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KATP channel pharmacology in the pancreas and the cardiovascular system
International Congress Series 1253 (2003) 279 – 287
KATP channel pharmacology in the pancreas and the cardiovascular system Fiona M. Gribble *, Frank Reimann Department of Clinical Biochemistry, University of Cambridge, Addenbrooke’s Hospital, Box 232, Hills Road, Cambridge CB2 2QR, UK Received 22 October 2002; accepted 7 February 2003
Abstract ATP-sensitive K (KATP) channels are the targets for a number of therapeutic drugs, including the sulphonylureas used to treat type 2 diabetes, and KATP channel openers used to treat hypertension and angina. The tissue selectivity of agents that modulate KATP channels is due to the differential expression of sulphonylurea receptor (SUR) subunits (SUR1 in pancreatic h-cells, SUR2A in cardiac and skeletal muscles, and SUR2B in smooth muscle). Simple sulphonylureas such as tolbutamide and gliclazide block channels containing SUR1, but not those containing SUR2, with high affinity. Glibenclamide, glimepiride and repaglinide, by contrast, inhibit both SUR1- and SUR2-containing channels. Differences in drug selectivity and reversibility have provided insight into the nature of the drug binding pocket on the sulphonylurea receptor subunits. This will be the basis for the development of more tissue-specific drugs, reducing the risk of unwanted side-effects. D 2003 Elsevier Science B.V. All rights reserved. Keywords: KATP channel; Sulphonylurea receptor; Kir6.2; Sulphonylureas
1. Introduction ATP-sensitive K+ (KATP) channels are found in a number of tissues including pancreatic h-cells, cardiac, smooth and skeletal muscles, and neurones of the central nervous system. They are the targets for several clinically important groups of drugs, including the sulphonylureas used to treat type 2 diabetes, and the KATP channel openers used to treat angina and hypertension [1,2]. The normal physiological role of KATP channels is to
* Corresponding author. Tel.: +44-1223-762626; fax: +44-1223-330598. E-mail address: [email protected] (F.M. Gribble). 0531-5131/03 D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0531-5131(03)00132-8
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couple the electrical activity of a cell to its metabolic rate, and it is believed that they do so by sensing changes in the concentrations of adenine nucleotides (ATP and ADP). In pancreatic h-cells the concentration of ATP ([ATP]) increases when the plasma glucose level rises, and the resulting closure of KATP channels in the plasma membrane allows the cells to depolarise, triggering calcium entry and insulin release [3]. The reciprocal fall in [ADP] contributes to KATP channel closure, as MgADP is a potent activator of these channels. In cardiac myocytes, ATP concentrations fall during hypoxia, resulting in opening of sarcolemmal KATP channels and thereby shortening the cardiac action potential and reducing cardiac work [4]. KATP channels in vascular smooth muscle sense changes in oxygen or neurotransmitters, and their opening results in vasodilatation [5]. There is also evidence for an inner mitochondrial membrane KATP channel which may be involved in cardiac ischaemic preconditioning [6].
2. Structural features of KATP channels KATP channels are formed from K+ channel (Kir6.x) subunits and regulatory sulphonylurea receptors (SUR), which are arranged in a stoichiometry of 4:4 (Fig. 1) [7,8]. Two genes encoding Kir6.x subunits have been cloned: Kir6.2 is widely expressed and forms the KATP channel pore in pancreatic h-cells, cardiac and skeletal muscles, and neurones [9,10]. Kir6.1, by contrast, acts as the pore-forming subunit in vascular smooth muscle and astrocytes [11– 14]. SUR is a member of the ATP binding cassette (ABC) transporter family and is predicted to possess two intracellular nucleotide binding domains (NBD1 and NBD2) and three transmembrane domains (TMD0, TMD1 and TMD2) containing 5, 6 and 6 transmembrane helices (TMs), respectively [15,16]. Two genes for sulphonylurea receptors have been identified, encoding the proteins SUR1 and SUR2 which are expressed in different tissues [17 – 19]. SUR1 is predominantly found in pancreatic h-cells and neurones. Alternative splicing of SUR2
Fig. 1. Schematic representation of KATP channel structure, showing Kir6.2 and SUR1 subunits. Nucleotide binding domains (NBDs) and the position of residue S1237 are indicated.
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produces a cardiac/skeletal muscle isoform (SUR2A) and a smooth muscle isoform (SUR2B) [18,19].
3. Mutations in KATP channel subunits cause congenital hyperinsulinism Congenital hyperinsulinism (CHI), or persistent hyperinsulinaemic hypoglycaemia of infancy (PHHI), is a condition of unregulated excessive insulin release resulting in persistent life-threatening hypoglycaemia. Inheritance of familial forms is usually autosomal recessive, but may be autosomal dominant, and sporadic cases are common [20 – 22]. Familial forms predominate in isolated populations, such as remote areas of Finland, or in ethnic groups such as the Ashkenazi Jews, in which consanguinous marriages are more common. The genetic basis has been established in f 50% of cases, the majority of mutations occurring in genes encoding the h-cell KATP channel subunits, SUR1 and Kir6.2, but a smaller number having been located in genes encoding glutamate dehydrogenase and glucokinase [20,21]. Subjects with mutations in glutamate dehydrogenase usually present with a condition of hyperinsulinism and hyperammonaemia. Over 40 disease-causing mutations in SUR1 have now been identified, resulting in premature stop codons, trafficking defects or loss of sensitivity to the endogenous channel activator, MgADP [20,22]. Loss of functional KATP channels in the plasma membrane causes continuous depolarisation of the h-cell membrane, persistently raised intracellular calcium concentrations and unregulated insulin release [23]. Importantly, the failure of KATP channels to open at low glucose concentrations prevents insulin secretion from being switched off, even at dangerous levels of hypoglycaemia.
4. Therapeutic modulation of KATP channel activity 4.1. Introduction Pharmacological inhibitors of h-cell KATP channels have been used for many years to treat type 2 diabetes [1]. Closing KATP channels in the h-cell induces membrane depolarisation and triggers insulin secretion, even when there is no increase in the metabolic rate. The classical therapeutic KATP channel inhibitors are the sulphonylureas, a group which includes tolbutamide, glibenclamide, gliclazide and glimepiride. Members of a new therapeutic class of agents also target the KATP channel, yet do not possess a sulphonylurea group. This group includes repaglinide, nateglinide and mitiglinide, which are structurally related to the benzamido compound, meglitinide. KATP channel openers targeting the pancreatic h-cell are used therapeutically to reduce insulin secretion, e.g. in some subjects with congenital hyperinsulinism [20 – 22]. Agents such as nicorandil, which open KATP channels in vascular smooth muscle, by contrast, are increasingly used in the treatment of angina and hypertension [24]. The therapeutic usefulness of such drugs relies on their selectivity for muscle types of KATP channel because cross-reactivity with h-cell KATP channels would impair insulin release.
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4.2. KATP channel inhibitors 4.2.1. Selectivity The underlying basis for the different pharmacological properties of channels containing SUR1, SUR2A and SUR2B has been studied by expressing cloned KATP channels in Xenopus oocytes. Following microinjection of oocytes with mRNAs encoding Kir6.2 and different SUR subunits, functional KATP channel complexes in the plasma membrane can be studied by the patch-clamp technique. Macroscopic currents, reflecting the flow of K+ ions through thousands of open KATP channels in a single giant membrane patch, provide a robust method to compare drug sensitivities under identical experimental conditions. In the Xenopus oocyte, as in other expression systems, the inhibition of KATP channels by sulphonylureas and related agents is best described by the effects of high and low affinity binding sites [25 –31]. The high affinity component of channel inhibition occurs at therapeutic drug concentrations and is mediated by drug binding to the sulphonylurea receptor [17]. The low affinity component, which is only observed at suprapharmacological drug levels, is independent of SUR and may involve direct drug interaction with the Kir subunit [25,32]. Whilst the low affinity component is of no clinical significance, it is important in the interpretation of dose –response curves measured in vitro, which extend to very high drug concentrations. The high and low affinity components of channel inhibition are clearly identifiable on the gliclazide dose – response curve for Kir6.2/SUR1 currents (Fig. 2), as the IC50s of the two sites are widely separated for this drug [28]. Another feature of the dose –response curve is that high affinity interaction of sulphonylureas with SUR only reduces the current amplitude by f 50% in excised membrane patches. This incomplete block of Kir6.2/ SUR1 currents is a consistent finding for all KATP channel inhibitors that bind to the sulphonylurea receptor, and is also observed with Kir6.2/SUR2 currents if the agent interacts at high affinity with SUR2 (see below) [25 –31]. Analysis of single channel
Fig. 2. Dose – response relation for gliclazide inhibition of Kir6.2/SUR1 currents. The conductance in the presence of drug ( G) is expressed relative to that in control solution lacking drug ( Gc). Error bars indicate 1 S.E.M. From Ref. [28].
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kinetics suggests that this incomplete block arises because KATP channels can still open even when sulphonylurea is bound to SUR [33]. Whilst a number of different agents have been shown to block KATP channels in vitro, those in clinical use do so by interacting with the sulphonylurea receptor [1]. They may be divided into those that are selective for SUR1, and those that target both SUR1 and SUR2. Studies on recombinant KATP channels have shown that gliclazide, tolbutamide and mitiglinide inhibit Kir6.2/SUR1 currents with high affinity, but show only low affinity block of Kir6.2/SUR2 currents [27,28,31]. By contrast, glibenclamide, glimepiride, meglitinide (a molecule corresponding to the non-sulphonylurea half of glibenclamide) and repaglinide block both Kir6.2/SUR1 and Kir6.2/SUR2 channels with similar high affinity [27,29,30]. 4.2.2. Reversibility KATP channel inhibitors exhibit different degrees of reversibility [25 – 31]. Inhibition of Kir6.2/SUR1 channel by gliclazide and tolbutamide is readily reversible, whereas the block by glibenclamide, glimepiride and repaglinide is only very slowly reversible. Inhibition of Kir6.2/SUR2 channels by repaglinide was also found to be slowly reversible, whereas block by glibenclamide and glimepiride was rapidly reversible. Drugs that inhibit SUR1 reversibly (tolbutamide, gliclazide, meglitinide) are smaller and possess fewer hydrophobic rings than those which are only slowly reversible (glibenclamide, glimepiride, repaglinide). An explanation for the selectivity and reversibility data may be that simple sulphonylureas (tolbutamide, gliclazide) associate with slightly different parts of the binding site on SUR1 compared with the benzamido derivative, meglitinide. The binding site on SUR2 appears to accommodate benzamido derivatives but not sulphonylureas. As glibenclamide contains both sulphonylurea and meglitinide moieties, it may bind to SUR1 with both halves of the drug, but to SUR2 only by association of the meglitinide moiety. Binding of both halves of glibenclamide to SUR1 may be responsible for its slow dissociation. The extra hydrophobic rings of repaglinide, which lacks a sulphonylurea group, may be responsible for its slow dissociation from both SUR1 and SUR2. 4.2.3. Location of the KATP channel inhibitor binding site The different sensitivities of SUR1 and SUR2 to tolbutamide and gliclazide made it possible to use SUR1/SUR2 chimeras to identify regions of SUR1 which are important for sulphonylurea action [34]. Functional studies in Xenopus oocytes showed that high affinity inhibition by tolbutamide and gliclazide was only observed when TMs 13 –16 originated from SUR1 [34,35]. Glibenclamide and glimepiride, which block both SUR1 and SUR2, were effective on all the chimeras but were irreversible only in those channels that were blocked by tolbutamide [34,35]. Such findings are consistent with the model suggested above, as glibenclamide and glimepiride would be expected to bind more tightly when the sulphonylurea moiety is accommodated in the binding site. The underlying basis for the different sulphonylurea sensitivities of SUR1 and SUR2 was addressed by mutational analysis of TMs 13– 16 [34]. When residue S1237 in SUR1 (Fig. 1) was mutated to its SUR2A counterpart (tyrosine), high affinity block by
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tolbutamide was abolished [34]. The mutation also caused glibenclamide block to become reversible and reduced the binding of [3H] glibenclamide, but did not affect current inhibition by meglitinide. Channels containing the reverse mutation (Y1206S) in SUR2B are more potently blocked by glibenclamide than those containing wild type SUR2B, and exhibit increased [3H] glibenclamide binding affinity [36]. The results suggest that S1237 in SUR1 may form part of the sulphonylurea binding pocket. The location of S1237 in an intracellular loop of SUR1 is consistent with a previous report that sulphonylureas access their binding site from the intracellular aspect of the membrane [37]. KATP channel inhibitors can be grouped according to whether they are affected by the S1237Y mutation in SUR1. As discussed above, Kir6.2/SUR1 –S1237Y channels are blocked normally by meglitinide, are only reversibly blocked by glibenclamide, and are unaffected by tolbutamide or gliclazide. Further studies have shown that inhibition of Kir6.2/SUR1 channels by repaglinide is unaffected by mutation of S1237Y [30], but that high-affinity block by nateglinide and mitiglinide is impaired [31,38]. The results suggest that although nateglinide and mitiglinide do not possess a sulphonylurea group, their binding site on SUR1 overlaps significantly with that of simple sulphonylureas such as tolbutamide and gliclazide. 4.2.4. Modulation of KATP channel inhibition by nucleotides The extent of KATP channel inhibition by sulphonylureas is modified in vivo by the presence of nucleotides. As discussed above, KATP channels containing either SUR1 or SUR2 are blocked f 50% by glibenclamide in excised patches in the absence of added nucleotide. Physiological concentrations of MgADP (100 AM) enhance the block of Kir6.2/SUR1 currents but impair that of Kir6.2/SUR2A currents [25,27]. The physiological consequence of this is unclear, but the data suggest that glibenclamide would have little effect in vivo on KATP channels in the cardiovascular system activated by rising MgADP levels during metabolic stress. This contrasts with the ability of glibenclamide to inhibit cardiovascular-type KATP channels activated by nicorandil and related drugs [39 – 41]. The effect in vivo of therapeutic concentrations of glibenclamide on cardiovascular KATP channels may therefore depend on whether the channels have been activated by KATP channel opening drugs, MgADP (metabolic stress) or a separate mechanism (e.g. neurotransmitters). Further studies are still required to clarify the relative contributions of these pathways under pathophysiological conditions.
5. Conclusion Modulation of KATP channel activity is an important therapeutic tool for the treatment of type 2 diabetes and cardiovascular disease. Drugs have been developed which selectively inhibit KATP channels in pancreatic h-cells, or open those in vascular smooth and cardiac muscles. This tissue selectivity is due to the expression of different types of sulphonylurea receptor. Studies on recombinant channels expressed in vitro have provided important insights into the molecular mechanisms of drug action on KATP channels.
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Acknowledgements We thank the Wellcome Trust and Diabetes UK for their support. FMG is a Wellcome Clinician Scientist Fellow, and FR is a Diabetes UK RD Lawrence Fellow. References [1] F.M. Ashcroft, F.M. Gribble, ATP-sensitive K+ channels in health and disease, Diabetologia 42 (1999) 903 – 919. [2] F.M. Ashcroft, F.M. Gribble, New windows on the mechanism of action of KATP channel openers, Trends Pharmacol. Sci. 21 (2000) 439 – 445. [3] P. Rorsman, The pancreatic beta-cell as a fuel sensor: an electrophysiologist’s viewpoint, Diabetologia 40 (1997) 487 – 495. [4] C.G. Nichols, W.J. Lederer, Adenosine triphosphate-sensitive potassium channels in the cardiovascular system, Am. J. Physiol. 261 (1991) H1675 – H1686. [5] J.M. Quayle, M.T. Nelson, N.B. Standen, ATP-sensitive and inwardly-rectifying potassium channels in smooth muscle, Physiol. Rev. 77 (1997) 1165 – 1232. [6] G.J. Grover, K.D. Garlid, ATP-sensitive potassium channels: a review of their cardioprotective pharmacology, J. Mol. Cell. Cardiol. 32 (2000) 677 – 695. [7] J.P. Clement, K. Kunjilwar, G. Gonzalez, M. Schwanstecher, U. Panten, L. Aguilar-Bryan, J. Bryan, Association and stoichiometry of KATP channel subunits, Neuron 18 (1997) 827 – 838. [8] S.-L. Shyng, C.G. Nichols, Octameric stochiometry of the KATP channel complex, J. Gen. Physiol. 110 (1997) 655 – 664. [9] H. Sakura, C. Ammala, P.A. Smith, F.M. Gribble, F.M. Ashcroft, Cloning and functional expression of the cDNA encoding a novel ATP-sensitive potassium channel subunit expressed in pancreatic beta-cells, brain, heart and skeletal muscle, FEBS Lett. 377 (1995) 338 – 344. [10] N. Inagaki, T. Gonoi, J.P. Clement, N. Namba, J. Inazawa, G. Gonzalez, L. Aguilar-Bryan, S. Seino, J. Bryan, Reconstitution of IKATP—an inward rectifier subunit plus the sulfonylurea receptor, Science 270 (1995) 1166 – 1170. [11] N. Inagaki, Y. Tsuura, N. Namba, K. Masuda, T. Gonoi, M. Horie, Y. Seino, M. Mizuta, S. Seino, Cloning and functional characterization of a novel ATP-sensitive potassium channel ubiquitously expressed in rat tissues, including pancreatic islets, pituitary, skeletal muscle, and heart, J. Biol. Chem. 270 (1995) 5691 – 5694. [12] M. Yamada, S. Isomoto, S. Matsumoto, C. Kondo, T. Shindo, Y. Horio, Y. Kurachi, Sulphonylurea receptor 2B and Kir6.1 form a sulphonylurea-sensitive but ATP-insensitive K+ channel, J. Physiol. 499 (1997) 715 – 720. [13] T. Miki, M. Suzuki, T. Shibasaki, H. Uemura, T. Sato, K. Yamaguchi, H. Koseki, T. Iwanaga, H. Nakaya, S. Seino, Mouse model of Prinzmetal angina by disruption of the inward rectifier Kir6.1, Nat. Med. 8 (2002) 466 – 472. [14] A. Thomzig, M. Wenzel, C. Karschin, M.J. Eaton, S.N. Skatchkov, A. Karschin, R.W. Veh, Kir6.1 is the principal pore-forming subunit of astrocyte but not neuronal plasma membrane K-ATP channels, Mol. Cell. Neurosci. 18 (2001) 671 – 690. [15] G.E. Tusnady, E. Bakos, A. Varadi, B. Sarkadi, Membrane topology distinguishes a subfamily of the ATPbinding cassette (ABC) transporters, FEBS Lett. 402 (1997) 1 – 3. [16] L.R. Conti, C.M. Radeke, S.L. Shyng, C.A. Vandenberg, Transmembrane topology of the sulfonylurea receptor SUR1, J. Biol. Chem. 276 (2001) 41270 – 41278. [17] L. Aguilar-Bryan, C.G. Nichols, S.W. Wechsler, J.P. Clement, A.E. Boyd, G. Gonzalez, S.H. Herrera, K. Nguy, J. Bryan, D.A. Nelson, Cloning of the beta cell high-affinity sulfonylurea receptor: a regulator of insulin secretion, Science 268 (1995) 423 – 426. [18] N. Inagaki, T. Gonoi, J.P. Clement, C.Z. Wang, L. Aguilar-Bryan, J. Bryan, S. Seino, A family of sulfonylurea receptors determines the pharmacological properties of ATP-sensitive K+ channels, Neuron 16 (1996) 1011 – 1017.
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[19] S. Isomoto, C. Kondo, M. Yamada, S. Matsumoto, O. Higashiguchi, Y. Horio, Y. Matsuzawa, Y. Kurachi, A novel sulfonylurea receptor forms with BIR (Kir6.2) a smooth muscle type ATP-sensitive K+ channel, J. Biol. Chem. 271 (1996) 24321 – 24324. [20] B. Glaser, P. Thornton, T. Otonkoski, C. Junien, Genetics of neonatal hyperinsulinism, Arch. Dis. Child., Fetal Neonatal Ed. 82 (2000) F79 – F86. [21] P. de Lonlay, J.C. Fournet, G. Touati, M.S. Groos, D. Martin, C. Sevin, V. Delagne, C. Mayaud, V. Chigot, C. Sempoux, M.C. Brusset, K. Laborde, C. Bellane-Chantelot, A. Vassault, J. Rahier, C. Junien, F. Brunelle, C. Nihoul-Fekete, J.M. Saudubray, J.J. Robert, Heterogeneity of persistent hyperinsulinaemic hypoglycaemia. A series of 175 cases, Eur. J. Pediatr. 161 (2002) 37 – 48. [22] N. Sharma, A. Crane, G. Gonzalez, J. Bryan, L. Aguilar-Bryan, Familial hyperinsulinism and pancreatic hcell ATP-sensitive potassium channels, Kidney Int. 57 (2000) 803 – 808. [23] C. Kane, R.M. Shepherd, P.E. Squires, P.R. Johnson, R.F. James, P.J. Milla, A. Aynsley-Green, K.J. Lindley, M.J. Dunne, Loss of functional KATP channels in pancreatic h-cells causes persistent hyperinsulinemic hypoglycemia of infancy, Nat. Med. 2 (1996) 1344 – 1347. [24] D.J. Patel, H.J. Purcell, K.M. Fox, Cardioprotection by opening of the KATP channel in unstable angina. Is this a clinical manifestation of myocardial preconditioning? Results of a randomized study with nicorandil. CESAR 2 investigation. Clinical European studies in angina and revascularization, Eur. Heart J. 20 (1999) 51 – 57. [25] F.M. Gribble, S.J. Tucker, F.M. Ashcroft, The interaction of nucleotides with the tolbutamide block of KATP currents: a reinterpretation, J. Physiol. 504 (1997) 35 – 45. [26] J.C. Koster, Q. Sha, C.G. Nichols, Sulfonylurea and K+-channel opener sensitivity of KATP channels. Functional coupling of Kir6.2 and SUR1 subunits, J. Gen. Physiol. 114 (1999) 203 – 213. [27] F.M. Gribble, S.J. Tucker, S. Seino, F.M. Ashcroft, Tissue specificity of sulphonylureas: studies on cloned cardiac and beta-cell KATP channels, Diabetes 47 (1998) 1412 – 1418. [28] F.M. Gribble, F.M. Ashcroft, Differential sensitivity of h-cell and extrapancreatic KATP channels to gliclazide, Diabetologia 42 (1999) 845 – 848. [29] D.K. Song, F.M. Ashcroft, Glimepiride block of cloned h-cell, cardiac and smooth muscle K-ATP channels, Br. J. Pharmacol. 133 (2001) 193 – 199. [30] M. Dabrowski, P. Wahl, W.E. Holmes, F.M. Ashcroft, Effect of repaglinide on cloned beta cell, cardiac and smooth muscle types of ATP-sensitive potassium channel, Diabetologia 44 (2001) 747 – 756. [31] F. Reimann, P. Proks, F.M. Ashcroft, Effects of mitiglinide (S 21403) on Kir6.2/SUR1, Kir6.2/ SUR2A and Kir6.2/SUR2B types of ATP-sensitive potassium channel, Br. J. Pharmacol. 132 (2001) 1542 – 1548. [32] L. Gros, A. Virsolvy, G. Salazar, D. Bataille, P. Blache, Characterization of low-affinity binding sites for glibenclamide on the Kir6.2 subunit of the beta-cell KATP channel, Biochem. Biophys. Res. Commun. 257 (1999) 766 – 770. [33] P. Proks, F. Reimann, N. Green, F.M. Gribble, F.M. Ashcroft, Sulphonylurea stimulation of insulin secretion, Diabetes 51 (Suppl. 3) (2002) S368 – S376. [34] R. Ashfield, F.M. Gribble, S.J.H. Ashcroft, F.M. Ashcroft, Identification of the high-affinity tolbutamide site on the SUR1 subunit of the KATP channel, Diabetes 48 (1999) 1341 – 1347. [35] F.M. Gribble, F. Reimann, Pharmacological modulation of KATP channels, Biochem. Soc. Trans. 30 (2002) 333 – 339. [36] A. Hambrock, C. Lo¨ffler-Walz, U. Russ, U. Lange, U. Quast, Characterization of a mutant sulfonylurea receptor SUR2B with high affinity for sulfonylureas and openers: differences in the coupling to Kir6.x subtypes, Mol. Pharmacol. 60 (2001) 190 – 199. [37] M. Schwanstecher, C. Schwanstecher, C. Dickel, F. Chudziak, A. Moshiri, U. Panten, Location of the sulphonylurea receptor at the cytoplasmic face of the beta-cell membrane, Br. J. Pharmacol. 113 (1994) 903 – 911. [38] A.M. Hansen, I.T. Christensen, J.B. Hansen, R.D. Carr, F.M. Ashcroft, P. Wahl, Differential interactions of nateglinide repaglinide on the human beta-cell sulphonylurea receptor 1, Diabetes 51 (2002) 2789 – 2795. [39] F. Reimann, F.M. Ashcroft, F.M. Gribble, Structural basis for the interference between nicorandil and sulfonylurea action, Diabetes 50 (2001) 2253 – 2259.
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[40] P.J. Bijlstra, J.A. Lutterman, F.G.M. Russel, T. Thien, P. Smits, Interaction of sulphonylurea derivatives with vascular ATP-sensitive potassium channels in humans, Diabetologia 39 (1996) 1083 – 1090. [41] C.L. Lawrence, P. Proks, G.C. Rodrigo, P. Jones, Y. Hayabuchi, N.B. Standen, F.M. Ashcroft, Gliclazide produces high-affinity block of KATP channels in mouse isolated pancreatic h-cells but not rat heart or arterial smooth muscle cells, Diabetologia 44 (2001) 1019 – 1025.
KATP channel pharmacology in the pancreas and the cardiovascular system Fiona M. Gribble *, Frank Reimann Department of Clinical Biochemistry, University of Cambridge, Addenbrooke’s Hospital, Box 232, Hills Road, Cambridge CB2 2QR, UK Received 22 October 2002; accepted 7 February 2003
Abstract ATP-sensitive K (KATP) channels are the targets for a number of therapeutic drugs, including the sulphonylureas used to treat type 2 diabetes, and KATP channel openers used to treat hypertension and angina. The tissue selectivity of agents that modulate KATP channels is due to the differential expression of sulphonylurea receptor (SUR) subunits (SUR1 in pancreatic h-cells, SUR2A in cardiac and skeletal muscles, and SUR2B in smooth muscle). Simple sulphonylureas such as tolbutamide and gliclazide block channels containing SUR1, but not those containing SUR2, with high affinity. Glibenclamide, glimepiride and repaglinide, by contrast, inhibit both SUR1- and SUR2-containing channels. Differences in drug selectivity and reversibility have provided insight into the nature of the drug binding pocket on the sulphonylurea receptor subunits. This will be the basis for the development of more tissue-specific drugs, reducing the risk of unwanted side-effects. D 2003 Elsevier Science B.V. All rights reserved. Keywords: KATP channel; Sulphonylurea receptor; Kir6.2; Sulphonylureas
1. Introduction ATP-sensitive K+ (KATP) channels are found in a number of tissues including pancreatic h-cells, cardiac, smooth and skeletal muscles, and neurones of the central nervous system. They are the targets for several clinically important groups of drugs, including the sulphonylureas used to treat type 2 diabetes, and the KATP channel openers used to treat angina and hypertension [1,2]. The normal physiological role of KATP channels is to
* Corresponding author. Tel.: +44-1223-762626; fax: +44-1223-330598. E-mail address: [email protected] (F.M. Gribble). 0531-5131/03 D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0531-5131(03)00132-8
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couple the electrical activity of a cell to its metabolic rate, and it is believed that they do so by sensing changes in the concentrations of adenine nucleotides (ATP and ADP). In pancreatic h-cells the concentration of ATP ([ATP]) increases when the plasma glucose level rises, and the resulting closure of KATP channels in the plasma membrane allows the cells to depolarise, triggering calcium entry and insulin release [3]. The reciprocal fall in [ADP] contributes to KATP channel closure, as MgADP is a potent activator of these channels. In cardiac myocytes, ATP concentrations fall during hypoxia, resulting in opening of sarcolemmal KATP channels and thereby shortening the cardiac action potential and reducing cardiac work [4]. KATP channels in vascular smooth muscle sense changes in oxygen or neurotransmitters, and their opening results in vasodilatation [5]. There is also evidence for an inner mitochondrial membrane KATP channel which may be involved in cardiac ischaemic preconditioning [6].
2. Structural features of KATP channels KATP channels are formed from K+ channel (Kir6.x) subunits and regulatory sulphonylurea receptors (SUR), which are arranged in a stoichiometry of 4:4 (Fig. 1) [7,8]. Two genes encoding Kir6.x subunits have been cloned: Kir6.2 is widely expressed and forms the KATP channel pore in pancreatic h-cells, cardiac and skeletal muscles, and neurones [9,10]. Kir6.1, by contrast, acts as the pore-forming subunit in vascular smooth muscle and astrocytes [11– 14]. SUR is a member of the ATP binding cassette (ABC) transporter family and is predicted to possess two intracellular nucleotide binding domains (NBD1 and NBD2) and three transmembrane domains (TMD0, TMD1 and TMD2) containing 5, 6 and 6 transmembrane helices (TMs), respectively [15,16]. Two genes for sulphonylurea receptors have been identified, encoding the proteins SUR1 and SUR2 which are expressed in different tissues [17 – 19]. SUR1 is predominantly found in pancreatic h-cells and neurones. Alternative splicing of SUR2
Fig. 1. Schematic representation of KATP channel structure, showing Kir6.2 and SUR1 subunits. Nucleotide binding domains (NBDs) and the position of residue S1237 are indicated.
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produces a cardiac/skeletal muscle isoform (SUR2A) and a smooth muscle isoform (SUR2B) [18,19].
3. Mutations in KATP channel subunits cause congenital hyperinsulinism Congenital hyperinsulinism (CHI), or persistent hyperinsulinaemic hypoglycaemia of infancy (PHHI), is a condition of unregulated excessive insulin release resulting in persistent life-threatening hypoglycaemia. Inheritance of familial forms is usually autosomal recessive, but may be autosomal dominant, and sporadic cases are common [20 – 22]. Familial forms predominate in isolated populations, such as remote areas of Finland, or in ethnic groups such as the Ashkenazi Jews, in which consanguinous marriages are more common. The genetic basis has been established in f 50% of cases, the majority of mutations occurring in genes encoding the h-cell KATP channel subunits, SUR1 and Kir6.2, but a smaller number having been located in genes encoding glutamate dehydrogenase and glucokinase [20,21]. Subjects with mutations in glutamate dehydrogenase usually present with a condition of hyperinsulinism and hyperammonaemia. Over 40 disease-causing mutations in SUR1 have now been identified, resulting in premature stop codons, trafficking defects or loss of sensitivity to the endogenous channel activator, MgADP [20,22]. Loss of functional KATP channels in the plasma membrane causes continuous depolarisation of the h-cell membrane, persistently raised intracellular calcium concentrations and unregulated insulin release [23]. Importantly, the failure of KATP channels to open at low glucose concentrations prevents insulin secretion from being switched off, even at dangerous levels of hypoglycaemia.
4. Therapeutic modulation of KATP channel activity 4.1. Introduction Pharmacological inhibitors of h-cell KATP channels have been used for many years to treat type 2 diabetes [1]. Closing KATP channels in the h-cell induces membrane depolarisation and triggers insulin secretion, even when there is no increase in the metabolic rate. The classical therapeutic KATP channel inhibitors are the sulphonylureas, a group which includes tolbutamide, glibenclamide, gliclazide and glimepiride. Members of a new therapeutic class of agents also target the KATP channel, yet do not possess a sulphonylurea group. This group includes repaglinide, nateglinide and mitiglinide, which are structurally related to the benzamido compound, meglitinide. KATP channel openers targeting the pancreatic h-cell are used therapeutically to reduce insulin secretion, e.g. in some subjects with congenital hyperinsulinism [20 – 22]. Agents such as nicorandil, which open KATP channels in vascular smooth muscle, by contrast, are increasingly used in the treatment of angina and hypertension [24]. The therapeutic usefulness of such drugs relies on their selectivity for muscle types of KATP channel because cross-reactivity with h-cell KATP channels would impair insulin release.
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4.2. KATP channel inhibitors 4.2.1. Selectivity The underlying basis for the different pharmacological properties of channels containing SUR1, SUR2A and SUR2B has been studied by expressing cloned KATP channels in Xenopus oocytes. Following microinjection of oocytes with mRNAs encoding Kir6.2 and different SUR subunits, functional KATP channel complexes in the plasma membrane can be studied by the patch-clamp technique. Macroscopic currents, reflecting the flow of K+ ions through thousands of open KATP channels in a single giant membrane patch, provide a robust method to compare drug sensitivities under identical experimental conditions. In the Xenopus oocyte, as in other expression systems, the inhibition of KATP channels by sulphonylureas and related agents is best described by the effects of high and low affinity binding sites [25 –31]. The high affinity component of channel inhibition occurs at therapeutic drug concentrations and is mediated by drug binding to the sulphonylurea receptor [17]. The low affinity component, which is only observed at suprapharmacological drug levels, is independent of SUR and may involve direct drug interaction with the Kir subunit [25,32]. Whilst the low affinity component is of no clinical significance, it is important in the interpretation of dose –response curves measured in vitro, which extend to very high drug concentrations. The high and low affinity components of channel inhibition are clearly identifiable on the gliclazide dose – response curve for Kir6.2/SUR1 currents (Fig. 2), as the IC50s of the two sites are widely separated for this drug [28]. Another feature of the dose –response curve is that high affinity interaction of sulphonylureas with SUR only reduces the current amplitude by f 50% in excised membrane patches. This incomplete block of Kir6.2/ SUR1 currents is a consistent finding for all KATP channel inhibitors that bind to the sulphonylurea receptor, and is also observed with Kir6.2/SUR2 currents if the agent interacts at high affinity with SUR2 (see below) [25 –31]. Analysis of single channel
Fig. 2. Dose – response relation for gliclazide inhibition of Kir6.2/SUR1 currents. The conductance in the presence of drug ( G) is expressed relative to that in control solution lacking drug ( Gc). Error bars indicate 1 S.E.M. From Ref. [28].
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kinetics suggests that this incomplete block arises because KATP channels can still open even when sulphonylurea is bound to SUR [33]. Whilst a number of different agents have been shown to block KATP channels in vitro, those in clinical use do so by interacting with the sulphonylurea receptor [1]. They may be divided into those that are selective for SUR1, and those that target both SUR1 and SUR2. Studies on recombinant KATP channels have shown that gliclazide, tolbutamide and mitiglinide inhibit Kir6.2/SUR1 currents with high affinity, but show only low affinity block of Kir6.2/SUR2 currents [27,28,31]. By contrast, glibenclamide, glimepiride, meglitinide (a molecule corresponding to the non-sulphonylurea half of glibenclamide) and repaglinide block both Kir6.2/SUR1 and Kir6.2/SUR2 channels with similar high affinity [27,29,30]. 4.2.2. Reversibility KATP channel inhibitors exhibit different degrees of reversibility [25 – 31]. Inhibition of Kir6.2/SUR1 channel by gliclazide and tolbutamide is readily reversible, whereas the block by glibenclamide, glimepiride and repaglinide is only very slowly reversible. Inhibition of Kir6.2/SUR2 channels by repaglinide was also found to be slowly reversible, whereas block by glibenclamide and glimepiride was rapidly reversible. Drugs that inhibit SUR1 reversibly (tolbutamide, gliclazide, meglitinide) are smaller and possess fewer hydrophobic rings than those which are only slowly reversible (glibenclamide, glimepiride, repaglinide). An explanation for the selectivity and reversibility data may be that simple sulphonylureas (tolbutamide, gliclazide) associate with slightly different parts of the binding site on SUR1 compared with the benzamido derivative, meglitinide. The binding site on SUR2 appears to accommodate benzamido derivatives but not sulphonylureas. As glibenclamide contains both sulphonylurea and meglitinide moieties, it may bind to SUR1 with both halves of the drug, but to SUR2 only by association of the meglitinide moiety. Binding of both halves of glibenclamide to SUR1 may be responsible for its slow dissociation. The extra hydrophobic rings of repaglinide, which lacks a sulphonylurea group, may be responsible for its slow dissociation from both SUR1 and SUR2. 4.2.3. Location of the KATP channel inhibitor binding site The different sensitivities of SUR1 and SUR2 to tolbutamide and gliclazide made it possible to use SUR1/SUR2 chimeras to identify regions of SUR1 which are important for sulphonylurea action [34]. Functional studies in Xenopus oocytes showed that high affinity inhibition by tolbutamide and gliclazide was only observed when TMs 13 –16 originated from SUR1 [34,35]. Glibenclamide and glimepiride, which block both SUR1 and SUR2, were effective on all the chimeras but were irreversible only in those channels that were blocked by tolbutamide [34,35]. Such findings are consistent with the model suggested above, as glibenclamide and glimepiride would be expected to bind more tightly when the sulphonylurea moiety is accommodated in the binding site. The underlying basis for the different sulphonylurea sensitivities of SUR1 and SUR2 was addressed by mutational analysis of TMs 13– 16 [34]. When residue S1237 in SUR1 (Fig. 1) was mutated to its SUR2A counterpart (tyrosine), high affinity block by
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tolbutamide was abolished [34]. The mutation also caused glibenclamide block to become reversible and reduced the binding of [3H] glibenclamide, but did not affect current inhibition by meglitinide. Channels containing the reverse mutation (Y1206S) in SUR2B are more potently blocked by glibenclamide than those containing wild type SUR2B, and exhibit increased [3H] glibenclamide binding affinity [36]. The results suggest that S1237 in SUR1 may form part of the sulphonylurea binding pocket. The location of S1237 in an intracellular loop of SUR1 is consistent with a previous report that sulphonylureas access their binding site from the intracellular aspect of the membrane [37]. KATP channel inhibitors can be grouped according to whether they are affected by the S1237Y mutation in SUR1. As discussed above, Kir6.2/SUR1 –S1237Y channels are blocked normally by meglitinide, are only reversibly blocked by glibenclamide, and are unaffected by tolbutamide or gliclazide. Further studies have shown that inhibition of Kir6.2/SUR1 channels by repaglinide is unaffected by mutation of S1237Y [30], but that high-affinity block by nateglinide and mitiglinide is impaired [31,38]. The results suggest that although nateglinide and mitiglinide do not possess a sulphonylurea group, their binding site on SUR1 overlaps significantly with that of simple sulphonylureas such as tolbutamide and gliclazide. 4.2.4. Modulation of KATP channel inhibition by nucleotides The extent of KATP channel inhibition by sulphonylureas is modified in vivo by the presence of nucleotides. As discussed above, KATP channels containing either SUR1 or SUR2 are blocked f 50% by glibenclamide in excised patches in the absence of added nucleotide. Physiological concentrations of MgADP (100 AM) enhance the block of Kir6.2/SUR1 currents but impair that of Kir6.2/SUR2A currents [25,27]. The physiological consequence of this is unclear, but the data suggest that glibenclamide would have little effect in vivo on KATP channels in the cardiovascular system activated by rising MgADP levels during metabolic stress. This contrasts with the ability of glibenclamide to inhibit cardiovascular-type KATP channels activated by nicorandil and related drugs [39 – 41]. The effect in vivo of therapeutic concentrations of glibenclamide on cardiovascular KATP channels may therefore depend on whether the channels have been activated by KATP channel opening drugs, MgADP (metabolic stress) or a separate mechanism (e.g. neurotransmitters). Further studies are still required to clarify the relative contributions of these pathways under pathophysiological conditions.
5. Conclusion Modulation of KATP channel activity is an important therapeutic tool for the treatment of type 2 diabetes and cardiovascular disease. Drugs have been developed which selectively inhibit KATP channels in pancreatic h-cells, or open those in vascular smooth and cardiac muscles. This tissue selectivity is due to the expression of different types of sulphonylurea receptor. Studies on recombinant channels expressed in vitro have provided important insights into the molecular mechanisms of drug action on KATP channels.
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Acknowledgements We thank the Wellcome Trust and Diabetes UK for their support. FMG is a Wellcome Clinician Scientist Fellow, and FR is a Diabetes UK RD Lawrence Fellow. References [1] F.M. Ashcroft, F.M. Gribble, ATP-sensitive K+ channels in health and disease, Diabetologia 42 (1999) 903 – 919. [2] F.M. Ashcroft, F.M. Gribble, New windows on the mechanism of action of KATP channel openers, Trends Pharmacol. Sci. 21 (2000) 439 – 445. [3] P. Rorsman, The pancreatic beta-cell as a fuel sensor: an electrophysiologist’s viewpoint, Diabetologia 40 (1997) 487 – 495. [4] C.G. Nichols, W.J. Lederer, Adenosine triphosphate-sensitive potassium channels in the cardiovascular system, Am. J. Physiol. 261 (1991) H1675 – H1686. [5] J.M. Quayle, M.T. Nelson, N.B. Standen, ATP-sensitive and inwardly-rectifying potassium channels in smooth muscle, Physiol. Rev. 77 (1997) 1165 – 1232. [6] G.J. Grover, K.D. Garlid, ATP-sensitive potassium channels: a review of their cardioprotective pharmacology, J. Mol. Cell. Cardiol. 32 (2000) 677 – 695. [7] J.P. Clement, K. Kunjilwar, G. Gonzalez, M. Schwanstecher, U. Panten, L. Aguilar-Bryan, J. Bryan, Association and stoichiometry of KATP channel subunits, Neuron 18 (1997) 827 – 838. [8] S.-L. Shyng, C.G. Nichols, Octameric stochiometry of the KATP channel complex, J. Gen. Physiol. 110 (1997) 655 – 664. [9] H. Sakura, C. Ammala, P.A. Smith, F.M. Gribble, F.M. Ashcroft, Cloning and functional expression of the cDNA encoding a novel ATP-sensitive potassium channel subunit expressed in pancreatic beta-cells, brain, heart and skeletal muscle, FEBS Lett. 377 (1995) 338 – 344. [10] N. Inagaki, T. Gonoi, J.P. Clement, N. Namba, J. Inazawa, G. Gonzalez, L. Aguilar-Bryan, S. Seino, J. Bryan, Reconstitution of IKATP—an inward rectifier subunit plus the sulfonylurea receptor, Science 270 (1995) 1166 – 1170. [11] N. Inagaki, Y. Tsuura, N. Namba, K. Masuda, T. Gonoi, M. Horie, Y. Seino, M. Mizuta, S. Seino, Cloning and functional characterization of a novel ATP-sensitive potassium channel ubiquitously expressed in rat tissues, including pancreatic islets, pituitary, skeletal muscle, and heart, J. Biol. Chem. 270 (1995) 5691 – 5694. [12] M. Yamada, S. Isomoto, S. Matsumoto, C. Kondo, T. Shindo, Y. Horio, Y. Kurachi, Sulphonylurea receptor 2B and Kir6.1 form a sulphonylurea-sensitive but ATP-insensitive K+ channel, J. Physiol. 499 (1997) 715 – 720. [13] T. Miki, M. Suzuki, T. Shibasaki, H. Uemura, T. Sato, K. Yamaguchi, H. Koseki, T. Iwanaga, H. Nakaya, S. Seino, Mouse model of Prinzmetal angina by disruption of the inward rectifier Kir6.1, Nat. Med. 8 (2002) 466 – 472. [14] A. Thomzig, M. Wenzel, C. Karschin, M.J. Eaton, S.N. Skatchkov, A. Karschin, R.W. Veh, Kir6.1 is the principal pore-forming subunit of astrocyte but not neuronal plasma membrane K-ATP channels, Mol. Cell. Neurosci. 18 (2001) 671 – 690. [15] G.E. Tusnady, E. Bakos, A. Varadi, B. Sarkadi, Membrane topology distinguishes a subfamily of the ATPbinding cassette (ABC) transporters, FEBS Lett. 402 (1997) 1 – 3. [16] L.R. Conti, C.M. Radeke, S.L. Shyng, C.A. Vandenberg, Transmembrane topology of the sulfonylurea receptor SUR1, J. Biol. Chem. 276 (2001) 41270 – 41278. [17] L. Aguilar-Bryan, C.G. Nichols, S.W. Wechsler, J.P. Clement, A.E. Boyd, G. Gonzalez, S.H. Herrera, K. Nguy, J. Bryan, D.A. Nelson, Cloning of the beta cell high-affinity sulfonylurea receptor: a regulator of insulin secretion, Science 268 (1995) 423 – 426. [18] N. Inagaki, T. Gonoi, J.P. Clement, C.Z. Wang, L. Aguilar-Bryan, J. Bryan, S. Seino, A family of sulfonylurea receptors determines the pharmacological properties of ATP-sensitive K+ channels, Neuron 16 (1996) 1011 – 1017.
286
F.M. Gribble, F. Reimann / International Congress Series 1253 (2003) 279–287
[19] S. Isomoto, C. Kondo, M. Yamada, S. Matsumoto, O. Higashiguchi, Y. Horio, Y. Matsuzawa, Y. Kurachi, A novel sulfonylurea receptor forms with BIR (Kir6.2) a smooth muscle type ATP-sensitive K+ channel, J. Biol. Chem. 271 (1996) 24321 – 24324. [20] B. Glaser, P. Thornton, T. Otonkoski, C. Junien, Genetics of neonatal hyperinsulinism, Arch. Dis. Child., Fetal Neonatal Ed. 82 (2000) F79 – F86. [21] P. de Lonlay, J.C. Fournet, G. Touati, M.S. Groos, D. Martin, C. Sevin, V. Delagne, C. Mayaud, V. Chigot, C. Sempoux, M.C. Brusset, K. Laborde, C. Bellane-Chantelot, A. Vassault, J. Rahier, C. Junien, F. Brunelle, C. Nihoul-Fekete, J.M. Saudubray, J.J. Robert, Heterogeneity of persistent hyperinsulinaemic hypoglycaemia. A series of 175 cases, Eur. J. Pediatr. 161 (2002) 37 – 48. [22] N. Sharma, A. Crane, G. Gonzalez, J. Bryan, L. Aguilar-Bryan, Familial hyperinsulinism and pancreatic hcell ATP-sensitive potassium channels, Kidney Int. 57 (2000) 803 – 808. [23] C. Kane, R.M. Shepherd, P.E. Squires, P.R. Johnson, R.F. James, P.J. Milla, A. Aynsley-Green, K.J. Lindley, M.J. Dunne, Loss of functional KATP channels in pancreatic h-cells causes persistent hyperinsulinemic hypoglycemia of infancy, Nat. Med. 2 (1996) 1344 – 1347. [24] D.J. Patel, H.J. Purcell, K.M. Fox, Cardioprotection by opening of the KATP channel in unstable angina. Is this a clinical manifestation of myocardial preconditioning? Results of a randomized study with nicorandil. CESAR 2 investigation. Clinical European studies in angina and revascularization, Eur. Heart J. 20 (1999) 51 – 57. [25] F.M. Gribble, S.J. Tucker, F.M. Ashcroft, The interaction of nucleotides with the tolbutamide block of KATP currents: a reinterpretation, J. Physiol. 504 (1997) 35 – 45. [26] J.C. Koster, Q. Sha, C.G. Nichols, Sulfonylurea and K+-channel opener sensitivity of KATP channels. Functional coupling of Kir6.2 and SUR1 subunits, J. Gen. Physiol. 114 (1999) 203 – 213. [27] F.M. Gribble, S.J. Tucker, S. Seino, F.M. Ashcroft, Tissue specificity of sulphonylureas: studies on cloned cardiac and beta-cell KATP channels, Diabetes 47 (1998) 1412 – 1418. [28] F.M. Gribble, F.M. Ashcroft, Differential sensitivity of h-cell and extrapancreatic KATP channels to gliclazide, Diabetologia 42 (1999) 845 – 848. [29] D.K. Song, F.M. Ashcroft, Glimepiride block of cloned h-cell, cardiac and smooth muscle K-ATP channels, Br. J. Pharmacol. 133 (2001) 193 – 199. [30] M. Dabrowski, P. Wahl, W.E. Holmes, F.M. Ashcroft, Effect of repaglinide on cloned beta cell, cardiac and smooth muscle types of ATP-sensitive potassium channel, Diabetologia 44 (2001) 747 – 756. [31] F. Reimann, P. Proks, F.M. Ashcroft, Effects of mitiglinide (S 21403) on Kir6.2/SUR1, Kir6.2/ SUR2A and Kir6.2/SUR2B types of ATP-sensitive potassium channel, Br. J. Pharmacol. 132 (2001) 1542 – 1548. [32] L. Gros, A. Virsolvy, G. Salazar, D. Bataille, P. Blache, Characterization of low-affinity binding sites for glibenclamide on the Kir6.2 subunit of the beta-cell KATP channel, Biochem. Biophys. Res. Commun. 257 (1999) 766 – 770. [33] P. Proks, F. Reimann, N. Green, F.M. Gribble, F.M. Ashcroft, Sulphonylurea stimulation of insulin secretion, Diabetes 51 (Suppl. 3) (2002) S368 – S376. [34] R. Ashfield, F.M. Gribble, S.J.H. Ashcroft, F.M. Ashcroft, Identification of the high-affinity tolbutamide site on the SUR1 subunit of the KATP channel, Diabetes 48 (1999) 1341 – 1347. [35] F.M. Gribble, F. Reimann, Pharmacological modulation of KATP channels, Biochem. Soc. Trans. 30 (2002) 333 – 339. [36] A. Hambrock, C. Lo¨ffler-Walz, U. Russ, U. Lange, U. Quast, Characterization of a mutant sulfonylurea receptor SUR2B with high affinity for sulfonylureas and openers: differences in the coupling to Kir6.x subtypes, Mol. Pharmacol. 60 (2001) 190 – 199. [37] M. Schwanstecher, C. Schwanstecher, C. Dickel, F. Chudziak, A. Moshiri, U. Panten, Location of the sulphonylurea receptor at the cytoplasmic face of the beta-cell membrane, Br. J. Pharmacol. 113 (1994) 903 – 911. [38] A.M. Hansen, I.T. Christensen, J.B. Hansen, R.D. Carr, F.M. Ashcroft, P. Wahl, Differential interactions of nateglinide repaglinide on the human beta-cell sulphonylurea receptor 1, Diabetes 51 (2002) 2789 – 2795. [39] F. Reimann, F.M. Ashcroft, F.M. Gribble, Structural basis for the interference between nicorandil and sulfonylurea action, Diabetes 50 (2001) 2253 – 2259.
F.M. Gribble, F. Reimann / International Congress Series 1253 (2003) 279–287
287
[40] P.J. Bijlstra, J.A. Lutterman, F.G.M. Russel, T. Thien, P. Smits, Interaction of sulphonylurea derivatives with vascular ATP-sensitive potassium channels in humans, Diabetologia 39 (1996) 1083 – 1090. [41] C.L. Lawrence, P. Proks, G.C. Rodrigo, P. Jones, Y. Hayabuchi, N.B. Standen, F.M. Ashcroft, Gliclazide produces high-affinity block of KATP channels in mouse isolated pancreatic h-cells but not rat heart or arterial smooth muscle cells, Diabetologia 44 (2001) 1019 – 1025.