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Bousquet P, Hudson A, García-Sevilla JA, Li JX. Imidazoline Receptor System: The Past, the Present, and the Future. Pharmacol Rev 2020; 72:50-79. [PMID: 31819014 DOI: 10.1124/pr.118.016311] [Citation(s) in RCA: 53] [Impact Index Per Article: 13.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022] Open
Abstract
Imidazoline receptors historically referred to a family of nonadrenergic binding sites that recognize compounds with an imidazoline moiety, although this has proven to be an oversimplification. For example, none of the proposed endogenous ligands for imidazoline receptors contain an imidazoline moiety but they are diverse in their chemical structure. Three receptor subtypes (I1, I2, and I3) have been proposed and the understanding of each has seen differing progress over the decades. I1 receptors partially mediate the central hypotensive effects of clonidine-like drugs. Moxonidine and rilmenidine have better therapeutic profiles (fewer side effects) than clonidine as antihypertensive drugs, thought to be due to their higher I1/α 2-adrenoceptor selectivity. Newer I1 receptor agonists such as LNP599 [3-chloro-2-methyl-phenyl)-(4-methyl-4,5-dihydro-3H-pyrrol-2-yl)-amine hydrochloride] have little to no activity on α 2-adrenoceptors and demonstrate promising therapeutic potential for hypertension and metabolic syndrome. I2 receptors associate with several distinct proteins, but the identities of these proteins remain elusive. I2 receptor agonists have demonstrated various centrally mediated effects including antinociception and neuroprotection. A new I2 receptor agonist, CR4056 [2-phenyl-6-(1H-imidazol-1yl) quinazoline], demonstrated clear analgesic activity in a recently completed phase II clinical trial and holds great promise as a novel I2 receptor-based first-in-class nonopioid analgesic. The understanding of I3 receptors is relatively limited. Existing data suggest that I3 receptors may represent a binding site at the Kir6.2-subtype ATP-sensitive potassium channels in pancreatic β-cells and may be involved in insulin secretion. Despite the elusive nature of their molecular identities, recent progress on drug discovery targeting imidazoline receptors (I1 and I2) demonstrates the exciting potential of these compounds to elicit neuroprotection and to treat various disorders such as hypertension, metabolic syndrome, and chronic pain.
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Affiliation(s)
- Pascal Bousquet
- Faculty of Medicine, University of Strasbourg, Strasbourg, France (P.B.); Department of Pharmacology, University of Alberta, Edmonton, Alberta, Canada (A.H.); Laboratory of Neuropharmacology, University Research Institute on Health Sciences, University of the Balearic Islands, Palma de Malllorca, Spain (J.A.G.-S.); and Department of Pharmacology and Toxicology, University at Buffalo, Buffalo, New York (J.-X.L.)
| | - Alan Hudson
- Faculty of Medicine, University of Strasbourg, Strasbourg, France (P.B.); Department of Pharmacology, University of Alberta, Edmonton, Alberta, Canada (A.H.); Laboratory of Neuropharmacology, University Research Institute on Health Sciences, University of the Balearic Islands, Palma de Malllorca, Spain (J.A.G.-S.); and Department of Pharmacology and Toxicology, University at Buffalo, Buffalo, New York (J.-X.L.)
| | - Jesús A García-Sevilla
- Faculty of Medicine, University of Strasbourg, Strasbourg, France (P.B.); Department of Pharmacology, University of Alberta, Edmonton, Alberta, Canada (A.H.); Laboratory of Neuropharmacology, University Research Institute on Health Sciences, University of the Balearic Islands, Palma de Malllorca, Spain (J.A.G.-S.); and Department of Pharmacology and Toxicology, University at Buffalo, Buffalo, New York (J.-X.L.)
| | - Jun-Xu Li
- Faculty of Medicine, University of Strasbourg, Strasbourg, France (P.B.); Department of Pharmacology, University of Alberta, Edmonton, Alberta, Canada (A.H.); Laboratory of Neuropharmacology, University Research Institute on Health Sciences, University of the Balearic Islands, Palma de Malllorca, Spain (J.A.G.-S.); and Department of Pharmacology and Toxicology, University at Buffalo, Buffalo, New York (J.-X.L.)
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Wu JX, Ding D, Wang M, Chen L. Structural Insights into the Inhibitory Mechanism of Insulin Secretagogues on the Pancreatic ATP-Sensitive Potassium Channel. Biochemistry 2019; 59:18-25. [PMID: 31566370 DOI: 10.1021/acs.biochem.9b00692] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
Abstract
Sulfonylureas and glinides are commonly used oral insulin secretagogues (ISs) that act on the pancreatic ATP-sensitive potassium (KATP) channel to promote insulin secretion in order to lower the blood glucose level. Physiologically, KATP channels are inhibited by intracellular ATP and activated by Mg-ADP. Therefore, they sense the cellular energy status to regulate the permeability of potassium ions across the plasma membrane. The pancreatic KATP channel is composed of the pore-forming Kir6.2 subunits and the regulatory SUR1 subunits. Previous electrophysiological studies have established that ISs bind to the SUR1 subunit and inhibit the channel activity primarily by two mechanisms. First, ISs prevent Mg-ADP activation. Second, ISs inhibit the channel activity of Kir6.2 directly. Several cryo-EM structures of the pancreatic KATP channel determined recently have provided remarkable structural insights into these two mechanisms.
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Affiliation(s)
- Jing-Xiang Wu
- State Key Laboratory of Membrane Biology, Beijing Key Laboratory of Cardiometabolic Molecular Medicine, Institute of Molecular Medicine , Peking University , Beijing 100871 , China.,Peking-Tsinghua Center for Life Sciences , Peking University , Beijing 100871 , China
| | - Dian Ding
- State Key Laboratory of Membrane Biology, Beijing Key Laboratory of Cardiometabolic Molecular Medicine, Institute of Molecular Medicine , Peking University , Beijing 100871 , China.,Peking-Tsinghua Center for Life Sciences , Peking University , Beijing 100871 , China.,Academy for Advanced Interdisciplinary Studies , Peking University , Beijing 100871 , China
| | - Mengmeng Wang
- State Key Laboratory of Membrane Biology, Beijing Key Laboratory of Cardiometabolic Molecular Medicine, Institute of Molecular Medicine , Peking University , Beijing 100871 , China.,Peking-Tsinghua Center for Life Sciences , Peking University , Beijing 100871 , China.,Academy for Advanced Interdisciplinary Studies , Peking University , Beijing 100871 , China
| | - Lei Chen
- State Key Laboratory of Membrane Biology, Beijing Key Laboratory of Cardiometabolic Molecular Medicine, Institute of Molecular Medicine , Peking University , Beijing 100871 , China.,Peking-Tsinghua Center for Life Sciences , Peking University , Beijing 100871 , China
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3
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Ostrovskaya RU, Ivanov SV, Voronin MV, Ozerova IV, Zolotov NN, Seredenin SB. Antidiabetic Activity of Afobazole in Wistar Rats. Bull Exp Biol Med 2018; 165:649-652. [DOI: 10.1007/s10517-018-4233-4] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/22/2018] [Indexed: 12/15/2022]
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Paniagua N, Girón R, Goicoechea C, López‐Miranda V, Vela J, Merlos M, Martín Fontelles M. Blockade of sigma 1 receptors alleviates sensory signs of diabetic neuropathy in rats. Eur J Pain 2017; 21:61-72. [PMID: 27341510 PMCID: PMC5215451 DOI: 10.1002/ejp.897] [Citation(s) in RCA: 19] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 04/12/2016] [Indexed: 12/11/2022]
Abstract
BACKGROUND E-52862 (S1RA, 4-[2-[[5-methyl-1-(2-naphthalenyl)-1H-pyrazol-3-yl]oxy]ethyl]-morpholine), a novel selective sigma 1 receptor (σ1R) antagonist, has demonstrated efficacy in nociceptive and neuropathic pain models. Our aim was to test if σ1R blockade with E-52862 may modify the signs of neuropathy in Zucker diabetic fatty (ZDF) rats, a type 2 diabetes model. METHODS Mechanical and thermal response thresholds were tested on 7-, 13-, 14- and 15-week-old ZDF rats treated with saline or with E-52862 acutely administered on week 13, followed by sub-chronic administration (14 days). Axonal peripheral activity (skin-saphenous nerve preparation) and isolated aorta or mesenteric bed reactivity were analysed in 15-week-old ZDF rats treated with saline or E-52862 and in LEAN rats. RESULTS Zucker diabetic fatty rats showed significantly decreased thermal withdrawal latency and threshold to mechanical stimulation on week 13 compared to week 7 (prediabetes) and with LEAN animals; single-dose and sub-chronic E-52862 administration restored both parameters to those recorded on week 7. Regarding axonal peripheral activity, E-52862 treatment increased the mean mechanical threshold (77.3 ± 21 mN vs. 19.6 ± 1.5 mN, saline group) and reduced the response evoked by mechanical increasing stimulation (86.4 ± 36.5 vs. 352.8 ± 41.4 spikes) or by repeated mechanical supra-threshold steps (39.4 ± 1.4 vs. 83.5 ± 0.9). E-52862 treatment also restored contractile response to phenylephrine in aorta and mesenteric bed. CONCLUSIONS E-52862 administration reverses neuropathic (behavioural and electrophysiological) and vascular signs in the ZDF rat. SIGNIFICANCE Blockade of σ1R avoids the development of diabetic neuropathy in rats, and may represent a potentially useful therapeutic approach to peripheral neuropathies in diabetic patients. WHAT DOES THIS STUDY ADD?: This study presents evidences for the potential usefulness of sigma receptor blockade on diabetic neuropathy in rats. The methodology includes behavioural evidences, electrophysiological data and vascular-isolated models.
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Affiliation(s)
- N. Paniagua
- Farmacología y NutriciónFacultad de Ciencias de la SaludUniversidad Rey Juan CarlosUnidad Asociada CSIC‐IQMAlcorcónSpain
| | - R. Girón
- Farmacología y NutriciónFacultad de Ciencias de la SaludUniversidad Rey Juan CarlosUnidad Asociada CSIC‐IQMAlcorcónSpain
| | - C. Goicoechea
- Farmacología y NutriciónFacultad de Ciencias de la SaludUniversidad Rey Juan CarlosUnidad Asociada CSIC‐IQMAlcorcónSpain
| | - V. López‐Miranda
- Farmacología y NutriciónFacultad de Ciencias de la SaludUniversidad Rey Juan CarlosUnidad Asociada CSIC‐IQMAlcorcónSpain
| | - J.M. Vela
- Drug Discovery & Preclinical ResearchEsteveBarcelonaSpain
| | - M. Merlos
- Drug Discovery & Preclinical ResearchEsteveBarcelonaSpain
| | - M.I. Martín Fontelles
- Farmacología y NutriciónFacultad de Ciencias de la SaludUniversidad Rey Juan CarlosUnidad Asociada CSIC‐IQMAlcorcónSpain
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Gasparik V, Greney H, Schann S, Feldman J, Fellmann L, Ehrhardt JD, Bousquet P. Synthesis and Biological Evaluation of 2-Aryliminopyrrolidines as Selective Ligands for I1 Imidazoline Receptors: Discovery of New Sympatho-Inhibitory Hypotensive Agents with Potential Beneficial Effects in Metabolic Syndrome. J Med Chem 2014; 58:878-87. [DOI: 10.1021/jm501456p] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/24/2022]
Affiliation(s)
- Vincent Gasparik
- Laboratoire
de Neurobiologie
et Pharmacologie Cardiovasculaire, Faculté de Médecine,
EA 7296, Fédération de Médecine Translationnelle, Université de Strasbourg, 11 rue Humann, 67000 Strasbourg, France
| | - Hugues Greney
- Laboratoire
de Neurobiologie
et Pharmacologie Cardiovasculaire, Faculté de Médecine,
EA 7296, Fédération de Médecine Translationnelle, Université de Strasbourg, 11 rue Humann, 67000 Strasbourg, France
| | - Stephan Schann
- Laboratoire
de Neurobiologie
et Pharmacologie Cardiovasculaire, Faculté de Médecine,
EA 7296, Fédération de Médecine Translationnelle, Université de Strasbourg, 11 rue Humann, 67000 Strasbourg, France
| | - Josiane Feldman
- Laboratoire
de Neurobiologie
et Pharmacologie Cardiovasculaire, Faculté de Médecine,
EA 7296, Fédération de Médecine Translationnelle, Université de Strasbourg, 11 rue Humann, 67000 Strasbourg, France
| | - Lyne Fellmann
- Laboratoire
de Neurobiologie
et Pharmacologie Cardiovasculaire, Faculté de Médecine,
EA 7296, Fédération de Médecine Translationnelle, Université de Strasbourg, 11 rue Humann, 67000 Strasbourg, France
| | - Jean-Daniel Ehrhardt
- Laboratoire
de Neurobiologie
et Pharmacologie Cardiovasculaire, Faculté de Médecine,
EA 7296, Fédération de Médecine Translationnelle, Université de Strasbourg, 11 rue Humann, 67000 Strasbourg, France
| | - Pascal Bousquet
- Laboratoire
de Neurobiologie
et Pharmacologie Cardiovasculaire, Faculté de Médecine,
EA 7296, Fédération de Médecine Translationnelle, Université de Strasbourg, 11 rue Humann, 67000 Strasbourg, France
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Rybczynska AA, de Bruyn M, Ramakrishnan NK, de Jong JR, Elsinga PH, Helfrich W, Dierckx RA, van Waarde A. In Vivo Responses of Human A375M Melanoma to a σ Ligand: 18F-FDG PET Imaging. J Nucl Med 2013; 54:1613-20. [DOI: 10.2967/jnumed.113.122655] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022] Open
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Fischer S, Wiese C, Maestrup EG, Hiller A, Deuther-Conrad W, Scheunemann M, Schepmann D, Steinbach J, Wünsch B, Brust P. Molecular imaging of σ receptors: synthesis and evaluation of the potent σ1 selective radioligand [18F]fluspidine. Eur J Nucl Med Mol Imaging 2011; 38:540-51. [PMID: 21072511 DOI: 10.1007/s00259-010-1658-z] [Citation(s) in RCA: 58] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/09/2010] [Accepted: 10/20/2010] [Indexed: 11/28/2022]
Abstract
PURPOSE Neuroimaging of σ(1) receptors in the human brain has been proposed for the investigation of the pathophysiology of neurodegenerative and psychiatric diseases. However, there is a lack of suitable (18)F-labelled PET radioligands for that purpose. METHODS The selective σ(1) receptor ligand [(18)F]fluspidine (1'-benzyl-3-(2-[(18)F]fluoroethyl)-3H-spiro[[2]benzofuran-1,4'-piperidine]) was synthesized by nucleophilic (18)F(-) substitution of the tosyl precursor. In vitro receptor binding affinity and selectivity were assessed by radioligand competition in tissue homogenate and autoradiographic approaches. In female CD-1 mice, in vivo properties of [(18)F]fluspidine were evaluated by ex vivo brain section imaging and organ distribution of intravenously administered radiotracer. Target specificity was validated by organ distribution of [(18)F]fluspidine after treatment with 1 mg/kg i.p. of the σ receptor antagonist haloperidol or the emopamil binding protein (EBP) inhibitor tamoxifen. In vitro metabolic stability and in vivo metabolism were investigated by LC-MS(n) and radio-HPLC analysis. RESULTS [(18)F]Fluspidine was obtained with a radiochemical yield of 35-45%, a radiochemical purity of ≥ 99.6% and a specific activity of 150-350 GBq/μmol (n = 6) within a total synthesis time of 90-120 min. In vitro, fluspidine bound specifically and with high affinity to σ(1) receptors (K (i) = 0.59 nM). In mice, [(18)F]fluspidine rapidly accumulated in brain with uptake values of 3.9 and 4.7%ID/g and brain to blood ratios of 7 and 13 at 5 and 30 min after intravenous application of the radiotracer, respectively. By ex vivo autoradiography of brain slices, resemblance between binding site occupancy of [(18)F]fluspidine and the expression of σ(1) receptors was shown. The radiotracer uptake in the brain as well as in peripheral σ(1) receptor expressing organs was significantly inhibited by haloperidol but not by tamoxifen. Incubation with rat liver microsomes led to a fast biotransformation of fluspidine. After an incubation period of 30 min only 13% of the parent compound was left. Seven metabolites were identified by HPLC-UV and LC-MS(n) techniques. However, [(18)F]fluspidine showed a higher metabolic stability in vivo. In plasma samples ∼ 94% of parent compound remained at 30 min and ∼ 67% at 60 min post-injection. Only one major radiometabolite was detected. None of the radiometabolites crossed the blood-brain barrier. CONCLUSION [(18)F]Fluspidine demonstrated favourable target affinity and specificity as well as metabolic stability both in vitro and in animal experiments. The in vivo properties of [(18)F]fluspidine offer a high potential of this radiotracer for neuroimaging and quantitation of σ(1) receptors in vivo.
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Affiliation(s)
- Steffen Fischer
- Forschungszentrum Dresden-Rossendorf, Research Site Leipzig, Institute of Radiopharmacy, Leipzig, Germany
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8
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Souza A, Mbatchi B, Herchuelz A. Induction of insulin secretion by an aqueous extract of Tabernanhte iboga Baill. (Apocynaceae) in rat pancreatic islets of Langerhans. JOURNAL OF ETHNOPHARMACOLOGY 2011; 133:1015-1020. [PMID: 21094242 DOI: 10.1016/j.jep.2010.11.028] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/03/2010] [Revised: 09/16/2010] [Accepted: 11/12/2010] [Indexed: 05/30/2023]
Abstract
The effect of an aqueous extract of Tabernanthe iboga (TBEt) was studied in the rat islets insulin secretion based on its use in traditional medicine for the treatment of diabetes. Rats islets were isolated by collagenase digestion. In insulin release experiments, the insulin content was determined by Enzyme-Link Immunosorbent Assay (ELISA). For experiments on ⁴⁵Ca(2+) Uptake, the radioactive content was determined using a liquid scintillation analyzer. The extract (10⁻³ μg/ml-100 μg/ml) did not exert a significant increase of insulin secretion (p>0.05) in the presence of 2.8 mM of glucose (a none stimulatory concentration). Whereas, in the presence of 11.1 mM of glucose (stimulatory concentration), TBEt augmented glucose-stimulated insulin secretion in a dose-dependent manner. Interestingly, the secretory effect of the extract was glucose-dependent (5.6-16.7 mM). Furthermore, the insulinotropic effect of TBEt (1 μg/ml) was significantly potentiated (p<0.001) in K(+)-depolarised media as well as in the presence of 2.8 mM and 16.8 mM of glucose concentrations. In contrast, in the same conditions, TBEt failed to stimulate the high K(+) medium-induced insulin release. The extract significantly amplified (p<0.001 and p<0.05) the insulin secretion induced by either IBMX or tolbutamide. Diazoxide, cobalt or calcium removal inhibited the insulinotropic effect of the extract. TBEt increased glucose-induced ⁴⁵Ca(2+) uptake in rat islets. Overall, our findings suggest that Tabernanthe iboga contains water soluble insulinotropic compounds. The insulin secretion of TBEt's active principles might involve the closure of K(+)-ATP and the intensification of calcium influx through voltage-sensitive Ca(2+) channels.
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Affiliation(s)
- Alain Souza
- Laboratory of Pharmacology and Toxicology, Institute of Pharmacopeia and Traditional Medicine (IPHAMETRA), PO Box 1156, Libreville, Gabon.
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9
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Abstract
Chaperones are proteins that assist the correct folding of other protein clients either when the clients are being synthesized or at their functional localities. Chaperones are responsible for certain diseases. The sigma-1 receptor is recently identified as a receptor chaperone whose activity can be activated/deactivated by specific ligands. Under physiological conditions, the sigma-1 receptor chaperones the functional IP3 receptor at the endoplasmic reticulum and mitochondrion interface to ensure proper Ca(2+) signaling from endoplasmic reticulum into mitochondrion. However, under pathological conditions whereby cells encounter enormous stress that results in the endoplasmic reticulum losing its global Ca(2+) homeostasis, the sigma-1 receptor translocates and counteracts the arising apoptosis. Thus, the sigma-1 receptor is a receptor chaperone essential for the metabotropic receptor signaling and for the survival against cellular stress. The sigma-1 receptor has been implicated in many diseases including addiction, pain, depression, stroke, and cancer. Whether the chaperone activity of the sigma-1 receptor attributes to those diseases awaits further investigation.
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Affiliation(s)
- Shang-Yi Tsai
- Cellular Pathobiology Section, Cellular Neurobiology Research Branch, IRP, NIDA, NIH, DHHS, 333 Cassell Drive, Baltimore, MD 21224, USA
| | - Teruo Hayashi
- Cellular Pathobiology Section, Cellular Neurobiology Research Branch, IRP, NIDA, NIH, DHHS, 333 Cassell Drive, Baltimore, MD 21224, USA
| | - Tomohisa Mori
- Cellular Pathobiology Section, Cellular Neurobiology Research Branch, IRP, NIDA, NIH, DHHS, 333 Cassell Drive, Baltimore, MD 21224, USA
| | - Tsung-Ping Su
- Cellular Pathobiology Section, Cellular Neurobiology Research Branch, IRP, NIDA, NIH, DHHS, 333 Cassell Drive, Baltimore, MD 21224, USA
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Monassier L, Bousquet P. Sigma receptors: from discovery to highlights of their implications in the cardiovascular system. Fundam Clin Pharmacol 2002; 16:1-8. [PMID: 11903506 DOI: 10.1046/j.1472-8206.2002.00063.x] [Citation(s) in RCA: 40] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Sigma receptors are the targets of many ligands, of which some (the haloperidol for instance) are psychoactive, and of substances known to have antiarrhythmic effects (amiodarone and clofilium). They are involved in a variety of cardiovascular functions, such as the regulation of cardiac contractility and rhythm and the regulation of coronary and peripheral arterial vasomotricity. This short review will focus on some aspects regarding the ligands, the binding sites, the intracellular coupling and the cardiovascular functions of these enigmatic receptors.
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Affiliation(s)
- Laurent Monassier
- Laboratoire de Neurobiologie et Pharmacologie Cardiovasculaire, Faculté de Médecine, 11 rue Humann, 67085 Strasbourg Cedex, France.
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11
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Rustenbeck I, Winkler M, Jörns A. Desensitization of insulin secretory response to imidazolines, tolbutamide, and quinine. I. Secretory and morphological studies. Biochem Pharmacol 2001; 62:1685-94. [PMID: 11755122 DOI: 10.1016/s0006-2952(01)00792-4] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
The desensitization of pancreatic B-cells against stimulation by insulin secretagogues that inhibit ATP-dependent K(+) channels (K(ATP) channels) was investigated by measuring insulin secretion of perifused pancreatic islets. Additionally, the islet insulin content and the number of secretory granules per B-cell were determined. Prior to the measurement of secretion, islets were cultured for 18 h in the presence or absence of the test agents in a cell-culture medium containing 5 mM glucose. The effects of three imidazolines, phentolamine, alinidine, and idazoxan (100 microM each) were compared with those of the well-characterized sulfonylurea, tolbutamide (500 microM), and those of the ion channel-blocking alkaloid, quinine (100 microM). Insulin secretion was strongly reduced upon re-exposure to phentolamine, alinidine, tolbutamide, and quinine, whereas idazoxan, which stimulated secretion only weakly, had no significant effect. The imidazoline secretagogues phentolamine and alinidine induced a cross-desensitization against the stimulatory effect of tolbutamide and quinine. A long-term depolarization with 40 mM KCl was also able to induce a significant reduction of the secretory response to all of the above secretagogues. The insulin content of cultured islets was moderately, but significantly reduced by alinidine, whereas the reduction by phentolamine, tolbutamide, and quinine was not significant. In contrast to these observations, the ultrastructural examination revealed that tolbutamide-treated B-cells had a high degree of degranulation, whereas the other test agents and 40 mM KCl produced only a partial degranulation, except for phentolamine, which produced no significant degranulation at all. These results suggest that the desensitization of insulin secretion is a common property of all agents that stimulate insulin secretion by depolarisation of the plasma membrane. Depending on the specific secretagogue, additional mechanisms, proximal and distal to Ca(2+) influx, appear to contribute to the desensitization (see Rustenbeck et al., pages 1695-1703, this issue).
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Affiliation(s)
- I Rustenbeck
- Institute of Clinical Biochemistry, Hannover Medical School, D-30623, Hannover, Germany.
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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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13
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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
Imidazoline binding sites are now generally accepted as being receptors. Despite this acceptance, the molecular structure and signal transduction mechanisms of these receptors are still poorly understood. The I1-imidazoline binding site (I1-receptor) is localized to the plasma membrane, but it is not clear if this represents a conventional receptor. It is also not clear if there are multiple forms of the I1-receptor. The signal transduction mechanisms of I1-receptors are similarly unclear, but much progress has been made. Evidence clearly indicates that ligands with high affinity for I1-receptors stimulate a novel signal transduction pathway, phosphatidylcholine-selective phospholipase C, in the rat adrenal medullary tumor cell line PC-12. However, this may not be the case in all cell types as microphysiometry, a novel technique for determining cellular activation, could not detect receptor activation in cultured bovine adrenal medullary cells exposed to a number of imidazolines considered to be agonists at the I1-receptor. This suggests that there is no I1-receptor-mediated stimulation of phosphatidylcholine-specific phospholipase C in these cells. By contrast, nicotine-stimulated increases in ion entry were blocked by clonidine. Ion channels have been suggested as another possible I1-imidazoline "receptor" family and may represent the low affinity I1-receptor. I1-Receptor ligands can be shown to bind to, or block, the following members of the ligand-gated ion channel super family, the 5HT3, K+ATP, NMDA, and nicotinic acetylcholine receptors. The site of action appears to be the phencyclidine binding site in these channels, but other possibilities cannot be excluded. Molecular modeling suggests that I1-receptor-selective ligands share a common three-dimensional structure with phencyclidine, providing a basis for these actions. This suggests that a phencyclidine-binding site motif may represent a novel site of action for I1-receptor ligands and that searches for receptors based on this motif may reveal novel imidazoline "receptors."
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Affiliation(s)
- I F Musgrave
- Department of Medical Laboratory Science, RMIT University, Melbourne, Victoria, Australia.
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