1
|
Stadlbauer K, Lehner Z, Stamenkovic N, Rustenbeck I, Surman L, Luger A, Fürnsinn C. Dissection of mechanisms that account for imidazoline-induced lowering of blood glucose in mice. Eur J Pharmacol 2014; 741:178-85. [PMID: 25149664 DOI: 10.1016/j.ejphar.2014.08.003] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/08/2014] [Revised: 07/25/2014] [Accepted: 08/08/2014] [Indexed: 10/24/2022]
Abstract
Multiple mechanisms have been suggested to be responsible for the insulinotropic and blood glucose lowering effects of imidazoline compounds. This study was to unravel which mechanism predominantly accounts for glucose lowering by the prototypical imidazolines idazoxan and phentolamine. To this end, an α2-adrenoceptor agonist (UK14,304) and a KATP channel opener (diazoxide) were used to inhibit insulin release from isolated perifused mouse islets and to induce hyperglycaemia in conscious mice. Potentials of idazoxan and phentolamine to counteract these effects were examined in a comparative manner. In perifused islets, idazoxan increased insulin release only in the presence of the α2-agonist, whereas phentolamine strongly counteracted both inhibitors of insulin release. In vivo, a lower dose of idazoxan was necessary to ameliorate hyperglycaemia induced by the α2-agonist than by the KATP channel opener, indicating α2A-antagonism as the predominant mechanism of action (decrease in incremental area under the glucose curve induced by 0.1mg/kg idazoxan: under diazoxide, -3±7%, vs. under UK14,304, -34±9%, P<0.02). In contrast, identical doses of phentolamine were required to counteract hyperglycaemia induced by the two inhibitors of insulin release, implicating involvement of another mechanism beside α2A-antagonism (2mg/kg phentolamine: diazoxide, -11±8%, vs. UK14,304, -15±9%, ns; 4mg/kg phentolamine: diazoxide, -48±6%, vs. UK14,304, -48±8%, ns). The results show that imidazolines can lower blood glucose via more than one mechanism of action, with the relative contributions of the mechanisms varying considerably between individual compounds. Dissection of the involved mechanisms could help to develop imidazoline drugs for the treatment of type 2 diabetes.
Collapse
Affiliation(s)
- Karin Stadlbauer
- Division of Endocrinology & Metabolism, Department of Medicine III, Medical University of Vienna, Währinger Gürtel 18-20, 1090 Vienna, Austria.
| | - Zsuzsanna Lehner
- Division of Endocrinology & Metabolism, Department of Medicine III, Medical University of Vienna, Währinger Gürtel 18-20, 1090 Vienna, Austria.
| | - Natasa Stamenkovic
- Division of Endocrinology & Metabolism, Department of Medicine III, Medical University of Vienna, Währinger Gürtel 18-20, 1090 Vienna, Austria.
| | - Ingo Rustenbeck
- Institute of Pharmacology and Toxicology, University of Braunschweig, Mendelssohnstraße 1, 38106 Braunschweig, Germany.
| | - Lidia Surman
- Division of Endocrinology & Metabolism, Department of Medicine III, Medical University of Vienna, Währinger Gürtel 18-20, 1090 Vienna, Austria.
| | - Anton Luger
- Division of Endocrinology & Metabolism, Department of Medicine III, Medical University of Vienna, Währinger Gürtel 18-20, 1090 Vienna, Austria.
| | - Clemens Fürnsinn
- Division of Endocrinology & Metabolism, Department of Medicine III, Medical University of Vienna, Währinger Gürtel 18-20, 1090 Vienna, Austria.
| |
Collapse
|
2
|
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
|
3
|
Anderson RL, Randall MD, Chan SLF. The complex effects of cannabinoids on insulin secretion from rat isolated islets of Langerhans. Eur J Pharmacol 2013; 706:56-62. [PMID: 23499687 DOI: 10.1016/j.ejphar.2013.02.034] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/15/2012] [Revised: 02/14/2013] [Accepted: 02/24/2013] [Indexed: 12/12/2022]
Abstract
Recent interest in the endocrine pancreas has revealed the presence of a functional endocannabinoid system in pancreatic islets, however, the effects of endocannabinoids and cannabinoid CB receptor activation on downstream signalling and on insulin release still remains unclear. In the current study, a variety of purported cannabinoid CB receptor agonists and antagonists were evaluated for their effects on insulin secretion. In fresh rat isolated islets, the endocannabinoid anandamide caused a glucose-dependent, concentration-dependent inhibition of insulin release, with two populations of islets being identified based on their sensitivity to anandamide. Methanandamide (a non-hydrolysable analogue of anandamide) elicited similar inhibition of insulin secretion, comparable to the responses obtained with anandamide-sensitive islets, suggesting that the islet responsiveness may be due to differences in local metabolism of anandamide. The antagonists O-2050 (CB1) and AM630 (CB2) failed to reveal the involvement of cannabinoid receptors in the inhibitory activity of anandamide on insulin release. Inhibition of fatty acid amide hydrolase (FAAH) with URB597 did not alter basal or glucose-induced insulin secretion, suggesting that endogenous islet endocannabinoids do not affect insulin release, or that islet FAAH content is low. URB597 also failed to affect the inhibitory actions of anandamide on insulin release in fresh isolated islets. However, in islets following overnight culture, anandamide caused augmentation of basal and glucose-mediated insulin release. The effects of cannabinoid agents on insulin secretion described in this study does not identify a precise mode of action but points to important modulation which may be dependent on local metabolism and prevailing cellular conditions.
Collapse
Affiliation(s)
- Richard L Anderson
- School of Biomedical Sciences, University of Nottingham Medical School, Queen's Medical Centre, Nottingham, NG7 2UH, United Kingdom
| | | | | |
Collapse
|
4
|
Lehner Z, Stadlbauer K, Adorjan I, Rustenbeck I, Belz M, Fenzl A, de Cillia VAM, Gruber D, Bauer L, Frobel K, Brunmair B, Luger A, Fürnsinn C. Mechanisms of antihyperglycaemic action of efaroxan in mice: time for reappraisal of α2A-adrenergic antagonism in the treatment of type 2 diabetes? Diabetologia 2012; 55:3071-82. [PMID: 22898767 DOI: 10.1007/s00125-012-2679-x] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 03/07/2012] [Accepted: 07/20/2012] [Indexed: 10/28/2022]
Abstract
AIMS/HYPOTHESIS Inspired by recent speculation about the potential utility of α(2A)-antagonism in the treatment of type 2 diabetes, the study examined the contribution of α(2)-antagonism vs other mechanisms to the antihyperglycaemic activity of the imidazoline (±)-efaroxan. METHODS Effects of the racemate and its pure enantiomers on isolated pancreatic islets and beta cells in vitro, as well as on hyperglycaemia in vivo, were investigated in a comparative manner in mice. RESULTS In isolated perifused islets, the two enantiomers of efaroxan were equally potent in counteracting inhibition of insulin release by the ATP-dependent K(+) (K(ATP)) channel-opener diazoxide but (+)-efaroxan, the presumptive carrier of α(2)-antagonistic activity, was by far superior in counteracting inhibition of insulin release by the α(2)-agonist UK14,304. In vivo, (+)-efaroxan improved oral glucose tolerance at 100-fold lower doses than (-)-efaroxan and, in parallel with observations made in vitro, was more effective in counteracting UK14,304-induced than diazoxide-induced hyperglycaemia. The antihyperglycaemic activity of much higher doses of (-)-efaroxan was associated with an opposing pattern (i.e. with stronger counteraction of diazoxide-induced than UK14,304-induced hyperglycaemia), which implicates a different mechanism of action. CONCLUSIONS/INTERPRETATION The antihyperglycaemic potency of (±)-efaroxan in mice is almost entirely due to α(2)-antagonism, but high doses can also lower blood glucose via another mechanism. Our findings call for reappraisal of the possible clinical utility of α(2A)-antagonistic compounds in recently identified subpopulations of patients in which a congenitally higher level of α(2A)-adrenergic activation contributes to the development and pathophysiology of type 2 diabetes.
Collapse
Affiliation(s)
- Z Lehner
- Division of Endocrinology and Metabolism, Department of Medicine III, Medical University of Vienna, Währinger Gürtel 18-20, A-1090 Vienna, Austria
| | | | | | | | | | | | | | | | | | | | | | | | | |
Collapse
|
5
|
Brozzi F, Lajus S, Diraison F, Rajatileka S, Hayward K, Regazzi R, Molnár E, Váradi A. MyRIP interaction with MyoVa on secretory granules is controlled by the cAMP-PKA pathway. Mol Biol Cell 2012; 23:4444-55. [PMID: 22993210 PMCID: PMC3496617 DOI: 10.1091/mbc.e12-05-0369] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022] Open
Abstract
Myosin- and Rab-interacting protein is not a classic receptor for MyoVa on large, dense-core secretory granules (SGs), but it aids in PKA-dependent phosphorylation of MyoVa-associated proteins on SGs in endocrine and neuroendocrine cells. Myosin- and Rab-interacting protein (MyRIP), which belongs to the protein kinase A (PKA)–anchoring family, is implicated in hormone secretion. However, its mechanism of action is not fully elucidated. Here we investigate the role of MyRIP in myosin Va (MyoVa)-dependent secretory granule (SG) transport and secretion in pancreatic beta cells. These cells solely express the brain isoform of MyoVa (BR-MyoVa), which is a key motor protein in SG transport. In vitro pull-down, coimmunoprecipitation, and colocalization studies revealed that MyRIP does not interact with BR-MyoVa in glucose-stimulated pancreatic beta cells, suggesting that, contrary to previous notions, MyRIP does not link this motor protein to SGs. Glucose-stimulated insulin secretion is augmented by incretin hormones, which increase cAMP levels and leads to MyRIP phosphorylation, its interaction with BR-MyoVa, and phosphorylation of the BR-MyoVa receptor rabphilin-3A (Rph-3A). Rph-3A phosphorylation on Ser-234 was inhibited by small interfering RNA knockdown of MyRIP, which also reduced cAMP-mediated hormone secretion. Demonstrating the importance of this phosphorylation, nonphosphorylatable and phosphomimic Rph-3A mutants significantly altered hormone release when PKA was activated. These data suggest that MyRIP only forms a functional protein complex with BR-MyoVa on SGs when cAMP is elevated and under this condition facilitates phosphorylation of SG-associated proteins, which in turn can enhance secretion.
Collapse
Affiliation(s)
- Flora Brozzi
- Centre for Research in Biomedicine, Faculty of Health and Life Sciences, University of the West of England, Bristol BS16 1QY, United Kingdom
| | | | | | | | | | | | | | | |
Collapse
|
6
|
Nisr RB, Russell MA, Chrachri A, Moody AJ, Gilpin ML. Effects of the microbial secondary metabolites pyrrolnitrin, phenazine and patulin on INS-1 rat pancreatic β-cells. ACTA ACUST UNITED AC 2012; 63:217-27. [PMID: 22077225 DOI: 10.1111/j.1574-695x.2011.00844.x] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/01/2022]
Abstract
The effects on pancreatic β-cell viability and function of three microbial secondary metabolites pyrrolnitrin, phenazine and patulin were investigated, using the rat clonal pancreatic β-cell line, INS-1. Cells were exposed to 10-fold serial dilutions (range 0-10 μg mL(-1)) of the purified compounds for 2, 24 and 72 h. After 2 h exposure, only patulin (10 μg mL(-1)) was cytotoxic. All compounds showed significant cytotoxicity after 24 h. None of the compounds altered insulin secretion with 2 and 20 mM glucose after 2 h. However, after 24 h treatment, phenazine and pyrrolnitrin (10 and 100 ng mL(-1)) potentiated insulin production and glucose-stimulated insulin secretion, whereas patulin had no effect. Exposure (24 h) to either phenazine (100 ng mL(-1)) or pyrrolnitrin (10 ng mL(-1)) caused similar increases in the Ca(2+) content of INS-1 cells. The outward membrane current was inhibited after 24 h exposure to either phenazine (100 ng mL(-1)) or pyrrolnitrin (10 or 100 ng mL(-1)). This study presents novel data suggesting that high concentrations of pyrrolnitrin and phenazine are cytotoxic to pancreatic β-cells and thus possibly diabetogenic, whereas at lower concentrations these agents are nontoxic and may be insulinotropic. The possible role of such agents in the development of cystic fibrosis-related diabetes is discussed.
Collapse
Affiliation(s)
- Raid B Nisr
- Centre for Research in Translational Biomedicine, School of Biomedical and Biological Sciences, University of Plymouth, Plymouth, UK.
| | | | | | | | | |
Collapse
|
7
|
Hatlapatka K, Wienbergen A, Kühne C, Jörns A, Willenborg M, Rustenbeck I. Selective Enhancement of Nutrient-Induced Insulin Secretion by ATP-Sensitive K+ Channel-Blocking Imidazolines. J Pharmacol Exp Ther 2009; 331:1033-41. [DOI: 10.1124/jpet.109.152751] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
|
8
|
Ning Y, O'Neill K, Lan H, Pang L, Shan LX, Hawes BE, Hedrick JA. Endogenous and synthetic agonists of GPR119 differ in signalling pathways and their effects on insulin secretion in MIN6c4 insulinoma cells. Br J Pharmacol 2008; 155:1056-65. [PMID: 18724386 PMCID: PMC2528830 DOI: 10.1038/bjp.2008.337] [Citation(s) in RCA: 76] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022] Open
Abstract
Background and purpose: GPR119 is a G protein-coupled receptor that is preferentially expressed in islet cells and mediates insulin secretion. Oleoyl-lysophosphatidylcholine and oleoylethanolamide (OEA) act as endogenous ligands for this receptor, whereas PSN375963 and PSN632408 are two recently reported synthetic agonists. In this study, we explored mechanisms underlying GPR119-induced insulin secretion. In addition, we assessed the potential utility of the synthetic agonists as tools for exploring GPR119 biology. Experimental approach: We examined natural and synthetic GPR119 agonist activity at GPR119 in MIN6c4 and RINm5f insulinoma cells. We evaluated insulin secretion, intracellular calcium [Ca2+]i, ion channel involvement and levels of cAMP. Key results: We report that increases in insulin secretion induced by OEA were associated with increased cAMP and a potentiation of glucose-stimulated increases in [Ca2+]i. We also demonstrate that ATP-sensitive K+ and voltage-dependent calcium channels were required for GPR119-mediated increases in glucose-stimulated insulin secretion. In contrast to OEA, the synthetic GPR119 agonist PSN375963 and PSN632408 have divergent effects on insulin secretion, cAMP and intracellular calcium in MIN6c4 cells. Conclusions and implications: The endogenous ligand OEA signals through GPR119 in a manner similar to glucagon-like peptide-1 (GLP-1) and its receptor with respect to insulin secretion, [Ca2+]i and cAMP. In addition, PSN375963 and PSN632408 substantially differ from OEA and from one another. These studies suggest that the commercially available synthetic agonists, although they do activate GPR119, may also activate GPR119-independent pathways and are thus unsuitable as GPR119-specific pharmacological tools.
Collapse
Affiliation(s)
- Y Ning
- Department of Cardiovascular and Metabolic Disease, Schering-Plough Research Institute, Kenilworth, NJ 07033, USA
| | | | | | | | | | | | | |
Collapse
|
9
|
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]
|
10
|
Wienbergen A, Bleck C, Lackmann TG, Rustenbeck I. Antagonism of the insulinotropic action of first generation imidazolines by openers of KATP channels. Biochem Pharmacol 2007; 73:94-102. [PMID: 17056015 DOI: 10.1016/j.bcp.2006.09.017] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/17/2006] [Revised: 09/13/2006] [Accepted: 09/15/2006] [Indexed: 10/24/2022]
Abstract
The antagonism between K(ATP) channel-blocking insulinotropic imidazolines - phentolamine, alinidine, idazoxan and efaroxan - and K(ATP) channel openers, diazoxide and nucleoside diphosphates, was studied in mouse pancreatic islets and B-cells. In inside-out patches from B-cells, 500muM MgGDP abolished the inhibitory effect of the imidazolines. 300muM diazoxide further increased channel activity. The depolarizing effect of all imidazolines (100muM) on the B-cell membrane potential was practically completely antagonized by 300muM diazoxide. In contrast, diazoxide was unable to decrease the cytosolic Ca(2+) concentration ([Ca(2+)](i)) which was elevated by phentolamine, whereas the [Ca(2+)](i) increases induced by the other imidazolines were promptly antagonized. The effects on [Ca(2+)](i) were reflected by the secretory activity in that the stimulatory effects of alinidine, idazoxan and efaroxan, but not that of phentolamine were antagonized by diazoxide. Metabolic inhibition of intact B-cells by 250muM NaCN, most likely by a decrease of the ATP/ADP ratio, significantly diminished the K(ATP) channel-blocking effect of a low concentration of alinidine (10muM), whereas efaroxan proved to be susceptible even at a highly effective concentration (100muM). This may explain the oscillatory pattern of the [Ca(2+)](i) increase typically produced by efaroxan in pancreatic B-cells. In conclusion, the inhibitory effect of imidazolines on K(ATP) channels, which is exerted at the pore-forming subunit, Kir6.2, is susceptible to the action of endogenous and exogenous K(ATP) channel openers acting at the regulatory subunit SUR, which confers tissue specificity. With intact cells this antagonism can be obscured, possibly by intracellular accumulation of some imidazolines.
Collapse
Affiliation(s)
- Antje Wienbergen
- Institute of Pharmacology and Toxicology, Technical University of Braunschweig, D-38106 Braunschweig, Germany
| | | | | | | |
Collapse
|
11
|
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.
Collapse
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
| | | | | | | |
Collapse
|
12
|
Ernsberger P, Koletsky RJ. Metabolic effects of antihypertensive agents: role of sympathoadrenal and renin-angiotensin systems. Naunyn Schmiedebergs Arch Pharmacol 2006; 373:245-58. [PMID: 16783586 DOI: 10.1007/s00210-006-0080-3] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/04/2006] [Accepted: 05/09/2006] [Indexed: 01/01/2023]
Abstract
Reports of beneficial, neutral and adverse impacts of antihypertensive drug classes on glucose and lipid metabolism can be found in human data. Furthermore, mechanisms for these diverse effects are often speculative and controversial. Clinical trial data on the metabolic effects of antihypertensive agents are highly contradictory. Comparisons of clinical trials involving different agents are complicated by differences in the spectrum of metabolic disturbances that accompany hypertension in different groups of patients. Two physiological systems are predominant at the interface between metabolic and cardiovascular regulation: the sympathetic nervous system (SNS) and the renin-angiotensin system (RAS). These two systems are major targets of antihypertensive drug actions, and also mediate many of the beneficial and adverse effects of antihypertensive agents on glucose and lipid metabolism. Thiazides and beta-adrenergic antagonists can adversely affect glucose and lipid metabolism, which are frequently compromised in human essential hypertension, and increase the incidence of new cases of diabetes. Laboratory studies confirm these effects, and suggest that compensatory activation of the SNS and RAS may be one mechanism. Other antihypertensives directly targeting the SNS and RAS may have beneficial effects on glucose and lipid metabolism, and may prevent diabetes. Resolution of the controversies surrounding the metabolic effects of antihypertensive agents can only be resolved by further laboratory studies, in addition to controlled clinical trials.
Collapse
|
13
|
Velliquette RA, Kossover R, Previs SF, Ernsberger P. Lipid-lowering actions of imidazoline antihypertensive agents in metabolic syndrome X. Naunyn Schmiedebergs Arch Pharmacol 2006; 372:300-12. [PMID: 16416266 DOI: 10.1007/s00210-005-0024-3] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/19/2005] [Accepted: 11/21/2005] [Indexed: 01/15/2023]
Abstract
Agonists active at I1-imidazoline receptors (I1R) not only lower blood pressure but also ameliorate glucose intolerance, insulin resistance, and hyperlipidemia with long-term treatment. We sought to determine the possible mechanism for the lipid-lowering actions of imidazolines in a model of metabolic Syndrome X, the spontaneously-hypertensive obese (SHROB) rat. The acute actions of moxonidine and rilmenidine, selective I1R agonists, were compared to a specific alpha2-adrenergic receptor agonist, guanabenz, with and without selective receptor blockers. Moxonidine and rilmenidine rapidly reduced plasma triglyceride (20+/-4% and 21+/-5%, respectively) and cholesterol (29+/-9% and 27+/-9%). In contrast, the specific alpha2-adrenergic receptor agonist guanabenz failed to reduce plasma lipids. Blocking experiments showed that moxonidine's actions were mediated by I1R and not alpha2-adrenergic receptors. To evaluate a hepatic site of action, radioligand binding studies with liver plasma membranes confirmed the presence of I1R. Intraportal moxonidine reduced plasma triglycerides by 23+/-3% within 10 min. Moxonidine inhibited hepatic triglyceride secretion by 75% compared to vehicle treatment. Tracer studies with 2H2O suggested that moxonidine inhibits de novo fatty acid synthesis. Thus, activation of I1R lowers plasma lipids, with the main site of action probably within the liver to reduce synthesis and secretion of triglycerides. More selective I1R agonists might provide monotherapy for hyperlipidemic hypertension.
Collapse
Affiliation(s)
- Rodney A Velliquette
- Department of Nutrition, Case Western Reserve University School of Medicine, 10900 Euclid Ave, Cleveland, OH 44106-4906, USA
| | | | | | | |
Collapse
|
14
|
Mori K, Takasaki K, Katoh Y, Yano H, Ueno K, Ichimura M, Kusaka H, Nomoto Y, Higo K, Nakanishi S. Glucose concentration-dependent potentiation of insulin secretion by a new chemical entity, KCP256. Eur J Pharmacol 2005; 528:176-82. [PMID: 16316644 DOI: 10.1016/j.ejphar.2005.10.033] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/20/2005] [Revised: 10/14/2005] [Accepted: 10/21/2005] [Indexed: 11/30/2022]
Abstract
The insulinotropic activity of KCP256 [(R)-8-benzyl-2-cyclopentyl-7, 8-dihydro-4-propyl-1H-imidazo[2,1-i]purin-5(4H)-one hydrochloride] was examined using MIN6 cells (a pancreatic beta-cell line) and pancreatic islets isolated from rats. Unlike sulfonylurea anti-diabetic drugs, KCP256 dose-dependently (0.1-10 microM) enhanced insulin secretion from MIN6 cells and its insulinotropic effect was exerted only at high concentrations of glucose (8.3-22 mM) but not at low concentrations of glucose (3.3-5.5 mM). Furthermore, the action mechanism of KCP256 was different because, unlike sulfonylurea drugs, KCP256 did not displace the binding of [3H]glibenclamide, and did not inhibit the 86Rb+ efflux nor K(ATP) channel activity. In isolated islets, KCP256 also enhanced insulin secretion in a dose- and a glucose-concentration-dependent manner. Plasma levels of insulin after glucose challenge in KCP256-administrated rats were higher than those in vehicle-administrated animals, indicating that KCP256 can enhance insulin secretion in vivo. Since the insulinotropic activity of KCP256 only occurs at high concentrations of glucose, this novel drug may exhibit a decreased risk of drug-induced hypoglycemia compared with sulfonylurea drugs when treating patients with diabetes.
Collapse
Affiliation(s)
- Kiyotoshi Mori
- Drug Discovery Research Laboratories, Pharmaceutical Research Center, Kyowa Hakko Kogyo Co., Ltd., 1188 Shimotogari, Nagaizumi-cho, Shizuoka, Japan.
| | | | | | | | | | | | | | | | | | | |
Collapse
|
15
|
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.
Collapse
Affiliation(s)
- C Bleck
- Institute of Pharmacology and Toxicology, Technical University of Brunswick, Brunswick, Germany
| | | | | |
Collapse
|
16
|
Sharoyko VV, Zaitseva II, Varsanyi M, Portwood N, Leibiger B, Leibiger I, Berggren PO, Efendić S, Zaitsev SV. Monomeric G-protein, Rhes, is not an imidazoline-regulated protein in pancreatic beta-cells. Biochem Biophys Res Commun 2005; 338:1455-9. [PMID: 16277977 DOI: 10.1016/j.bbrc.2005.10.145] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/03/2005] [Accepted: 10/23/2005] [Indexed: 11/18/2022]
Abstract
The monomeric G-protein, Rhes, is a candidate imidazoline-regulated molecule involved in mediating the insulin secretory response to efaroxan [S.L. Chan, L.K. Monks, H. Gao, P. Deaville, N.G. Morgan, Identification of the monomeric G-protein, Rhes, as an efaroxan-regulated protein in the pancreatic beta-cell, Br. J. Pharmacol. 136 (1) (2002) 31-36]. This suggestion was based on observations regarding changes in Rhes mRNA expression in rat islets and pancreatic beta-cells after prolonged culture with efaroxan, leading to desensitization of the insulin response to the compound. To verify this report, we have evaluated the effects of the imidazoline compounds efaroxan and BL11282 on Rhes mRNA expression in isolated rat pancreatic islets maintained in conditions identical to those used by Chan et al. The results demonstrate that desensitization of the insulin response to efaroxan, or to another imidazoline, BL11282, does not change Rhes mRNA expression levels. Transfection of MIN6 cells with plasmids containing Rhes or Rhes-antisense also does not alter efaroxan- or BL11282-induced insulin secretion. Together, these data do not support the hypothesis that Rhes is an imidazoline-regulated protein.
Collapse
Affiliation(s)
- Vladimir V Sharoyko
- The Rolf Luft Research Center for Diabetes and Endocrinology, Karolinska Institutet, Karolinska University Hospital, Solna, SE-17176 Stockholm, Sweden
| | | | | | | | | | | | | | | | | |
Collapse
|
17
|
Bleck C, Wienbergen A, Rustenbeck I. Glucose dependence of imidazoline-induced insulin secretion: different characteristics of two ATP-Sensitive K+ channel-blocking compounds. Diabetes 2004; 53 Suppl 3:S135-9. [PMID: 15561901 DOI: 10.2337/diabetes.53.suppl_3.s135] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/13/2022]
Abstract
The glucose dependence of the insulinotropic action of KATP channel-blocking imidazoline compounds was investigated. Administration of 100 micromol/l phentolamine, but not 100 micromol/l efaroxan, markedly increased insulin secretion of freshly isolated mouse islets when the perifusion medium contained 5 mmol/l glucose. When the glucose concentration was raised to 10 mmol/l in the continued presence of either imidazoline, a clear potentiation of secretion occurred as compared with 10 mmol/l glucose alone. In the presence of efaroxan, a brisk first-phase-like increase was followed by a sustained phase, whereas a more gradual increase resulted in the presence of phentolamine. Administration of 100 micromol/l phentolamine was somewhat more effective than 100 micromol/l efaroxan to inhibit KATP channel activity in intact cultured beta-cells (reduction by 96 vs. 83%). Both compounds were similarly effective to depolarize the beta-cells. When measured by the perforated patch-technique, the depolarization by efaroxan was often oscillatory, whereas that by phentolamine was sustained. In perifused cultured islets, both compounds increased the cytosolic calcium concentration ([Ca2+]c) in the presence of 5 and 10 mmol/l glucose. Efaroxan induced large amplitude oscillations of [Ca2+]c, whereas phentolamine induced a sustained increase. It appears that a KATP channel block by imidazolines is not incompatible with a glucose-selective enhancement of insulin secretion. The glucose selectivity of efaroxan may involve an inhibitory effect distal to [Ca2+]c increase and/or the generation of [Ca2+]c oscillations.
Collapse
Affiliation(s)
- Claudia Bleck
- Institute of Pharmacology and Toxicology, University of Braunschweig, Mendelssohnstrasse 1, D-38106 Braunschweig, Germany
| | | | | |
Collapse
|
18
|
Abstract
Physiologically, insulin secretion is subject to a dual, hierarchal control by triggering and amplifying pathways. By closing ATP-sensitive K+ channels (KATP channels) in the plasma membrane, glucose and other metabolized nutrients depolarize beta-cells, stimulate Ca2+ influx, and increase the cytosolic concentration of free Ca2+ ([Ca2+]i), which constitutes the indispensable triggering signal to induce exocytosis of insulin granules. The increase in beta-cell metabolism also generates amplifying signals that augment the efficacy of Ca2+ on the exocytotic machinery. Stimulatory hormones and neurotransmitters modestly increase the triggering signal and strongly activate amplifying pathways biochemically distinct from that set into operation by nutrients. Many drugs can increase insulin secretion in vitro, but only few have a therapeutic potential. This review identifies six major pathways or sites of stimulus-secretion coupling that could be aimed by potential insulin-secreting drugs and describes several strategies to reach these targets. It also discusses whether these perspectives are realistic or theoretical only. These six possible beta-cell targets are 1) stimulation of metabolism, 2) increase of [Ca2+]i by closure of K+ ATP channels, 3) increase of [Ca2+]i by other means, 4) stimulation of amplifying pathways, 5) action on membrane receptors, and 6) action on nuclear receptors. The theoretical risk of inappropriate insulin secretion and, hence, of hypoglycemia linked to these different approaches is also envisaged.
Collapse
Affiliation(s)
- Jean-Claude Henquin
- Unité d'Endocrinologie et Métabolisme, UCL 55.30, avenue Hippocrate 55, B-1200 Brussels, Begium.
| |
Collapse
|
19
|
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.
Collapse
Affiliation(s)
- Paul E Squires
- Molecular Physiology, Biomedical Research Institute, Department of Biological Sciences, University of Warwick, Coventry CV4 7AL, UK
| | | | | | | | | | | |
Collapse
|
20
|
Prell GD, Martinelli GP, Holstein GR, Matulić-Adamić J, Watanabe KA, Chan SLF, Morgan NG, Haxhiu MA, Ernsberger P. Imidazoleacetic acid-ribotide: an endogenous ligand that stimulates imidazol(in)e receptors. Proc Natl Acad Sci U S A 2004; 101:13677-82. [PMID: 15365189 PMCID: PMC518812 DOI: 10.1073/pnas.0404846101] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/01/2004] [Indexed: 11/18/2022] Open
Abstract
We identified the previously unknown structures of ribosylated imidazoleacetic acids in rat, bovine, and human tissues to be imidazole-4-acetic acid-ribotide (IAA-RP) and its metabolite, imidazole-4-acetic acid-riboside. We also found that IAA-RP has physicochemical properties similar to those of an unidentified substance(s) extracted from mammalian tissues that interacts with imidazol(in)e receptors (I-Rs). ["Imidazoline," by consensus (International Union of Pharmacology), includes imidazole, imidazoline, and related compounds. We demonstrate that the imidazole IAA-RP acts at I-Rs, and because few (if any) imidazolines exist in vivo, we have adopted the term "imidazol(in)e-Rs."] The latter regulate multiple functions in the CNS and periphery. We now show that IAA-RP (i) is present in brain and tissue extracts that exhibit I-R activity; (ii) is present in neurons of brainstem areas, including the rostroventrolateral medulla, a region where drugs active at I-Rs are known to modulate blood pressure; (iii) is present within synaptosome-enriched fractions of brain where its release is Ca(2+)-dependent, consistent with transmitter function; (iv) produces I-R-linked effects in vitro (e.g., arachidonic acid and insulin release) that are blocked by relevant antagonists; and (v) produces hypertension when microinjected into the rostroventrolateral medulla. Our data also suggest that IAA-RP may interact with a novel imidazol(in)e-like receptor at this site. We propose that IAA-RP is a neuroregulator acting via I-Rs.
Collapse
Affiliation(s)
- George D Prell
- Department of Pharmacology and Biological Chemistry, Mount Sinai School of Medicine, New York, NY 10029, USA.
| | | | | | | | | | | | | | | | | |
Collapse
|
21
|
Cooper EJ, Hudson AL, Parker CA, Morgan NG. Effects of the beta-carbolines, harmane and pinoline, on insulin secretion from isolated human islets of Langerhans. Eur J Pharmacol 2004; 482:189-96. [PMID: 14660022 DOI: 10.1016/j.ejphar.2003.09.039] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
It is well known that certain imidazoline compounds can stimulate insulin secretion and this has been attributed to the activation of imidazoline I(3) binding sites in the pancreatic beta-cell. Recently, it has been proposed that beta-carbolines may be endogenous ligands having activity at imidazoline sites and we have, therefore, studied the effects of beta-carbolines on insulin secretion. The beta-carbolines harmane, norharmane and pinoline increased insulin secretion two- to threefold from isolated human islets of Langerhans. The effects of harmane and pinoline were dose-dependent (EC(50): 5 and 25 microM, respectively) and these agents also blocked the inhibitory effects of the potassium channel agonist, diazoxide, on glucose-induced insulin release. Stimulation of insulin secretion by harmane was glucose-dependent but, unlike the imidazoline I(3) receptor agonist efaroxan, it increased the rate of insulin release beyond that elicited by 20 mM glucose (20 mM glucose alone: 253+/-34% vs. basal; 20 mM glucose plus 100 microM harmane: 327+/-15%; P<0.01). Stimulation of insulin secretion by harmane was attenuated by the imidazoline I(3) receptor antagonist KU14R (2 (2-ethyl 2,3-dihydro-2-benzofuranyl)-2-imidazole) and was reduced when islets were treated with efaroxan for 18 h, prior to the addition of harmane. The results reveal that beta-carbolines can potentiate the rate of insulin secretion from human islets and suggest that these agents may be useful prototypes for the development of novel insulin secretagogues.
Collapse
Affiliation(s)
- E Jane Cooper
- Cellular Pharmacology Group, School of Life Sciences, Keele University, Staffs ST5 5BG, UK
| | | | | | | |
Collapse
|
22
|
Rustenbeck I, Krautheim A, Jörns A, Steinfelder HJ. β-Cell toxicity of ATP-sensitive K+ channel-blocking insulin secretagogues. Biochem Pharmacol 2004; 67:1733-41. [PMID: 15081872 DOI: 10.1016/j.bcp.2004.01.016] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/18/2003] [Accepted: 01/16/2004] [Indexed: 11/27/2022]
Abstract
A prolonged exposure of isolated pancreatic islets to insulin secretagogues, the imidazolines phentolamine, alinidine and idazoxan (100microM each), the sulfonylurea tolbutamide (500microM), or the alkaloid quinine (100microM) resulted in morphological damage of 4-18% of beta-cells compared to less than 2% in controls. Thus, the question arose whether K(ATP) channel-blocking insulin secretagogues are beta-cell toxic as has already been suggested for sulfonylureas. The concentration- and time-dependency of the secretagogue-associated toxicity was documented by viability assays in insulin-secreting HIT T15 cells. Treatment for 24h with idazoxan reduced MTT conversion by 50% at 100microM and by 98% at 1000microM. Phentolamine and quinine reduced viability comparably at 1000microM, but were less toxic at 100microM. On the other hand, the imidazoline alinidine and the sulfonylurea tolbutamide were only moderately toxic (less than 40% viability loss at 1000microM). The imidazoline efaroxan appeared even to be non-toxic. Apoptotic DNA fragmentation and DEVD-caspase activation was observed at 100microM of idazoxan and phentolamine, whereas at 1000microM signs of necrosis predominated. Alinidine, tolbutamide and quinine treatment did not increase markers of apoptotic cell death. Blocking Ca(2+) influx by D600 did not diminish secretagogue-associated toxicity. Electron microscopy confirmed the validity of these observations for beta-cells in intact mouse islets. In summary, beta-cell toxicity of the tested insulin secretagogues varied widely and did not depend on a prolonged Ca(2+) influx via L-type Ca(2+) channels. Thus, secretagogue-mediated closure of K(ATP) channels is apparently not per se beta-cell toxic.
Collapse
Affiliation(s)
- Ingo Rustenbeck
- Institute of Pharmacology and Toxicology, Technical University of Braunschweig, D-38106 Braunschweig, Germany.
| | | | | | | |
Collapse
|
23
|
Velliquette RA, Ernsberger P. The role of I(1)-imidazoline and alpha(2)-adrenergic receptors in the modulation of glucose metabolism in the spontaneously hypertensive obese rat model of metabolic syndrome X. J Pharmacol Exp Ther 2003; 306:646-57. [PMID: 12756274 DOI: 10.1124/jpet.103.050468] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
We examined glucose metabolism after I1-imidazoline (I1R) and alpha2-adrenergic receptor (alpha2AR) activation in an animal model of metabolic syndrome X. Fasted spontaneously hypertensive obese rats (SHROB) were given the I1R/alpha2AR agonists moxonidine and rilmenidine or the alpha2AR agonist guanabenz. Because of the dual specificity of moxonidine, its actions were split into adrenergic and nonadrenergic components by using selective antagonists: rauwolscine (alpha2AR) efaroxan (I1R/alpha2AR), or 2-endo-amino-3-exo-isopropylbicyclo[2.2.1.]heptane (AGN 192403) (I1R). Hyperglycemia induced by moxonidine, rilmenidine, and guanabenz resulted from inhibition of insulin secretion. Similar responses were observed after oral dosing and in lean littermates. Glucagon was reduced by the I1R agonists (moxonidine, 32 +/- 5%; rilmenidine, 24 +/- 7%) but elevated by guanabenz (71 +/- 32%). The hyperglycemic and hypoinsulinemic responses to moxonidine were blocked by rauwolscine. In contrast, rauwolscine potentiated the reduction in glucagon (39 +/- 6%). AGN 193402 blocked the glucagon response without affecting hyperglycemia and hypoinsulinemia. Efaroxan blocked all responses to moxonidine. When SHROB rats were treated with moxonidine 15 min before an oral glucose tolerance test, the glucose area under the curve (AUC) was increased. Antagonizing the alpha2AR component of moxonidine's action with rauwolscine improved glucose AUC 3-fold and facilitated the insulin secretory response and reduced glucagon secretion. Testing fasting glucose and insulin during 3 weeks of oral moxonidine revealed early hyperglycemia that later faded, and a progressive drop in fasting insulin. The acute hyperglycemia and hypoinsulinemia elicited by moxonidine and rilmenidine was mediated by alpha2AR, whereas I1R may reduce glucagon and increase insulin, particularly after a glucose load.
Collapse
Affiliation(s)
- Rodney A Velliquette
- Department of Nutrition, Case Western Reserve University School of Medicine, Cleveland, OH 44106-4906, USA
| | | |
Collapse
|
24
|
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.
Collapse
Affiliation(s)
- Gaëll Mayer
- Department of Pharmacology, University of Bristol, School of Medical Sciences, University Walk, Bristol BS8 1TD, UK
| | | |
Collapse
|
25
|
Greney H, Urosevic D, Schann S, Dupuy L, Bruban V, Ehrhardt JD, Bousquet P, Dontenwill M. [125I]2-(2-chloro-4-iodo-phenylamino)-5-methyl-pyrroline (LNP 911), a high-affinity radioligand selective for I1 imidazoline receptors. Mol Pharmacol 2002; 62:181-91. [PMID: 12065769 DOI: 10.1124/mol.62.1.181] [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: 11/22/2022] Open
Abstract
The I1 subtype of imidazoline receptors (I1R) is a plasma membrane protein that is involved in diverse physiological functions. Available radioligands used so far to characterize the I(1)R were able to bind with similar affinities to alpha2-adrenergic receptors (alpha2-ARs) and to I1R. This feature was a major drawback for an adequate characterization of this receptor subtype. New imidazoline analogs were therefore synthesized and the present study describes one of these compounds, 2-(2-chloro-4-iodo-phenylamino)-5-methyl-pyrroline (LNP 911), which was of high affinity and selectivity for the I1R. LNP 911 was radioiodinated and its binding properties characterized in different membrane preparations. Saturation experiments with [125I]LNP 911 revealed a single high affinity binding site in PC-12 cell membranes (K(D) = 1.4 nM; B(max) = 398 fmol/mg protein) with low nonspecific binding. [125I]LNP 911 specific binding was inhibited by various imidazolines and analogs but was insensitive to guanosine-5'-O-(3-thio)triphosphate. The rank order of potency of some competing ligands [LNP 911, PIC, rilmenidine, 4-chloro-2-(imidazolin-2-ylamino)-isoindoline (BDF 6143), lofexidine, and clonidine] was consistent with the definition of [125I]LNP 911 binding sites as I1R. However, other high-affinity I1R ligands (moxonidine, efaroxan, and benazoline) exhibited low affinities for these binding sites in standard binding assays. In contrast, when [125I]LNP 911 was preincubated at 4 degrees C, competition curves of moxonidine became biphasic. In this case, moxonidine exhibited similar high affinities on [125I]LNP 911 binding sites as on I1R defined with [125I]PIC. Moxonidine proved also able to accelerate the dissociation of [125I]LNP 911 from its binding sites. These results suggest the existence of an allosteric modulation at the level of the I1R, which seems to be corroborated by the dose-dependent enhancement by LNP 911 of the agonist effects on the adenylate cyclase pathway associated to I1R. Because [125I]LNP 911 was unable to bind to the I2 binding site and alpha2AR, our data indicate that [125I]LNP 911 is the first highly selective radioiodinated probe for I1R with a nanomolar affinity. This new tool should facilitate the molecular characterization of the I1 imidazoline receptor.
Collapse
Affiliation(s)
- Hugues Greney
- Laboratoire de Neurobiologie et Pharmacologie Cardiovasculaire, Faculté de Medecine, Strasbourg, France
| | | | | | | | | | | | | | | |
Collapse
|
26
|
Abstract
Desensitization of insulin secretion describes a reversible state of decreased secretory responsiveness of the pancreatic beta-cell, induced by a prolonged exposure to a multitude of stimuli. These include the main physiological stimulator, glucose, but also other nutrients like free fatty acids and practically all pharmacological stimulators acting by depolarization and Ca2+ influx into the beta-cell. Desensitization of insulin secretion appears to be an important step in the manifestation of type 2 diabetes and in the secondary failure of oral antidiabetic treatment. In this commentary, the basic concepts and the controversial issues in the field will be outlined. With regard to glucose-induced desensitization, two fundamentally opposing concepts have emerged. The first is that desensitization is the consequence of functional changes in the beta-cell that impair glucose-recognition. The second is that long-term increased secretory activity leads to a depletion of releasable insulin, often in spite of increased insulin synthesis. The latter concept is more appropriately termed beta-cell exhaustion. The same dichotomy applies to the desensitization evoked by pharmacological stimuli: again the relative contributions of a decreased insulin content versus alterations in signal transduction are in dispute. The action of tolbutamide on beta-cells may be an example of desensitization caused by a lack of releasable insulin since the signaling mechanisms are nearly unchanged, whereas the action of phentolamine, an imidazoline, induces a strong desensitization without reducing insulin content or secretory granules, apparently by abolishing Ca2+ influx. With pharmacological agents it seems that both, alterations in signal transduction and decreased availability of releasable insulin, can contribute to the desensitized state of the beta-cell, the relative contribution being variable depending upon the exact nature of the secretory stimulus.
Collapse
Affiliation(s)
- Ingo Rustenbeck
- Institute of Pharmacology and Toxicology, Technical University of Braunschweig, Mendelssohnstr. 1, D-38106, Braunschweig, Germany.
| |
Collapse
|
27
|
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.
Collapse
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:
| |
Collapse
|
28
|
Komatsu M, Sato Y, Yamada S, Yamauchi K, Hashizume K, Aizawa T. Triggering of insulin release by a combination of cAMP signal and nutrients: an ATP-sensitive K+ channel-independent phenomenon. Diabetes 2002; 51 Suppl 1:S29-32. [PMID: 11815454 DOI: 10.2337/diabetes.51.2007.s29] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/13/2022]
Abstract
Nutrient augmentation of Ca(2+)-triggered insulin release occurs in an ATP-sensitive K(+) (K(ATP)) channel--independent manner. Here, using rat islets, we explored the possibility of the K(ATP) channel-independent nutrient triggering of insulin release. In the presence of 250 micromol/l diazoxide, simultaneous application of forskolin and 16.7 mmol/l glucose strongly stimulated insulin release: fourfold and eightfold increases with 1 and 30 micromol/l forskolin, respectively. alpha-Ketoisocaproate (KIC) and 3-isobutylmethylxanthine (IBMX) could be used in place of glucose and forskolin, respectively, to trigger insulin release in the presence of diazoxide. Triggering of insulin release by a combination of nutrients and forskolin was not attenuated by 10 micromol/l nifedipine (a blocker of voltage-dependent Ca(2+) channels) and 2 micromol/l thapsigargin (an inhibitor of intracellular Ca(2+)-ATPase), ascertaining independence of this phenomenon from Ca(2+) entry and from intracellular Ca(2+) liberation. As anticipated, the action of glucose and KIC was greatly (>80%) suppressed by inhibition of mitochondrial metabolism by 2 mmol/l sodium azide (NaN(3)). A combination of palmitate and dimethyl glutamate (a cell-permeable glutamate donor), but not either one alone, weakly but unequivocally triggered insulin release when applied simultaneously with forskolin. In this case, however, mitochondrial poisoning by azide was without effect. The finding suggests that a combination of induced palmitoylation and cytosolic glutamate accumulation partially reconstituted signaling beyond mitochondrial metabolism in the beta-cell upon glucose stimulation. In conclusion, a combination of cAMP signal and nutrients potently triggers insulin release under full activation of the K(ATP) channel, indicating the multiplicity of driving force for insulin exocytosis.
Collapse
Affiliation(s)
- Mitsuhisa Komatsu
- Department of Aging Medicine and Geriatrics, Shinshu University School of Medicine, Matsumoto, Japan.
| | | | | | | | | | | |
Collapse
|