A transmembrane domain of the sulfonylurea receptor mediates activation of ATP-sensitive K(+) channels by K(+) channel openers

Structure-Activity Relationships, Pharmacokinetics, and Pharmacodynamics of the Kir6.2/SUR1-Specific Channel Opener VU0071063

Glucose-stimulated insulin secretion from pancreatic β-cells is controlled by ATP-regulated potassium (K_ATP) channels composed of Kir6.2 and sulfonylurea receptor 1 (SUR1) subunits. The K_ATP channel-opener diazoxide is FDA-approved for treating hyperinsulinism and hypoglycemia but suffers from off-target effects on vascular K_ATP channels and other ion channels. The development of more specific openers would provide critically needed tool compounds for probing the therapeutic potential of Kir6.2/SUR1 activation. Here, we characterize a novel scaffold activator of Kir6.2/SUR1 that our group recently discovered in a high-throughput screen. Optimization efforts with medicinal chemistry identified key structural elements that are essential for VU0071063-dependent opening of Kir6.2/SUR1. VU0071063 has no effects on heterologously expressed Kir6.1/SUR2B channels or ductus arteriole tone, indicating it does not open vascular K_ATP channels. VU0071063 induces hyperpolarization of β-cell membrane potential and inhibits insulin secretion more potently than diazoxide. VU0071063 exhibits metabolic and pharmacokinetic properties that are favorable for an in vivo probe and is brain penetrant. Administration of VU0071063 inhibits glucose-stimulated insulin secretion and glucose-lowering in mice. Taken together, these studies indicate that VU0071063 is a more potent and specific opener of Kir6.2/SUR1 than diazoxide and should be useful as an in vitro and in vivo tool compound for investigating the therapeutic potential of Kir6.2/SUR1 expressed in the pancreas and brain.

Lean and Obese Coronary Perivascular Adipose Tissue Impairs Vasodilation via Differential Inhibition of Vascular Smooth Muscle K+ Channels

OBJECTIVE

The effects of coronary perivascular adipose tissue (PVAT) on vasomotor tone are influenced by an obese phenotype and are distinct from other adipose tissue depots. The purpose of this investigation was to examine the effects of lean and obese coronary PVAT on end-effector mechanisms of coronary vasodilation and to identify potential factors involved.

APPROACH AND RESULTS

Hematoxylin and eosin staining revealed similarities in coronary perivascular adipocyte size between lean and obese Ossabaw swine. Isometric tension studies of isolated coronary arteries from Ossabaw swine revealed that factors derived from lean and obese coronary PVAT attenuated vasodilation to adenosine. Lean coronary PVAT inhibited K(Ca) and KV7, but not KATP channel-mediated dilation in lean arteries. In the absence of PVAT, vasodilation to K(Ca) and KV7 channel activation was impaired in obese arteries relative to lean arteries. Obese PVAT had no effect on K(Ca) or KV7 channel-mediated dilation in obese arteries. In contrast, obese PVAT inhibited KATP channel-mediated dilation in both lean and obese arteries. The differential effects of obese versus lean PVAT were not associated with changes in either coronary KV7 or K(ATP) channel expression. Incubation with calpastatin attenuated coronary vasodilation to adenosine in lean but not in obese arteries.

CONCLUSIONS

These findings indicate that lean and obese coronary PVAT attenuates vasodilation via inhibitory effects on vascular smooth muscle K(+) channels and that alterations in specific factors such as calpastatin are capable of contributing to the initiation or progression of smooth muscle dysfunction in obesity.

Aortic aneurysm and craniosynostosis in a family with Cantu syndrome

Cantu syndrome is an autosomal dominant overgrowth syndrome associated with facial dysmorphism, congenital hypertrichosis, and cardiomegaly. Some affected individuals show bone undermodeling of variable severity. Recent investigations revealed that the disorder is caused by a mutation in ABCC9, encoding a regulatory SUR2 subunit of an ATP-sensitive potassium channel mainly expressed in cardiac and skeletal muscle as well as vascular smooth muscle. We report here on a Japanese family with this syndrome. An affected boy and his father had a novel missense mutation in ABCC9. Each patient had a coarse face and hypertrichosis. However, cardiomegaly was seen only in the boy, and macrosomia only in the father. Skeletal changes were not evident in either patient. Craniosynostosis in the boy and the development of aortic aneurysm in the father are previously undescribed associations with Cantu syndrome.

Glucose use in fasted rats under sevoflurane anesthesia and propofol anesthesia

BACKGROUND

We previously reported the marked differences in the effects of sevoflurane anesthesia and propofol anesthesia on glucose use in fed rats; however, we could not elucidate mechanisms underlying the differences.

METHODS

We used fasted rats in this study. After surgical preparation under sevoflurane anesthesia, rats were divided into 3 groups: awake rats, rats under sevoflurane anesthesia, and rats under propofol anesthesia. All rats underwent the IV glucose tolerance test (IVGTT); 0.5 g/kg glucose was administered IV to rats. Just before IVGTT, some rats were pretreated with glibenclamide or diazoxide. We measured glucose, insulin, tumor necrosis factor-α (TNF-α), and high molecular weight adiponectin levels during IVGTT and calculated the quantitative insulin sensitivity check index (QUICKI) using glucose and insulin levels before glucose administration in each rat.

RESULTS

Before glucose administration, rats under sevoflurane anesthesia showed similar glucose and insulin levels with significantly higher QUICKI compared with awake rats, while rats under propofol anesthesia showed similar glucose levels and significantly higher insulin levels with significantly lower QUICKI compared with awake rats. After glucose administration, rats under sevoflurane anesthesia showed significantly higher glucose levels and similar insulin levels compared with awake rats, while rats under propofol anesthesia showed similar glucose levels and significantly higher insulin levels compared with awake rats. Before glucose administration, TNF-α levels in rats under sevoflurane anesthesia and rats under propofol anesthesia were similar to those in awake rats. After glucose administration, TNF-α was undetectable in all awake rats and all rats under sevoflurane anesthesia, whereas TNF-α was detectable in all rats under propofol anesthesia; TNF-α levels in rats under propofol anesthesia were significantly higher than those in awake rats. High molecular weight adiponectin levels in rats under sevoflurane anesthesia and rats under propofol anesthesia were similar to those in awake rats throughout the experimental period. In rats under sevoflurane anesthesia, glibenclamide significantly decreased glucose levels and significantly increased insulin levels; however, diazoxide produced no significant effects on glucose and insulin levels. In rats under propofol anesthesia, glibenclamide significantly decreased glucose levels and significantly increased insulin levels, while diazoxide significantly decreased glucose levels without changing insulin levels.

CONCLUSIONS

Sevoflurane anesthesia attenuates glucose-induced insulin secretion without affecting basic insulin secretion, while propofol anesthesia enhances insulin secretion. Propofol anesthesia exaggerates insulin-resistive conditions, whereas sevoflurane anesthesia dose not impair insulin sensitivity; there may be a possible association of TNF-α with insulin-resistive conditions under propofol anesthesia.

The involvement of adenosine triphosphate-sensitive potassium channels in the different effects of sevoflurane and propofol on glucose metabolism in fed rats

BACKGROUND

Recently, we reported marked differences in the effects of sevoflurane and propofol on glucose metabolism; glucose use is impaired by sevoflurane, but not by propofol. Opening of adenosine triphosphate-sensitive potassium channels (K(ATP) channels) in β islet cells attenuates insulin secretion, while inhibition of K(ATP) channels in β islet cells increases insulin secretion. It is reported that volatile anesthetics open K(ATP) channels, whereas propofol inhibits K(ATP) channels. In this study, we examined the effects of sevoflurane and propofol on glucose metabolism under normovolemic and hypovolemic conditions, focusing on insulin secretion.

METHODS

Anesthesia was induced with sevoflurane (3% in 1 L/min oxygen) in all rats. After surgical preparation, rats were assigned to 2 groups. Anesthesia was maintained with sevoflurane (2% in 1 L/min oxygen) in the 1st group, and with propofol (a bolus dose of 30 mg/kg followed by continuous infusion at a rate of 30 mg · kg(-1) · h(-1)) in the 2nd group. Each group was divided into 3 subgroups: rats without pretreatment, rats pretreated with glibenclamide, and rats pretreated with nicorandil. After a 30-minute stabilization period, we withdrew 15 mL/kg of blood to induce hypovolemia. We evaluated glucose metabolism under both normovolemic and hypovolemic conditions by measuring blood glucose levels and plasma insulin levels.

RESULTS

Under both normovolemia and hypovolemia, glucose levels in rats anesthetized with sevoflurane were significantly higher than those in rats anesthetized with propofol, and insulin levels in rats anesthetized with sevoflurane were significantly lower than those in rats anesthetized with propofol. Glibenclamide, a K(ATP) channel inhibitor, significantly decreased glucose levels and significantly increased insulin levels under sevoflurane anesthesia, suggesting that sevoflurane decreases insulin secretion by opening K(ATP) channels in β islet cells. Glibenclamide significantly decreased glucose levels and significantly increased insulin levels under propofol anesthesia as well; however, insulin levels in rats pretreated with glibenclamide under propofol anesthesia were much higher than those in rats pretreated with glibenclamide under sevoflurane anesthesia. Furthermore, insulin levels in rats without pretreatment under propofol anesthesia seemed to be equal to or higher than those in rats pretreated with glibenclamide under sevoflurane anesthesia. These results suggest that there are marked differences in the effects of sevoflurane and propofol on insulin secretion regulated by K(ATP) channels in β islet cells. Nicorandil, a K(ATP) channel opener, produced no significant effects on glucose metabolism under both sevoflurane and propofol anesthesia.

CONCLUSIONS

Insulin secretion regulated by K(ATP) channels in β islet cells is involved, at least in part, in the different effects of sevoflurane and propofol on glucose metabolism.

Ontogeny of sulfonylurea-binding regulatory subunits of K(ATP) channels in the pregnant rat myometrium

ATP-sensitive potassium channels (K(ATP) channels) are composed of sulfonylurea receptors (SURs) and potassium inward rectifiers (Kir(6.x)) that assemble to form a large octameric channel. This study was designed to examine the expression and role of sulfonylurea-binding regulatory subunits 1 (SUR1 (ABCC8)) and 2 (SUR2 (ABCC9)) of the K(ATP) channels in the pregnant rat myometrium with particular regard to the contractility. RT-PCR and western blot analyses were performed to detect the presence of SUR1 and SUR2. The SUR1 levels were markedly increased in the early stages of pregnancy. The highest level was detected on day 6 of pregnancy, whereas in the late stages, the levels of SUR1 were significantly decreased. The SUR2 level remained unchanged throughout pregnancy. The SUR non-selective diazoxide and the SUR2-selective pinacidil inhibited oxytocin-induced contractions. Glibenclamide, a K(ATP) channel blocker, antagonized both pinacidil- and diazoxide-induced relaxations. It was established that SURs are responsible for pharmacological reactivity of K(ATP) channel openers. We conclude that both SURs are involved in the K(ATP) channel in the pregnant rat myometrium. It may further be concluded that 'pinacidil-like' K(ATP) channel openers may be of therapeutic relevance as tocolytic agents in the future.

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.

Quantitative structure-activity relationship study of ATP-sensitive potassium channel openers: derivatives of 3-alkylamino-4H-1,2,4-benzothiadiazine 1,1-dioxide

The inhibitory activity of glucose-induced insulin secretion on isolated rat pancreatic islets and the contractile activity of KCl-depolarized rat aorta rings of the derivatives of 3-alkylamino-4H-1,2,4-benzothiadiazine 1,1-dioxide are quantitatively analyzed using multiple regression analysis. The study has helped to ascertain the role of different substituents in explaining these observed inhibitory activities. From a derived most significant correlation equation, it was concluded that a less hydrophobic 3-substituent and a less bulky 7-substituent in addition to a 3-aminoisopropyl and a 6-chloro substituent are advantageous to enhance the inhibitory action of a compound towards rat pancreatic islets. On the other hand, the more hydrophobic 6- and 7-substituents augment the contractile activity. The analysis, in this way, provided the grounds for rationalizing the substituent selection in designing the improved potency compounds in the series.

Identification of two types of ATP-sensitive K+ channels in rat ventricular myocytes

The ATP-sensitive K(+) (K(ATP)) channels are known to provide a functional linkage between the electrical activity of the cell membrane and metabolism. Two types of inwardly rectifying K(+) channel subunits (i.e., Kir6.1 and Kir6.2) with which sulfonylurea receptors are associated were reported to constitute the K(ATP) channels. In this study, we provide evidence to show two types of K(ATP) channels with different biophysical properties functionally expressed in isolated rat ventricular myocytes. Using patch-clamp technique, we found that single-channel conductance for the different two types of K(ATP) channels in these cells was 57 and 21 pS. The kinetic properties, including mean open time and bursting kinetics, did not differ between these two types of K(ATP) channels. Diazoxide only activated the small-conductance K(ATP) channel, while pinacidil and dinitrophenol stimulated both channels. Both of these K(ATP) channels were sensitive to block by glibenclamide. Additionally, western blotting, immunochemistry, and RT-PCR revealed two types of Kir6.X channels, i.e., Kir6.1 and Kir6.2, in rat ventricular myocytes. Single-cell Ca(2+) imaging also revealed that similar to dinitrophenol, diazoxide reduced the concentration of intracellular Ca(2+). The present results suggest that these two types of K(ATP) channels may functionally be related to the activity of heart cells.

ABCC8 and ABCC9: ABC transporters that regulate K+ channels

The sulfonylurea receptors (SURs) ABCC8/SUR1 and ABCC9/SUR2 are members of the C-branch of the transport adenosine triphosphatase superfamily. Unlike their brethren, the SURs have no identified transport function; instead, evolution has matched these molecules with K(+) selective pores, either K(IR)6.1/KCNJ8 or K(IR)6.2/KCNJ11, to assemble adenosine triphosphate (ATP)-sensitive K(+) channels found in endocrine cells, neurons, and both smooth and striated muscle. Adenine nucleotides, the major regulators of ATP-sensitive K(+) (K(ATP)) channel activity, exert a dual action. Nucleotide binding to the pore reduces the activity or channel open probability, whereas Mg-nucleotide binding and/or hydrolysis in the nucleotide-binding domains of SUR antagonize this inhibitory action to stimulate channel openings. Mutations in either subunit can alter this balance and, in the case of the SUR1/KIR6.2 channels found in neurons and insulin-secreting pancreatic beta cells, are the cause of monogenic forms of hyperinsulinemic hypoglycemia and neonatal diabetes. Additionally, the subtle dysregulation of K(ATP) channel activity by a K(IR)6.2 polymorphism has been suggested as a predisposing factor in type 2 diabetes mellitus. Studies on K(ATP) channel null mice are clarifying the roles of these metabolically sensitive channels in a variety of tissues.

Augmented activity of adenosine triphosphate-sensitive K+ channels induced by droperidol in the rat aorta

Droperidol produces the inhibition of K+ channels in cardiac myocytes. However, the effects of droperidol on K+ channels have not been studied in blood vessels. Therefore, we designed the present study to determine whether droperidol modulates the activity of adenosine triphosphate (ATP)-sensitive K+ channels in vascular smooth muscle cells. Rat aortic rings without endothelium were suspended or used for isometric force and membrane potential recordings, respectively. Vasorelaxation and hyperpolarization induced by levcromakalim (10(-8) to 10(-5) M or 10(-5) M, respectively) were completely abolished by the ATP-sensitive K+ channel antagonist glibenclamide (10(-5) M). Droperidol (10(-7) M) and an alpha-adrenergic receptor antagonist phentolamine (3 x 10(-9) M) caused a similar vasodilator effect (approximately 20% of vasorelaxation compared with maximal vasorelaxation induced by papaverine [3 x 10(-4) M]), whereas glibenclamide did not alter vasorelaxation induced by droperidol. Droperidol (3 x 10(-8) M to 10(-7) M) augmented vasorelaxation and hyperpolarization produced by levcromakalim, whereas phentolamine (3 x 10(-9) M) did not alter this vasorelaxation. Glibenclamide (10(-5) M) abolished the vasodilating and hyperpolarizing effects of levcromakalim in the aorta treated with droperidol (10(-7) M). These results suggest that droperidol augments vasodilator activity via ATP-sensitive K+ channels. However, it is unlikely that this augmentation is mediated by the inhibition of alpha-adrenergic receptors in vascular smooth muscles.

Characterization of K(ATP)-channels in rat basilar and middle cerebral arteries: studies of vasomotor responses and mRNA expression

Changes in the activity of K+ channels represent a major mechanism that regulates vascular tone. Cerebrovascular adenosine 5'-triphosphate-sensitive K+(K(ATP)) channels were characterized in studies of the molecular expression and vasomotor reactivity to different K(ATP) channel openers in rat basilar and middle cerebral arteries. Both arteries showed strong mRNA expression of the subunits of the pore-forming inward-rectifying K+ channel type 6.1 (Kir6.1), Kir6.2 and the connected sulfonylurea receptor (SUR) subunits, SUR1 and SUR2B, while only weak bands for SUR2A were seen. The K(ATP) channel openers induced relaxation of prostaglalndin F2alpha-precontracted isolated basilar and middle cerebral arteries with the order of potency N-Cyano-N-(1,1-dimethylpropyl)-N''-3pyridylguanidine (P-1075)>levcromakalim>N-(4-Phenylsulfonylphenyl)-3,3,3-trifluoro-2-hydroxy-2-methylpropanamide (ZM226600)>pinacidil>diazoxide. The responses induced by levcromakalim, ZM226600 and diazoxide were significantly more potent in basilar arteries than in middle cerebral arteries, while pinacidil and P-1075 were equipotent. Endothelium removal decreased (P<0.05) the sensitivity (pIC50) of basilar arteries, but not of middle cerebral arteries, to pinacidil, levcromakalim, P-1075 and ZM226600. The maximum relaxant response to P-1075 was stronger (P<0.005) in basilar arteries with endothelium than without endothelium. Correlation of the relaxant potency of K(ATP) channel openers in rat basilar and middle cerebral arteries with historical measurements of affinity obtained in COS-7 cell lines expressing either SUR1, SUR2A or SUR2B showed that vasodilatation by K(ATP) channel openers correlated with binding to either the SUR2A or the SUR2B subunit. Glibenclamide was a blocker of relaxation induced by pinacidil, levcromakalim, P-1075 and ZM226600 in basilar arteries. Only a weak antagonistic effect of glibenclamide on pinacidil-, levcromakalim- and ZM226600-induced relaxations was found in middle cerebral arteries. The subunit profile and the observed pharmacological properties suggest that the K(ATP) channels expressed in rat basilar and middle cerebral artery are likely to be composed of SUR2B co-associated with Kir6.1 or Kir6.2. In basilar arteries, but not in middle cerebral arteries, endothelial K(ATP) channels may be involved.

Activation of KATP channels by H2S in rat insulin-secreting cells and the underlying mechanisms

H2S is an important gasotransmitter, generated in mammalian cells from L-cysteine metabolism. As it stimulates K(ATP) channels in vascular smooth muscle cells, H2S may also function as an endogenous opener of K(ATP) channels in INS-1E cells, an insulin-secreting cell line. In the present study, K(ATP) channel currents in INS-1E cells were recorded using the whole-cell and single-channel recording configurations of the patch-clamp technique. K(ATP) channels in INS-1E cells have a single-channel conductance of 78 pS. These channels were activated by diazoxide and inhibited by gliclazide. ATP (3 mm) in the pipette solution inhibited K(ATP) channels in INS-1E cells. Significant amount of H2S was produced from INS-1E cells in which the expression of cystathinonie gamma-lyase (CSE) was confirmed. After INS-1E cells were transfected with CSE-targeted short interfering RNA (CSE-siRNA) or treated with DL-propargylglycine (PPG; 1-5 mm) to inhibit CSE, endogenous production of H2S was abolished. Increase in extracellular glucose concentration significantly decreased endogenous production of H2S in INS-1E cells, and increased insulin secretion. After transfection of INS-1E cells with adenovirus containing the CSE gene (Ad-CSE) to overexpress CSE, high glucose-stimulated insulin secretion was virtually abolished. Basal K(ATP) channel currents were significantly reduced after incubating INS-1E cells with a high glucose concentration (16 mm) or lowering endogenous H2S level by CSE-siRNA transfection. Under these conditions, exogenously applied H2S significantly increased whole-cell K(ATP) channel currents at concentrations equal to or lower than 100 microm. H2S (100 microm) markedly increased open probability by more than 2-fold of single K(ATP) channels (inside-out recording) in native INS-1E cells (n = 4, P < 0.05). Single-channel conductance and ATP sensitivity of K(ATP) channels were not changed by H2S. In conclusion, endogenous H2S production from INS-1E cells varies with in vivo conditions, which significantly affects insulin secretion from INS-1E cells. H2S stimulates K(ATP) channels in INS-1E cells, independent of activation of cytosolic second messengers, which may underlie H2S-inhibited insulin secretion from these cells. Interaction among H2S, glucose and the K(ATP) channel may constitute an important and novel mechanism for the fine control of insulin secretion from pancreatic beta-cells.

The size of a single residue of the sulfonylurea receptor dictates the effectiveness of K ATP channel openers

K(ATP) channel openers are a diverse group of molecules able to activate ATP-sensitive K(+) channels in a tissue-dependent manner by binding to the channel regulatory subunit, the sulfonylurea receptor (SUR), an ATP-binding cassette protein. Residues crucial to this action were previously identified in the last transmembrane helix of SUR, transmembrane helix 17. This study examined the residue at the most important position, 1253 in the muscle isoform SUR2A and the matching 1290 in the pancreatic/neuronal isoform SUR1 (rat numbering). At this position in either isoform, a threonine enables action of openers, whereas a methionine prohibits it. Using single-point mutagenesis, we have examined the physicochemical basis of this phenomenon and discovered that it relied uniquely on side chain volume and not on shape, polarity, or hydrogen-bonding capacity of the residue. Moreover, the aromatic nature of neighboring residues conserved in SUR1 and SUR2A was found necessary for SUR2A to sustain the wild-type levels of channel activation by the openers tested, the cromakalim analog SR47063 [4-(2-cyanimino-1,2-dihydro-1-pyridyl)-2,2-dimethyl-6-nitrochromene] and the pinacidil analog P1075 [N-cyano-N'-(1,1-dimethylpropyl)-N'-3-pyridylguanidine]. These observations suggest that these residues can interact with openers via nonspecific stacking interactions provided that the adjacent 1253/1290 residue does not obstruct access. The smaller Thr1253 of SUR2A would permit activation, whereas the bulky Met1290 of SUR1 would not. This hypothesis is discussed in the context of a simple molecular model of transmembrane helix 17.

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.

Inhibitory effect of high concentration of glucose on relaxations to activation of ATP-sensitive K+ channels in human omental artery

OBJECTIVE

The present study was designed to examine in the human omental artery whether high concentrations of D-glucose inhibit the activity of ATP-sensitive K+ channels in the vascular smooth muscle and whether this inhibitory effect is mediated by the production of superoxide.

METHODS AND RESULTS

Human omental arteries without endothelium were suspended for isometric force recording. Changes in membrane potentials were recorded and production of superoxide was evaluated. Glibenclamide abolished vasorelaxation and hyperpolarization in response to levcromakalim. D-glucose (10 to 20 mmol/L) but not l-glucose (20 mmol/L) reduced these vasorelaxation and hyperpolarization. Tiron and diphenyleneiodonium, but not catalase, restored vasorelaxation and hyperpolarization in response to levcromakalim in arteries treated with D-glucose. Calphostin C and Gö6976 simultaneously recovered these vasorelaxation and hyperpolarization in arteries treated with D-glucose. Phorbol 12-myristate 13 acetate (PMA) inhibited the vasorelaxation and hyperpolarization, which are recovered by calphostin C as well as Gö6976. D-glucose and PMA, but not l-glucose, significantly increased superoxide production from the arteries, whereas such increased production was reversed by Tiron.

CONCLUSIONS

These results suggest that in the human visceral artery, acute hyperglycemia modulates vasodilation mediated by ATP-sensitive K+ channels via the production of superoxide possibly mediated by the activation of protein kinase C.

Zinc is both an intracellular and extracellular regulator of KATP channel function

Extracellular Zn(2+) has been identified as an activator of pancreatic K(ATP) channels. We further examined the action of Zn(2+) on recombinant K(ATP) channels formed with the inward rectifier K(+) channel subunit Kir6.2 associated with either the pancreatic/neuronal sulphonylurea receptor 1 (SUR1) subunit or the cardiac SUR2A subunit. Zn(2+), applied at either the extracellular or intracellular side of the membrane appeared as a potent, reversible activator of K(ATP) channels. External Zn(2+), at micromolar concentrations, activated SUR1/Kir6.2 but induced a small inhibition of SUR2A/Kir6.2 channels. Cytosolic Zn(2+) dose-dependently stimulated both SUR1/Kir6.2 and SUR2A/Kir6.2 channels, with half-maximal effects at 1.8 and 60 microm, respectively, but it did not affect the Kir6.2 subunit expressed alone. These observations point to an action of both external and internal Zn(2+) on the SUR subunit. Effects of internal Zn(2+) were not due to Zn(2+) leaking out, since they were unaffected by the presence of a Zn(2+) chelator on the external side. Similarly, internal chelators did not affect activation by external Zn(2+). Therefore, Zn(2+) is an endogenous K(ATP) channel opener being active on both sides of the membrane, with potentially distinct sites of action located on the SUR subunit. These findings uncover a novel regulatory pathway targeting K(ATP) channels, and suggest a new role for Zn(2+) as an intracellular signalling molecule.

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.

Diazoxide affects the IF1 inhibitor protein binding to F1 sector of beef heart F0F1ATPsynthase

Diazoxide, a selective opener of the mitochondrial ATP-sensitive K+ channel (mitoK(ATP)), has been reported to enhance F(0)F(1)ATPsynthase inhibition during ischemia, but the underlying mechanisms are still unclear. Here, we demonstrate that diazoxide directly interacts with the F(1) sector of beef heart F(0)F(1)ATPsynthase markedly promoting the binding of the inhibitor protein (IF(1)) to beta subunit. More specifically, the treatment of soluble F(1) with one equivalent of diazoxide was sufficient to decrease the K(d) of IF(1)-F(1) complex at low pH. Such effect was revealed only on the cycling enzyme, while no effect was observed in the absence of Mg-ATP. However, diazoxide binding occurred independently from the catalysis, as shown by the structural changes induced by the drug in not catalytically active F(1) and revealed by CD spectra. In addition, kinetic analysis of ATP hydrolysis demonstrated that diazoxide exerts a stabilising role on Mg-ADP bound in the catalytic site of the beta subunit adopting the tight conformation (beta(DP)). In accordance, a stabilising effect of Mg-ADP at the nucleotide binding domain (NBD) has been reported also for K(ATP) channel. These results suggest that diazoxide binds to beta subunit at NBD, which is highly conserved in the ATP-binding cassette protein family, thus inducing nucleotide stabilisation and favouring F(1) conformation suitable for IF(1) binding. Finally, diazoxide also increased IF(1) binding to membrane bound F(1), while it did not influence the energisation-dependent IF(1) release. As IF(1) binding mediates the F(0)F(1)ATPsynthase inhibition, we suggest that such mechanism may contribute to cardioprotection during ischemia.

Functional distinctions between the mitochondrial ATP-dependent K+ channel (mitoKATP) and its inward rectifier subunit (mitoKIR)

The ATP-sensitive potassium channel from the inner mitochondrial membrane (mitoK(ATP)) is a highly selective conductor of K(+) ions. When isolated in the presence of nonionic detergent and reconstituted in liposomes, mitoK(ATP) is inhibited with high affinity by ATP (K((1/2)) = 20-30 microM). We have suggested that holo-mitoK(ATP) is a heteromultimer consisting of an inwardly rectifying K(+) channel (mitoKIR) and a sulfonylurea receptor (Grover, G. J., and Garlid, K. D. (2000) J. Mol. Cell. Cardiol. 32, 677-695). Here, we show that a 55-kDa protein isolated by ethanol extraction and reconstituted in bilayer lipid membranes and liposomes is the mitoKIR. This protein, which lacks the sulfonylurea receptor subunit, is inhibited with low affinity by ATP, with K(1/2) approximately 550 microM. ATP inhibition of both mitoKIR and holo-mitoK(ATP) is reversed by UDP (K((1/2))1/2 = 10-15 microM). Holo-mitoK(ATP) is and diazoxide, and the opened by cromakalim flux through the open channel is inhibited by glibenclamide and 5-hydroxydecanoate. None of these agents has any effect upon mitoKIR. We have identified two compounds that act specifically on mitoKIR. p-diethylaminoethylbenzoate reverses inhibition of mitoKIR by ATP and ADP at micromolar concentrations and also opens mitoK(ATP) in isolated mitochondria. Tetraphenylphosphonium inhibits K(+) flux through both mitoKIR and mitoK(ATP) with the same apparent affinity. These findings support the hypothesis that the 55-kDa mitoKIR is the channel component of mitoK(ATP).

Mitochondrial potassium transport: the K(+) cycle

Potassium transport plays three distinct roles in mitochondria. Volume homeostasis to prevent excess matrix swelling is a housekeeping function that is essential for maintaining the structural integrity of the organelle. This function is mediated by the K(+)/H(+) antiporter and was first proposed by Peter Mitchell. Volume homeostasis to prevent excess matrix contraction is a recently discovered function that maintains a fully expanded matrix when diffusive K(+) influx declines due to membrane depolarization caused by high rates of electron transport. Maintaining matrix volume under these conditions is important because matrix contraction inhibits electron transport and also perturbs the structure-function of the intermembrane space (IMS). This volume regulation is mediated by the mitochondrial ATP-sensitive K(+) channel (mitoK(ATP)). Cell signaling functions to protect the cell from ischemia-reperfusion injury and also to trigger transcription of genes required for cell growth. This function depends on the ability of mitoK(ATP) opening to trigger increased mitochondrial production of reactive oxygen species (ROS). This review discusses the properties of the mitochondrial K(+) cycle that help to understand the basis of these diverse effects.

Potent and selective activation of the pancreatic beta-cell type K(ATP) channel by two novel diazoxide analogues

AIMS/HYPOTHESIS

We investigated the pharmacological properties of two novel ATP sensitive potassium (K(ATP)) channel openers, 6-Chloro-3-isopropylamino-4 H-thieno[3,2- e]-1,2,4-thiadiazine 1,1-dioxide (NNC 55-0118) and 6-chloro-3-(1-methylcyclopropyl)amino-4 H-thieno[3,2-e]-1,2,4-thiadiazine 1,1-dioxide (NN414), on the cloned cardiac (Kir6.2/SUR2A), smooth muscle (Kir6.2/SUR2B) and pancreatic beta cell (Kir6.2/SUR1) types of K(ATP) channel.

METHODS

We studied the effects of these compounds on whole-cell currents through cloned K(ATP) channels expressed in Xenopus oocytes or mammalian cells (HEK293). We also used inside-out macropatches excised from Xenopus oocytes.

RESULTS

In HEK 293 cells, NNC 55-0118 and NN414 activated Kir6.2/SUR1 currents with EC(50) values of 0.33 micromol/l and 0.45 micromol/l, respectively, compared with that of 31 micro mol/l for diazoxide. Neither compound activated Kir6.2/SUR2A or Kir6.2/SUR2B channels expressed in oocytes, nor did they activate Kir6.2 expressed in the absence of SUR. Current activation was dependent on the presence of intracellular MgATP, but was not supported by MgADP.

CONCLUSION/INTERPRETATION

Both NNC 55-0118 and NN414 selectively stimulate the pancreatic beta-cell type of K(ATP) channel with a higher potency than diazoxide, by interaction with the SUR1 subunit. The high selectivity and efficacy of the compounds could prove useful for treatment of disease states where inhibition of insulin secretion is beneficial.

[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.

Dualistic actions of cromakalim and new potent 2H-1,4-benzoxazine derivatives on the native skeletal muscle K ATP channel

1 New 2H-1,4-benzoxazine derivatives were synthesized and tested for their agonist properties on the ATP-sensitive K(+) channels (K(ATP)) of native rat skeletal muscle fibres by using the patch-clamp technique. The novel modifications involved the introduction at position 2 of the benzoxazine ring of alkyl substituents such as methyl (-CH(3)), ethyl (-C(2)H(5)) or propyl (-C(3)H(7)) groups, while maintaining pharmacophore groups critical for conferring agonist properties. 2 The effects of these molecules were compared with those of cromakalim in the presence or absence of internal ATP (10(-4) M). In the presence of internal ATP, all the compounds increased the macropatch K(ATP) currents. The order of potency of the molecules as agonists was -C(3)H(7) (DE(50)=1.63 x 10(-8) M) >-C(2)H(5) (DE(50)=1.11 x 10(-7) M)>-CH(3) (DE(50)=2.81 x 10(-7) M)>cromak-slim (DE(50)= 1.42 x 10(-5) M). Bell-shaped dose-response curves were observed for these compounds and cromakalim indicating a downturn in response when a certain dose was exceeded. 3 In contrast, in the absence of internal ATP, all molecules including cromakalim inhibited the K(ATP) currents. The order of increasing potency as antagonists was cromakalim (IC(50)=1.15 x 10(-8) M)> or =-CH(3) (IC(50)=2.6 x 10(-8) M)>-C(2)H(5) (IC(50)=4.4 x 10(-8) M)>-C(3)H(7) (IC(50)=1.68 x 10(-7) M) derivatives. 4 These results suggest that the newly synthesized molecules and cromakalim act on muscle K(ATP) channel by binding on two receptor sites that have opposite actions. Alternatively, a more simple explanation is to consider the existence of a single site for potassium channel openers regulated by ATP which favours the transduction of the channel opening. The alkyl chains at position 2 of the 2H-1,4-benzoxazine nucleus is pivotal in determining the potency of benzoxazine derivatives as agonists or antagonists.

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.

Physiological and pathophysiological roles of ATP-sensitive K+ channels

ATP-sensitive potassium (K(ATP)) channels are present in many tissues, including pancreatic islet cells, heart, skeletal muscle, vascular smooth muscle, and brain, in which they couple the cell metabolic state to its membrane potential, playing a crucial role in various cellular functions. The K(ATP) channel is a hetero-octamer comprising two subunits: the pore-forming subunit Kir6.x (Kir6.1 or Kir6.2) and the regulatory subunit sulfonylurea receptor SUR (SUR1 or SUR2). Kir6.x belongs to the inward rectifier K(+) channel family; SUR belongs to the ATP-binding cassette protein superfamily. Heterologous expression of differing combinations of Kir6.1 or Kir6.2 and SUR1 or SUR2 variant (SUR2A or SUR2B) reconstitute different types of K(ATP) channels with distinct electrophysiological properties and nucleotide and pharmacological sensitivities corresponding to the various K(ATP) channels in native tissues. The physiological and pathophysiological roles of K(ATP) channels have been studied primarily using K(ATP) channel blockers and K(+) channel openers, but there is no direct evidence on the role of the K(ATP) channels in many important cellular responses. In addition to the analyses of naturally occurring mutations of the genes in humans, determination of the phenotypes of mice generated by genetic manipulation has been successful in clarifying the function of various gene products. Recently, various genetically engineered mice, including mice lacking K(ATP) channels (knockout mice) and mice expressing various mutant K(ATP) channels (transgenic mice), have been generated. In this review, we focus on the physiological and pathophysiological roles of K(ATP) channels learned from genetic manipulation of mice and naturally occurring mutations in humans.

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.

Characterization of two novel forms of the rat sulphonylurea receptor SUR1A2 and SUR1BDelta31

1. The ATP-sensitive potassium channel (K(ATP)) of pancreatic beta-cells is composed of the sulphonylurea-binding protein, SUR1, and the inwardly rectifying K(+) channel subunit, Kir6.2. We have characterized two novel isoforms of rat SUR1 in the RINm5F insulin-secreting cell line. 2. SUR1A2 is an allelic variant with a single amino acid change in the first nucleotide-binding domain. Coinjection of SUR1A2 plus Kir6.2 into Xenopus oocytes or expression of a SUR1A2-Kir6.2 tandem in HEK-293 cells yielded large currents with characteristics similar to the wild-type K(ATP) channel. 3. SUR1BDelta31, detected in several human tissues, is a splice variant of the rat SUR1 gene that lacks exon 31 of the corresponding human SUR1 gene. SUR1BDelta31 lacks the TM16-TM17 transmembrane-spanning helices leading to a protein with a different transmembrane topology. Coinjection of SUR1BDelta31 plus Kir6.2 into Xenopus oocytes or expression of the Kir6.2/SUR1BDelta31 tandem construct in HEK-293 cells did not result in any current, and a surface expression assay indicated that this channel does not reach the plasma membrane. 4. SUR1A2 and SUR1A1 proteins expressed in HEK-293 cells display similar binding affinities for [(3)H]-glibenclamide, while SUR1BDelta31 shows a 500-fold lower affinity. 5. These findings confirm that TM16-TM17 of SUR1 are important for high-affinity glibenclamide binding and that their deletion impairs trafficking of the K(ATP) channel to the surface membrane.

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.

Structural basis for the interference between nicorandil and sulfonylurea action

Nicorandil is a new antianginal agent that potentially may be used to treat the cardiovascular side effects of diabetes. It is both a nitric oxide donor and an opener of ATP-sensitive K(+) (K(ATP)) channels in muscle and thereby causes vasodilation of the coronary vasculature. The aim of this study was to investigate the domains of the K(ATP) channel involved in nicorandil activity and to determine whether nicorandil interacts with hypoglycemic sulfonylureas that target K(ATP) channels in pancreatic beta-cells. K(ATP) channels in muscle and beta-cells share a common pore-forming subunit, Kir6.2, but possess alternative sulfonylurea receptors (SURs; SUR1 in beta-cells, SUR2A in cardiac muscle, and SUR2B in smooth muscle). We expressed recombinant K(ATP) channels in Xenopus oocytes and measured the effects of drugs and nucleotides by recording macroscopic currents in excised membrane patches. Nicorandil activated Kir6.2/SUR2A and Kir6.2/SUR2B but not Kir6.2/SUR1 currents, consistent with its specificity for cardiac and smooth muscle K(ATP) channels. Drug activity depended on the presence of intracellular nucleotides and was impaired when the Walker A lysine residues were mutated in either nucleotide-binding domain of SUR2. Chimeric studies showed that the COOH-terminal group of transmembrane helices (TMs), especially TM 17, is responsible for the specificity of nicorandil for channels containing SUR2. The splice variation between SUR2A and SUR2B altered the off-rate of the nicorandil response. Finally, we showed that nicorandil activity was unaffected by gliclazide, which specifically blocks SUR1-type K(ATP) channels, but was severely impaired by glibenclamide and glimepiride, which target both SUR1 and SUR2-type K(ATP) channels.

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.

Cibenzoline has an inhibitory effect on vasorelaxation mediated by adenosine triphosphate-sensitive K(+) channels in the rat carotid artery

Studies in cardiac myocytes have shown that cibenzoline reduces adenosine triphosphate (ATP)-sensitive K(+) currents, suggesting that this class Ia antiarrhythmic drug may modify the activity of ATP-sensitive K(+) channels in these preparations. The effects of class Ia antiarrhythmic drugs on vasodilation mediated by ion channels have not been studied. Therefore, we designed this study to examine whether cibenzoline may produce changes in vasorelaxation in response to a selective ATP-sensitive K(+) channel opener, levcromakalim, in the isolated rat carotid artery. Rings of rat carotid arteries without endothelium were suspended for isometric force recording. Concentration-response curves were obtained in a cumulative fashion. During submaximal contraction to phenylephrine (3 x 10(-7) M), vasorelaxation in response to levcromakalim (10(-8) to 10(-5) M) or 1-hydroxy-2-oxo-3-(N-methyl-3-aminopropyl)-3-methyl-1-triazene (NOC-7; 10(-10) to 10(-5) M) was obtained. During contraction to phenylephrine, levcromakalim induced concentration-dependent vasorelaxation. A selective ATP-sensitive K(+) channel antagonist, glibenclamide (5 x 10(-6) M), completely abolished vasorelaxation in response to levcromakalim, whereas a selective Ca(2+)-dependent K(+) channel antagonist, iberiotoxin (5 x 10(-8) M), did not affect the relaxation. Cibenzoline (10(-6) to 10(-5) M) significantly reduced vasorelaxation to levcromakalim in a concentration-dependent fashion. In contrast, cibenzoline (10(-5) M) did not alter vasorelaxation to a nitric oxide donor, NOC-7. These results suggest that from the clinically relevant concentrations, a novel class Ia antiarrhythmic drug, cibenzoline, impairs carotid vasodilation mediated by ATP-sensitive K(+) channels.

IMPLICATIONS

In isolated rat carotid artery, cibenzoline (10(-6) to 10(-5) M) reduced vasorelaxation to levcromakalim in a concentration-dependent fashion. These results suggest that from the clinically relevant concentrations, a novel class Ia antiarrhythmic drug, cibenzoline, impairs carotid vasodilation mediated by adenosine triphosphate-sensitive K(+) channels.

The molecular basis of the specificity of action of K(ATP) channel openers

K(ATP) channels incorporate a regulatory subunit of the ATP-binding cassette (ABC) transporter family, the sulfonylurea receptor (SUR), which defines their pharmacology. The therapeutically important K(+) channel openers (e.g. pinacidil, cromakalim, nicorandil) act specifically on the SUR2 muscle isoforms but, except for diazoxide, remain ineffective on the SUR1 neuronal/pancreatic isoform. This SUR1/2 dichotomy underpinned a chimeric strategy designed to identify the structural determinants of opener action, which led to a minimal set of two residues within the last transmembrane helix of SUR. Transfer of either residue from SUR2A to SUR1 conferred opener sensitivity to SUR1, while the reverse operation abolished SUR2A sensitivity. It is therefore likely that these residues form part of the site of interaction of openers with the channel. Thus, openers would target a region that, in other ABC transporters, is known to be tightly involved with the binding of substrates and other ligands. This first glimpse of the site of action of pharmacological openers should permit rapid progress towards understanding the structural determinants of their affinity and specificity.

Differential response of K(ATP) channels containing SUR2A or SUR2B subunits to nucleotides and pinacidil

ATP-sensitive K-channels (K(ATP) channels) are the target for K(ATP)-channel openers (KCOs), such as pinacidil and P1075. These channels are formed from pore-forming Kir6.2 and regulatory sulfonylurea receptors (SUR2A in heart and skeletal muscle; SUR2B in smooth muscle). The two isoforms of SUR2 differ only in their final 42 amino acids, a region that includes neither the Walker A and B nucleotide binding motifs nor the proposed KCO binding site, yet channels containing SUR2A or SUR2B respond differently to both nucleotides and KCOs. We explored the basis for this difference by expressing Kir6.2/SUR2A and Kir6.2/SUR2B currents in Xenopus laevis oocytes. Kir6.2/SUR2B but not Kir6.2/SUR2A currents were activated by the Mg-nucleoside triphosphates MgATP and MgGTP, whereas both channel types responded to the diphosphates MgADP and MgGDP. This activation of Kir6.2/SUR2B currents by MgATP explains how the ATP concentration-response curve is shifted to the right in the presence of Mg(2+). In the absence of nucleotide, pinacidil and P1075 activated Kir6.2/SUR2B and Kir6.2/SUR2A currents, but the presence of nucleotide slowed the drug off-rates. In the presence of MgATP, the response to pinacidil reversed approximately 14 times more slowly with SUR2B than SUR2A. The EC(50) for ATP, measured by its ability to slow the pinacidil off-rate, was also approximately 20 times higher for channels containing SUR2A than SUR2B. Our findings suggest that nucleotide binding and/or hydrolysis is enhanced in SUR2B compared with SUR2A, and that the greater KCO-affinities of SUR2B compared with SUR2A may be a consequence of this altered nucleotide handling.

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.

ATPase activity of the sulfonylurea receptor: a catalytic function for the KATP channel complex

ATP-sensitive K+ (KATP) channels are unique metabolic sensors formed by association of Kir6.2, an inwardly rectifying K+ channel, and the sulfonylurea receptor SUR, an ATP binding cassette protein. We identified an ATPase activity in immunoprecipitates of cardiac KATP channels and in purified fusion proteins containing nucleotide binding domains NBD1 and NBD2 of the cardiac SUR2A isoform. NBD2 hydrolyzed ATP with a twofold higher rate compared to NBD1. The ATPase required Mg2+ and was insensitive to ouabain, oligomycin, thapsigargin, or levamisole. K1348A and D1469N mutations in NBD2 reduced ATPase activity and produced channels with increased sensitivity to ATP. KATP channel openers, which bind to SUR, promoted ATPase activity in purified sarcolemma. At higher concentrations, openers reduced ATPase activity, possibly through stabilization of MgADP at the channel site. K1348A and D1469N mutations attenuated the effect of openers on KATP channel activity. Opener-induced channel activation was also inhibited by the creatine kinase/creatine phosphate system that removes ADP from the channel complex. Thus, the KATP channel complex functions not only as a K+ conductance, but also as an enzyme regulating nucleotide-dependent channel gating through an intrinsic ATPase activity of the SUR subunit. Modulation of the channel ATPase activity and/or scavenging the product of the ATPase reaction provide novel means to regulate cellular functions associated with KATP channel opening.

Different binding properties and affinities for ATP and ADP among sulfonylurea receptor subtypes, SUR1, SUR2A, and SUR2B

ATP-sensitive potassium (K(ATP)) channels, composed of sulfonylurea receptor (SURx) and Kir6.x, play important roles by linking cellular metabolic state to membrane potential in various tissues. Pancreatic, cardiac, and vascular smooth muscle K(ATP) channels, which consist of different subtypes of SURx, differ in their responses to cellular metabolic state. To explore the possibility that different interactions of SURx with nucleotides cause differential regulation of K(ATP) channels, we analyzed the properties of nucleotide-binding folds (NBFs) of SUR1, SUR2A, and SUR2B. SURx in crude membrane fractions was incubated with 8-azido-[alpha-(32)P]ATP or 8-azido-[gamma-(32)P]ATP under various conditions and was photoaffinity-labeled. Then, SURx was digested mildly with trypsin, and partial tryptic fragments were immunoprecipitated with antibodies against NBF1 and NBF2. Some nucleotide-binding properties were different among SUR subtypes as follows. 1) Mg(2+) dependence of nucleotide binding of NBF2 of SUR1 was high, whereas those of SUR2A and SUR2B were low. 2) The affinities of NBF1 of SUR1 for ATP and ADP, especially for ATP, were significantly higher than those of SUR2A and SUR2B. 3) The affinities of NBF2 of SUR2B for ATP and ADP were significantly higher than those of SUR2A. This is the first biochemical study to analyze and compare the nucleotide-binding properties of NBFs of three SUR subtypes, and our results suggest that their different properties may explain, in part, the differential regulation of K(ATP) channel subtypes. The high nucleotide-binding affinities of SUR1 may explain the high ability of SUR1 to stimulate pancreatic K(ATP) channels. It is also suggested that the C-terminal 42 amino acids affect the physiological roles of SUR2A and SUR2B by changing the nucleotide-binding properties of their NBFs.

KATP channels regulate mitogenically induced proliferation in primary rat hepatocytes and human liver cell lines. Implications for liver growth control and potential therapeutic targeting

To determine whether K(ATP) channels control liver growth, we used primary rat hepatocytes and several human cancer cell lines for assays. K(ATP) channel openers (minoxidil, cromakalim, and pinacidil) increased cellular DNA synthesis, whereas K(ATP) channel blockers (quinidine and glibenclamide) attenuated DNA synthesis. The channel inhibitor glibenclamide decreased the clonogenicity of HepG2 cells without inducing cytotoxicity or apoptosis. To demonstrate the specificity of drugs for K(+) channels, whole-cell patch-clamp recordings were made. Hepatocytes revealed K(+) currents with K(ATP) channel properties. These K(+) currents were augmented by minoxidil and pinacidil and attenuated by glibenclamide as well as tetraethylammonium, in agreement with established responses of K(ATP) channels. Reverse transcription of total cellular RNA followed by polymerase chain reaction showed expression of K(ATP) channel-specific subunits in rat hepatocytes and human liver cell lines. Calcium fluxes were unperturbed in glibenclamide-treated HepG2 cells and primary rat hepatocytes following induction with ATP and hepatocyte growth factor, respectively, suggesting that the effect of K(ATP) channel activity upon hepatocyte proliferation was not simply due to indirect modulation of intracellular calcium. The regulation of mitogen-related hepatocyte proliferation by K(ATP) channels advances our insights into liver growth control. The findings have implications in mechanisms concerning liver development, regeneration, and oncogenesis.

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.

Pharmacological plasticity of cardiac ATP-sensitive potassium channels toward diazoxide revealed by ADP

The pharmacological phenotype of ATP-sensitive potassium (K(ATP)) channels is defined by their tissue-specific regulatory subunit, the sulfonylurea receptor (SUR), which associates with the pore-forming channel core, Kir6.2. The potassium channel opener diazoxide has hyperglycemic and hypotensive properties that stem from its ability to open K(ATP) channels in pancreas and smooth muscle. Diazoxide is believed not to have any significant action on cardiac sarcolemmal K(ATP) channels. Yet, diazoxide can be cardioprotective in ischemia and has been found to bind to the presumed cardiac sarcolemmal K(ATP) channel-regulatory subunit, SUR2A. Here, in excised patches, diazoxide (300 microM) activated pancreatic SUR1/Kir6.2 currents and had little effect on native or recombinant cardiac SUR2A/Kir6.2 currents. However, in the presence of cytoplasmic ADP (100 microM), SUR2A/Kir6.2 channels became as sensitive to diazoxide as SUR1/Kir6. 2 channels. This effect involved specific interactions between MgADP and SUR, as it required Mg(2+), but not ATP, and was abolished by point mutations in the second nucleotide-binding domain of SUR, which impaired channel activation by MgADP. At the whole-cell level, in cardiomyocytes treated with oligomycin to block mitochondrial function, diazoxide could also activate K(ATP) currents only after cytosolic ADP had been raised by a creatine kinase inhibitor. Thus, ADP serves as a cofactor to define the responsiveness of cardiac K(ATP) channels toward diazoxide. The present demonstration of a pharmacological plasticity of K(ATP) channels identifies a mechanism for the control of channel activity in cardiac cells depending on the cellular ADP levels, which are elevated under ischemia.