The interaction of nucleotides with the tolbutamide block of cloned ATP-sensitive K+ channel currents expressed in Xenopus oocytes: a reinterpretation

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

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

Comparative aspects of the function and mechanism of SUR1 and MDR1 proteins

ATP-binding cassette (ABC) superfamily proteins have divergent functions and can be classified as transporters, channels, and receptors, although their predicted secondary structures are very much alike. Prominent members include the sulfonylurea receptor (SUR1) and the multidrug transporter (MDR1). SUR1 is a subunit of the pancreatic beta-cell K(ATP) channel and plays a key role in the regulation of glucose-induced insulin secretion. SUR1 binds ATP at NBF1, and ADP at NBF2 and the two NBFs work cooperatively. The pore-forming subunit of the pancreatic beta-cell K(ATP) channel, Kir6.2, is a member of the inwardly rectifying K(+) channel family, and also binds ATP. In this article, we present a model in which the activity of the K(ATP) channel is determined by the balance of the action of ADP, which activates the channel through SUR1, and the action of ATP, which stabilizes the long closed state by binding to Kir6.2. The concentration of ATP could also affect the channel activity through binding to NBF1 of SUR1. MDR1, on the other hand, is an ATP-dependent efflux pump which extrudes cytotoxic drugs from cells before they can reach their intracellular targets, and in this way confers multidrug resistance to cancer cells. Both NBFs of MDR1 can hydrolyze nucleotides, and their ATPase activity is necessary for drug transport. The interaction of SUR1 with nucleotides is quite different from that of MDR1. Variations in the interactions with nucleotides of ABC proteins may account for the differences in their functions.

Altered functional properties of KATP channel conferred by a novel splice variant of SUR1

1. ATP-sensitive potassium (KATP) channels are composed of pore-forming (Kir6.x) and regulatory sulphonylurea receptor (SURx) subunits. We have isolated a novel SUR variant (SUR1bDelta33) from a hypothalamic cDNA library. This variant lacked exon 33 and introduced a frameshift that produced a truncated protein lacking the second nucleotide binding domain (NBD2). It was expressed at low levels in hypothalamus, midbrain, heart and the insulin-secreting beta-cell line MIN6. 2. We examined the properties of KATP channels composed of Kir6.2 and SUR1bDelta33 by recording macroscopic currents in membrane patches excised from Xenopus oocytes expressing these subunits. We also investigated the effect of truncating SUR1 at either the start (SUR1bT1) or end (SUR1bT2) of exon 33 on KATP channel properties. 3. Kir6.2/SUR1bDelta33 showed an enhanced open probability (Po = 0.6 at -60 mV) and a reduced ATP sensitivity (Ki, 86 microM), when compared with wild-type channels (Po = 0.3; Ki, 22 microM). However, Kir6.2/SUR1bT1 and Kir6.2/SUR1bT2 resembled the wild-type channel in their Po and ATP sensitivity. 4. Neither MgADP, nor the K+ channel opener diazoxide, enhanced Kir6.2/SUR1bDelta33, Kir6.2/SUR1bT1 or Kir6.2/SUR1bT2 currents, consistent with the idea that these agents require an intact NBD2 for their action. Sulphonylureas blocked KATP channels containing any of the three SUR variants, but in excised patches the extent of block was less than that for the wild-type channel. In intact cells, the extent of sulphonylurea block of Kir6.2/SUR1bDelta33 was greater than that in excised patches and was comparable to that found for wild-type channels. 5. Our results demonstrate that NBD2 is not essential for functional expression or sulphonylurea block, but is required for KATP channel activation by K+ channel openers and nucleotides. Some of the unusual properties of Kir6.2/SUR1bDelta33 resemble those reported for the KATP channel of ventromedial hypothalamic (VMH) neurones, but the fact that this mRNA is expressed at low levels in many other tissues makes it less likely that SUR1bDelta33 serves as the SUR subunit for the VMH KATP channel.

In vitro mechanism of action on insulin release of S-22068, a new putative antidiabetic compound

1. The MIN6 cell line derived from in vivo immortalized insulin-secreting pancreatic beta cells was used to study the insulin-releasing capacity and the cellular mode of action of S-22068, a newly synthesized imidazoline compound known for its antidiabetic effect in vivo. 2. S-22068, was able to release insulin from MIN6 cells in a dose-dependent manner with a half-maximal stimulation at 100 micronM. Its efficacy (8 fold over the basal value), which did not differ whatever the glucose concentration (stimulatory or not), was intermediate between that of sulphonylurea and that of efaroxan. 3. Similarly to sulphonylureas and classical imidazolines, S-22068 blocked K(ATP) channels and, in turn, opened nifedipine-sensitive voltage-dependent Ca2+ channels, triggering Ca2+ entry. 4. Similarly to other imidazolines, S-22068 induced a closure of cloned K(ATP) channels injected to Xenopus oocytes by interacting with the pore-forming Kir6.2 moiety. 5. S-22068 did not interact with the sulphonylurea binding site nor with the non-I1 and non-I2 imidazoline site evidenced in the beta cells that is recognized by the imidazoline compounds efaroxan, phentolamine and RX821002. 6. We conclude that S-22068 is a novel imidazoline compound which stimulates insulin release via interaction with an original site present on the Kir6.2 moiety of the beta cell K(ATP) channels.

The tolbutamide site of SUR1 and a mechanism for its functional coupling to K(ATP) channel closure

Micromolar concentrations of tolbutamide will inhibit (SUR1/K(IR)6. 2)(4) channels in pancreatic beta-cells, but not (SUR2A/K(IR)6.2)(4) channels in cardiomyocytes. Inhibition does not require Mg(2+) or nucleotides and is enhanced by intracellular nucleotides. Using chimeras between SUR1 and SUR2A, we show that transmembrane domains 12-17 (TMD12-17) are required for high-affinity tolbutamide inhibition of K(ATP) channels. Deletions demonstrate involvement of the cytoplasmic N-terminus of K(IR)6.2 in coupling sulfonylurea-binding with SUR1 to the stabilization of an interburst closed configuration of the channel. The increased efficacy of tolbutamide by nucleotides results from an impairment of their stimulatory action on SUR1 which unmasks their inhibitory effects. The mechanism of inhibition of beta-cell K(ATP) channels by sulfonylureas during treatment of non-insulin-dependent diabetes mellitus thus involves two components, drug-binding and conformational changes within SUR1 which are coupled to the pore subunit through its N-terminus and the disruption of nucleotide-dependent stimulatory effects of the regulatory subunit on the pore. These findings uncover a molecular basis for an inhibitory influence of SUR1, an ATP-binding cassette (ABC) protein, on K(IR)6.2, a ion channel subunit.

Inhibition of heterologously expressed cystic fibrosis transmembrane conductance regulator Cl- channels by non-sulphonylurea hypoglycaemic agents

1. Hypoglycaemia-inducing sulphonylureas, such as glibenclamide, inhibit cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channels. In search of modulators of CFTR, we investigated the effects of the non-sulphonylurea hypoglycaemic agents meglitinide, repaglinide, and mitiglinide (KAD-1229) on CFTR Cl- channels in excised inside-out membrane patches from C127 cells expressing wild-type human CFTR. 2. When added to the intracellular solution, meglitinide and mitiglinide inhibited CFTR Cl- currents with half-maximal concentrations of 164+/-19 microM and 148+/-36 microM, respectively. However, repaglinide only weakly inhibited CFTR Cl- currents. 3. To understand better how non-sulphonylurea hypoglycaemic agents inhibit CFTR, we studied single channels. Channel blockade by both meglitinide and mitiglinide was characterized by flickery closures and a significant decrease in open probability (Po). In contrast, repaglinide was without effect on either channel gating or Po, but caused a small decrease in single-channel current amplitude. 4. Analysis of the dwell time distributions of single channels indicated that both meglitinide and mitiglinide greatly decreased the open time of CFTR. Mitiglinide-induced channel closures were about 3-fold longer than those of meglitinide. 5. Inhibition of CFTR by meglitinide and mitiglinide was voltage-dependent: at positive voltages channel blockade was relieved. 6. The data demonstrate that non-sulphonylurea hypoglycaemic agents inhibit CFTR. This indicates that these agents have a wider specificity of action than previously recognized. Like glibenclamide, non-sulphonylurea hypoglycaemic agents may inhibit CFTR by occluding the channel pore and preventing Cl- permeation.

The molecular assembly of ATP-sensitive potassium channels. Determinants on the pore forming subunit

ATP-sensitive potassium channels form a link between membrane excitability and cellular metabolism. These channels are important in physiological processes such as insulin release and they are an important site of drug action. They are an octomeric complex comprised of four sulfonylurea receptors, a member of the ATP-binding cassette family of proteins, and four Kir 6.0 subunits from the inward rectifier family of potassium channels. We have investigated the nature of the interaction between SUR1 and Kir 6.2 and the domains on the channel responsible for the biochemical and functional manifestations of coupling. The results point to the proximal C terminus determining biochemical interaction in a region that also largely governs homotypic and heterotypic interaction between different Kir family members. While this domain may be necessary for functional communication between the two proteins, it is not sufficient since relative modifications of either the N or C terminus are able to disrupt many aspects of functional coupling mediated by the sulfonylurea receptor.

Sulfonylurea and K(+)-channel opener sensitivity of K(ATP) channels. Functional coupling of Kir6.2 and SUR1 subunits

The sensitivity of K(ATP) channels to high-affinity block by sulfonylureas and to stimulation by K(+) channel openers and MgADP (PCOs) is conferred by the regulatory sulfonylurea receptor (SUR) subunit, whereas ATP inhibits the channel through interaction with the inward rectifier (Kir6.2) subunit. Phosphatidylinositol 4, 5-bisphosphate (PIP(2)) profoundly antagonized ATP inhibition of K(ATP) channels expressed from cloned Kir6.2+SUR1 subunits, but also abolished high affinity tolbutamide sensitivity. By stabilizing the open state of the channel, PIP(2) drives the channel away from closed state(s) that are preferentially affected by high affinity tolbutamide binding, thereby producing an apparent loss of high affinity tolbutamide inhibition. Mutant K(ATP) channels (Kir6. 2[DeltaN30] or Kir6.2[L164A], coexpressed with SUR1) also displayed an "uncoupled" phenotype with no high affinity tolbutamide block and with intrinsically higher open state stability. Conversely, Kir6. 2[R176A]+SUR1 channels, which have an intrinsically lower open state stability, displayed a greater high affinity fraction of tolbutamide block. In addition to antagonizing high-affinity block by tolbutamide, PIP(2) also altered the stimulatory action of the PCOs, diazoxide and MgADP. With time after PIP(2) application, PCO stimulation first increased, and then subsequently decreased, probably reflecting a common pathway for activation of the channel by stimulatory PCOs and PIP(2). The net effect of increasing open state stability, either by PIP(2) or mutagenesis, is an apparent "uncoupling" of the Kir6.2 subunit from the regulatory input of SUR1, an action that can be partially reversed by screening negative charges on the membrane with poly-L-lysine.

Involvement of the n-terminus of Kir6.2 in coupling to the sulphonylurea receptor

1. ATP-sensitive potassium (KATP) channels are composed of pore-forming Kir6.2 and regulatory SUR subunits. ATP inhibits the channel by interacting with Kir6.2, while sulphonylureas block channel activity by interaction with a high-affinity site on SUR1 and a low-affinity site on Kir6.2. MgADP and diazoxide interact with SUR1 to promote channel activity. 2. We examined the effect of N-terminal deletions of Kir6.2 on the channel open probability, ATP sensitivity and sulphonylurea sensitivity by recording macroscopic currents in membrane patches excised from Xenopus oocytes expressing wild-type or mutant Kir6.2/SUR1. 3. A 14 amino acid N-terminal deletion (DeltaN14) did not affect the gating, ATP sensitivity or tolbutamide block of a truncated isoform of Kir6.2, Kir6.2DeltaC26, expressed in the absence of SUR1. Thus, the N-terminal deletion does not alter the intrinsic properties of Kir6.2. 4. When Kir6.2DeltaN14 was coexpressed with SUR1, the resulting KATP channels had a higher open probability (Po = 0.7) and a lower ATP sensitivity (Ki = 196 microM) than wild-type (Kir6.2/SUR1) channels (Po = 0.32, Ki = 28 microM). High-affinity tolbutamide block was also abolished. 5. Truncation of five or nine amino acids from the N-terminus of Kir6.2 also enhanced the open probability, and reduced both the ATP sensitivity and the fraction of high-affinity tolbutamide block, although to a lesser extent than for the DeltaN14 deletion. Site-directed mutagenesis suggests that hydrophobic residues in Kir6. 2 may be involved in this effect. 6. The reduced ATP sensitivity of Kir6.2DeltaN14 may be explained by the increased Po. However, when the Po was decreased (by ATP), tolbutamide was unable to block Kir6. 2DeltaN14/SUR1-K719A,K1385M currents, despite the fact that the drug inhibited Kir6.2-C166S/SUR1-K719A,K1385M currents (which in the absence of ATP have a Po of > 0.8 and are not blocked by tolbutamide). Thus the N-terminus of Kir6.2 may be involved in coupling sulphonylurea binding to SUR1 to closure of the Kir6.2 pore.

Imidazolines and the pancreatic B-cell. Actions and binding sites

Stimulation of insulin secretion by imidazoline compounds displays variable characteristics. Phentolamine (10-100 microM) increased secretion of perifused mouse islets at nonstimulatory glucose concentrations (5 mM) and even in the absence of glucose. Idazoxan (20-100 microM) elicited a moderate increase in insulin secretion, which required the presence of a stimulatory glucose concentration (10 mM). Phentolamine is therefore a stimulator of secretion in its own right, whereas idazoxan may be termed an enhancer of secretion. Both compounds inhibited the activity of ATP-dependent K+ channels in inside-out patches from B-cells; however, idazoxan achieved only an incomplete block. Both compounds depolarized the B-cell plasma membrane to an extent that permitted the opening of voltage-dependent Ca2+ channels (-40 to -30 mV). An increase in cytoplasmic Ca2+ concentration was induced by phentolamine and much less so by idazoxan. Activation of protein kinase C, a possible mechanism to amplify Ca(2+)-induced secretion, could not be verified for phentolamine. It thus appears that stimulation of insulin secretion by phentolamine is due to its blocking effect on KATP channels, which may be the correlate of non-adrenergic imidazoline binding sites which were characterized in insulin-secreting HIT cells. Whether incomplete closure of KATP channels by idazoxan or additional effects are responsible for the requirement of high glucose to stimulate secretion remains to be clarified.

Characterization of low-affinity binding sites for glibenclamide on the Kir6.2 subunit of the beta-cell KATP channel

The ATP-sensitive K+ channel, an octameric complex of two structurally unrelated types of subunits, SUR1 and Kir6.2, plays a central role in the physiological regulation of insulin secretion. The sulfonylurea glibenclamide, which trigger insulin secretion by blocking the ATP-sensitive K+ channel, interacts with both high and low affinity binding sites present on beta-cells. The high affinity binding site has been localized on SUR1 but the molecular nature of the low affinity site is still uncertain. In this study, we analyzed the pharmacology of glibenclamide in a transformed COS-7 cell line expressing the rat Kir6.2 cDNA and compared with that of the MIN6 beta cell line expressing natively both the Kir6.2 and the SUR1 subunits. Binding studies and Scatchard analysis revealed the presence of a single class of low affinity binding sites for glibenclamide on the COS/Kir6.2 cells with characteristics similar to that observed for the low affinity site of the MIN6 beta cells.

Cooperative binding of ATP and MgADP in the sulfonylurea receptor is modulated by glibenclamide

The ATP-sensitive potassium (KATP) channels in pancreatic beta cells are critical in the regulation of glucose-induced insulin secretion. Although electrophysiological studies provide clues to the complex control of KATP channels by ATP, MgADP, and pharmacological agents, the molecular mechanism of KATP-channel regulation remains unclear. The KATP channel is a heterooligomeric complex of SUR1 subunits of the ATP-binding-cassette superfamily with two nucleotide-binding folds (NBF1 and NBF2) and the pore-forming Kir6.2 subunits. Here, we report that MgATP and MgADP, but not the Mg salt of gamma-thio-ATP, stabilize the binding of prebound 8-azido-[alpha-32P]ATP to SUR1. Mutation in the Walker A and B motifs of NBF2 of SUR1 abolished this stabilizing effect of MgADP. These results suggest that SUR1 binds 8-azido-ATP strongly at NBF1 and that MgADP, either by direct binding to NBF2 or by hydrolysis of bound MgATP at NBF2, stabilizes prebound 8-azido-ATP binding at NBF1. The sulfonylurea glibenclamide caused release of prebound 8-azido-[alpha-32P]ATP from SUR1 in the presence of MgADP or MgATP in a concentration-dependent manner. This direct biochemical evidence of cooperative interaction in nucleotide binding of the two NBFs of SUR1 suggests that glibenclamide both blocks this cooperative binding of ATP and MgADP and, in cooperation with the MgADP bound at NBF2, causes ATP to be released from NBF1.

Pharmacology of CFTR chloride channel activity

Pharmacology of CFTR Chloride Channel Activity. Physiol. Rev. 79, Suppl.: S109-S144, 1999. - The pharmacology of cystic fibrosis transmembrane conductance regulator (CFTR) is at an early stage of development. Here we attempt to review the status of those compounds that modulate the Cl- channel activity of CFTR. Three classes of compounds, the sulfonylureas, the disulfonic stilbenes, and the arylaminobenzoates, have been shown to directly interact with CFTR to cause channel blockade. Kinetic analysis has revealed the sulfonylureas and arylaminobenzoates interact with the open state of CFTR to cause blockade. Suggestive evidence indicates the disulfonic stilbenes act by a similar mechanism but only from the intracellular side of CFTR. Site-directed mutagenesis studies indicate the involvement of specific amino acid residues in the proposed transmembrane segment 6 for disulfonic stilbene blockade and segments 6 and 12 for arylaminobenzoate blockade. Unfortunately, these compounds (sulfonylureas, disulfonic stilbenes, arylaminobenzoate) also act at a number of other cellular sites that can indirectly alter the activity of CFTR or the transepithelial secretion of Cl-. The nonspecificity of these compounds has complicated the interpretation of results from cellular-based experiments. Compounds that increase the activity of CFTR include the alkylxanthines, phosphodiesterase inhibitors, phosphatase inhibitors, isoflavones and flavones, benzimidazolones, and psoralens. Channel activation can arise from the stimulation of the cAMP signal transduction cascade, the inhibition of inactivating enzymes (phosphodiesterases, phosphatases), as well as the direct binding to CFTR. However, in contrast to the compounds that block CFTR, a detailed understanding of how the above compounds increase the activity of CFTR has not yet emerged.

ATP-sensitive potassium channels: structures, functions, and pathophysiology

ATP-sensitive potassium channels (KATP channels) play important roles in various tissues by coupling cell metabolic status to electrical activity. Recently, molecular biological and electrophysiological techniques have revealed the molecular basis of the KATP channels to be a complex of the Kir6.0 subunit, a member of the inwardly rectifying K+ channel subfamily Kir6.0, and the sulfonylurea receptor (SUR) subunit, a member of ATP-binding cassette (ABC) superfamily; the functional diversity of the various KATP channels is being determined by a combination of the Kir6.0 subunit (Kir6.1 or Kir6.2) and the SUR subunit (SUR1 or SUR2) comprising it. Recent studies of the KATP channels have suggested mechanisms of KATP channel regulation and pathophysiology and also a new model in which ABC proteins regulate the functional expression of ion channels.

The antiarrhythmic agent cibenzoline inhibits KATP channels by binding to Kir6.2

We reported previously that cibenzoline, an antiarrhythmic agent, inhibits the ATP-sensitive K+ (KATP) channels of pancreatic beta-cells through a binding site distinct from that for glibenclamide. In the present study, we have determined the locus of the action of cibenzoline on KATP channels reconstituted with mutant Kir6.2 and SUR1. We expressed a C-terminal truncated Kir6.2 (Kir6. 2DeltaC26) with and without SUR1 in COS7 cells. Both Kir6.2DeltaC26 and Kir6.2DeltaC26 + SUR1 formed functional KATP channels. Glibenclamide inhibited Kir6.2DeltaC26 + SUR1 channels but failed to inhibit Kir6.2DeltaC26. In contrast, cibenzoline inhibited equally Kir6.2DeltaC26 and Kir6.2DeltaC26 + SUR1 channels, in a dose-dependent manner, the half-maximal concentrations of channel inhibition being 22.2 +/- 6.1 and 30.9 +/- 9.4 microM, respectively. Furthermore, we determined also that [3H]cibenzoline bound to Kir6. 2DeltaC26. These findings confirm that cibenzoline inhibits KATP channels by a novel inhibitory mechanism in which cibenzoline directly affects the pore-forming Kir6.2 subunit rather than the SUR1 subunit.

Glibenclamide blocks volume-sensitive Cl- channels by dual mechanisms

To study the mechanisms of glibenclamide actions on volume-sensitive Cl- channels, whole cell patch-clamp studies were performed at various pH levels in human epithelial Intestine 407 cells. Extracellular application of glibenclamide reversibly suppressed volume-sensitive Cl- currents in the entire range of voltage examined (-100 to +100 mV) and accelerated the depolarization-induced inactivation at pH 7.5. When glibenclamide was applied from the intracellular side, in contrast, no effect was observed. At acidic pH, at which the weak acid glibenclamide exists largely in the uncharged form, the instantaneous current was, in a voltage-independent manner, suppressed by the extracellular drug at micromolar concentrations without significantly affecting the depolarization-induced inactivation. At alkaline pH, at which almost all of the drug is in the charged form, glibenclamide speeded the inactivation time course and induced a leftward shift of the steady-state inactivation curve at much higher concentrations. Thus it is concluded that glibenclamide exerts inhibiting actions on swelling-activated Cl- channels from the extracellular side and that the uncharged form is mainly responsible for voltage-independent inhibition of instantaneous currents, whereas the anionic form facilitates voltage-dependent channel inactivation in human epithelial Intestine 407 cells.

Human alpha-endosulfine, a possible regulator of sulfonylurea-sensitive KATP channel: molecular cloning, expression and biological properties

Sulfonylureas are a class of drugs commonly used in the management of non-insulin-dependent diabetes mellitus. Their therapeutic action results primarily from their ability to inhibit ATP-sensitive potassium (KATP) channels in the plasma membrane of pancreatic beta cells and thereby stimulate insulin release. A key question is whether an endogenous ligand for the KATP channel exists that is able to mimic the inhibitory effects of sulfonylureas. We describe here the cloning of the cDNA encoding human alpha-endosulfine, a 13-kDa peptide that is a putative candidate for such a role. alpha-Endosulfine is expressed in a wide range of tissues including muscle, brain, and endocrine tissues. The recombinant protein displaces binding of the sulfonylurea [3H]glibenclamide to beta cell membranes, inhibits cloned KATP channel currents, and stimulates insulin secretion. We propose that endosulfine is an endogenous regulator of the KATP channel, which has a key role in the control of insulin release and, more generally, couples cell metabolism to electrical activity.

MgATP activates the beta cell KATP channel by interaction with its SUR1 subunit

ATP-sensitive potassium (KATP) channels in the pancreatic beta cell membrane mediate insulin release in response to elevation of plasma glucose levels. They are open at rest but close in response to glucose metabolism, producing a depolarization that stimulates Ca2+ influx and exocytosis. Metabolic regulation of KATP channel activity currently is believed to be mediated by changes in the intracellular concentrations of ATP and MgADP, which inhibit and activate the channel, respectively. The beta cell KATP channel is a complex of four Kir6.2 pore-forming subunits and four SUR1 regulatory subunits: Kir6.2 mediates channel inhibition by ATP, whereas the potentiatory action of MgADP involves the nucleotide-binding domains (NBDs) of SUR1. We show here that MgATP (like MgADP) is able to stimulate KATP channel activity, but that this effect normally is masked by the potent inhibitory effect of the nucleotide. Mg2+ caused an apparent reduction in the inhibitory action of ATP on wild-type KATP channels, and MgATP actually activated KATP channels containing a mutation in the Kir6.2 subunit that impairs nucleotide inhibition (R50G). Both of these effects were abolished when mutations were made in the NBDs of SUR1 that are predicted to abolish MgATP binding and/or hydrolysis (D853N, D1505N, K719A, or K1384M). These results suggest that, like MgADP, MgATP stimulates KATP channel activity by interaction with the NBDs of SUR1. Further support for this idea is that the ATP sensitivity of a truncated form of Kir6.2, which shows functional expression in the absence of SUR1, is unaffected by Mg2+.

Photoinactivation of fluorescein isothiocyanate-modified Na,K-ATPase by 2'(3')-O-(2,4,6-trinitrophenyl)8-azidoadenosine 5'-diphosphate. Abolition of E1 and E2 partial reactions by sequential block of high and low affinity nucleotide sites

The Na,K-ATPase activity of the sodium pump exhibits apparent multisite kinetics toward ATP, a feature that is inherent to the minimal enzyme unit, the alpha beta protomer. We have argued that this should arise from separate catalytic and noncatalytic sites on the alpha beta protomer as fluorescein isothiocyanate (FITC) blocks a high affinity ATP site on all alpha subunits and yet the modified Na, K-ATPase retains a low affinity response to nucleotides (Ward, D. G., and Cavieres, J. D. (1996) J. Biol. Chem. 271, 12317-12321). We now find that 2'(3')-O-(2,4,6-trinitrophenyl)8-azido-adenosine 5'-diphosphate (TNP-8N3-ADP), a high affinity photoactivatable analogue of ATP, can inhibit the K+-phosphatase activity of the FITC-modified enzyme during assays in dimmed light. The inhibition occurs with a Ki of 140 microM at 20 mM K+; it requires the adenine ring as 2'(3')-O-(2,4 6-trinitrophenyl) (TNP)-UDP or TNP-uridine are less potent and 2,4,6-trinitrobenzene-sulfonate is ineffective. Under irradiation with UV light, TNP-8N3-ADP inactivates the K+-phosphatase activity of the fluorescein-enzyme and also its phosphorylation by [32P]Pi. The photoinactivation process is stimulated by Na+ or Mg2+, and is inhibited by K+ or excess TNP-ADP. In the presence of 50 mM Na+ and 1 mM Mg2+, TNP-8N3-ADP photoinactivates with a K0.5 of 15 microM. Furthermore, TNP-8N3-ADP photoinactivates the FITC-modified, solubilized alpha beta protomers, even more effectively than the membrane-bound fluorescein-enzyme. These results strongly suggest that catalytic and allosteric ATP sites coexist on the alpha beta protomer of Na,K-ATPase.

Inhibition of the ATP-sensitive potassium channel from mouse pancreatic beta-cells by surfactants

1. We have used patch-clamp methods to study the effects of the detergents, Cremophor, Tween 80 and Triton X100 on the K(ATP) channel in the pancreatic beta-cell from mouse. 2. All three detergents blocked K(ATP) channel activity with the following order of potency: Tween 80 (Ki< approximately 83 nM)>Triton X100 (Ki=350 nM)>Cremophor. In all cases the block was poorly reversible. 3. Single-channel studies suggested that at low doses, the detergents act as slow blockers of the K(ATP) channel. 4. Unlike the block produced by tolbutamide, that produced by detergent was not affected by intracellular Mg2+-nucleotide, diazoxide or trypsin treatment, nor did it involve an acceleration of rundown or increase in ATP sensitivity of the chanel. 5. The detergents could block the pore-forming subunit, Kir6.2deltaC26, which can be expressed independently of SUR1 (the regulatory subunit of the K(ATP) channel). These data suggest that the detergents act on Kir6.2 and not SUR1. 6. The detergents had no effect on another member of the inward rectifier family: Kir1.1a (ROMK1). 7. Voltage-dependent K-currents in the beta-cell were reversibly blocked by the detergents with a far lower potency than that found for the K(ATP) channel. 8. Like other insulin secretagogues that act by blocking the K(ATP) channel, Cremophor elevated intracellular Ca2+ in single beta-cells to levels that would be expected to elicit insulin secretion. 9. Given the role of the K(ATP) channel in many physiological processes, we conclude that plasma borne detergent may have pharmacological actions mediated through blockage of the K(ATP) channel.

A touching case of channel regulation: the ATP-sensitive K+ channel

The classical type of KATP channel is an octameric (4:4) complex of two structurally unrelated subunits, Kir6.2 and SUR. The former serves as an ATP-inhibitable pore, while SUR is a regulatory subunit endowing sensitivity to sulphonylurea and K+ channel opener drugs, and the potentiatory action of MgADP. Both subunits are required to form a functional channel.

Characterization of K(ATP) channels in intact mammalian skeletal muscle fibres

1. The aim of this study was to characterize the K(ATP) channel of intact rat skeletal muscle (rat flexor digitorum brevis muscle). Changes in membrane currents were recorded with two-electrode voltage-clamp of whole fibres. 2. The K(ATP) channel openers, levcromakalim and pinacidil (10-400 microM), caused a concentration-dependent increase in whole-cell chord conductance (up to approximately 1.5 mScm(-2)). The activated current had a weak inwardly rectifying current-voltage relation, a reversal potential near E(K) and nanomolar sensitivity to glibenclamide--characteristic of a K(ATP) channel current. Concentration-effect analysis revealed that levcromakalim and pinacidil were not particularly potent (EC50 approximately 186 microM, approximately 30 microM, respectively), but diazoxide was completely inactive. 3. The ability of both classical K(ATP) channel inhibitors (glibenclamide, tolbutamide, glipizide and 5-hydroxydecanoic acid) and a number of structurally related glibenclamide analogues to antagonize the levcromakalim-induced current was determined. Glibenclamide was the most potent compound with an IC50 of approximately 5 nM. However, the non-sulphonylurea (but cardioactive) compound 5-hydroxydecanoic acid was inactive in this preparation. 4. Regression analysis showed that the glibenclamide analogues used have a similar rank order of potency to that observed previously in vascular smooth muscle and cerebral tissue. However, two compounds (glipizide and DK13) were found to have unexpectedly low potency in skeletal muscle. 5. These experiments revealed K(ATP) channels of skeletal muscle to be at least 10x more sensitive to glibenclamide than previously found; this may be because of the requirement for an intact intracellular environment for the full effect of sulphonylureas to be realised. Pharmacologically, K(ATP) channels of mammalian skeletal muscle appear to resemble most closely K(ATP) channels of cardiac myocytes.