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Lin MH, Hsu CC, Lin J, Cheng JT, Wu MC. Investigation of morin-induced insulin secretion in cultured pancreatic cells. Clin Exp Pharmacol Physiol 2017; 44:1254-1262. [PMID: 28699234 DOI: 10.1111/1440-1681.12815] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/04/2017] [Revised: 06/26/2017] [Accepted: 07/06/2017] [Indexed: 12/25/2022]
Abstract
Morin is a flavonoid contained in guava that is known to reduce hyperglycemia in diabetes. Insulin secretion has been demonstrated to increase following the administration of morin. The present study is designed to investigate the potential mechanism(s) of morin-induced insulin secretion in the MIN6 cell line. First, we identified that morin induced a dose-dependent increase in insulin secretion and intracellular calcium content in MIN6 cells. Morin potentiated glucose-stimulated insulin secretion (GSIS). Additionally, we used siRNA for the ablation of imidazoline receptor protein (NISCH) expression in MIN6 cells. Interestingly, the effects of increased insulin secretion by morin and canavanine were markedly reduced in Si-NISCH cells. Moreover, we used KU14R to block imidazoline I3 receptor (I-3R) that is known to enhance insulin release from the pancreatic β-cells. Without influence on the basal insulin secretion, KU14R dose-dependently inhibited the increased insulin secretion induced by morin or efaroxan in MIN6 cells. Additionally, effects of increased insulin secretion by morin or efaroxan were reduced by diazoxide at the dose sufficient to open KATP channels and attenuated by nifedipine at the dose used to inhibit L-type calcium channels. Otherwise, phospholipase C (PLC) is introduced to couple with imidazoline receptor (I-R). The PLC inhibitor dose-dependently inhibited the effects of morin in MIN6 cells. Similar blockade was also observed in protein kinase C (PKC) inhibitor-treated cells. Taken together, we found that morin increases insulin secretion via the activation of I-R in pancreatic cells. Therefore, morin would be useful to develop in the research and treatment of diabetic disorders.
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Affiliation(s)
- Mang Hung Lin
- Department of Food Science, College of Agriculture, National Pingtung University of Science and Technology, Pingtung, Taiwan.,Chief Secretary's Office, Chiayi Hospital, Ministry of Health and Welfare, Chiayi, Taiwan
| | - Chia-Chen Hsu
- Department of Food Science, College of Agriculture, National Pingtung University of Science and Technology, Pingtung, Taiwan
| | - Jenshinn Lin
- Department of Food Science, College of Agriculture, National Pingtung University of Science and Technology, Pingtung, Taiwan
| | - Juei Tang Cheng
- Department of Medical Research, Chi-Mei Medical Center, Tainan, Taiwan.,Institute of Medical Science, College of Health Science, Chang Jung Christian University, Tainan, Taiwan
| | - Ming Chang Wu
- Department of Food Science, College of Agriculture, National Pingtung University of Science and Technology, Pingtung, Taiwan
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Lin MH, Hsu CC, Lin J, Cheng JT, Wu MC. Identification of morin as an agonist of imidazoline I-3 receptor for insulin secretion in diabetic rats. Naunyn Schmiedebergs Arch Pharmacol 2017; 390:997-1003. [PMID: 28689255 DOI: 10.1007/s00210-017-1399-7] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/12/2017] [Accepted: 06/28/2017] [Indexed: 10/19/2022]
Abstract
Morin is a flavonoid contained in guava that is known to reduce hyperglycemia in diabetics. Morin has been demonstrated to increase plasma insulin. However, the mechanism(s) remains unknown. The present study is designed to investigate the effect of morin on the imidazoline receptor (I-R) that regulates insulin secretion. We used Chinese hamster ovary (CHO) cells transfected with an I-R expression construct (NISCH-CHO-K1 cells) to identify the direct effect of morin on the I-R. Moreover, the imidazoline I3 receptor (I-3R) is known to be present in pancreatic β cells and involved in insulin secretion. Therefore, we applied a specific antagonist (KU14R) to block I-3R in diabetic rats. Additionally, the effect of morin on insulin secretion was characterized in isolated pancreatic islets. Morin decreased blood glucose levels by increasing plasma insulin levels in diabetic rats. In CHO cells expressing an I-R, morin increased calcium influx in a dose-dependent manner. Additionally, KU14R dose-dependently inhibited the morin-induced effects, including hypoglycemia and the increase in insulin secretion and plasma C-peptide levels, in diabetic rats. Furthermore, morin enhanced insulin secretion from isolated pancreatic islets, and this effect was also dose-dependently inhibited by KU14R. Phospholipase C (PLC) is known to couple with the I-R, and a PLC inhibitor dose-dependently attenuated the insulin secretion induced by morin in isolated pancreatic islets. Taken together, these data suggest that morin can activate I-3R to enhance insulin secretion. Therefore, it would be useful to develop morin into a treatment for diabetic disorders.
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Affiliation(s)
- Mang Hung Lin
- Department of Food Science, College of Agriculture, National Pingtung University of Science and Technology, Pingtung City, Taiwan, 90801.,Chief Secretary's Office, Chiayi Hospital, Ministry of Health and Welfare, Chiayi City, Taiwan, 60001
| | - Chia-Chen Hsu
- Department of Food Science, College of Agriculture, National Pingtung University of Science and Technology, Pingtung City, Taiwan, 90801
| | - Jenshinn Lin
- Department of Food Science, College of Agriculture, National Pingtung University of Science and Technology, Pingtung City, Taiwan, 90801
| | - Juei-Tang Cheng
- Department of Medical Research, Chi-Mei Medical Center, Yong Kang, Tainan City, Taiwan, 73101. .,Institute of Medical Science, College of Health Science, Chang Jung Christian University, Guei-Ren, Tainan City, Taiwan, 71101.
| | - Ming Chang Wu
- Department of Food Science, College of Agriculture, National Pingtung University of Science and Technology, Pingtung City, Taiwan, 90801.
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Yang TT, Niu HS, Chen LJ, Ku PM, Lin KC, Cheng JT. Canavanine induces insulin release via activation of imidazoline I3 receptors. Clin Exp Pharmacol Physiol 2014; 42:263-8. [PMID: 25482045 DOI: 10.1111/1440-1681.12348] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/15/2014] [Revised: 11/18/2014] [Accepted: 11/26/2014] [Indexed: 12/01/2022]
Abstract
The aim of the present study was to identify the effect of canavanine on the imidazoline receptor because canavanine is a guanidinium derivative that has a similar structure to imidazoline receptor ligands. Transfected Chinese hamster ovary-K1 cells expressing imidazoline receptors (nischarin (NISCH)-CHO-K1 cells) were used to elucidate the direct effects of canavanine on imidazoline receptors. In addition, the imidazoline I3 receptor has been implicated in stimulation of insulin secretion from pancreatic β-cells. Wistar rats were used to investigate the effects of canavanine (0.1, 1 and 2.5 mg/kg, i.v.) on insulin secretion. In addition the a specific I3 receptor antagonist KU14R (4 or 8 mg/kg, i.v.) was used to block I3 receptors. Canavanine decreased blood glucose by increasing plasma insulin in rats. In addition, canavanine increased calcium influx into NISCH-CHO-K1 cells in a manner similar to agmatine, the endogenous ligand of imidazoline receptors. Moreover, KU12R dose-dependently attenuated canavanine-induced insulin secretion in HIT-T15 pancreatic β-cells and in the plasma of rats. The data suggest that canavanine is an agonist of I3 receptors both in vivo and in vitro. Thus, canavanine would be a useful tool in imidazoline receptor research.
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Affiliation(s)
- Ting-Ting Yang
- The School of Chinese Medicine for Post-Baccalaureate, I-Shou University, Yanchao, Kaohsiung City, Taiwan
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Tsai CC, Chen LJ, Niu HS, Chung KM, Cheng JT, Lin KC. Allantoin activates imidazoline I-3 receptors to enhance insulin secretion in pancreatic β-cells. Nutr Metab (Lond) 2014. [DOI: 10.1186/1743-7075-11-41] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/10/2023] Open
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5
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Farn RD, Ramsden CA, Morgan NG. Preparation of analogues of efaroxan and KU14R as potential imidazoline receptor subtype 3 ligands. J Heterocycl Chem 2008. [DOI: 10.1002/jhet.5570450338] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
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Taylor JP, Jackson DA, Morgan NG, Chan SLF. Rhes expression in pancreatic beta-cells is regulated by efaroxan in a calcium-dependent process. Biochem Biophys Res Commun 2006; 349:809-15. [PMID: 16945334 DOI: 10.1016/j.bbrc.2006.08.102] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/17/2006] [Accepted: 08/18/2006] [Indexed: 11/22/2022]
Abstract
The monomeric G-protein Rhes has been described to be present in pancreatic beta-cells, and a putative role in the control of insulin release has been proposed. Here, we show that treatment of beta-cells with the imidazoline insulin secretagogue efaroxan resulted in a concentration- and time-dependent increase in the expression of Rhes, which peaked after 4h of efaroxan exposure; thereafter, Rhes mRNA levels decreased. Marked stereoselectivity was displayed, with (-)-efaroxan (the selectively insulinotropic enantiomer) being much more effective than (+)-efaroxan at raising Rhes transcript levels. The mechanism by which Rhes gene expression is activated in beta-cells appears to require the influx of extracellular calcium and de novo protein synthesis, and is not directly associated with the release of insulin. The present results confirm our earlier proposal that Rhes is an imidazoline-regulated transcript in pancreatic beta-cells. Studies to understand the role of Rhes as a regulator of beta-cell function are, thus, warranted.
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Affiliation(s)
- James P Taylor
- Institute of Cell Signalling, School of Biomedical Sciences, University of Nottingham, The Medical School, Queen's Medical Centre, Nottingham NG7 2UH, UK
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Bleck C, Wienbergen A, Rustenbeck I. Essential role of the imidazoline moiety in the insulinotropic effect but not the KATP channel-blocking effect of imidazolines; a comparison of the effects of efaroxan and its imidazole analogue, KU14R. Diabetologia 2005; 48:2567-75. [PMID: 16283242 DOI: 10.1007/s00125-005-0031-4] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 05/18/2005] [Accepted: 07/18/2005] [Indexed: 10/25/2022]
Abstract
AIMS/HYPOTHESIS Imidazolines are a class of investigational antidiabetic drugs. It is still unclear whether the imidazoline ring is decisive for insulinotropic characteristics. MATERIALS AND METHODS We studied the imidazoline efaroxan and its imidazole analogue, KU14R, which is currently classified as an imidazoline antagonist. The effects of both on stimulus secretion-coupling in normal mouse islets and beta cells were compared by measuring KATP channel activity, plasma membrane potential, cytosolic calcium concentration ([Ca2+]c) and dynamic insulin secretion. RESULTS In the presence of 10 mmol/l but not of 5 mmol/l glucose, efaroxan (100 micromol/l) strongly enhanced insulin secretion by freshly isolated perifused islets, whereas KU14R (30, 100 or 300 micromol/l) was ineffective at both glucose concentrations. Surprisingly, the insulinotropic effect of efaroxan was not antagonised by KU14R. KATP channels were blocked by efaroxan (IC50 8.8 micromol/l, Hill slope -1.1) and by KU14R (IC50 31.9 micromol/l, Hill slope -1.5). Neither the KATP channel-blocking effect nor the depolarising effect of efaroxan was antagonised by KU14R. Rather, both compounds strongly depolarised the beta cell membrane potential and induced action potential spiking. However, KU14R was clearly less efficient than efaroxan in raising [Ca2+]c in single beta cells and whole islets at 5 mmol/l glucose. The increase in [Ca2+]c induced by 10 mmol/l glucose was affected neither by efaroxan nor by KU14R. Again, KU14R did not antagonise the effects of efaroxan. CONCLUSIONS/INTERPRETATION The presence of an imidazole instead of an imidazoline ring leads to virtually complete loss of the insulinotropic effect in spite of a preserved ability to block KATP channels. The imidazole compound is less efficient in raising [Ca2+]c; in particular, it lacks the ability of the imidazoline to potentiate the enhancing effect of energy metabolism on Ca2+-induced insulin secretion.
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Affiliation(s)
- C Bleck
- Institute of Pharmacology and Toxicology, Technical University of Brunswick, Brunswick, Germany
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Kino Y, Tanabe M, Honda M, Ono H. Involvement of Supraspinal Imidazoline Receptors and Descending Monoaminergic Pathways in Tizanidine-Induced Inhibition of Rat Spinal Reflexes. J Pharmacol Sci 2005; 99:52-60. [PMID: 16127244 DOI: 10.1254/jphs.fp0050520] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/25/2022] Open
Abstract
The neuronal pathways involved in the muscle relaxant effect of tizanidine were examined by measurement of spinal reflexes in rats. Tizanidine (i.v. and intra-4th ventricular injection) decreased the mono- and disynaptic (the fastest polysynaptic) reflexes (MSR and DSR, respectively) in non-spinalized rats. Depletion of central noradrenaline by 6-hydroxydopamine abolished the depressant effect of tizanidine on the MSR almost completely and attenuated the effect on the DSR. Co-depletion of serotonin by 5,6-dihydroxytryptamine and noradrenaline resulted in more prominent attenuation of tizanidine-induced inhibition of the DSR. Supraspinal receptors were then studied using yohimbine- and some imidazoline-receptor ligands containing an imidazoline moiety. Idazoxan (I1, I2, I3, and alpha2), efaroxan (I1, I3, and alpha2), and RX821002 (I3 and alpha2), but not yohimbine, an alpha2-adrenergic receptor antagonist with no affinity for I receptors, antagonized the inhibitory effects of tizanidine. Thus, supraspinal I receptors (most likely I3) and descending monoaminergic influences are necessary for tizanidine-induced inhibition of spinal segmental reflexes.
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Affiliation(s)
- Yurika Kino
- Laboratory of CNS Pharmacology, Graduate School of Pharmaceutical Sciences, Nagoya City University, Nagoya, Japan
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Squires PE, Hills CE, Rogers GJ, Garland P, Farley SR, Morgan NG. The putative imidazoline receptor agonist, harmane, promotes intracellular calcium mobilisation in pancreatic β-cells. Eur J Pharmacol 2004; 501:31-9. [PMID: 15464060 DOI: 10.1016/j.ejphar.2004.08.018] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/08/2004] [Revised: 08/04/2004] [Accepted: 08/09/2004] [Indexed: 10/26/2022]
Abstract
beta-Carbolines (including harmane and pinoline) stimulate insulin secretion by a mechanism that may involve interaction with imidazoline I(3)-receptors but which also appears to be mediated by actions that are additional to imidazoline receptor agonism. Using the MIN6 beta-cell line, we now show that both the imidazoline I(3)-receptor agonist, efaroxan, and the beta-carboline, harmane, directly elevate cytosolic Ca(2+) and increase insulin secretion but that these responses display different characteristics. In the case of efaroxan, the increase in cytosolic Ca(2+) was readily reversible, whereas, with harmane, the effect persisted beyond removal of the agonist and resulted in the development of a repetitive train of Ca(2+)-oscillations whose frequency, but not amplitude, was concentration-dependent. Initiation of the Ca(2+)-oscillations by harmane was independent of extracellular calcium but was sensitive to both dantrolene and high levels (20 mM) of caffeine, suggesting the involvement of ryanodine receptor-gated Ca(2+)-release. The expression of ryanodine receptor-1 and ryanodine receptor-2 mRNA in MIN6 cells was confirmed using reverse transcription-polymerase chain reaction (RT-PCR) and, since low concentrations of caffeine (1 mM) or thimerosal (10 microM) stimulated increases in [Ca(2+)](i), we conclude that ryanodine receptors are functional in these cells. Furthermore, the increase in insulin secretion induced by harmane was attenuated by dantrolene, consistent with the involvement of ryanodine receptors in mediating this response. By contrast, the smaller insulin secretory response to efaroxan was unaffected by dantrolene. Harmane-evoked changes in cytosolic Ca(2+) were maintained by nifedipine-sensitive Ca(2+)-influx, suggesting the involvement of L-type voltage-gated Ca(2+)-channels. Taken together, these data imply that harmane may interact with ryanodine receptors to generate sustained Ca(2+)-oscillations in pancreatic beta-cells and that this effect contributes to the insulin secretory response.
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Affiliation(s)
- Paul E Squires
- Molecular Physiology, Biomedical Research Institute, Department of Biological Sciences, University of Warwick, Coventry CV4 7AL, UK
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11
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Mayer G, Taberner PV. Effects of the imidazoline ligands efaroxan and KU14R on blood glucose homeostasis in the mouse. Eur J Pharmacol 2002; 454:95-102. [PMID: 12409010 DOI: 10.1016/s0014-2999(02)02473-1] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
The putative imidazoline I(3) receptor antagonist 2-(2-ethyl-2,3-dihydrobenzo[b]furan-2-yl)-1H-imidazole (KU14R) has been shown to block the effects of the atypical I(3) agonist efaroxan at the level of the ATP-sensitive K(+) (K(ATP)) channel in isolated pancreatic islet beta cells, but its effects in vivo are not known. We have therefore investigated the effects of KU14R on blood glucose and insulin level in vivo. When KU14R was administered before or after a hypoglycaemic dose of efaroxan, the fall in blood glucose was at least additive. When the antihyperglycaemic imidazoline ligand S22068 was administered after a dose of KU14R, it did not alter the hypoglycaemic response. In the mouse isolated vas deferens preparation, neither rauwolscine (at concentrations which competitively antagonised the inhibitory response to 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK14304)) nor KU14R affected inhibition produced by S22068. At 10(-4) M, KU14R had weak alpha(2)-adrenoceptor antagonist activity. We conclude that KU14R does not act as an antagonist of either efaroxan or S22068 at an imidazoline site in vivo.
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Affiliation(s)
- Gaëll Mayer
- Department of Pharmacology, University of Bristol, School of Medical Sciences, University Walk, Bristol BS8 1TD, UK
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12
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Slough S, Guillaumet G, Taberner PV. Evidence that the novel imidazoline compound FT005 is an alpha(2)-adrenoceptor agonist. Br J Pharmacol 2002; 136:1049-57. [PMID: 12145105 PMCID: PMC1573438 DOI: 10.1038/sj.bjp.0704810] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022] Open
Abstract
1: The aim of this study was to determine whether the hyperglycaemic action of the novel imidazoline compound FT005 could be mediated by activation of alpha(2)-adrenoceptors, using a variety of in vivo and in vitro methods including radioligand binding. 2: FT005 produced a dose-dependent increase in blood glucose levels of CBA/Ca mice (0.125-25 mg kg(-1), i.p.). The time course of this hyperglycaemic effect matched that of adrenaline (1 mg kg(-1)) more closely than glucagon (1 mg kg(-1)) or the K(ATP) channel opener diazoxide (25 mg kg(-1)). The hyperglycaemic effect of FT005 (1 mg kg(-1)) was significantly reduced by the alpha(2)-adrenoceptor antagonist rauwolscine (0.5 mg kg(-1)). 3: FT005 produced a significant reduction in plasma insulin levels of mice 30 min after administration. The hyperglycaemic effect of FT005 (25 mg kg(-1)), although still present, was significantly less in fasted mice in which insulin levels are lower, suggesting that a reduction of insulin secretion contributes to the action of FT005. 4: When studied in the mouse isolated vas deferens preparation, FT005 produced a complete inhibition of neurogenic contractions, which was blocked by rauwolscine. This is consistent with activation of pre-synaptic alpha(2)-adrenoceptors. 5: In radioligand binding studies FT005 completely displaced the alpha(2)-adrenoceptor antagonist [(3)H]-RX821002 from mouse whole brain homogenates. The displacement was best described by a two-site model of interaction comprising high and low affinity components. 6: The results indicate that FT005 is an agonist at alpha(2)-adrenoceptors. A reduction in insulin secretion contributes to the hyperglycaemic action of FT005, although an additional mechanism can not be excluded.
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Affiliation(s)
- Scott Slough
- Department of Pharmacology, University of Bristol, School of Medical Sciences, University Walk, Bristol BS8 1TD
| | - Gerald Guillaumet
- Institut de Chimie Organique et Analytique, UMR CNRS 6005, Université d'Orléans, BP 6759, 45067 ORLEANS, Cedex 2, France
| | - Peter V Taberner
- Department of Pharmacology, University of Bristol, School of Medical Sciences, University Walk, Bristol BS8 1TD
- Author for correspondence:
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Chan SLF, Monks LK, Gao H, Deaville P, Morgan NG. Identification of the monomeric G-protein, Rhes, as an efaroxan-regulated protein in the pancreatic beta-cell. Br J Pharmacol 2002; 136:31-6. [PMID: 11976265 PMCID: PMC1762110 DOI: 10.1038/sj.bjp.0704680] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022] Open
Abstract
Efaroxan induces membrane depolarization by interaction with the pore forming subunit of the ATP-sensitive potassium channel, Kir6.2. However, this effect is not responsible for its full secretory activity. In this study we have used an anti-idiotypic approach to generate antibodies that recognize additional proteins that may be regulated by efaroxan in pancreatic beta-cells. Using these antisera in an expression cloning strategy we have identified a monomeric GTP-binding protein, Rhes, as a potential target for regulation by imidazoline ligands. Rhes is shown to be expressed in beta-cells and its expression is regulated by efaroxan under conditions when a structurally related molecule, KU14R, is ineffective. The results reveal that beta-cells express Rhes and suggest that changes in the expression of this molecule may regulate the sensitivity of beta-cells to imidazoline secretagogues.
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Affiliation(s)
- Sue L F Chan
- Institute of Cell Signalling, University of Nottingham, Queen's Medical Centre, Nottingham NG7 2UH
| | - Lara K Monks
- Cellular Pharmacology Group, School of Life Sciences, Keele University, Staffs ST5 5BG
| | - Hongwei Gao
- Cellular Pharmacology Group, School of Life Sciences, Keele University, Staffs ST5 5BG
| | - Pamela Deaville
- Cellular Pharmacology Group, School of Life Sciences, Keele University, Staffs ST5 5BG
| | - Noel G Morgan
- Cellular Pharmacology Group, School of Life Sciences, Keele University, Staffs ST5 5BG
- Author for correspondence:
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Abstract
The site of the hypotensive action of imidazoline compounds, such as clonidine, was first identified within the rostroventrolateral part of the brainstem. Afterwards, it was shown that imidazolines reduced blood pressure when applied in this area, whereas no catecholamine was capable of such an effect. These data led us to suggest the existence of receptors specific for imidazolines different from the alpha-adrenergic receptors. Soon after, the existence of imidazoline binding sites (IBS) was reported in the brain and in a variety of peripheral tissues including pancreatic gland and kidney. As expected, these specific binding sites do not bind the catecholamines. The IBS are classified in two groups: the I1 type, sensitive to clonidine and idazoxan; and the I2 type, sensitive to idazoxan and largely insensitive to clonidine. Imidazoline receptors were shown to be involved in several physiological regulations and pathological processes such as hypertension, diabetes mellitus and some mood disorders. Evidence for their implication in the nervous regulation of blood pressure and in the insulin secretion control will be presented. The hypotensive effects of clonidine-like drugs involve imidazoline receptors (I1Rs), while their most frequent side-effects only involve alpha2-adrenergic receptors. A new class of centrally acting antihypertensive drugs selective for I1Rs is now available. At hypotensive doses, these drugs are devoid of significant side effects. It was shown that the good acceptability of these drugs is likely due to their selectivity for I1Rs.
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Affiliation(s)
- P Bousquet
- Laboratoire de Neurobiologie et Pharmacologie Cardiovasculaire, Faculté de Médecine, Université Louis Pasteur, Strasbourg, France
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15
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Minyan W, Dunn WR, Blaylock NA, Chan SLF, Wilson VG. Evidence for a non-adrenoceptor, imidazoline-mediated contractile response to oxymetazoline in the porcine isolated rectal artery. Br J Pharmacol 2001; 132:1359-63. [PMID: 11264227 PMCID: PMC1572690 DOI: 10.1038/sj.bjp.0703949] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022] Open
Abstract
Imidazoline derivatives are known to elicit responses through both alpha(2)-adrenoceptor and non-adrenoceptor, imidazoline sites, though as yet there are no examples of the latter on vascular smooth muscle. In the presence of 0.3 microM prazosin, neither UK-14304 (0.01 - 3 microM) nor oxymetazoline (0.01 - 30 microM) caused a significant contraction of the porcine isolated rectal artery, a preparation with a low density of alpha(2)-adrenoceptors. In the presence of a combination of U46619 and forskolin, however, both agonists produced concentration-dependent contractions. Pretreatment with phenoxybenzamine (3 microM) abolished responses to UK-14304, but left those elicited by oxymetazoline largely unaffected. The putative I(3) imidazoline antagonist 2-(2,3 dihydro-2-benzofuranyl)-2-imidazole (KU-14R, 10 microM) caused a 6 fold rightward displacement of the phenoxybenzamine-insensitive concentration - response curve to oxymetazoline. Our data indicates that non-adrenoceptor, imidazoline sites, pharmacologically similar to the I(3) imidazoline site on islet cells, mediate vasoconstriction in the porcine isolated rectal artery.
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Affiliation(s)
- Wang Minyan
- School of Biomedical Sciences, Queen's Medical Centre, Clifton Boulevard, Nottingham, NG7 2UH
| | - W R Dunn
- School of Biomedical Sciences, Queen's Medical Centre, Clifton Boulevard, Nottingham, NG7 2UH
| | - N A Blaylock
- School of Biomedical Sciences, Queen's Medical Centre, Clifton Boulevard, Nottingham, NG7 2UH
| | | | - V G Wilson
- School of Biomedical Sciences, Queen's Medical Centre, Clifton Boulevard, Nottingham, NG7 2UH
- Author for correspondence:
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Clews PJ, Ramsden CA, Morgan NG. Preparation of novel 2-(benzo[b]furan-2-yl)-1H-imidazolines for photoaffinity labelling and affinity isolation of imidazoline binding. J Heterocycl Chem 2001. [DOI: 10.1002/jhet.5570380237] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
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Chan SL, Mourtada M, Morgan NG. Characterization of a KATP channel-independent pathway involved in potentiation of insulin secretion by efaroxan. Diabetes 2001; 50:340-7. [PMID: 11272145 DOI: 10.2337/diabetes.50.2.340] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/13/2022]
Abstract
Efaroxan, like several other imidazoline reagents, elicits a glucose-dependent increase in insulin secretion from pancreatic beta-cells. This response has been attributed to efaroxan-mediated blockade of KATP channels, with the subsequent gating of voltage-sensitive calcium channels. However, increasing evidence suggests that, at best, this mechanism can account for only part of the secretory response to the imidazoline. In support of this, we now show that efaroxan can induce functional changes in the secretory pathway of pancreatic beta-cells that are independent of KATP channel blockade. In particular, efaroxan was found to promote a sustained sensitization of glucose-induced insulin release that persisted after removal of the drug and to potentiate Ca2+-induced insulin secretion from electropermeabilized islets. To investigate the mechanisms involved, we studied the effects of the efaroxan antagonist KU14R. This agent is known to selectively inhibit insulin secretion induced by efaroxan, without altering the secretory response to glucose or KCl. Surprisingly, however, KU14R markedly impaired the potentiation of insulin secretion mediated by agents that raise cAMP, including the adenylate cyclase activator, forskolin, and the phosphodiesterase inhibitor isobutylmethyl xanthine (IBMX). These effects were not accompanied by any reduction in cAMP levels, suggesting an antagonistic action of KU14R at a more distal point in the pathway of potentiation. In accord with our previous work, islets that were exposed to efaroxan for 24 h became selectively desensitized to this agent, but they still responded normally to glucose. Unexpectedly, however, the ability of either forskolin or IBMX to potentiate glucose-induced insulin secretion was severely impaired in these islets. By contrast, the elevation of cAMP was unaffected by culture of islets with efaroxan. Taken together, the data suggest that, in addition to effects on the KATP channel, imidazolines also interact with a more distal component that is crucial to the potentiation of insulin secretion. This component is not required for Ca2+-dependent secretion per se but is essential to the mechanism by which cAMP potentiates insulin release. Overall, the results indicate that the actions of efaroxan at this distal site may be more important for control of insulin secretion than its effects on the KATP channel.
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Affiliation(s)
- S L Chan
- Institute of Cell Signalling, University of Nottingham, UK
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Ball AJ, Flatt PR, McClenaghan NH. Stimulation of insulin secretion in clonal BRIN-BD11 cells by the imidazoline derivatives KU14r and RX801080. Pharmacol Res 2000; 42:575-9. [PMID: 11058411 DOI: 10.1006/phrs.2000.0739] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The imidazoline derivatives KU14R and RX801080 have each been reported to antagonize imidazoline-stimulated insulin secretion. This study investigated the effects of a range of concentrations of both KU14R and RX801080 on insulin secretion from the clonal pancreatic beta cell line, BRIN-BD11. In the presence of a stimulatory (8.4 m m) glucose concentration, both KU14R (50-200 microm;P< 0.01 to P< 0.001) and RX801080 (50-200 microm;P< 0.01 to P< 0.001) were found to dose-dependently stimulate insulin secretion. The imidazoline efaroxan (200 microm) stimulated insulin secretion (P< 0.001) from BRIN-BD11 cells. This insulinotropic effect was significantly augmented by KU14R (100-200 microm;P< 0.01 to P< 0.001) and RX801080 (200 microm;P< 0.05). Insulin secretion from BRIN-BD11 cells was also stimulated by the novel guanidine derivative BTS 67 582 (200 microm;P< 0.001). This secretagogue action was augmented both by KU14R (25-200 microm;P< 0.001) and by RX801080 (25-200 microm;P< 0.05 to P< 0.001). It is concluded that, rather than acting as antagonists of imidazoline-induced insulin secretion, the imidazoline derivatives KU14R and RX801080 are themselves potent insulinotropic agents.
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Affiliation(s)
- A J Ball
- School of Biomedical Sciences, University of Ulster, Coleraine, Northern Ireland, BT52 1SA, UK
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Mourtada M, Chan SLF, Smith SA, Morgan NG. Multiple effector pathways regulate the insulin secretory response to the imidazoline RX871024 in isolated rat pancreatic islets. Br J Pharmacol 1999; 127:1279-87. [PMID: 10455276 PMCID: PMC1566128 DOI: 10.1038/sj.bjp.0702656] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
When isolated rat islets were cultured for 18 h prior to use, the putative imidazoline binding site ligand, RX871024 caused a dose-dependent increase in insulin secretion at both 6 mM and 20 mM glucose. By contrast, a second ligand, efaroxan, was ineffective at 20 mM glucose whereas it did stimulate insulin secretion in response to 6 mM glucose. Exposure of islets to RX871024 (50 microM) for 18 h, resulted in loss of responsiveness to this reagent upon subsequent re-exposure. However, islets that were unresponsive to RX871024 still responded normally to efaroxan. The imidazoline antagonist, KU14R, blocked the insulin secretory response to efaroxan, but failed to prevent the stimulatory response to RX871024. By contrast with its effects in cultured islets, RX871024 inhibited glucose-induced insulin release from freshly isolated islets. Efaroxan did not inhibit insulin secretion under any conditions studied. In freshly isolated islets, the effects of RX871024 on insulin secretion could be converted from inhibitory to stimulatory, by starvation of the animals. Inhibition of insulin secretion by RX871024 in freshly isolated islets was prevented by the cyclo-oxygenase inhibitors indomethacin or flurbiprofen. Consistent with this, RX871024 caused a marked increase in islet PGE2 formation. Efaroxan did not alter islet PGE2 levels. The results suggest that RX871024 exerts multiple effects in the pancreatic beta-cell and that its effects on insulin secretion cannot be ascribed only to interaction with a putative imidazoline binding site.
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Affiliation(s)
- Mirna Mourtada
- Cellular Pharmacology Group, Department of Biological Sciences, Keele University, Staffs ST5 5BG
| | - Sue L F Chan
- Cellular Pharmacology Group, Department of Biological Sciences, Keele University, Staffs ST5 5BG
| | - Stephen A Smith
- Department of Vascular Biology, SmithKline Beecham Pharmaceuticals, Harlow, Essex, CM19 5AD
| | - Noel G Morgan
- Cellular Pharmacology Group, Department of Biological Sciences, Keele University, Staffs ST5 5BG
- Author for correspondence:
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Abstract
A range of imidazoline derivatives are known to be effective stimulators of insulin secretion, and this response correlates with closure of ATP-sensitive potassium channels in the pancreatic beta-cell. However, mounting evidence indicates that potassium channel blockade may form only part of the mechanism by which imidazolines exert their effects on insulin secretion. Additionally, it remains unclear whether members of this class of drugs can bind directly to potassium channel components and whether occupation of a single binding site accounts for their functional activity. This review considers recent developments in the field and highlights evidence that does not fit readily with the concept that a single mechanism of action is sufficient to mediate the effects of imidazolines on pancreatic hormone secretion.
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Affiliation(s)
- N G Morgan
- Department of Biological Sciences, Keele University, Staffs, UK.
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Monks LK, Cosgrove KE, Dunne MJ, Ramsden CA, Morgan NG, Chan SL. Affinity isolation of imidazoline binding proteins from rat brain using 5-amino-efaroxan as a ligand. FEBS Lett 1999; 447:61-4. [PMID: 10218583 DOI: 10.1016/s0014-5793(99)00264-1] [Citation(s) in RCA: 21] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/17/2022]
Abstract
We have employed an amino derivative of the imidazoline ligand, efaroxan, to isolate imidazoline binding proteins from solubilised extracts of rat brain, by affinity chromatography. A number of proteins were specifically retained on the affinity column and one of these was immunoreactive with an antiserum raised against the ion conducting pore component of the ATP-sensitive potassium channel. Patch clamp experiments confirmed that, like its parent compound, amino-efaroxan blocks ATP-sensitive potassium channels in human pancreatic beta-cells and can stimulate the insulin secretion from these cells. The results reveal that a member of the ion conducting pore component family is strongly associated with imidazoline binding proteins in brain and in the endocrine pancreas.
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Affiliation(s)
- L K Monks
- Cellular Pharmacology Group, School of Life Sciences, Keele University, Staffordshire, UK
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