Ca²⁺]_c, which was followed by a decrease in [Ca²⁺]_c, and after a certain lag period it induced large oscillatory elevations of [Ca²⁺]_c. Initial rapid peak and subsequent reduction of [Ca²⁺]_c were independent of glucose metabolism and reproduced by a nonmetabolizable glucose analogue. These signals were also blocked by an inhibitor of T1R3, a subunit of the glucose-sensing receptor, and by deletion of the T1R3 gene. Besides Ca²⁺, glucose also induced an immediate and sustained elevation of intracellular cAMP ([cAMP]_c). The elevation of [cAMP]_c was blocked by transduction of the dominant-negative G_s, and deletion of the T1R3 gene. These results indicate that glucose induces rapid changes in [Ca²⁺]_c and [cAMP]_c by activating the cell-surface glucose-sensing receptor. Hence, glucose generates rapid intracellular signals by activating the cell-surface receptor.     

Glucose-Dependent Insulin Secretion in Pancreatic β-Cell Islets from Male Rats Requires Ca2+ Release via ROS-Stimulated Ryanodine Receptors

Glucose-stimulated insulin secretion (GSIS) from pancreatic β-cells requires an increase in intracellular free Ca²⁺ concentration ([Ca²⁺]). Glucose uptake into β-cells promotes Ca²⁺ influx and reactive oxygen species (ROS) generation. In other cell types, Ca²⁺ and ROS jointly induce Ca²⁺ release mediated by ryanodine receptor (RyR) channels. Therefore, we explored here if RyR-mediated Ca²⁺ release contributes to GSIS in β-cell islets isolated from male rats. Stimulatory glucose increased islet insulin secretion, and promoted ROS generation in islets and dissociated β-cells. Conventional PCR assays and immunostaining confirmed that β-cells express RyR2, the cardiac RyR isoform. Extended incubation of β-cell islets with inhibitory ryanodine suppressed GSIS; so did the antioxidant N-acetyl cysteine (NAC), which also decreased insulin secretion induced by glucose plus caffeine. Inhibitory ryanodine or NAC did not affect insulin secretion induced by glucose plus carbachol, which engages inositol 1,4,5-trisphosphate receptors. Incubation of islets with H₂O₂ in basal glucose increased insulin secretion 2-fold. Inhibitory ryanodine significantly decreased H₂O₂-stimulated insulin secretion and prevented the 4.5-fold increase of cytoplasmic [Ca²⁺] produced by incubation of dissociated β-cells with H₂O₂. Addition of stimulatory glucose or H₂O₂ (in basal glucose) to β-cells disaggregated from islets increased RyR2 S-glutathionylation to similar levels, measured by a proximity ligation assay; in contrast, NAC significantly reduced the RyR2 S-glutathionylation increase produced by stimulatory glucose. We propose that RyR2-mediated Ca²⁺ release, induced by the concomitant increases in [Ca²⁺] and ROS produced by stimulatory glucose, is an essential step in GSIS.     

Mitochondrial Ca2+ Uptake 1 (MICU1) and Mitochondrial Ca2+ Uniporter (MCU) Contribute to Metabolism-Secretion Coupling in Clonal Pancreatic ��-Cells

In pancreatic β-cells, uptake of Ca²⁺ into mitochondria facilitates metabolism-secretion coupling by activation of various matrix enzymes, thus facilitating ATP generation by oxidative phosphorylation and, in turn, augmenting insulin release. We employed an siRNA-based approach to evaluate the individual contribution of four proteins that were recently described to be engaged in mitochondrial Ca²⁺ sequestration in clonal INS-1 832/13 pancreatic β-cells: the mitochondrial Ca²⁺ uptake 1 (MICU1), mitochondrial Ca²⁺ uniporter (MCU), uncoupling protein 2 (UCP2), and leucine zipper EF-hand-containing transmembrane protein 1 (LETM1). Using a FRET-based genetically encoded Ca²⁺ sensor targeted to mitochondria, we show that a transient knockdown of MICU1 or MCU diminished mitochondrial Ca²⁺ uptake upon both intracellular Ca²⁺ release and Ca²⁺ entry via L-type channels. In contrast, knockdown of UCP2 and LETM1 exclusively reduced mitochondrial Ca²⁺ uptake in response to either intracellular Ca²⁺ release or Ca²⁺ entry, respectively. Therefore, we further investigated the role of MICU1 and MCU in metabolism-secretion coupling. Diminution of MICU1 or MCU reduced mitochondrial Ca²⁺ uptake in response to d-glucose, whereas d-glucose-triggered cytosolic Ca²⁺ oscillations remained unaffected. Moreover, d-glucose-evoked increases in cytosolic ATP and d-glucose-stimulated insulin secretion were diminished in MICU1- or MCU-silenced cells. Our data highlight the crucial role of MICU1 and MCU in mitochondrial Ca²⁺ uptake in pancreatic β-cells and their involvement in the positive feedback required for sustained insulin secretion.     

Store-operated Ca2+ Entry (SOCE) Induced by Protease-activated Receptor-1 Mediates STIM1 Protein Phosphorylation to Inhibit SOCE in Endothelial Cells through AMP-activated Protein Kinase and p38β Mitogen-activated Protein Kinase

The Ca²⁺ sensor STIM1 is crucial for activation of store-operated Ca²⁺ entry (SOCE) through transient receptor potential canonical and Orai channels. STIM1 phosphorylation serves as an “off switch” for SOCE. However, the signaling pathway for STIM1 phosphorylation is unknown. Here, we show that SOCE activates AMP-activated protein kinase (AMPK); its effector p38β mitogen-activated protein kinase (p38β MAPK) phosphorylates STIM1, thus inhibiting SOCE in human lung microvascular endothelial cells. Activation of AMPK using 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR) resulted in STIM1 phosphorylation on serine residues and prevented protease-activated receptor-1 (PAR-1)-induced Ca²⁺ entry. Furthermore, AICAR pretreatment blocked PAR-1-induced increase in the permeability of mouse lung microvessels. Activation of SOCE with thrombin caused phosphorylation of isoform α1 but not α2 of the AMPK catalytic subunit. Moreover, knockdown of AMPKα1 augmented SOCE induced by thrombin. Interestingly, SB203580, a selective inhibitor of p38 MAPK, blocked STIM1 phosphorylation and led to sustained STIM1-puncta formation and Ca²⁺ entry. Of the three p38 MAPK isoforms expressed in endothelial cells, p38β knockdown prevented PAR-1-mediated STIM1 phosphorylation and potentiated SOCE. In addition, inhibition of the SOCE downstream target CaM kinase kinase β (CaMKKβ) or knockdown of AMPKα1 suppressed PAR-1-mediated phosphorylation of p38β and hence STIM1. Thus, our findings demonstrate that SOCE activates CaMKKβ-AMPKα1-p38β MAPK signaling to phosphorylate STIM1, thereby suppressing endothelial SOCE and permeability responses.     

Control by Ca2+ of mitochondrial structure and function in pancreatic β-cells

Mitochondria play a central role in glucose metabolism and the stimulation of insulin secretion from pancreatic β-cells. In this review, we discuss firstly the regulation and roles of mitochondrial Ca²⁺ transport in glucose-regulated insulin secretion, and the molecular machinery involved. Next, we discuss the evidence that mitochondrial dysfunction in β-cells is associated with type 2 diabetes, from a genetic, functional and structural point of view, and then the possibility that these changes may in part be mediated by dysregulation of cytosolic Ca²⁺. Finally, we review the importance of preserved mitochondrial structure and dynamics for mitochondrial gene expression and their possible relevance to the pathogenesis of type 2 diabetes.     

Preserving Insulin Secretion in Diabetes by Inhibiting VDAC1 Overexpression and Surface Translocation in β Cells

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Potentiation by Isoproterenol on Carbachol-induced K+ and Cl− Currents and Fluid Secretion in Rat Parotid

Isoproterenol (IPR) and 8-(4-chlorophenylthio)-cyclic AMP (cpt-cAMP) enhanced carbachol (CCh)-induced fluid secretion from rat parotid glands, but had no effect by themselves. The enhancement by IPR was blocked by propranolol. In dispersed parotid acinar cells, IPR and cpt-cAMP potentiated CCh-induced K⁺ and Cl⁻ currents (I _K and I _Cl). IPR at the concentration of 0.1 μm significantly potentiated the CCh-induced increase in intracellular Ca²⁺ concentration ([Ca²⁺] _i ), but 1 mm cpt-cAMP did not. The incidence of the potentiation by IPR in CCh-induced Mn²⁺ entry was 31% and that by cpt-cAMP was 21%. The potentiation by IPR in the ionic currents and the [Ca²⁺] _i was suppressed by propranolol. These results suggest that the CCh-induced fluid secretion from rat parotid glands is enhanced by IPR through the potentiation of I _K and I _Cl mainly by the increased cyclic AMP level and partially by the potentiated Ca²⁺ influx and [Ca²⁺] _i increase, and that IPR is more effective than cpt-cAMP in the enhancement of the CCh-induced [Ca²⁺] _i increase. Received: 6 October 1997/Revised: 16 April 1998     

Preconditioning with Associated Blocking of Ca2+ Inflow Alleviates Hypoxia-Induced Damage to Pancreatic β-Cells

Objective

Beta cells of pancreatic islets are susceptible to functional deficits and damage by hypoxia. Here we aimed to characterize such effects and to test for and pharmacological means to alleviate a negative impact of hypoxia.

Methods and Design

Rat and human pancreatic islets were subjected to 5.5 h of hypoxia after which functional and viability parameters were measured subsequent to the hypoxic period and/or following a 22 h re-oxygenation period. Preconditioning with diazoxide or other agents was usually done during a 22 h period prior to hypoxia.

Results

Insulin contents decreased by 23% after 5.5 h of hypoxia and by 61% after a re-oxygenation period. Preconditioning with diazoxide time-dependently alleviated these hypoxia effects in rat and human islets. Hypoxia reduced proinsulin biosynthesis (³H-leucine incorporation into proinsulin) by 35%. Preconditioning counteracted this decrease by 91%. Preconditioning reduced hypoxia-induced necrosis by 40%, attenuated lowering of proteins of mitochondrial complexes I–IV and enhanced stimulation of HIF-1-alpha and phosphorylated AMPK proteins. Preconditioning by diazoxide was abolished by co-exposure to tolbutamide or elevated potassium (i.e. conditions which increase Ca²⁺ inflow). Preconditioning with nifedipine, a calcium channel blocker, partly reproduced effects of diazoxide. Both diazoxide and nifedipine moderately reduced basal glucose oxidation whereas glucose-induced oxygen consumption (tested with diazoxide) was unaffected. Preconditioning with diaxoxide enhanced insulin contents in transplants of rat islets to non-diabetic rats and lowered hyperglycemia vs. non-preconditioned islets in streptozotocin-diabetic rats. Preconditioning of human islet transplants lowered hyperglycemia in streptozotocin-diabetic nude mice.

Conclusions

1) Prior blocking of Ca²⁺ inflow associates with lesser hypoxia-induced damage, 2) preconditioning affects basal mitochondrial metabolism and accelerates activation of hypoxia-reactive and potentially protective factors, 3) results indicate that preconditioning by K⁺-ATP-channel openers has therapeutic potential for islet transplantations.     

?-Blocker Timolol Prevents Arrhythmogenic Ca2+ Release and Normalizes Ca2+ and Zn2+ Dyshomeostasis in Hyperglycemic Rat Heart

Defective cardiac mechanical activity in diabetes results from alterations in intracellular Ca²⁺ handling, in part, due to increased oxidative stress. Beta-blockers demonstrate marked beneficial effects in heart dysfunction with scavenging free radicals and/or acting as an antioxidant. The aim of this study was to address how β-blocker timolol-treatment of diabetic rats exerts cardioprotection. Timolol-treatment (12-week), one-week following diabetes induction, prevented diabetes-induced depressed left ventricular basal contractile activity, prolonged cellular electrical activity, and attenuated the increase in isolated-cardiomyocyte size without hyperglycemic effect. Both in vivo and in vitro timolol-treatment of diabetic cardiomyocytes prevented the altered kinetic parameters of Ca²⁺ transients and reduced Ca²⁺ loading of sarcoplasmic reticulum (SR), basal intracellular free Ca²⁺ and Zn²⁺ ([Ca²⁺]_i and [Zn²⁺]_i), and spatio-temporal properties of the Ca²⁺ sparks, significantly. Timolol also antagonized hyperphosphorylation of cardiac ryanodine receptor (RyR2), and significantly restored depleted protein levels of both RyR2 and calstabin2. Western blot analysis demonstrated that timolol-treatment also significantly normalized depressed levels of some [Ca²⁺]_i-handling regulators, such as Na⁺/Ca²⁺ exchanger (NCX) and phospho-phospholamban (pPLN) to PLN ratio. Incubation of diabetic cardiomyocytes with 4-mM glutathione exerted similar beneficial effects on RyR2-macromolecular complex and basal levels of both [Ca²⁺]_i and [Zn²⁺]_i, increased intracellular Zn²⁺ hyperphosphorylated RyR2 in a concentration-dependent manner. Timolol also led to a balanced oxidant/antioxidant level in both heart and circulation and prevented altered cellular redox state of the heart. We thus report, for the first time, that the preventing effect of timolol, directly targeting heart, seems to be associated with a normalization of macromolecular complex of RyR2 and some Ca²⁺ handling regulators, and prevention of Ca²⁺ leak, and thereby normalization of both [Ca²⁺]_i and [Zn²⁺]_i homeostasis in diabetic rat heart, at least in part by controlling the cellular redox status of hyperglycemic cardiomyocytes.