Mg2+ and ATP dependence of K(ATP) channel modulator binding to the recombinant sulphonylurea receptor, SUR2B

ATP binding without hydrolysis switches sulfonylurea receptor 1 (SUR1) to outward-facing conformations that activate KATP channels

Neuroendocrine-type ATP-sensitive K⁺ (K_ATP) channels are metabolite sensors coupling membrane potential with metabolism, thereby linking insulin secretion to plasma glucose levels. They are octameric complexes, (SUR1/Kir6.2)₄, comprising sulfonylurea receptor 1 (SUR1 or ABCC8) and a K⁺-selective inward rectifier (Kir6.2 or KCNJ11). Interactions between nucleotide-, agonist-, and antagonist-binding sites affect channel activity allosterically. Although it is hypothesized that opening these channels requires SUR1-mediated MgATP hydrolysis, we show here that ATP binding to SUR1, without hydrolysis, opens channels when nucleotide antagonism on Kir6.2 is minimized and SUR1 mutants with increased ATP affinities are used. We found that ATP binding is sufficient to switch SUR1 alone between inward- or outward-facing conformations with low or high dissociation constant, K_D , values for the conformation-sensitive channel antagonist [³H]glibenclamide ([³H]GBM), indicating that ATP can act as a pure agonist. Assembly with Kir6.2 reduced SUR1's K_D for [³H]GBM. This reduction required the Kir N terminus (KNtp), consistent with KNtp occupying a "transport cavity," thus positioning it to link ATP-induced SUR1 conformational changes to channel gating. Moreover, ATP/GBM site coupling was constrained in WT SUR1/WT Kir6.2 channels; ATP-bound channels had a lower K_D for [³H]GBM than ATP-bound SUR1. This constraint was largely eliminated by the Q1179R neonatal diabetes-associated mutation in helix 15, suggesting that a "swapped" helix pair, 15 and 16, is part of a structural pathway connecting the ATP/GBM sites. Our results suggest that ATP binding to SUR1 biases K_ATP channels toward open states, consistent with SUR1 variants with lower K_D values causing neonatal diabetes, whereas increased K_D values cause congenital hyperinsulinism.

Two neonatal diabetes mutations on transmembrane helix 15 of SUR1 increase affinity for ATP and ADP at nucleotide binding domain 2

K(ATP) channels, (SUR1/Kir6.2)(4) (sulfonylurea receptor type 1/potassium inward rectifier type 6.2) respond to the metabolic state of pancreatic β-cells, modulating membrane potential and insulin exocytosis. Mutations in both subunits cause neonatal diabetes by overactivating the pore. Hyperactive channels fail to close appropriately with increased glucose metabolism; thus, β-cell hyperpolarization limits insulin release. K(ATP) channels are inhibited by ATP binding to the Kir6.2 pore and stimulated, via an uncertain mechanism, by magnesium nucleotides at SUR1. Glibenclamide (GBC), a sulfonylurea, was used as a conformational probe to compare nucleotide action on wild type versus Q1178R and R1182Q SUR1 mutants. GBC binds with high affinity to aporeceptors, presumably in the inward facing ATP-binding cassette configuration; MgATP reduces binding affinity via a shift to the outward facing conformation. To determine nucleotide affinities under equilibrium, non-hydrolytic conditions, Mg(2+) was eliminated. A four-state equilibrium model describes the allosteric linkage. The K(D) for ATP(4-) is ~1 versus 12 mM, Q1178R versus wild type, respectively. The linkage constant is ~10, implying that outward facing conformations bind GBC with a lower affinity, 9-10 nM for Q1178R. Thus, nucleotides cannot completely inhibit GBC binding. Binding of channel openers is reported to require ATP hydrolysis, but diazoxide, a SUR1-selective agonist, concentration-dependently augments ATP(4-) action. An eight-state model describes linkage between diazoxide and ATP(4-) binding; diazoxide markedly increases the affinity of Q1178R for ATP(4-) and ATP(4-) augments diazoxide binding. NBD2, but not NBD1, has a higher affinity for ATP (and ADP) in mutant versus wild type (with or without Mg(2+)). Thus, the mutants spend more time in nucleotide-bound conformations, with reduced affinity for GBC, that activate the pore.

Muscle KATP channels: recent insights to energy sensing and myoprotection

ATP-sensitive potassium (K(ATP)) channels are present in the surface and internal membranes of cardiac, skeletal, and smooth muscle cells and provide a unique feedback between muscle cell metabolism and electrical activity. In so doing, they can play an important role in the control of contractility, particularly when cellular energetics are compromised, protecting the tissue against calcium overload and fiber damage, but the cost of this protection may be enhanced arrhythmic activity. Generated as complexes of Kir6.1 or Kir6.2 pore-forming subunits with regulatory sulfonylurea receptor subunits, SUR1 or SUR2, the differential assembly of K(ATP) channels in different tissues gives rise to tissue-specific physiological and pharmacological regulation, and hence to the tissue-specific pharmacological control of contractility. The last 10 years have provided insights into the regulation and role of muscle K(ATP) channels, in large part driven by studies of mice in which the protein determinants of channel activity have been deleted or modified. As yet, few human diseases have been correlated with altered muscle K(ATP) activity, but genetically modified animals give important insights to likely pathological roles of aberrant channel activity in different muscle types.

Substitution of the Walker A lysine by arginine in the nucleotide-binding domains of sulphonylurea receptor SUR2B: effects on ligand binding and channel activity

Sulphonylurea receptors (SURs) serve as regulatory subunits of ATP-sensitive K(+) channels. SURs are members of the ATP-binding cassette (ABC) protein superfamily and contain two conserved nucleotide-binding domains (NBDs) which bind and hydrolyse MgATP; in addition, they carry the binding sites for the sulphonylureas like glibenclamide (GBC) which close the channel and for the K(ATP) channel openers such as P1075. Here we have exchanged the conserved Lys in the Walker A motif by Arg in both NBDs of SUR2B, the regulatory subunit of the vascular K(ATP) channel. Then the effect of the mutation on the ATPase-dependent binding of GBC and P1075 to SUR2B and on the activity of the recombinant vascular (Kir6.1/SUR2B) channel was assessed. Surprisingly, in the absence of MgATP, the mutation weakened binding of P1075 and the extent of allosteric inhibition of GBC binding by P1075. The mutation abolished most, but not all, of the MgATP effects on the binding of GBC and P1075 and prevented nucleotide-induced activation of the channel which relies on SUR reaching the posthydrolytic (MgADP-bound) state; the mutant channel was, however, opened by P1075 at higher concentrations. The data provide evidence that mutant SUR2B binds MgATP but that the posthydrolytic state is insufficiently populated. This suggests that the mutation locks SUR2B in an MgATP-binding prehydrolytic-like state; binding of P1075 may induce a posthydrolytic-like conformation to open the channel.

Inwardly rectifying potassium channels: their structure, function, and physiological roles

Inwardly rectifying K(+) (Kir) channels allow K(+) to move more easily into rather than out of the cell. They have diverse physiological functions depending on their type and their location. There are seven Kir channel subfamilies that can be classified into four functional groups: classical Kir channels (Kir2.x) are constitutively active, G protein-gated Kir channels (Kir3.x) are regulated by G protein-coupled receptors, ATP-sensitive K(+) channels (Kir6.x) are tightly linked to cellular metabolism, and K(+) transport channels (Kir1.x, Kir4.x, Kir5.x, and Kir7.x). Inward rectification results from pore block by intracellular substances such as Mg(2+) and polyamines. Kir channel activity can be modulated by ions, phospholipids, and binding proteins. The basic building block of a Kir channel is made up of two transmembrane helices with cytoplasmic NH(2) and COOH termini and an extracellular loop which folds back to form the pore-lining ion selectivity filter. In vivo, functional Kir channels are composed of four such subunits which are either homo- or heterotetramers. Gene targeting and genetic analysis have linked Kir channel dysfunction to diverse pathologies. The crystal structure of different Kir channels is opening the way to understanding the structure-function relationships of this simple but diverse ion channel family.

Analysis of two KCNJ11 neonatal diabetes mutations, V59G and V59A, and the analogous KCNJ8 I60G substitution: differences between the channel subtypes formed with SUR1

beta-Cell-type K(ATP) channels are octamers assembled from Kir6.2/KCNJ11 and SUR1/ABCC8. Adenine nucleotides play a major role in their regulation. Nucleotide binding to Kir6.2 inhibits channel activity, whereas ATP binding/hydrolysis on sulfonylurea receptor 1 (SUR1) opposes inhibition. Segments of the Kir6.2 N terminus are important for open-to-closed transitions, form part of the Kir ATP, sulfonylurea, and phosphoinositide binding sites, and interact with L0, an SUR cytoplasmic loop. Inputs from these elements link to the pore via the interfacial helix, which forms an elbow with the outer pore helix. Mutations that destabilize the interfacial helix increase channel activity, reduce sensitivity to inhibitory ATP and channel inhibitors, glibenclamide and repaglinide, and cause neonatal diabetes. We compared Kir6.x/SUR1 channels carrying the V59G substitution, a cause of the developmental delay, epilepsy, and neonatal diabetes syndrome, with a V59A substitution and the equivalent I60G mutation in the related Kir6.1 subunit from vascular smooth muscle. The substituted channels have increased P(O) values, decreased sensitivity to inhibitors, and impaired stimulation by phosphoinositides but retain sensitivity to Ba(2+)-block. The V59G and V59A channels are either not, or poorly, stimulated by phosphoinositides, respectively. Inhibition by sequestrating phosphatidylinositol 4,5-bisphosphate with neomycin and polylysine is reduced in V59A, and abolished in V59G channels. Stimulation by SUR1 is intact, and increasing the concentration of inhibitory ATP restores the sensitivity of Val-59-substituted channels to glibenclamide. The I60G channels, strongly dependent on SUR stimulation, remain sensitive to sulfonylureas. The results suggest the interfacial helix dynamically links inhibitory inputs from the Kir N terminus to the gate and that sulfonylureas stabilize an inhibitory configuration.

Incomplete dissociation of glibenclamide from wild-type and mutant pancreatic K ATP channels limits their recovery from inhibition

BACKGROUND AND PURPOSE

The antidiabetic sulphonylurea, glibenclamide, acts by inhibiting the pancreatic ATP-sensitive K(+) (K(ATP)) channel, a tetradimeric complex of K(IR)6.2 and sulphonylurea receptor 1 (K(IR)6.2/SUR1)(4). At room temperature, recovery of channel activity following washout of glibenclamide is very slow and cannot be measured. This study investigates the relation between the recovery of channel activity from glibenclamide inhibition and the dissociation rate of [(3)H]-glibenclamide from the channel at 37 degrees C.

EXPERIMENTAL APPROACH

K(IR)6.2, K(IR)6.2DeltaN5 or K(IR)6.2DeltaN10 (the latter lacking amino-terminal residues 2-5 or 2-10 respectively) were coexpressed with SUR1 in HEK cells. Dissociation of [(3)H]-glibenclamide from the channel and recovery of channel activity from glibenclamide inhibition were determined at 37 degrees C.

KEY RESULTS

The dissociation kinetics of [(3)H]-glibenclamide from the wild-type channel followed an exponential decay with a dissociation half-time, t(1/2)(D) = 14 min; however, only limited and slow recovery of channel activity was observed. t(1/2)(D) for K(IR)6.2DeltaN5/SUR1 channels was 5.3 min and recovery of channel activity exhibited a sluggish sigmoidal time course with a half-time, t(1/2)(R) = 12 min. t(1/2)(D) for the DeltaN10 channel was 2.3 min; recovery kinetics were again sigmoidal with t(1/2)(R) approximately 4 min.

CONCLUSIONS AND IMPLICATIONS

The dissociation of glibenclamide from the truncated channels is the rate-limiting step of channel recovery. The sigmoidal recovery kinetics are in quantitative agreement with a model where glibenclamide must dissociate from all four (or at least three) sites before the channel reopens. It is argued that these conclusions hold also for the wild-type (pancreatic) K(ATP) channel.

K(ATP) channel expression and pharmacological in vivo and in vitro studies of the K(ATP) channel blocker PNU-37883A in rat middle meningeal arteries

BACKGROUND AND PURPOSE

Dilatation of cerebral and dural arteries causes a throbbing, migraine-like pain, indicating that these structures are involved in migraine. Clinical trials suggest that adenosine 5'-triphosphate-sensitive K(+) (K(ATP)) channel opening may cause migraine by dilatating intracranial arteries, including the middle meningeal artery (MMA). We studied the K(ATP) channel expression profile in rat MMA and examined the potential inhibitory effects of the K(ATP) channel blocker PNU-37883A on K(ATP) channel opener-induced relaxation of the rat MMA, using the three K(ATP) channel openers levcromakalim, pinacidil and P-1075.

EXPERIMENTAL APPROACH

mRNA and protein expression of K(ATP) channel subunits in the rat MMA were studied by quantitative real-time PCR and western blotting, respectively. The in vivo and in vitro effects of the K(ATP) channel drugs on rat MMA were studied in the genuine closed cranial window model and in myograph baths, respectively.

KEY RESULTS

Expression studies indicate that inwardly rectifying K(+) (Kir)6.1/sulphonylurea receptor (SUR)2B is the major K(ATP) channel complex in rat MMA. PNU-37883A (0.5 mg kg(-1)) significantly inhibited the in vivo dilatory effect of levcromakalim (0.025 mg kg(-1)), pinacidil (0.38 mg kg(-1)) and P-1075 (0.016 mg kg(-1)) in rat MMA. In vitro PNU-37883A significantly inhibited the dilatory responses of the three K(ATP) channel openers in rat MMA at 10(-7) and 3 x 10(-7) M.

CONCLUSIONS AND IMPLICATIONS

We suggest that Kir6.1/SUR2B is the major functional K(ATP) channel complex in the rat MMA. Furthermore, we demonstrate the potent in vivo and in vitro blocking potentials of PNU-37883A on K(ATP) channel opener-induced relaxation of the rat MMA.

The actions of a novel potent islet beta-cell specific ATP-sensitive K+ channel opener can be modulated by syntaxin-1A acting on sulfonylurea receptor 1

Islet beta-cell-specific ATP-sensitive K(+) (K(ATP)) channel openers thiadiazine dioxides induce islet rest to improve insulin secretion, but their molecular basis of action remains unclear. We reported that syntaxin-1A binds nucleotide binding folds of sulfonylurea receptor 1 (SUR1) in beta-cells to inhibit K(ATP) channels. As a strategy to elucidate the molecular mechanism of action of these K(ATP) channel openers, we explored the possibility that 6-chloro-3-(1-methylcyclobutyl)amino-4H-thieno[3,2-e]-1,2,4-thiadiazine 1,1-dioxide (NNC55-0462) might influence syntaxin-1A-SUR1 interactions or vice versa. Whole-cell and inside-out patch-clamp electrophysiology was used to examine the effects of glutathione S-transferase (GST)-syntaxin-1A dialysis or green fluorescence protein/syntaxin-1A cotransfection on NNC55-0462 actions. In vitro pull-down binding studies were used to examine NNC55-0462 influence on syntaxin-1A-SUR1 interactions. Dialysis of GST-syntaxin-1A into the cell cytoplasm reduced both potency and efficacy of extracellularly perfused NNC55-0462 in a HEK cell line stably expressing Kir6.2/SUR1 (BA8 cells) and in rat islet beta-cells. Moreover, inside-out membrane patches excised from BA8 cells showed that both GST-syntaxin-1A and its H3 domain inhibited K(ATP) channels previously activated by NNC55-0462. This action on K(ATP) channels is isoform-specific to syntaxin-1A because syntaxin-2 was without effect. Furthermore, the parent compound diazoxide showed similar sensitivity to GST-syntaxin-1A inhibition. NNC55-0462, however, did not influence syntaxin-1A-SUR1 binding interaction. Our results demonstrated that syntaxin-1A interactions with SUR1 at its cytoplasmic domains can modulate the actions of the K(ATP) channel openers NNC55-0462 and diazoxide on K(ATP) channels. The reduced levels of islet syntaxin-1A in diabetes would thus be expected to exert a positive influence on the therapeutic effects of this class of K(ATP) channel openers.

Testing the bipartite model of the sulfonylurea receptor binding site: binding of A-, B-, and A + B-site ligands

ATP-sensitive K(+) (K(ATP)) channels are composed of pore-forming subunits (Kir6.x) and of regulatory subunits, the sulfonylurea receptors (SURx). Subtypes of K(ATP) channels are expressed in different organs. The sulfonylureas and glinides (insulinotropes) close the K(ATP) channel in pancreatic beta-cells and stimulate insulin secretion. The insulinotrope binding site of the pancreatic channel (Kir6.2/SUR1) consists of two overlapping (sub)-sites, site A, located on SUR1 and containing Ser1237 (which in SUR2 is replaced by Tyr1206), and site B, formed by SUR1 and Kir6.2. Insulinotropes bind to the A-, B-, or A + B-site(s) and are grouped accordingly. A-ligands are highly selective in closing the pancreatic channel, whereas B-ligands are nonselective and insensitive to the mutation S1237Y. We have examined the binding of insulinotropes representative of the three groups in [(3)H]glibenclamide competition experiments to determine the contribution of Kir6.x to binding affinity, the effect of the mutation Y1206S in site A of SUR2, and the subtype selectivity of the compounds. The results show that the bipartite nature of the SUR1 binding site applies also to SUR2. Kir6.2 as part of the B-site may interact directly or allosterically with structural elements common to all insulinotropes, i.e., the negative charge and/or the adjacent phenyl ring. The B-site confers a moderate subtype selectivity on B-ligands. The affinity of B-ligands is altered by the mutation SUR2(Y1206S), suggesting that the mutation affects the binding chamber of SUR2 as a whole or subsite A, including the region where the subsites overlap.

Regulation of cation channels in cardiac and smooth muscle cells by intracellular magnesium

Magnesium regulates various ion channels in many tissues, including those of the cardiovascular system. General mechanisms by which intracellular Mg(2+) (Mg(i)(2+)) regulates channels are presented. These involve either a direct interaction with the channel, or an indirect modification of channel function via other proteins, such as enzymes or G proteins, or via membrane surface charges and phospholipids. To provide an insight into the role of Mg(i)(2+) in the cardiovascular system, effects of Mg(i)(2+) on major channels in cardiac and smooth muscle cells and the underlying mechanisms are then reviewed. Although Mg(i)(2+) concentrations are known to be stable, conditions under which they may change exist, such as following stimulation of beta-adrenergic receptors and of insulin receptors, or during pathophysiological conditions such as ischemia, heart failure or hypertension. Modifications of cardiovascular electrical or mechanical function, possibly resulting in arrhythmias or hypertension, may result from such changes of Mg(i)(2+) and their effects on cation channels.

Selectivity of repaglinide and glibenclamide for the pancreatic over the cardiovascular K(ATP) channels

AIMS/HYPOTHESIS

Sulfonylureas and glinides close beta cell ATP-sensitive K(+) (K(ATP)) channels to increase insulin release; the concomitant closure of cardiovascular K(ATP) channels, however, leads to complications in patients with cardiac ischaemia. The insulinotrope repaglinide is successful in therapy, but has been reported to inhibit the recombinant K(ATP) channels of beta cells, cardiocytes and non-vascular smooth muscle cells with similar potencies, suggesting that the (patho-)physiological role of the cardiovascular K(ATP) channels may be overstated. We therefore re-examined repaglinide's potency at and affinity for the recombinant pancreatic, myocardial and vascular K(ATP) channels in comparison with glibenclamide.

METHODS

K(ATP) channel subunits (i.e. inwardly rectifying K(+) channels [Kir6.x] and sulfonylurea receptors [SURx]) were expressed in intact human embryonic kidney cells and assayed in whole-cell patch-clamp and [(3)H]glibenclamide binding experiments at 37 degrees C.

RESULTS

Repaglinide and glibenclamide, respectively, were >or=30 and >or=1,000 times more potent in closing the pancreatic than the cardiovascular channels and they did not lead to complete inhibition of the myocardial channel. Binding assays showed that the selectivity of glibenclamide was essentially based on high affinity for the pancreatic SUR, whereas binding of repaglinide to the SUR subtypes was rather non-selective. After coexpression with Kir6.x to form the assembled channels, however, the affinity of the pancreatic channel for repaglinide was increased 130-fold, an effect much larger than with the cardiovascular channels. This selective effect of coexpression depended on the piperidino substituent in repaglinide.

CONCLUSIONS/INTERPRETATION

Repaglinide and glibenclamide show higher potency and efficacy in inhibiting the pancreatic than the cardiovascular K(ATP) channels, thus supporting their clinical use.

KATP channel openers of the benzopyran type reach their binding site via the cytosol

BACKGROUND AND PURPOSE

ATP-sensitive K+ (KATP) channels are composed of pore-forming subunits (Kir6.x) and of sulphonylurea receptors (SUR). Both sulphonylureas and K(ATP) channel openers act by binding to SUR. Sulphonylureas reach their binding site from the cytosol but it remains unknown whether this holds for openers too.

EXPERIMENTAL APPROACH

A poorly membrane-permeant sulphonic acid derivative of the benzopyran-type opener, bimakalim, was synthesized, descyano-bimakalim-6-sulphonic acid (BMSA). Binding of BMSA and bimakalim was compared in membranes and intact cells expressing the Kir6.2/SUR2B channel and channel opening was compared in inside-out patches and whole cells.

KEY RESULTS

In membranes, bimakalim and BMSA bound to Kir6.2/SUR2B with Ki values of 61 nM and 4.3 microM, showing that the negative charge decreased affinity 69-fold. In intact cells, however, binding of BMSA was much weaker than in membranes (75-fold) whereas that of bimakalim was unchanged. The Ki value of BMSA decreased with increasing incubation time. In inside-out patches, bimakalim (1 microM) and BMSA (100 microM) opened the Kir6.2/SUR2B channel closed by MgATP to a similar degree whereas in whole-cell experiments, only bimakalim was effective.

CONCLUSIONS AND IMPLICATIONS

Despite its negative charge, BMSA is an effective channel opener. The fact that BMSA binds and acts more effectively when applied to the inner side of the cell membrane shows that benzopyran openers reach their binding site at SUR from the cytosol. This suggests that the binding pocket of SUR is only open on the cytoplasmic side.

Lipids modulate ligand binding to sulphonylurea receptors

ATP-sensitive K(+) channels (K(ATP) channels) are complexes of inwardly rectifying K(+) channels (Kir6.x) and sulphonylurea receptors (SURs). Kir6.2-containing channels are closed by ATP binding to Kir6.2, and opened by MgADP binding to SUR. Channel activity is modulated by synthetic compounds such as the channel-blocking sulphonylureas and the K(ATP) channel openers, which both act by binding to SUR. By interacting with Kir6.2, phosphatidylinositol-4,5-bisphosphate (PIP(2)) and oleoyl-coenzyme A (OCoA) decrease the ATP-sensitivity of the channel and abolish the effect of the synthetic channel modulators. Here, we have investigated whether lipids and related compounds interfered with the binding of the sulphonylurea, glibenclamide (GBC) and of the opener, N-cyano-N'-(1,1-dimethylpropyl)-N''-3-pyridylguanidine (P1075), to the SUR subtypes. Lipids (100-300 microM) inhibited binding of [(3)H]GBC and [(3)H]P1075 to SUR subtypes in the rank order OCoA>dioleylglycerol-succinyl-nitriloacetic acid (DOGS-NTA)>oleate>malonyl-CoA>PIP(2.)OCoA inhibited radioligand binding to SUR completely, with IC(50) values ranging from 6 to 44 microM. Inhibition was reversed by increasing the concentration of the radioligands in agreement with an essentially competitive mechanism. MgATP and coexpression with Kir6.2 decreased the potency of OCoA. DOGS-NTA inhibited radioligand binding to SUR by 40-88%, with IC(50) values ranging from 38 to 120 microM. Poly-lysine increased radioligand binding to SUR by up to 30% but did not affect much the inhibition of ligand binding by OCoA and DOGS-NTA. Radioligand binding to SUR2A but not to the other SUR subtypes was slightly (10-20%) stimulated by lipids at concentrations approximately 10 x lower than required for inhibition. The data show that certain lipids, at high concentrations, interact with SUR and inhibit the binding of GBC and P1075; with SUR2A, a modest stimulation of ligand binding precedes inhibition. Regarding K(ATP) channel activity, these effects will be overruled by the interaction of the lipids with Kir6.2 at lower (physiological) concentrations. They are, however, of pharmacological importance and must be taken into account if high concentrations of these compounds (e.g. OCoA>10 microM) are used to interfere with the action of sulphonylureas and openers on K(ATP) channel activity.

The mutation Y1206S increases the affinity of the sulphonylurea receptor SUR2A for glibenclamide and enhances the effects of coexpression with Kir6.2

1. ATP-sensitive K(+) channels (K(ATP) channels) are tetradimeric complexes of inwardly rectifying K(+) channels (Kir6.x) and sulphonylurea receptors (SURs). The SURs SUR2A (cardiac) and SUR2B (smooth muscle) differ only in the last 42 amino acids. In SUR2B, the mutation Y1206S, located at intracellular loop 8, increases the affinity for glibenclamide (GBC) about 10-fold. Here, we examined whether the mutation Y1206S in SUR2A had effects similar to those in SUR2B.2. GBC bound to SUR2A with K(D)=20 nM; the mutation increased affinity approximately 5 x. 3. In cells, coexpression of SUR2A with Kir6.2 increased the affinity for GBC approximately 3 x; with the mutant, the increase was 9 x. 4. The mutation did not affect the affinity of SUR2A for openers; coexpression with Kir6.2 reduced opener affinity of wild-type and mutant SUR2A by about 2 x. 5. The negative allosteric interaction between the opener, P1075, and GBC at wild-type and mutant SUR2A was markedly affected by the presence of MgATP and by coexpression with Kir6.2. 6. In inside-out patches, GBC inhibited the wild-type Kir6.2/SUR2A and 2B channels with IC(50) values of 27 nM; the mutation shifted the IC(50) values to approximately 1 nM. 7. The data show that the mutation Y1206S increased the affinity of SUR2A for GBC and modulated the effects of coexpression. Overall, the changes were similar to those observed with SUR2B(Y1206S), suggesting that the differences in the last 42 carboxy-terminal amino acids of SUR2A and 2B are of limited influence on the binding of GBC and P1075 to the SUR2 isoforms.

Effect of two amino acids in TM17 of Sulfonylurea receptor SUR1 on the binding of ATP-sensitive K+ channel modulators

The sulfonylurea receptor (SUR) is the important regulatory subunit of ATP-sensitive K+ channels. It is an ATP-binding cassette protein comprising 17 transmembrane helices. SUR is endowed with binding sites for channel blockers like the antidiabetic sulfonylurea glibenclamide and for the chemically very heterogeneous channel openers. SUR1, the typical pancreatic SUR isoform, shows much higher affinity for glibenclamide but considerably lower affinity for most openers than SUR2. In radioligand binding assays, we investigated the role of two amino acids, T1285 and M1289, located in transmembrane helix (TM)-17, in opener binding to SUR1. These amino acids were exchanged for the corresponding amino acids of SUR2. In competition experiments using [3H]glibenclamide as radioligand, SUR1(T1285L, M1289T) showed much higher affinity toward the cyanoguanidine openers pinacidil and P1075 than SUR1 wild type. The affinity for the thioformamide aprikalim was also markedly increased. In contrast, the affinity for the benzopyrans rilmakalim and levcromakalim was unaffected; however, the amount of displaced [3H]glibenclamide binding was nearly doubled. The binding properties of the opener diazoxide and the blocker glibenclamide were unchanged. In conclusion, mutation of two amino acids in TM17 of SUR1, especially of M1289, leads to class-specific effects on opener binding by increasing opener affinity or by changing allosteric coupling between opener and glibenclamide binding.

Toward linking structure with function in ATP-sensitive K+ channels

Advances in understanding the overall structural features of inward rectifiers and ATP-binding cassette (ABC) transporters are providing novel insight into the architecture of ATP-sensitive K+ channels (KATP channels) (KIR6.0/SUR)4. The structure of the K(IR) pore has been modeled on bacterial K+ channels, while the lipid-A exporter, MsbA, provides a template for the MDR-like core of sulfonylurea receptor (SUR)-1. TMD0, an NH2-terminal bundle of five alpha-helices found in SURs, binds to and activates KIR6.0. The adjacent cytoplasmic L0 linker serves a dual function, acting as a tether to link the MDR-like core to the KIR6.2/TMD0 complex and exerting bidirectional control over channel gating via interactions with the NH2-terminus of the KIR. Homology modeling of the SUR1 core offers the possibility of defining the glibenclamide/sulfonylurea binding pocket. Consistent with 30-year-old studies on the pharmacology of hypoglycemic agents, the pocket is bipartite. Elements of the COOH-terminal half of the core recognize a hydrophobic group in glibenclamide, adjacent to the sulfonylurea moiety, to provide selectivity for SUR1, while the benzamido group appears to be in proximity to L0 and the KIR NH2-terminus.

KATP channel openers: structure-activity relationships and therapeutic potential

ATP-sensitive potassium channels (K(ATP) channels) are heteromeric complexes of pore-forming inwardly rectifying potassium channel subunits and regulatory sulfonylurea receptor subunits. K(ATP) channels were identified in a variety of tissues including muscle cells, pancreatic beta-cells, and various neurons. They are regulated by the intracellular ATP/ADP ratio; ATP induces channel inhibition and MgADP induces channel opening. Functionally, K(ATP) channels provide a means of linking the electrical activity of a cell to its metabolic state. Shortening of the cardiac action potential, smooth muscle relaxation, inhibition of both insulin secretion, and neurotransmitter release are mediated via K(ATP) channels. Given their many physiological functions, K(ATP) channels represent promising drug targets. Sulfonylureas like glibenclamide block K(ATP) channels; they are used in the therapy of type 2 diabetes. Openers of K(ATP) channels (KCOs), for example, relax smooth muscle and induce hypotension. KCOs are chemically heterogeneous and include as different classes as the benzopyrans, cyanoguanidines, thioformamides, thiadiazines, and pyridyl nitrates. Examples for new chemical entities more recently developed as KCOs include cyclobutenediones, dihydropyridine related structures, and tertiary carbinols.

Interaction of a novel dihydropyridine K+ channel opener, A-312110, with recombinant sulphonylurea receptors and KATP channels: comparison with the cyanoguanidine P1075

1. ATP-sensitive K(+) channels (K(ATP) channels) are composed of pore-forming subunits (Kir6.x) and of regulatory subunits, the sulphonylurea receptors (SURx). Synthetic openers of K(ATP) channels form a chemically heterogeneous class of compounds that are of interest in several therapeutic areas. We have investigated the interaction of a novel dihydropyridine opener, A-312110 ((9R)-9-(4-fluoro-3-iodophenyl)-2,3,5,9-tetrahydro-4H-pyrano[3,4-b]thieno [2,3-e]pyridin-8(7H)-one-1,1-dioxide), with SURs and Kir6/SUR channels in comparison to the cyanoguanidine opener P1075. 2. In the presence of 1 mM MgATP, A-312110 bound to SUR2A (the SUR in cardiac and skeletal muscle) and to SUR2B (smooth muscle) with K(i) values of 14 and 18 nM; the corresponding values for P1075 were 16 and 9 nM, respectively. Decreasing the MgATP concentration reduced the affinity of A312110 binding to SUR2A significantly more than that to SUR2B; for P1075, the converse was true. At SUR1 (pancreatic beta-cell), both openers showed little binding up to 100 microM. 3. In the presence of MgATP, both openers inhibited [(3)H]glibenclamide binding to the SUR2 subtypes in a biphasic manner. In the absence of MgATP, the high-affinity component of the inhibition curves was absent. 4. In inside-out patches, the two openers activated the Kir6.2/SUR2A and Kir6.2/SUR2B channels with similar potency (approximately 50 nm). Both were almost 2 x more efficacious in opening the Kir6.2/SUR2B than the Kir6.2/SUR2A channel. 5. The results show that the novel dihydropyridine A-312110 is a potent K(ATP) channel opener with binding and channel-opening properties similar to those of P1075.

Sulphonylurea action revisited: the post-cloning era

Hypoglycaemic agents such as sulphonylureas and the newer group of "glinides" stimulate insulin secretion by closing ATP-sensitive potassium (K(ATP)) channels in pancreatic beta cells, but have varying cross-reactivity with related channels in extrapancreatic tissues such as heart, vascular smooth and skeletal muscle. Experiments on the structure-function relationships of recombinant K(ATP) channels and the phenotypes of mice deficient in different K(ATP) channel subunits have provided important insights into the mechanisms underlying sulphonylurea selectivity, and the potential consequences of K(ATP) channel blockade outside the pancreatic beta cell. The different pharmacological properties of K(ATP) channels from beta cells compared with those from cardiac, smooth and skeletal muscle, are accounted for by the expression of alternative types of sulphonylurea receptor, with non-identical drug binding sites. The sulphonylureas and glinides are found to fall into two groups: one exhibiting selectivity for beta cell sulphonylurea receptors (SUR1), and the other blocking cardiovascular and skeletal muscle sulphonylurea receptors (SUR2) with potencies similar to their action on SUR1. In seeking potential side effects of K(ATP) channel inhibitors in humans, it is essential to take these drug differences into account, along with the probability (suggested by the studies on K(ATP) channel knockout mice) that the effects of extrapancreatic K(ATP) channel inhibition might be either subtle or rare. Further studies are still required before a final decision can be made on whether non-selective agents are appropriate for the therapy of Type 2 diabetes.

[125I]A-312110, a novel high-affinity 1,4-dihydropyridine ATP-sensitive K+ channel opener: characterization and pharmacology of binding

Although ATP-sensitive K+ channels continue to be explored for their therapeutic potential, developments in high-affinity radioligands to investigate native and recombinant KATP channels have been less forthcoming. This study reports the identification and pharmacological characterization of a novel iodinated 1,4-dihydropyridine KATP channel opener, [125I]A-312110 [(9R)-9-(4-fluoro-3-125iodophenyl)-2,3,5,9-tetrahydro-4H-pyrano[3,4-b]thieno[2,3-e]pyridin-8(7H)-one-1,1-dioxide]. Binding of [125I]A-312110 to guinea pig cardiac (KD = 5.8 nM) and urinary bladder (KD = 4.9 nM) membranes were of high affinity, saturable, and to a single set of binding sites. Displacement of [125I]A-312110 by structurally diverse potassium channel openers (KCOs) indicated a similar rank order of potency in both guinea pig cardiac and bladder membranes (Ki, heart): A-312110 (4.3 nM) > N-cyano-N'-(1,1-dimethylpropyl)-N"-3-pyridylguanidine (P1075) > (-)-N-(2-ethoxyphenyl)-N'-(1,2,3-trimethylpropyl)-2-nitroethene-1,1-diamine (Bay X 9228) > pinacidil > (-)-cromakalim > N-(4-benzoyl phenyl)-3,3,3-trifluro-2-hydroxy-2-methylpropionamine (ZD6169) > 9-(3-cyanophenyl)-3,4,6,7,9,10-hexahydro-1,8-(2H,5H)-acridinedione (ZM244085) >> diazoxide (16.7 microM). Displacement by KATP channel blockers, the sulfonylurea glyburide, and the cyanoguanidine N-[1-(3-chlorophenyl)cyclobutyl]-N'-cyano-N"-3-pyridinyl-guanidine (PNU-99963) were biphasic in the heart but monophasic in bladder with about a 100- to 500-fold difference in Ki values between high- and low-affinity sites. Good correlations were observed between cardiac or bladder-binding affinities of KCOs with functional activation as assessed by their respective potencies to either suppress action potential duration (APD) in Purkinje fibers or to relax electrical field-stimulated bladder contractions. Collectively, these results demonstrate that [125I]A-312110 binds with high affinity and has an improved activity profile compared with other radiolabeled KCOs. [125I]A-312110 is a useful tool for investigation of the molecular and functional properties of the KATP channel complex and for the identification, in a high throughput manner, of both novel channel blockers and openers that interact with cardiac/smooth muscle-type KATP channels.

Binding and effect of K ATP channel openers in the absence of Mg2+

1 Openers of ATP-sensitive K(+) channels (K(ATP) channels) are thought to act by enhancing the ATPase activity of sulphonylurea receptors (SURs), the regulatory channel subunits. At higher concentrations, some openers activate K(ATP) channels also in the absence of MgATP. Here, we describe binding and effect of structurally diverse openers in the absence of Mg(2+) and presence of EDTA. 2 Binding of openers to SUR2B was measured using a mutant with high affinity for [(3)H]glibenclamide ([(3)H]GBC). In the absence of Mg(2+), 'typical' openers (benzopyrans, cyanoguanidines and aprikalim) inhibited [(3)H]GBC binding with K(i) values approximately 200 x higher than in the presence of MgATP. Minoxidil sulphate and nicorandil were inactive, whereas binding of diazoxide was unaffected by MgATP. 3 In the absence/presence of MgATP, N-cyano-N'-(1,1-dimethylpropyl)-N"-3-pyridylguanidine (P1075) activated the Kir6.2/SUR2B channel in inside-out patches with EC(50)=2000/67nM and E(max)=32/134%. In the absence of Mg(2+), responses were variable with only a small part of the variability being explained by a decrease in channel responsiveness with time after patch excision and to differences in the ATP sensitivity between patches. 4 The rank order of efficacy of the openers was P1075>rilmakalim approximately nicorandil>diazoxide>minoxidil sulphate. 5 The data show that structurally diverse openers are able to bind to, and to activate the Kir6.2/SUR2B channel by a pathway independent of ATP hydrolysis. These effects are observed at concentrations used to define the biochemical mechanism of the openers in the presence of MgATP and allow the openers to be classified into 'typical' and 'atypical' KCOs with diazoxide standing apart.

Modulation of the trafficking efficiency and functional properties of ATP-sensitive potassium channels through a single amino acid in the sulfonylurea receptor

Mutations in the sulfonylurea receptor 1 (SUR1), a subunit of ATP-sensitive potassium (K(ATP)) channels, cause familial hyperinsulinism. One such mutation, deletion of phenylalanine 1388 (DeltaPhe-1388), leads to defects in both trafficking and MgADP response of K(ATP) channels. Here we investigated the biochemical features of Phe-1388 that control the proper trafficking and function of K(ATP) channels by substituting the residue with all other 19 amino acids. Whereas surface expression is largely dependent on hydrophobicity, channel response to MgADP is governed by multiple factors and involves the detailed architecture of the amino acid side chain. Thus, structural features in SUR1 required for proper channel function are distinct from those required for correct protein trafficking. Remarkably, replacing Phe-1388 by leucine profoundly alters the physiological and pharmacological properties of the channel. The F1388L-SUR1 channel has increased sensitivity to MgADP and metabolic inhibition, decreased sensitivity to glibenclamide, and responds to both diazoxide and pinacidil. Because this conservative amino acid substitution occurs in the SUR2A and SUR2B isoforms, the mutation provides a mechanism by which functional diversities in K(ATP) channels are generated.

The cytoplasmic C-terminus of the sulfonylurea receptor is important for KATP channel function but is not key for complex assembly or trafficking

ATP-sensitive K+ channels are an octameric assembly of two proteins, a sulfonylurea receptor (SUR1) and an ion conducting subunit (Kir 6.0). We have examined the role of the C-terminus of SUR1 by expressing a series of truncation mutants together with Kir6.2 stably in HEK293 cells. Biochemical analyses using coimmunoprecipitation indicate that SUR1 deletion mutants and Kir6.2 assemble and that a SUR1 deletion mutant binds glibenclamide with high affinity. Electrophysiological recordings indicate that ATP sensitivity is normal but the response of the mutant channel complexes to tolbutamide, MgADP and diazoxide is disturbed. Quantitative immunofluorescence and cell surface biotinylation supports the idea that there is little disturbance in the efficiency of trafficking. Our data show that deletions of the C-terminal most cytoplasmic domain of SUR1, can result in functional channels at the plasma membrane in mammalian cells that have an abnormal response to physiological and pharmacological agents.

The stereoenantiomers of a pinacidil analog open or close cloned ATP-sensitive K+ channels

ATP-dependent K(+) channels (K(ATP) channels) are composed of pore-forming subunits Kir6.x and sulfonylurea receptors (SURs). Cyanoguanidines such as pinacidil and P1075 bind to SUR and enhance MgATP binding to and hydrolysis by SUR, thereby opening K(ATP) channels. In the vasculature, openers of K(ATP) channels produce vasorelaxation. Some novel cyanoguanidines, however, selectively reverse opener-induced vasorelaxation, suggesting that they might be K(ATP) channel blockers. Here we have analyzed the interaction of the enantiomers of a racemic cyanoguanidine blocker, PNU-94750, with Kir6.2/SUR channels. In patch clamp experiments, the R-enantiomer (PNU-96293) inhibited Kir6.2/SUR2 channels (IC(50) approximately 50 nm in the whole cell configuration), whereas the S-enantiomer (PNU-96179) was a weak opener. Radioligand binding studies showed that the R-enantiomer was more potent and that it was negatively allosterically coupled to MgATP binding, whereas the S-enantiomer was weaker and positively coupled. Binding experiments also suggested that both enantiomers bound to the P1075 site of SUR. This is the first report to show that the enantiomers of a K(ATP) channel modulator affect channel activity and coupling to MgATP binding in opposite directions and that these opposite effects are apparently mediated by binding to the same (opener) site of SUR.

Mg2+ sensitizes KATP channels to inhibition by DIDS: dependence on the sulphonylurea receptor subunit

1. ATP-sensitive potassium channels (K(ATP) channels) consist of pore-forming Kir6.x subunits and of sulphonylurea receptors (SURs). In the absence of Mg(2+), the stilbene disulphonate, DIDS, irreversibly inhibits K(ATP) channels by binding to the Kir subunit. Here, the effects of Mg(2+) on the interaction of DIDS with recombinant K(ATP) channels were studied in electrophysiological and [(3)H]-glibenclamide binding experiments. 2. In inside-out macropatches, Mg(2+) (0.7 mM) increased the sensitivity of K(ATP) channels towards DIDS up to 70 fold (IC(50)=2.7 micro M for Kir6.2/SUR2B). Inhibition of current at DIDS concentrations > or =10 micro M was irreversible. 3. Mg(2+) sensitized the truncated Kir6.2Delta26 channel towards inhibition by DIDS only upon coexpression with a SUR subunit (SUR2B). The effect of Mg(2+) did not require the presence of nucleotides. 4. [(3)H]-glibenclamide binding to SUR2B(Y1206S), a mutant with improved affinity for glibenclamide, was inhibited by DIDS. The potency of inhibition was increased by Mg(2+) and by coexpression with Kir6.2. 5. In the presence of Mg(2+), DIDS inhibited binding of [(3)H]-glibenclamide to Kir6.2/SUR2B(Y1206S) with IC(50)=7.9 micro M by a non-competitive mechanism. Inhibition was fully reversible. 6. It is concluded that the binding site of DIDS on SUR that is sensed by glibenclamide does not mediate channel inhibition. Instead, Mg(2+) binding to SUR may allosterically increase the accessibility and/or reactivity of the DIDS site on Kir6.2. The fact that the Mg(2+) effect does not require the presence of nucleotides underlines the importance of this ion in modulating the properties of the K(ATP) channel.

Glibenclamide binding to sulphonylurea receptor subtypes: dependence on adenine nucleotides

1: ATP-sensitive K(+) channels are composed of pore-forming subunits Kir6.2 and of sulphonylurea receptors (SURs); the latter are the target of the hypoglycaemic sulphonylureas like glibenclamide. Here, we report on the negative allosteric modulation by MgATP and MgADP of glibenclamide binding to SUR1 and to SUR2 mutants with high glibenclamide affinity, SUR2A(Y1206S) and SUR2B(Y1206S). 2: ATP, in the presence of an ATP-regenerating system to oppose hydrolysis during incubation, inhibited glibenclamide binding to SUR1 and SUR2B(Y1206S) by approximately 60%, to SUR2A(Y1206S) by 21%). Inhibition curves for the SUR2(Y1206S) isoforms were monophasic with IC(50) values of 5-10 microM; the curve for SUR1 was biphasic (IC(50) values 4.7 and 1300 microM). 3: Glibenclamide inhibition curves for ADP, performed in the presence of an ATP-consuming system to oppose ATP formation from ADP, were generally shifted rightwards and showed positive cooperativity, in particular with the SUR2(Y1206S) isoforms. 4: In the absence of the coupled enzyme systems, inhibition curves of MgATP or MgADP were generally shifted leftwards. This indicated synergy of MgATP and MgATP in acting together. 5: Coexpression of SUR1 and SUR2B(Y1206S) with Kir6.2 reduced both potency and efficacy of ATP in inhibiting glibenclamide binding; this was particularly marked for Kir6.2/SUR1. 6: The data show (a) that the inhibitory effects of ATP and ADP on glibenclamide binding differ from one another, (b) that they depend on the SUR subtype, and (c) that they are weakened by coexpression with Kir6.2.

Four novel splice variants of sulfonylurea receptor 1

ATP-sensitive K(+) (K(ATP)) channels are composed of pore-forming Kir6.x subunits and regulatory sulfonylurea receptor (SUR) subunits. SURs are ATP-binding cassette proteins with two nucleotide-binding folds (NBFs) and binding sites for sulfonylureas, like glibenclamide, and for channel openers. Here we report the identification and functional characterization of four novel splice forms of guinea pig SUR1. Three splice forms originate from alternative splicing of the region coding for NBF1 and lack exons 17 (SUR1Delta17), 19 (SUR1Delta19), or both (SUR1Delta17Delta19). The fourth (SUR1C) is a COOH-terminal SUR1-fragment formed by exons 31-39 containing the last two transmembrane segments and the COOH terminus of SUR1. RT-PCR analysis showed that these splice forms are expressed in several tissues with strong expression of SUR1C in cardiomyocytes. Confocal microscopy using enhanced green fluorescent protein-tagged SUR or Kir6.x did not provide any evidence for involvement of these splice forms in the mitochondrial K(ATP) channel. Only SUR1 and SUR1Delta17 showed high-affinity binding of glibenclamide (K(d) approximately 2 nM in the presence of 1 mM ATP) and formed functional K(ATP) channels upon coexpression with Kir6.2.

The novel diazoxide analog 3-isopropylamino-7-methoxy-4H-1,2,4-benzothiadiazine 1,1-dioxide is a selective Kir6.2/SUR1 channel opener

ATP-sensitive K(+) (K(ATP)) channels are activated by a diverse group of compounds known as potassium channel openers (PCOs). Here, we report functional studies of the Kir6.2/SUR1 Selective PCO 3-isopropylamino-7-methoxy-4H-1,2,4-benzothiadiazine 1,1-dioxide (NNC 55-9216). We recorded cloned K(ATP) channel currents from inside-out patches excised from Xenopus laevis oocytes heterologously expressing Kir6.2/SUR1, Kir6.2/SUR2A, or Kir6.2/SUR2B, corresponding to the beta-cell, cardiac, and smooth muscle types of the K(ATP) channel. NNC 55-9216 reversibly activated Kir6.2/SUR1 currents (EC(50) = 16 micromol/l). This activation was dependent on intracellular MgATP and was abolished by mutation of a single residue in the Walker A motifs of either nucleotide-binding domain of SUR1. The drug had no effect on Kir6.2/SUR2A or Kir6.2/SUR2B currents. We therefore used chimeras of SUR1 and SUR2A to identify regions of SUR1 involved in the response to NNC 55-9216. Activation was completely abolished and significantly reduced by swapping transmembrane domains 8-11. The reverse chimera consisting of SUR2A with transmembrane domains 8-11 and NBD2 consisting SUR1 was activated by NNC 55-9216, indicating that these SUR1 regions are important for drug activation. [(3)H]glibenclamide binding to membranes from HEK293 cells transfected with SUR1 was displaced by NNC 55-9216 (IC(50) = 105 micromol/l), and this effect was impaired when NBD2 of SUR1 was replaced by that of SUR2A. These results suggest NNC 55-9216 is a SUR1-selective PCO that requires structural determinants, which differ from those needed for activation of the K(ATP) channel by pinacidil and cromakalim. The high selectivity of NNC 55-9216 may prove to be useful for studies of the molecular mechanism of PCO action.

Interaction of K(ATP) channel modulators with sulfonylurea receptor SUR2B: implication for tetramer formation and allosteric coupling of subunits

Sulfonylurea receptors (SURs) are subunits of ATP-sensitive K(+) channels (K(ATP) channels); they mediate the channel-closing effect of sulfonylureas such as glibenclamide and the channel-activating effect of K(ATP) channel openers such as the pinacidil analog P1075. We investigated the inhibition by MgATP and P1075 of glibenclamide binding to SUR2B, the SUR subtype in smooth muscle. To increase specific binding, experiments were also performed using SUR2B(Y1206S), a mutant with higher affinity for glibenclamide than for the wild-type (K(D )= 4 versus 22 nM, respectively) but otherwise exhibiting similar pharmacological properties. In the absence of MgATP, [(3)H]glibenclamide binding to both SURs was homogenous. MgATP inhibited [(3)H]glibenclamide binding to both SURs to 25% by reducing the apparent number of glibenclamide binding sites, leaving the affinity unchanged. In the absence of MgATP, P1075 inhibited [(3)H]glibenclamide binding in a monophasic manner with K(i) approximately 1 microM. In the presence of MgATP (1 mM), inhibition was biphasic with one K(i) value resembling the true affinity of P1075 for SUR2B (2-6 nM) and the other resembling K(i) in the absence of MgATP (approximately 1 microM). The data show that (1) MgATP induces heterogeneity in the glibenclamide sites; (2) the high-affinity glibenclamide sites remaining with MgATP are linked to two classes of P1075 sites; and (3) P1075 interacts specifically with SUR2B also in the absence of MgATP. The data are discussed with the assumption that SUR2B, expressed alone, forms tetramers; that MgATP induces allosteric interactions between the subunits; and that mixed SUR2B-glibenclamide-P1075 complexes can exist at equilibrium.

Synthesis and characterization of a novel tritiated KATP channel opener with a benzopyran structure

The synthesis of a tritiated benzopyran-type opener of the ATP-dependent K+ channel (KATP channel), [3H]-PKF217 - 744 (3S,4R)-N-[3,4-dihydro-2,2-dimethyl-3-hydroxy-6-(2-methyl-4-pyridinyl)-2H-1-benzopyran-4-yl]-3-[2,6-3H]pyridinecarboxamide with a specific activity of 50 Ci mmol(-1) is described. Binding of the ligand was studied in membranes from human embryonic kidney cells transfected with the sulphonylurea receptor isoforms, SUR2B and SUR2A, respectively. PKF217 - 744 was confirmed as being a KATP channel opener by its ability to open the Kir6.1/SUR2B channel, the recombinant form of the vascular KATP channel, and to inhibit binding of the pinacidil analogue, [3H]-P1075, to SUR2B (Ki=26 nM). The kinetics of [3H]-PKF217 - 744 binding to SUR2B was described by rate constants of association and dissociation of 6.9x10(6) M(-1) min(-1) and 0.09 min(-1), respectively. Binding of [3H]-PKF217 - 744 to SUR2B/2A was activated by MgATP (EC50 approximately 3 microM) and inhibited (SUR2B) or enhanced (SUR2A) by MGADP: Binding of [3H]-PKF217 - 744 to SUR2B was inhibited by representatives of the different structural classes of openers and sulphonylureas. Ki values were identical with those obtained using the opener [3H]-P1075 as the radioligand. Glibenclamide accelerated dissociation of the SUR2B-[3H]-PKF217 - 744 complex. The data show that the affinity of [3H]-PKF217 - 744 binding to SUR2B is approximately 6 times lower than that of [3H]-P1075. This is due to a surprisingly slow association rate of the benzopyran-type ligand, suggesting a complex mechanism of opener binding to SUR. The other pharmacological properties of the two opener radioligands are identical.

Characterization of a mutant sulfonylurea receptor SUR2B with high affinity for sulfonylureas and openers: differences in the coupling to Kir6.x subtypes

ATP-dependent K(+) channels are composed of pore-forming subunits of the Kir6.x family and of sulfonylurea receptors (SURs). SUR1, expressed in pancreatic beta-cells, has a higher affinity for sulfonylureas, such as glibenclamide, than SUR2B, expressed in smooth muscle. This difference is mainly caused by serine 1237 in SUR1 corresponding to tyrosine 1206 in SUR2B. To increase the affinity of SUR2B for glibenclamide, the mutant SUR2B(Y1206S) was constructed. In whole-cell patch-clamp experiments, glibenclamide inhibited the channel formed by coexpression of mutant SUR2B with Kir6.1 or 6.2 in human embryonic kidney cells with IC(50) values of 2.7 and 13 nM, respectively (wild-type, 43 and 167 nM). In intact cells, [(3)H]glibenclamide bound to mutant SUR2B with a K(D) value of 4.7 nM (wild-type, 32 nM); coexpression with Kir6.1 or 6.2 increased affinity by 4- and 8-fold, respectively. Binding of the opener [(3)H]P1075 to SUR2B(Y1206S) was the same as to wild-type and was unaffected by coexpression. In cells, the ratio of glibenclamide:P1075 sites was approximately 1:1; in membranes, it varied with the MgATP concentration. Heterologous competition curves were generally biphasic; the shape of the curve depended on the Kir-subtype. The effects of coexpression were weakened or abolished when binding assays were conducted in membranes. It is concluded that the mutation Y1206S increases the affinity of SUR2B for and the channel sensitivity toward glibenclamide by 7- to 15-fold. The interaction of glibenclamide (but not opener) with mutant SUR2B is modified by coexpression with Kir6.x in a manner depending on the Kir subtype and on the integrity of the cell.

New windows on the mechanism of action of K(ATP) channel openers

K(ATP) channel openers are a diverse group of drugs with a wide range of potential therapeutic uses. Their molecular targets, the K(ATP) channels, exhibit tissue-specific responses because they possess different types of regulatory sulfonylurea receptor subunits. It is well recognized that complex interactions occur between K(ATP) channel openers and nucleotides, but the cloning of the K(ATP) channel has introduced a new dimension to the study of these events and has furthered our understanding of the molecular basis of the action of K(ATP) channel openers.

Molecular characterization of human SUR2-containing K(ATP) channels

The distribution of human sulfonylurea receptor-2 (SUR2)-containing K(ATP) channels was investigated using reverse transcriptase-polymerase chain reaction (RT-PCR). mRNA for SUR2B was detected in a variety of tissues including brain, skeletal, cardiac and smooth muscle, whereas SUR2A message was restricted to cardiac and skeletal muscle. An additional splice variant of SUR2 that lacked exon 17 was also identified by RT-PCR in tissues expressing both SUR2A and SUR2B or SUR2B alone. Quantification of RNA for SUR2 exon 17+ and SUR2 exon 17- splice variants using real-time Taqman PCR indicated differential levels of expression in brain, kidney, skeletal muscle, heart and small intestine. Interestingly, the SUR2 exon 17+ variant is the major species expressed in all tissues examined in this study. Each of the SUR2 splice variants transiently expressed with the inward rectifier Kir 6.2 formed functional K(ATP) channels in HEK 293 cells as assessed either by changes in DiBAC(4)(3) fluorescence responses or glyburide-sensitive whole cell currents. Collectively, our findings demonstrate that various SUR2 splice variants have distinct expression patterns and can form functional K(ATP) channels.

On the mechanism of ADP-induced alteration of sulphonylurea sensitivity in cardiac ATP-sensitive K(+) channels

1. To study the mechanism of regulation of sulphonylurea sensitivity in ATP-sensitive K(+) (K(ATP)) channels, we used the inside-out patch clamp technique in guinea-pig ventricular myocytes. 2. In the absence of nucleotides, the half maximal concentration of tolbutamide inhibition of K(ATP) channels (IC(50)) was 0.4 mM, and it decreased to 0.1 mM when 0.1 mM ATP was added. 3. Increasing the ADP concentration from 0 to 0.1 and 0.3 mM in the absence of ATP shifted the IC(50) from 0.4 to 5.3 and 11.4 mM, respectively. Increasing the ADP concentration further to 1 and 3 mM conversely reduced the IC(50) to 9.5 and 4.4 mM, respectively. 4. In the absence of Mg(2+) and ADP, the IC(50) was calculated to 16.6 mM which was found to be less, 12.3, 5.1 and 2.5 mM, respectively, when the ADP concentration was increased to 0.1, 0.3 and 1 mM. 5. The IC(50)s for tolbutamide obtained at various concentrations of ADP in the presence of Mg(2+) were best fitted by equations reflecting a model that assumed two binding sites for ADP; one is a high affinity site that reduces the sensitivity to the sulphonylurea, while the other is a low affinity site that increases such sensitivity. Dissociation constants calculated for ADP to sites 1 and 2 were 2.6 microM and 46.7 mM, respectively. In the absence of Mg(2+), data were fitted by equations corresponding to a single site model (site 2); the dissociation constant for ADP was 25.0 mM. 6. It is concluded that ADP modifies tolbutamide sensitivity by binding to two sites. The high affinity site is strongly Mg(2+)-dependent, whereas the low affinity site is Mg(2+)-independent.

Pharmaco-topology of sulfonylurea receptors. Separate domains of the regulatory subunits of K(ATP) channel isoforms are required for selective interaction with K(+) channel openers

The differential responsiveness of (SUR1/K(IR)6.2)(4) pancreatic beta-cell versus (SUR2A/K(IR)6.2)(4) sarcolemmal or (SUR2B/K(IR)6. 0)(4) smooth muscle cell K(ATP) channels to K(+) channel openers (KCOs) is the basis for the selective prevention of hyperinsulinemia, myocardial infarction, and acute hypertension. KCO-stimulation of K(ATP) channels is a unique example of functional coupling between a transport ATPase and a K(+) inward rectifier. KCO binding to SUR is Mg-ATP-dependent and antagonizes the inhibition of (K(IR)6.0)(4) pore opening by nucleotides. Patch-clamping of matched chimeric human SUR1-SUR2A/K(IR)6.2 channels was used to identify the SUR regions that specify the selective response of sarcolemmal versus beta-cell channels to cromakalim or pinacidil versus diazoxide. The SUR2 segment containing the 12th through 17th predicted transmembrane domains, TMD12-17, confers sensitivity to the benzopyran, cromakalim, and the pyridine, pinacidil, whereas an SUR1 segment which includes TMD6-11 and the first nucleotide-binding fold, NBF1, controls responsiveness to the benzothiadiazine, diazoxide. These data are incorporated into a functional topology model for the regulatory SUR subunits of K(ATP) channels.

Sulfonylurea receptors: ABC transporters that regulate ATP-sensitive K(+) channels

The association of sulfonylurea receptors (SURs) with K(IR)6.x subunits to form ATP-sensitive K(+) channels presents perhaps the most unusual function known for members of the transport ATPase family. The integration of these two protein subunits extends well beyond conferring sensitivity to sulfonylureas. Recent studies indicate SUR-K(IR)6.x interactions are critical for all of the properties associated with native K(ATP) channels including quality control over surface expression, channel kinetics, inhibition and stimulation by Mg-nucleotides and response both to channel blockers like sulfonylureas and to potassium channel openers. K(ATP) channels are a unique example of the physiologic and medical importance of a transport ATPase and provide a paradigm for how other members of the family may interact with other ion channels.

Differential effects of pinacidil and levcromakalim on the contractions elicited electrically or by noradrenaline in the portal vein of the rabbit

The present study was undertaken to examine the antivasoconstrictor effects of pinacidil and levcromakalim, two potassium channel openers (PCOs), on the isolated rabbit portal vein and to define the role for different subtypes of pre- and/or post-synaptic K+ channels in the antivasoconstrictor action of the PCOs. The vein strips were contracted by electrical field stimulation (EFS) or by exogenous noradrenaline (NA). The results of this study showed that pinacidil produced a more potent inhibition of the neurogenic contractions (pD2 = 6.04 +/- 0.05) than of contractions induced by exogenous NA (pD2 = 4.90 +/- 0.10). Glibenclamide (1 microM), a selective blocker of adenosine triphosphate (ATP)-sensitive K+ channels (K(ATP)), did not affect the pinacidil-induced inhibition of contractions evoked by exogenous NA. In contrast, glibenclamide (0.1-10 microM) significantly antagonized the effect of pinacidil on EFS evoked contractions in a noncompetitive manner. There was no difference between the inhibitory effects of levcromakalim on neurogenic contractions (pD2 = 7.58 +/- 0.05) and contractions evoked by exogenous NA (pD2 = 7.64 +/- 0.08). Glibenclamide (1 microM) antagonized in the same manner the levcromakalim-induced inhibition of neurogenic contractions and contractions evoked by exogenous NA. Moreover, glibenclamide competitively antagonized the effect of levcromakalim on EFS induced contractions of the rabbit portal vein (pA2 = 6.40 +/- 0.10). Charybdotoxin (0.4 microM) and apamin (0.1 microM) did not influence the inhibitory effects of pinacidil and levcromakalim, both on contractions evoked by EFS and contractions evoked by exogenous NA. These results suggest that the antivasoconstrictor effect of levcromakalim might be postsynaptic and associated with opening of the smooth muscle K(ATP) channels. In contrast, it is hypothesized that the effect of pinacidil on neurogenic contractions is due to an interference with K(ATP) channels in the neuromuscular synapse. It seems that the action of pinacidil on the NA contractions is mediated by another still undefined mechanisms of pinacidil.

Identification of the Potassium Channel Opener Site on Sulfonylurea Receptors

Diversity of sulfonylurea receptor (SUR) subunits underlies tissue specific pharmacology of K(ATP) channels, which represent critical regulators of electrical activity in numerous cells. Notably, the neuronal/pancreatic beta-cell receptor, SUR1, imparts high sensitivity to hypoglycemic sulfonylureas (SUs; e.g. glibenclamide) and low to potassium channel openers (KCOs; e.g. P1075), whereas the opposite drug sensitivities are conferred by cardiovascular receptors, SUR2A and SUR2B. By exchanging domains between SUR1 and SUR2B, we identify two regions (KCO I: Thr(1059)-Leu(1087) and KCO II: Arg(1218)-Asn(1320); rat SUR2 numbering) within the second set of transmembrane domains (TMDII) as critical for KCO binding. Swapping both regions reconstitutes KCO affinities and sensitivities of the donor SUR isoform. High glibenclamide affinity of SUR1 is not reduced by transfer of KCO I plus II from SUR2B, demonstrating that high SU and KCO affinity can coexist in the same SUR molecule. Consistently, high SU affinity was imparted on SUR2B by substituting the region separating KCO I and II (Ile(1088)-Val(1217)) with the corresponding domain of SUR1. We infer the receptor sites for KCOs and SUs to be closely associated within a regulatory domain (Thr(1059)-Asn(1320)) in TMDII of SURs.

Structural requirements of sulphonylureas and analogues for interaction with sulphonylurea receptor subtypes

1. The structure-activity relationship for hypoglycaemic sulphonylureas and analogues was examined. Binding affinities were compared using membranes from HIT-T15 cells (beta-cell line) and from COS-7 cells transiently expressing sulphonylurea receptor subtypes (SUR1, SUR2A and SUR2B). Inhibition of adenosine-triphosphate-sensitive K+ channels (KATP-channels) was measured in mouse pancreatic beta-cells. 2. The tested compounds displayed similar binding affinities for SUR2A and SUR2B. 3. Meglitinide (benzoic acid derivative) bound to SUR1 and the SUR2 isoforms with similar affinities. Replacement of the carboxyl group of meglitinide by a methyl group significantly decreased the binding affinities for SUR1 and the SUR2 isoforms (>4 fold) and the potency to inhibit KATP-channel activity of beta-cells (24 fold). Replacement of the carboxyl group of meglitinide by a sulphonylurea group significantly increased the affinities for SUR1 (5 fold) and the SUR2 isoforms (13 - 16 fold). 4. Glibenclamide bound to the SUR2 isoforms with 300 - 500 fold lower affinity than to SUR1. Exchanging the cyclohexyl ring of glibenclamide by a methyl group or removal of the lipophilic side chain of glibenclamide (5-chloro-2-methoxy-benzamidoethyl chain) markedly reduced but did not abolish the selectivity for SUR1. 5. In conclusion, interaction of sulphonylureas and acidic analogues with SUR1, SUR2A and SUR2B is favoured by the anionic group of these drugs. Hypoglycaemic sulphonylureas (e.g. glibenclamide) owe selectivity for SUR1 to lipophilic substitution on their urea group. Sulphonylureas without lipophilic substitution on the urea group could represent lead compounds for the development of SUR2-selective drugs.

Relaxant activity in rat aorta and trachea, conversion to a muscarinic receptor antagonist and structure-activity relationships of new K(ATP) activating 6-varied benzopyrans

To characterize ATP-sensitive channels (K(ATP) channels) benzopyrans with different substituents at position 6 were synthesized as new K(ATP)-activators. Their relaxant potencies were determined in rat aorta and trachea. In aorta, pEC50-values (-log, M) ranged from 7.37 to 5.43; in trachea, pEC50-values were 0.3 to 0.8 log units lower. Functional data were compared with binding data obtained in calf tracheal cells using the cyanoguanidine [3H]P1075 (N-cyano-N'-1,1-dimethyl[2,3(n)-3H]propyl)-N11-(3-pyridinyl)guanidine) as radioligand. A high correlation (r = 0.96) between pEC50- and pKD-values indicated that tracheal relaxation produced by benzopyrans is mediated via K(ATP) channels without signal amplification. The permanently charged trimethylammonium derivative designed as a probe for the membrane site of action completely lost its affinity for K(ATP) channels, but converted to an antagonist for muscarinic acetylcholine receptors (pK(B) = 6.12+/-0.10), as confirmed in radioligand binding studies (pK(D) = 5.77+/-0.04). Structure-activity analyses revealed that the 6-substituent influences biological activity by a direct receptor interaction of its own and not indirectly by withdrawing electrons from the benzopyran nucleus. The variance of the biological activity is primarily determined by electrostatic properties, but desolvation energies additionally contribute.

Molecular biology of adenosine triphosphate-sensitive potassium channels

KATP channels are a newly defined class of potassium channels based on the physical association of an ABC protein, the sulfonylurea receptor, and a K+ inward rectifier subunit. The beta-cell KATP channel is composed of SUR1, the high-affinity sulfonylurea receptor with multiple TMDs and two NBFs, and KIR6.2, a weak inward rectifier, in a 1:1 stoichiometry. The pore of the channel is formed by KIR6.2 in a tetrameric arrangement; the overall stoichiometry of active channels is (SUR1/KIR6.2)4. The two subunits form a tightly integrated whole. KIR6.2 can be expressed in the plasma membrane either by deletion of an ER retention signal at its C-terminal end or by high-level expression to overwhelm the retention mechanism. The single-channel conductance of the homomeric KIR6.2 channels is equivalent to SUR/KIR6.2 channels, but they differ in all other respects, including bursting behavior, pharmacological properties, sensitivity to ATP and ADP, and trafficking to the plasma membrane. Coexpression with SUR restores the normal channel properties. The key role KATP channel play in the regulation of insulin secretion in response to changes in glucose metabolism is underscored by the finding that a recessive form of persistent hyperinsulinemic hypoglycemia of infancy (PHHI) is caused by mutations in KATP channel subunits that result in the loss of channel activity. KATP channels set the resting membrane potential of beta-cells, and their loss results in a constitutive depolarization that allows voltage-gated Ca2+ channels to open spontaneously, increasing the cytosolic Ca2+ levels enough to trigger continuous release of insulin. The loss of KATP channels, in effect, uncouples the electrical activity of beta-cells from their metabolic activity. PHHI mutations have been informative on the function of SUR1 and regulation of KATP channels by adenine nucleotides. The results indicate that SUR1 is important in sensing nucleotide changes, as implied by its sequence similarity to other ABC proteins, in addition to being the drug sensor. An unexpected finding is that the inhibitory action of ATP appears to be through a site located on KIR6.2, whose affinity for ATP is modified by SUR1. A PHHI mutation, G1479R, in the second NBF of SUR1 forms active KATP channels that respond normally to ATP, but fail to activate with MgADP. The result implies that ATP tonically inhibits KATP channels, but that the ADP level in a fasting beta-cell antagonizes this inhibition. Decreases in the ADP level as glucose is metabolized result in KATP channel closure. Although KATP channels are the target for sulfonylureas used in the treatment of NIDDM, the available data suggest that the identified KATP channel mutations do not play a major role in diabetes. Understanding how KATP channels fit into the overall scheme of glucose homeostasis, on the other hand, promises insight into diabetes and other disorders of glucose metabolism, while understanding the structure and regulation of these channels offers potential for development of novel compounds to regulate cellular electrical activity.