Molecular analysis of the subtype-selective inhibition of cloned KATP channels by PNU-37883A

Identification and characterisation of functional Kir6.1-containing ATP-sensitive potassium channels in the cardiac ventricular sarcolemmal membrane

BACKGROUND AND PURPOSE

The canonical Kir6.2/SUR2A ventricular KATP channel is highly ATP-sensitive and remains closed under normal physiological conditions. These channels activate only when prolonged metabolic compromise causes significant ATP depletion and then shortens the action potential to reduce contractile activity. Pharmacological activation of KATP channels is cardioprotective, but physiologically, it is difficult to understand how these channels protect the heart if they only open under extreme metabolic stress. The presence of a second KATP channel population could help explain this. Here, we characterise the biophysical and pharmacological behaviours of a constitutively active Kir6.1-containing KATP channel in ventricular cardiomyocytes.

EXPERIMENTAL APPROACH

Patch-clamp recordings from rat ventricular myocytes in combination with well-defined pharmacological modulators was used to characterise these newly identified K+ channels. Action potential recording, calcium (Fluo-4) fluorescence measurements and video edge detection of contractile function were used to assess functional consequences of channel modulation.

KEY RESULTS

Our data show a ventricular K+ conductance whose biophysical characteristics and response to pharmacological modulation were consistent with Kir6.1-containing channels. These Kir6.1-containing channels lack the ATP-sensitivity of the canonical channels and are constitutively active.

CONCLUSION AND IMPLICATIONS

We conclude there are two functionally distinct populations of ventricular KATP channels: constitutively active Kir6.1-containing channels that play an important role in fine-tuning the action potential and Kir6.2/SUR2A channels that activate with prolonged ischaemia to impart late-stage protection against catastrophic ATP depletion. Further research is required to determine whether Kir6.1 is an overlooked target in Comprehensive in vitro Proarrhythmia Assay (CiPA) cardiac safety screens.

Discovery and Characterization of VU0542270, the First Selective Inhibitor of Vascular Kir6.1/SUR2B KATP Channels

Vascular smooth muscle KATP channels critically regulate blood flow and blood pressure by modulating vascular tone and therefore represent attractive drug targets for treating several cardiovascular disorders. However, the lack of potent inhibitors that can selectively inhibit Kir6.1/SUR2B (vascular KATP) over Kir6.2/SUR1 (pancreatic KATP) has eluded discovery despite decades of intensive research. We therefore screened 47,872 chemically diverse compounds for novel inhibitors of heterologously expressed Kir6.1/SUR2B channels. The most potent inhibitor identified in the screen was an N-aryl-N'-benzyl urea compound termed VU0542270. VU0542270 inhibits Kir6.1/SUR2B with an IC50 of approximately 100 nM but has no apparent activity toward Kir6.2/SUR1 or several other members of the Kir channel family at doses up to 30 µM (>300-fold selectivity). By expressing different combinations of Kir6.1 or Kir6.2 with SUR1, SUR2A, or SUR2B, the VU0542270 binding site was localized to SUR2. Initial structure-activity relationship exploration around VU0542270 revealed basic texture related to structural elements that are required for Kir6.1/SUR2B inhibition. Analysis of the pharmacokinetic properties of VU0542270 showed that it has a short in vivo half-life due to extensive metabolism. In pressure myography experiments on isolated mouse ductus arteriosus vessels, VU0542270 induced ductus arteriosus constriction in a dose-dependent manner similar to that of the nonspecific KATP channel inhibitor glibenclamide. The discovery of VU0542270 provides conceptual proof that SUR2-specific KATP channel inhibitors can be developed using a molecular target-based approach and offers hope for developing cardiovascular therapeutics targeting Kir6.1/SUR2B. SIGNIFICANCE STATEMENT: Small-molecule inhibitors of vascular smooth muscle KATP channels might represent novel therapeutics for patent ductus arteriosus, migraine headache, and sepsis; however, the lack of selective channel inhibitors has slowed progress in these therapeutic areas. Here, this study describes the discovery and characterization of the first vascular-specific KATP channel inhibitor, VU0542270.

Rapid Characterization of the Functional and Pharmacological Consequences of Cantú Syndrome KATP Channel Mutations in Intact Cells

Gain-of-function of KATP channels, resulting from mutations in either KCNJ8 (encoding inward rectifier sub-family 6 [Kir6.1]) or ABCC9 (encoding sulphonylurea receptor [SUR2]), cause Cantú syndrome (CS), a channelopathy characterized by excess hair growth, coarse facial appearance, cardiomegaly, and lymphedema. Here, we established a pipeline for rapid analysis of CS mutation consequences in Landing pad HEK 293 cell lines stably expressing wild type (WT) and mutant human Kir6.1 and SUR2B. Thallium-influx and cell membrane potential, reported by fluorescent Tl-sensitive Fluozin-2 and voltage-sensitive bis-(1,3-dibutylbarbituric acid)trimethine oxonol (DiBAC4(3)) dyes, respectively, were used to assess channel activity. In the Tl-influx assay, CS-associated Kir6.1 mutations increased sensitivity to the ATP-sensitive potassium (KATP) channel activator, pinacidil, but there was strikingly little effect of pinacidil for any SUR2B mutations, reflecting unexpected differences in the molecular mechanisms of Kir6.1 versus SUR2B mutations. Compared with the Tl-influx assay, the DiBAC4(3) assay presents more significant signal changes in response to subtle KATP channel activity changes, and all CS mutants (both Kir6.1 and SUR2B), but not WT channels, caused marked hyperpolarization, demonstrating that all mutants were activated under ambient conditions in intact cells. Most SUR2 CS mutations were markedly inhibited by <100 nM glibenclamide, but sensitivity to inhibition by glibenclamide, repaglinide, and PNU37883A was markedly reduced for Kir6.1 CS mutations. Understanding functional consequences of mutations can help with disease diagnosis and treatment. The analysis pipeline we have developed has the potential to rapidly identify mutational consequences, aiding future CS diagnosis, drug discovery, and individualization of treatment. SIGNIFICANCE STATEMENT: We have developed new fluorescence-based assays of channel activities and drug sensitivities of Cantú syndrome (CS) mutations in human Kir6.1/SUR2B-dependent KATP channels, showing that Kir6.1 mutations increase sensitivity to potassium channel openers, while SUR2B mutations markedly reduce K channel opener (KCO) sensitivity. However, both Kir6.1 and SUR2B CS mutations are both more hyperpolarized than WT cells under basal conditions, confirming pathophysiologically relevant gain-of-function, validating DiBAC4(3) fluorescence to characterize hyperpolarization induced by KATP channel activity under basal, non KCO-activated conditions.

CL-705G: a novel chemical Kir6.2-specific KATP channel opener

Background: KATP channels have diverse roles, including regulation of insulin secretion and blood flow, and protection against biological stress responses and are excellent therapeutic targets. Different subclasses of KATP channels exist in various tissue types due to the unique assemblies of specific pore-forming (Kir6.x) and accessory (SURx) subunits. The majority of pharmacological openers and blockers act by binding to SURx and are poorly selective against the various KATP channel subclasses. Methods and Results: We used 3D models of the Kir6.2/SUR homotetramers based on existing cryo-EM structures of channels in both the open and closed states to identify a potential agonist binding pocket in a functionally critical area of the channel. Computational docking screens of this pocket with the Chembridge Core chemical library of 492,000 drug-like compounds yielded 15 top-ranked "hits", which were tested for activity against KATP channels using patch clamping and thallium (Tl+) flux assays with a Kir6.2/SUR2A HEK-293 stable cell line. Several of the compounds increased Tl+ fluxes. One of them (CL-705G) opened Kir6.2/SUR2A channels with a similar potency as pinacidil (EC50 of 9 µM and 11 μM, respectively). Remarkably, compound CL-705G had no or minimal effects on other Kir channels, including Kir6.1/SUR2B, Kir2.1, or Kir3.1/Kir3.4 channels, or Na+ currents of TE671 medulloblastoma cells. CL-705G activated Kir6.2Δ36 in the presence of SUR2A, but not when expressed by itself. CL-705G activated Kir6.2/SUR2A channels even after PIP2 depletion. The compound has cardioprotective effects in a cellular model of pharmacological preconditioning. It also partially rescued activity of the gating-defective Kir6.2-R301C mutant that is associated with congenital hyperinsulinism. Conclusion: CL-705G is a new Kir6.2 opener with little cross-reactivity with other channels tested, including the structurally similar Kir6.1. This, to our knowledge, is the first Kir-specific channel opener.

Omega-3 polyunsaturated fatty acid-induced vasodilation in mouse aorta and mesenteric arteries is not mediated by ATP-sensitive potassium channels

There is strong evidence that the omega-3 polyunsaturated fatty acids (n-3 PUFAs) docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) have cardioprotective effects. n-3 PUFAs cause vasodilation in hypertensive patients, in part controlled by increased membrane conductance to potassium. As KATP channels play a major role in vascular tone regulation and are involved in hypertension, we aimed to verify whether n-3 PUFA-mediated vasodilation involved the opening of KATP channels. We used a murine model in which the KATP channel pore subunit, Kir6.1, is deleted in vascular smooth muscle. The vasomotor response of preconstricted arteries to physiologically relevant concentrations of DHA and EPA was measured using wire myography, using the channel blocker PNU-37883A. The effect of n-3 PUFAs on potassium currents in wild-type native smooth muscle cells was investigated using whole-cell patch clamping. DHA and EPA induced vasodilation in mouse aorta and mesenteric arteries; relaxations in the aorta were sensitive to KATP blockade with PNU-37883A. Endothelium removal didn't affect relaxation to EPA and caused a small but significant inhibition of relaxation to DHA. In the knock-out model, relaxations to DHA and EPA were unaffected by channel knockdown but were still inhibited by PNU-37883A, indicating that the action of PNU-37883A on relaxation may not reflect inhibition of KATP. In native aortic smooth muscle cells DHA failed to activate KATP currents. We conclude that DHA and EPA cause vasodilation in mouse aorta and mesenteric arteries. Relaxations in blocker-treated arteries from knock-out mice demonstrate that KATP channels are not involved in the n-3 PUFA-induced relaxation.

Kir6.1 and SUR2B in Cantú syndrome

Kir6.1 and SUR2 are subunits of ATP-sensitive potassium (KATP) channels expressed in a wide range of tissues. Extensive study has implicated roles of these channel subunits in diverse physiological functions. Together they generate the predominant KATP conductance in vascular smooth muscle and are the target of vasodilatory drugs. Roles for Kir6.1/SUR2 dysfunction in disease have been suggested based on studies of animal models and human genetic discoveries. In recent years, it has become clear that gain-of-function (GoF) mutations in both genes result in Cantú syndrome (CS)-a complex, multisystem disorder. There is currently no targeted therapy for CS, but studies of mouse models of the disease reveal that pharmacological reversibility of cardiovascular and gastrointestinal pathologies can be achieved by administration of the KATP channel inhibitor, glibenclamide. Here we review the function, structure, and physiological and pathological roles of Kir6.1/SUR2B channels, with a focus on CS. Recent studies have led to much improved understanding of the underlying pathologies and the potential for treatment, but important questions remain: Can the study of genetically defined CS reveal new insights into Kir6.1/SUR2 function? Do these reveal new pathophysiological mechanisms that may be important in more common diseases? And is our pharmacological armory adequately stocked?

Kir Channel Molecular Physiology, Pharmacology, and Therapeutic Implications

For the past two decades several scholarly reviews have appeared on the inwardly rectifying potassium (Kir) channels. We would like to highlight two efforts in particular, which have provided comprehensive reviews of the literature up to 2010 (Hibino et al., Physiol Rev 90(1):291-366, 2010; Stanfield et al., Rev Physiol Biochem Pharmacol 145:47-179, 2002). In the past decade, great insights into the 3-D atomic resolution structures of Kir channels have begun to provide the molecular basis for their functional properties. More recently, computational studies are beginning to close the time domain gap between in silico dynamic and patch-clamp functional studies. The pharmacology of these channels has also been expanding and the dynamic structural studies provide hope that we are heading toward successful structure-based drug design for this family of K⁺ channels. In the present review we focus on placing the physiology and pharmacology of this K⁺ channel family in the context of atomic resolution structures and in providing a glimpse of the promising future of therapeutic opportunities.

The Pharmacology of ATP-Sensitive K+ Channels (KATP)

ATP-sensitive K⁺ channels (K_ATP) are inwardly-rectifying potassium channels, broadly expressed throughout the body. K_ATP is regulated by adenine nucleotides, characteristically being activated by falling ATP and rising ADP levels thus playing an important physiological role by coupling cellular metabolism with membrane excitability. The hetero-octameric channel complex is formed of 4 pore-forming inward rectifier Kir6.x subunits (Kir6.1 or Kir6.2) and 4 regulatory sulfonylurea receptor subunits (SUR1, SUR2A, or SUR2B). These subunits can associate in various tissue-specific combinations to form functional K_ATP channels with distinct electrophysiological and pharmacological properties. K_ATP channels play many important physiological roles and mutations in channel subunits can result in diseases such as disorders of insulin handling, cardiac arrhythmia, cardiomyopathy, and neurological abnormalities. The tissue-specific expression of K_ATP channel subunits coupled with their rich and diverse pharmacology makes K_ATP channels attractive therapeutic targets in the treatment of endocrine and cardiovascular diseases.

Insulin Release Mechanism Modulated by Toxins Isolated from Animal Venoms: From Basic Research to Drug Development Prospects

Venom from mammals, amphibians, snakes, arachnids, sea anemones and insects provides diverse sources of peptides with different potential medical applications. Several of these peptides have already been converted into drugs and some are still in the clinical phase. Diabetes type 2 is one of the diseases with the highest mortality rate worldwide, requiring specific attention. Diverse drugs are available (e.g., Sulfonylureas) for effective treatment, but with several adverse secondary effects, most of them related to the low specificity of these compounds to the target. In this context, the search for specific and high-affinity compounds for the management of this metabolic disease is growing. Toxins isolated from animal venom have high specificity and affinity for different molecular targets, of which the most important are ion channels. This review will present an overview about the electrical activity of the ion channels present in pancreatic β cells that are involved in the insulin secretion process, in addition to the diversity of peptides that can interact and modulate the electrical activity of pancreatic β cells. The importance of prospecting bioactive peptides for therapeutic use is also reinforced.

Role of ATP-sensitive potassium channels on hypoxic pulmonary vasoconstriction in endotoxemia

BACKGROUND

ATP-regulated potassium channels (KATP) regulate pulmonary vascular tone and are involved in hypoxic pulmonary vasoconstriction (HPV). In patients with inflammation like sepsis or ARDS, HPV is impaired, resulting in a ventilation-perfusion mismatch and hypoxia. Since increase of vascular KATP channel Kir6.1 has been reported in animal models of endotoxemia, we studied the expression and physiological effects of Kir6.1 in murine endotoxemic lungs. We hypothesized that inhibition of overexpressed Kir6.1 increases HPV in endotoxemia.

METHODS

Mice (C57BL/6; n = 55) with (n = 27) and without (n = 28) endotoxemia (35 mg/kg LPS i.p. for 18 h) were analyzed for Kir6.1 gene as well as protein expression and HPV was examined in isolated perfused mouse lungs with and without selective inhibition of Kir6.1 with PNU-37883A. Pulmonary artery pressure (PAP) and pressure-flow curves during normoxic (F_iO₂ 0.21) and hypoxic (F_iO₂ 0.01) ventilation were obtained. HPV was quantified as the increase in perfusion pressure in response to hypoxic ventilation in mmHg of baseline perfusion pressure (ΔPAP) in the presence and absence of PNU-37883A.

RESULTS

Endotoxemia increases pulmonary Kir6.1 gene (+ 2.8 ± 0.3-fold) and protein expression (+ 2.1 ± 0.3-fold). Hypoxia increases HPV in lungs of control animals, while endotoxemia decreases HPV (∆PAP control: 9.2 ± 0.9 mmHg vs. LPS: 3.0 ± 0.7 mmHg, p < 0.05, means ± SEM). Inhibition of Kir6.1 with 1 μM PNU-37883A increases HPV in endotoxemia, while not increasing HPV in controls (∆PAP PNU control: 9.3 ± 0.7 mmHg vs.

PNU LPS

8.3 ± 0.9 mmHg, p < 0.05, means ± SEM).

CONCLUSION

Endotoxemia increases pulmonary Kir6.1 gene and protein expression. Inhibition of Kir6.1 augments HPV in murine endotoxemic lungs.

Molecular analysis of ATP-sensitive K(+) channel subunits expressed in mouse portal vein

BACKGROUND

Several combinations of inwardly rectifying K(+) channel 6.x family pore-forming (KIR6.x) subunits associated with sulphonylurea receptor (SUR.x) subunits have been detected among ATP-sensitive K(+) (KATP) channels. It remains to be established which of these is expressed in native vascular smooth muscle.

METHODS

Pharmacological and electrophysiological properties of KATP channels in mouse portal vein were investigated using tension measurements and patch-clamp techniques. Molecular biological analyses were also performed to investigate the structural properties of these channels.

RESULTS

Spontaneous contractions in mouse portal vein were reversibly reduced by pinacidil and MCC-134, and the pinacidil-induced relaxation was antagonized by glibenclamide and U-37883A. In cell-attached mode, pinacidil activated glibenclamide-sensitive K(+) channels with a conductance (35 pS) similar to that of KIR6.1. RT-PCR analysis revealed the expression of KIR6.1, KIR6.2 and SUR2B transcripts. Using real-time PCR methods, the quantitative expression of KIR6.1 was much greater than that of KIR6.2. Immunohistochemical studies indicated the presence of KIR6.1 and SUR2B proteins in the smooth muscle layers of mouse portal vein and in single smooth muscle cells dispersed from mouse portal vein.

CONCLUSIONS

The results indicate that native KATP channels in mouse portal vein are likely to be composed of a heterocomplex of KIR6.1 and SUR2B subunits.

Pharmaceuticals that contain polycyclic hydrocarbon scaffolds

This review comprehensively explores approved pharmaceutical compounds that contain polycyclic scaffolds and the properties that these skeletons convey.

Molecular analysis of ATP-sensitive K⁺ channel subunits expressed in mouse vas deferens myocytes

BACKGROUND AND PURPOSE

ATP-sensitive K(+)(K(ATP)) channels, which are composed of K(IR)6.x associated with sulphonylurea receptor (SUR) subunits, have been detected in native smooth muscle cells, but it is currently not known which of these is expressed in mouse vas deferens myocytes.

EXPERIMENTAL APPROACH

Pharmacological and electrophysiological properties of K(ATP) channels in mouse vas deferens myocytes were investigated using patch clamp techniques. Molecular biological analyses were performed to examine the properties of these K(ATP) channels.

KEY RESULTS

During conventional whole-cell recording, pinacidil elicited an inward current that was suppressed by glibenclamide, a sulfonylurea agent, and by U-37883A, a selective K(IR)6.1 blocker. When 0.3 mM ATP was added to the pipette solution, the peak amplitude of the pinacidil-induced current was much smaller than that recorded in its absence. When 3 mM UDP, GDP or ADP was included in the pipette solution, an inward current was elicited after establishment of the conventional whole-cell configuration, with potency order being UDP > GDP > ADP. These nucleoside diphosphate-induced inward currents were suppressed by glibenclamide. MCC-134, a SUR modulator, induced glibenclamide-sensitive K(ATP) currents that were similar to those induced by 100 μM pinacidil. In the cell-attached configuration, pinacidil activated channels with a conductance similar to that of K(IR)6.1. Reverse transcription PCR analysis revealed the expression of K(IR)6.1 and SUR2B transcripts and immunohistochemical studies indicated the presence of K(IR)6.1 and SUR2B proteins in the myocytes.

CONCLUSIONS AND IMPLICATIONS

Our results indicate that native K(ATP) channels in mouse vas deferens myocytes are a heterocomplex of K(IR)6.1 channels and SUR2B subunits.

K(ATP) channel action in vascular tone regulation: from genetics to diseases

ATP-sensitive potassium (K(ATP)) channels are widely distributed in vasculatures, and play an important role in the vascular tone regulation. The K(ATP) channels consist of 4 pore-forming inward rectifier K(+) channel (Kir) subunits and 4 regulatory sulfonylurea receptors (SUR). The major vascular isoform of K(ATP) channels is composed of Kir6.1/SUR2B, although low levels of other subunits are also present in vascular beds. The observation from transgenic mice and humans carrying Kir6.1/SUR2B channel mutations strongly supports that normal activity of the Kir6.1/SUR2B channel is critical for cardiovascular function. The Kir6.1/SUR2B channel is regulated by intracellular ATP and ADP. The channel is a common target of several vasodilators and vasoconstrictors. Endogenous vasopressors such as arginine vasopressin and α-adrenoceptor agonists stimulate protein kinase C (PKC) and inhibit the K(ATP) channels, while vasodilators such as β-adrenoceptor agonists and vasoactive intestinal polypeptide increase K(ATP) channel activity by activating the adenylate cyclase-cAMP-protein kinase A (PKA) pathway. PKC phosphorylates a cluster of 4 serine residues at C-terminus of Kir6.1, whereas PKA acts on Ser1387 in the nucleotide binding domain 2 of SUR2B. The Kir6.1/SUR2B channel is also inhibited by oxidants including reactive oxygen species allowing vascular regulation in oxidative stress. The molecular basis underlying such a channel inhibition is likely to be mediated by S-glutathionylation at a few cysteine residues, especially Cys176, in Kir6.1. Furthermore, the channel activity is augmented in endotoxemia or septic shock, as a result of the upregulation of Kir6.1/SUR2B expression. Activation of the nuclear factor-κB dependent transcriptional mechanism contributes to the Kir6.1/SUR2B channel upregulation by lipopolysaccharides and perhaps other toll-like receptor ligands as well. In this review, we summarize the vascular K(ATP) channel regulation under physiological and pathophysiological conditions, and discuss the importance of K(ATP) channel as a potentially useful target in the treatment and prevention of cardiovascular diseases.

Presence and vascular pharmacology of KATP channel subtypes in rat central and peripheral tissues

K(ATP) channel openers are vasodilators and induce headache in normal subjects. We previously identified the Kir6.1/SUR2B K(ATP) channel subtype in major cerebral and dural arteries of rat, pig and man. We hypothesized that craniovascular Kir6.1/SUR2B K(ATP) channels mediate the headache-inducing effects of K(ATP) channel openers and that a Kir6.1/SUR2B specific blocker might be effective in the treatment of primary headaches such as migraine. Since K(ATP) channels are ubiquitous, we characterized the K(ATP) channel subtypes in major rat cranial and peripheral arteries and organs in order to understand the possible adverse effects of a Kir6.1/SUR2B blocker. We studied the mRNA expression of K(ATP) channel subunits in rat femoral, mesenteric, renal, coronary, basilar, middle cerebral and middle meningeal arteries and in tissue from rat heart, brain, liver, colon, lung, kidney and pancreas. We also studied the effects and potencies of a panel of synthetic K(ATP) channel openers and their potential inhibition by the Kir6.1 subunit-specific K(ATP) channel blocker PNU-37883A in segments of the arteries mounted in a wire myograph. Our studies suggest that Kir6.1/SUR2B forms the major functional K(ATP) channel complex in rat cranial and peripheral arteries. The mRNA transcripts of SUR1 and Kir6.2 subunits were predominantly found in brain, pancreas and heart, while SUR2A mRNA was merely detected within the heart. K(ATP) channel blockers highly specific for the SUR2B subunit may have no adverse CNS and cardiac effects and will not affect insulin release in the pancreas. However, a SUR2B blocker may not discriminate between cranial and peripheral arteries.

Phenformin has a direct inhibitory effect on the ATP-sensitive potassium channel

The biguanides, phenformin and metformin, are used in the treatment of type II diabetes mellitus, as well as being routinely used in studies investigating AMPK activity. We used the patch-clamp technique and rubidium flux assays to determine the role of these drugs in ATP-sensitive K+ channel (K(ATP)) regulation in cell lines expressing the cloned components of K(ATP) and the current natively expressed in vascular smooth muscle cells (VSMCs). Phenformin but not metformin inhibits a number of variants of K(ATP) including the cloned equivalents of currents present in vascular and non-vascular smooth muscle (Kir6.1/SUR2B and Kir6.2/SUR2B) and pancreatic beta-cells (Kir6.2/SUR1). However it does not inhibit the current potentially present in cardiac myocytes (Kir6.2/SUR2A). The highest affinity interaction is seen with Kir6.1/SUR2B (IC50=0.55 mM) and it also inhibits the current in native vascular smooth muscle cells. The extent and rate of inhibition are similar to that seen with the known K(ATP) blocker PNU 37883A. Additionally, phenformin inhibited the current elicited through the Kir6.2DeltaC26 (functional without SUR) channel with an IC50 of 1.78 mM. Phenformin reduced the open probability of Kir6.1/SUR2B channels by approximately 90% in inside-out patches. These findings suggest that phenformin interacts directly with the pore-forming Kir6.0 subunit however the sulphonylurea receptor is able to significantly modulate the affinity. It is likely to block from the intracellular side of the channel in a manner analogous to that of PNU 37883A.

K ATP channels in pig and human intracranial arteries

Clinical trials suggest that synthetic ATP-sensitive K(+) (K(ATP)) channel openers may cause headache and migraine by dilating cerebral and meningeal arteries. We studied the mRNA expression profile of K(ATP) channel subunits in the pig and human middle meningeal artery (MMA) and in the pig middle cerebral artery (MCA). We determined the order of potency of four K(ATP) channel openers when applied to isolated pig MMA and MCA, and we examined the potential inhibitory effects of the Kir6.1 subunit specific K(ATP) channel blocker PNU-37883A on K(ATP) channel opener-induced relaxation of the isolated pig MMA and MCA. Using conventional RT-PCR, we detected the mRNA transcripts of the K(ATP) channel subunits Kir6.1 and SUR2B in all the examined pig and human intracranial arteries. Application of K(ATP) channel openers to isolated pig MMA and MCA in myographs caused a concentration-dependent vasodilatation with an order of potency that supports the presence of functional SUR2B K(ATP) channel subunits. 10(-7) M PNU-37883A significantly inhibited the in vitro dilatory responses of the potent K(ATP) channel opener P-1075 in both pig MMA and MCA. In conclusion, our combined mRNA expression and pharmacological studies indicate that Kir6.1/SUR2B is the major functional K(ATP) channel complex in the pig MMA and MCA, and mRNA expression studies suggest that the human MMA shares this K(ATP) channel subunit profile. Specific blocking of Kir6.1 or SUR2B K(ATP) channel subunits in large cerebral and meningeal arteries may be a future anti-migraine strategy.

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.

Ion channels and calcium signaling in cerebral arteries following subarachnoid hemorrhage

Entry of Ca(2+) through voltage-dependent calcium channels (VDCCs) is critical to the regulation of intracellular free calcium concentration ([Ca(2+)](i)) in vascular smooth muscle and thus the control of cerebral artery diameter. Increased VDCC activity in cerebral artery myocytes may contribute to decreased cerebral blood flow and the accompanying neurological deficits associated with subarachnoid hemorrhage (SAH). This review will focus on the impact of SAH on VDCCs and K(+)-selective ion channels, two important classes of ion channels located in the plasma membrane of cerebral artery myocytes. SAH may act through a variety of direct and indirect mechanisms to increase the activity of VDCCs promoting cerebral artery constriction and reduced cerebral blood flow. Further, SAH may lead to suppression of K(+) channel activity to cause membrane potential depolarization to enhance VDCC activity. The ability of VDCC blockers or K(+) channel activators to alleviate SAH-induced vasospasm will also be examined.

Pharmacological Profile of U-37883A, a Channel Blocker of Smooth Muscle-Type ATP-Sensitive K Channels

U-37883A (PNU-37883A, guanidine; 4-morpholinecarboximidine-N-1-adamantyl-N'-cyclohexyl hydrochloride) was originally developed as a potential diuretic with specific binding in kidney and vascular smooth muscle rather than in brain or pancreatic beta cells. U-37883A inhibits ATP-sensitive K(+) channels (K(ATP) channels) in vascular smooth muscle at submicromolar concentrations whilst even at high concentrations (> or =10 microM) it has no inhibitory effect at pancreatic, cardiac or skeletal K(ATP) channels. Thus, it is generally thought that U-37883A is a selective inhibitor of vascular smooth muscle K(ATP) channels. Approximately one decade ago, K(ATP) channels were cloned and found to consist of at least two subunits: an inwardly-rectifying K(+) channel six family (K(ir)6.x; K(ir)6.1 and K(ir)6.2) which forms the ion conducting pore and a modulatory sulphonylurea receptor (SUR.x; SUR1, SUR2A, and SUR2B) that accounts for several pharmacological properties. It is generally believed that different combinations of K(ir)6.x and SUR.x determine the molecular properties of K(ATP) channels. Thus, K(ir)6.2/SUR1 channel represents the pancreatic beta-cell K(ATP) channel, K(ir)6.2/SUR2A channel is thought to represent the cardiac K(ATP) channel, whereas K(ir)6.1/SUR2B channel is likely to represent the vascular smooth muscle K(ATP) channel. Recent molecular studies have shown that U-37883A selectively suppresses the activity of recombinant K(ATP) channels which contain K(ir)6.1 subunits in the channel pore unit. It was thus thought that U-37883A was a selective pharmacological tool which could be used to investigate the activity of vascular smooth muscle K(ATP) channels. However, due to its multiple pharmacological actions on several ion channels and poor tissue selectivity, U-37883A should not be viewed as a selective blocker of smooth muscle K(ATP) channels.

Human ether-a-go-go-related (HERG) gene and ATP-sensitive potassium channels as targets for adverse drug effects

Torsades de pointes (TdP) arrhythmia is a potentially fatal form of ventricular arrhythmia that occurs under conditions where cardiac repolarization is delayed (as indicated by prolonged QT intervals from electrocardiographic recordings). A likely mechanism for QT interval prolongation and TdP arrhythmias is blockade of the rapid component of the cardiac delayed rectifier K+ current (IKr), which is encoded by human ether-a-go-go-related gene (HERG). Over 100 non-cardiovascular drugs have the potential to induce QT interval prolongations in the electrocardiogram (ECG) or TdP arrhythmias. The binding site of most HERG channel blockers is located inside the central cavity of the channel. An evaluation of possible effects on HERG channels during the development of novel drugs is recommended by international guidelines. During cardiac ischaemia activation of ATP-sensitive K+ (KATP) channels contributes to action potential (AP) shortening which is either cardiotoxic by inducing re-entrant ventricular arrhythmias or cardioprotective by inducing energy-sparing effects or ischaemic preconditioning (IPC). KATP channels are formed by an inward-rectifier K+ channel (Kir6.0) and a sulfonylurea receptor (SUR) subunit: Kir6.2 and SUR2A in cardiac myocytes, Kir6.2 and SUR1 in pancreatic beta-cells. Sulfonylureas and glinides stimulate insulin secretion via blockade of the pancreatic beta-cell KATP channel. Clinical studies about cardiotoxic effects of sulfonylureas are contradictory. Sulfonylureas and glinides differ in their selectivity for pancreatic over cardiovascular KATP channels, being either selective (tolbutamide, glibenclamide) or non-selective (repaglinide). The possibility exists that non-selective KATP channel inhibitors might have cardiovascular side effects. Blockers of the pore-forming Kir6.2 subunit are insulin secretagogues and might have cardioprotective or cardiotoxic effects during cardiac ischaemia.

Multiple actions of U-37883A, an ATP-sensitive K+ channel blocker, on membrane currents in pig urethra

The effects of U-37883A, a vascular ATP-sensitive K(+) channel (K(ATP) channel) blocker, on membrane currents were investigated in pig urethral myocytes by use of patch-clamp techniques (conventional whole-cell recordings, nystatin-perforated patches and cell-attached configuration). Tension measurement was also performed to study the effects of U-37883A on the levcromakalim-induced urethral relaxation and the urethral resting tone in the absence and presence of Bay K 8644. Although cumulative application of U-37883A produced a concentration-dependent inhibitory effect on the levcromakalim-induced urethral relaxation, U-37883A did not abolish the relaxation. In nystatin-perforated patch recording, K(ATP) currents activated by levcromakalim were inhibited by U-37883A in a concentration-dependent manner (K(i), 4.7 microM). Approximately 10% of the K(ATP) currents still remained even in the presence of 300 microM U-37883A. In cell-attached mode, extracellular application of U-37883A (100 microM) irreversibly inhibited the activity of the levcromakalim-induced K(ATP) channels. In whole-cell configuration, U-37883A suppressed the peak amplitude of voltage-dependent Ba(2+) currents in a concentration- and voltage-dependent manner, and at 30 microM, shifted the steady-state inactivation curve of the Ba(2+) currents to the left at -90 mV. These results demonstrate that U-37883A reduces not only the activities of K(ATP) channels but also voltage-dependent Ca(2+) channels. Therefore, it is not appropriate to define U-37883A as solely a vascular K(ATP) channel blocker.

Interaction of acetylcholine with Kir6.1 channels heterologously expressed in human embryonic kidney cells

Kir6.1 subunit is one of the pore-forming components of K(ATP) channel complex. The endogenous modulation of Kir6.1 subunit function has been largely unknown. Whether acetylcholine modulated the function of Kir6.1 subunit stably expressed in human embryonic kidney (HEK-293) cells was examined in the present study using the whole-cell patch-clamp technique. Acetylcholine from 1-100 microM concentration-dependently stimulated the heteologously expressed and PNU-37883A sensitive Kir6.1 channels (p<0.05). Co-expression of sulphonylurea receptor 1 subunit with Kir6.1 significantly inhibited the stimulatory effect of acetylcholine on K(ATP) currents. Pretreatment of the transfected HEK-293 cells with atropine, alpha-bungarotoxin, mecamylamine, prazocine, propranolol, or dihydro-beta-erythroidine hydrobromide did not alter the stimulatory effect of acetylcholine on Kir6.1 currents. When intracellular ATP was increased from 0.3 mM to 5 mM, acetylcholine at 10 microM still exhibited its stimulatory effect (-16.4+/-2.3 to -25.5+/-3.8 pA/pF, n=8, p<0.05). In conclusion, we have demonstrated an excitatory effect of acetylcholine on Kir6.1 channels, which is mediated neither by an acetylcholine receptor-dependent mechanism, nor by alteration in ATP metabolism.

Mediation of the Effect of Nicotine on Kir6.1 Channels by Superoxide Anion Production

KATP channels are a complex of regulatory sulfonylurea receptor subunits and the pore-forming inward rectifiers such as Kir6.1. Using the whole-cell patch-clamp technique, we investigated the interaction of nicotine with the Kir6.1 subunit as well as the underlying mechanism. Stable expression of Kir6.1 in HEK-293 cells yielded a detectable inward rectifier KATP current. This inward current was significantly inhibited by PNU-37883A and by a specific anti-Kir6.1 antibody. Nicotine at 30 and 100 microM increased Kir6.1 currents by 42 +/- 11.8% and 26.2 +/- 14.6%, respectively (n = 4-6, P < 0.05). In contrast, nicotine at 1-3 mM inhibited Kir6.1 currents (P < 0.05). Nicotine at 100 microM increased the production of superoxide anion (O2) by 20.3 +/- 5.7%, whereas at 1 mM it significantly decreased the production of O2 by 37.7 +/- 4.3%. Coapplication of hypoxanthine (HX) and xanthine oxidase (XO) to the transfected HEK-293 cells resulted in a significant and reproducible increase in Kir6.1 currents (P < 0.05). The stimulatory effect of HX/XO on Kir6.1 current was abolished by tempol, a scavenger of O2. Tempol also abolished the stimulatory effect of 30 muM nicotine on Kir6.1 currents. In conclusion, nicotine stimulates Kir6.1 channel at least in part through the production of O2.