Pharmacological properties of ATP-sensitive K+ channels in mammalian skeletal muscle cells

Skeletal muscle delimited myopathy and verapamil toxicity in SUR2 mutant mouse models of AIMS

ABCC9-related intellectual disability and myopathy syndrome (AIMS) arises from loss-of-function (LoF) mutations in the ABCC9 gene, which encodes the SUR2 subunit of ATP-sensitive potassium (KATP ) channels. KATP channels are found throughout the cardiovascular system and skeletal muscle and couple cellular metabolism to excitability. AIMS individuals show fatigability, muscle spasms, and cardiac dysfunction. We found reduced exercise performance in mouse models of AIMS harboring premature stop codons in ABCC9. Given the roles of KATP channels in all muscles, we sought to determine how myopathy arises using tissue-selective suppression of KATP and found that LoF in skeletal muscle, specifically, underlies myopathy. In isolated muscle, SUR2 LoF results in abnormal generation of unstimulated forces, potentially explaining painful spasms in AIMS. We sought to determine whether excessive Ca2+ influx through CaV 1.1 channels was responsible for myopathology but found that the Ca2+ channel blocker verapamil unexpectedly resulted in premature death of AIMS mice and that rendering CaV 1.1 channels nonpermeable by mutation failed to reverse pathology; results which caution against the use of calcium channel blockers in AIMS.

ATP sensitive potassium channel openers: A new class of ocular hypotensive agents

ATP sensitive potassium (K_ATP) channels connect the metabolic and energetic state of cells due to their sensitivity to ATP and ADP concentrations. K_ATP channels have been identified in multiple tissues and organs of the body including heart, pancreas, vascular smooth muscles and skeletal muscles. These channels are obligatory hetero-octamers and contain four sulfonylurea (SUR) and four potassium inward rectifier (K_ir) subunits. Based on the particular type of SUR and K_ir present, there are several tissue specific subtypes of K_ATP channels, each with their own unique set of functions. Recently, K_ATP channels have been reported in human and mouse ocular tissues. In ex vivo and in vivo model systems, K_ATP channel openers showed significant ocular hypotensive properties with no appearance of toxic side effects. Additionally, when used in conjunction with known intraocular pressure lowering drugs, an additive effect on IOP reduction was observed. These K_ATP channel openers have also been reported to protect the retinal ganglion cells during ischemic stress and glutamate induced toxicity suggesting a neuroprotective property for this drug class. Medications that are currently used for treating ocular hypertensive diseases like glaucoma do not directly protect the affected retinal cells, are sometimes ineffective and may show significant side effects. In light of this, K_ATP channel openers with both ocular hypotensive and neuroprotective properties, have the potential to develop into a new class of glaucoma therapeutics.

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.

Three-dimensional quantitative structure-activity relationships of ATP-sensitive potassium (KATP) channel openers belonging to the 3-alkylamino-4H-1,2,4-benzo- and 3-alkylamino-4H-1,2,4-pyridothiadiazine 1,1-dioxide families

Recent studies have demonstrated that selective activation of pancreatic ATP-sensitive potassium (KATP) channels could be of clinical value in the treatment of type I and type II diabetes, obesity, and hypersinsulinemia. Taking into account these promising therapeutic opportunities, we have explored the 3-alkylamino-4H-1,2,4-pyrido- and 3-alkylamino-4H-1,2,4-benzothiadiazine 1,1-dioxide families. Among these series, numerous drugs were identified as highly potent and selective openers of either the pancreatic or the aortic KATP channels. Thanks to comparative molecular field analysis (CoMFA) and comparative molecular similarity indices analysis (CoMSIA), quantitative structure-activity relationship approaches using more than 100 compounds, pharmacophoric models explaining the activity and selectivity of the drugs have been elaborated. These models highlighted the importance of several chemical regions for KATP channel activation and could be very helpful for future improvement of drug potency, selectivity, or both. Moreover, an original CoMSIA analysis, using a selectivity index (SI) as a dependent variable, was also performed with the aim of identifying the structural parameters influencing tissue selectivity.

Drugs to facilitate recovery of neuromuscular blockade and muscle strength

Several drugs that quicken recovery from neuromuscular blockade caused by vecuronium in anesthetized patients are reviewed. Ulinastatin, a protease inhibitor, is thought to promote the release of acetylcholine at the neuromuscular junction and increases hepatic blood flow and urine volume. For this reason, ulinastatin quickens recovery from neuromuscular blockade in anesthetized patients receiving vecuronium. Additionally, pretreatment with ulinastatin avoids prolongation of vecuronium-induced neuromuscular blockade in patients with hepatic cirrhosis. Gabexate mesilate is also a protease inhibitor. During a continuous infusion of gabexate mesilate, recovery from neuromuscular blockade was quickened. Amino acid-enriched solution supplies energy to the skeletal muscles and causes an increase in muscle strength. An infusion of amino acid-enriched solution hastens recovery from neuromuscular blockade in anesthetized patients. When amino acids supply energy to the skeletal muscles, they simultaneously produce heat in the skeletal muscles. This thermal generation may be closely related to fast recovery from neuromuscular blockade. Amino acid-enriched solution makes recovery from neuromuscular blockade quick and avoids hypothermia during general anesthesia. Milrinone, a phosphodiesterase III inhibitor, is supposed to increase the release of acetylcholine at the neuromuscular junction and make the neuromuscular junction sensitive to acetylcholine. Therefore, recovery from neuromuscular blockade is hastened. Nicorandil enhances membrane K+ conductance in skeletal muscle and increases contraction of the skeletal muscle. Thus, nicorandil quickens recovery from neuromuscular blockade.

Expression and characterization of delayed rectifying K+channels in anterior rat taste buds

Delayed rectifying K+(DRK) channels in taste cells have been implicated in the regulation of cell excitability and as potential targets for direct and indirect modulation by taste stimuli. In the present study, we have used patch-clamp recording to determine the biophysical properties and pharmacological sensitivity of DRK channels in isolated rat fungiform taste buds. Molecular biological assays at the taste bud and single-cell levels are consistent with the interpretation that taste cells express a variety of DRK channels, including members from each of the three major subfamilies: KCNA, KCNB, and KCNC. Real-time PCR assays were used to quantify expression of the nine DRK channel subtypes. While taste cells express a number of DRK channels, the electrophysiological and molecular biological assays indicate that the Shaker Kv1.5 channel (KCNA5) is the major functional DRK channel expressed in the anterior rat tongue.

3-Alkylamino-4H-1,2,4-benzothiadiazine 1,1-dioxides as ATP-sensitive potassium channel openers: effect of 6,7-disubstitution on potency and tissue selectivity

A series of 6,7-disubstituted 4H-1,2,4-benzothiadiazine 1,1-dioxides bearing a short alkylamino side chain in the 3-position were synthesized. These compounds were tested on rat pancreatic islets and on rat aorta rings. In vitro data indicated that in most cases substitution in the 6 and the 7 positions increased their activity as inhibitors of insulin secretion, while the myorelaxant potency of the drugs was maintained or enhanced according to the nature of the substituent in the 7-position. The presence of either chlorine or bromine atoms in the 6 and 7 positions did not improve the apparent selectivity of the drugs for the pancreatic tissue. By contrast, the introduction of one or two fluorine atoms, as well as the presence of a methoxy group in the 7-position, generated potent and selective inhibitors of insulin release. Radioisotopic and fluorimetric experiments performed with the most potent compound inhibiting insulin release (34, BPDZ 259, 6-chloro-7-fluoro-3-isopropylamino-4H-1,2,4-benzothiadiazine 1,1-dioxide) confirmed that the drug activated K(ATP) channels. 34 was found to be one of the most potent and selective pancreatic potassium channel openers yet described.

Effect on K(ATP) channel activation properties and tissue selectivity of the nature of the substituent in the 7- and the 3-position of 4H-1,2,4-benzothiadiazine 1,1-dioxides

The present work explored 3-alkylamino-4H-1,2,4-benzothiadiazine 1,1-dioxides diversely substituted in the 7-position. Those compounds, structurally related to previously described potassium channel openers such as the benzothiadiazine dioxide BPDZ 73, were tested as putative K(ATP) channel activators on the pancreatic endocrine tissue and on the vascular smooth muscle tissue. The nature of the substituent introduced in the 7-position as well as the nature of the alkylamino side chain in the 3-position strongly affected both potency and tissue selectivity of 4H-1,2,4-benzothiadiazine 1,1-dioxides. Thus, compounds bearing in the 7-position a methyl or a methoxy group or devoid of a substituent in this position, and bearing an ethyl, an isopropyl, or a cyclobutylamino group in the 3-position were found to be potent and selective inhibitors of insulin release from rat pancreatic B-cells (i.e. 10a, 10b, 12b, 12d, 22c). In contrast, 3-alkylamino-7-trifluoromethyl- (20a-c) and 3-alkylamino-7-pentyl-4H-1,2,4-benzothiadiazine 1,1-dioxides (11a,b) expressed a marked myorelaxant activity on rat aorta ring. Among the latter compounds, the 3-alkylamino-7-pentyl derivative (11a) showed a clear selectivity for the vascular smooth muscle tissue. The present work gives new insights into the role of the substituent in both the 7- and the 3-position for the design of 4H-1,2,4-benzothiadiazine 1,1-dioxide potassium channel openers exhibiting different tissue selectivity profiles.

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.

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.

4-aminopyridine- and dendrotoxin-sensitive potassium channels influence excitability of vagal mechano-sensitive endings in guinea-pig oesophagus

1. Distension-sensitive vagal afferent fibres from the guinea-pig oesophagus were recorded extracellularly in vitro. Most recorded units were spontaneously active firing at 3.2+/-0.3 Hz (n=41, N=41) and had low thresholds (less than 1 mm) to circumferential stretch. Dynamic and adapted phases of stretch-evoked firing, as well as a silent period were linearly dependent on the amplitude of stretch. 2. High K+ (7-12 mM) Krebs solution dose-dependently increased both spontaneous and stretch-evoked firing and reduced the duration of the silent period. 3. Charybdotoxin (ChTX, 100 nM) slightly increased spontaneous and stretch-evoked firing and decreased the silent period, while neither iberiotoxin (100 nM) nor apamin (0.5 microM) had significant effects. omega-Conotoxin GVIA (0.5 microM) did not significantly affect firing of vagal mechanoreceptors. 4. In the majority of single units, 4-aminopyridine (4-AP) concentration-dependently (EC(50) approximately 28 microM) increased spontaneous firing, strongly reduced the silent period but did not affect stretch (3 mm)-induced firing. Firing evoked by 1-2 mm was increased by 4-AP. 5. Alpha-dendrotoxin (DnTX, 300 nM) and DnTX K (30 nM) slightly increased spontaneous and stretch-evoked firing. There was no additive effect on spontaneous firing when ChTX and DnTX K were applied simultaneously. 6. Barium (100 microM) increased stretch-induced firing, probably due to an increase in intramural tension. Glibenclamide (10 microM) had no effect on spontaneous or stretch-induced firing. 7. The results indicate that voltage-gated 4-AP- and dendrotoxin-sensitive K+ channels are the main type of K+ channels that influence excitability of vagal mechano-sensitive endings of the guinea-pig oesophagus. They were involved in control of spontaneous firing and in stretch-induced firing evoked by moderate stretch, but none of the K+ channels appeared to be involved in adaptation to maintained stretch by their slowly adapting vagal mechanoreceptors.

ATP-sensitive potassium channels participate in glucose uptake in skeletal muscle and adipose tissue

ATP-sensitive potassium (K(ATP)) channels are known to be critical in the control of both insulin and glucagon secretion, the major hormones in the maintenance of glucose homeostasis. The involvement of K(ATP) channels in glucose uptake in the target tissues of insulin, however, is not known. We show here that Kir6.2(-/-) mice lacking Kir6.2, the pore-forming subunit of these channels, have no K(ATP) channel activity in their skeletal muscles. A 2-deoxy-[(3)H]glucose uptake experiment in vivo showed that the basal and insulin-stimulated glucose uptake in skeletal muscles and adipose tissues of Kir6.2(-/-) mice is enhanced compared with that in wild-type (WT) mice. In addition, in vitro measurement of glucose uptake indicates that disruption of the channel increases the basal glucose uptake in Kir6.2(-/-) extensor digitorum longus and the insulin-stimulated glucose uptake in Kir6.2(-/-) soleus muscle. In contrast, glucose uptake in adipose tissue, measured in vitro, was similar in Kir6.2(-/-) and WT mice, suggesting that the increase in glucose uptake in Kir6.2(-/-) adipocytes is mediated by altered extracellular hormonal or neuronal signals altered by disruption of the K(ATP) channels.

Denervation enhances the physiological effects of the K(ATP) channel during fatigue in EDL and soleus muscle

The objective was to determine whether denervation reduces or enhances the physiological effects of the K(ATP) channel during fatigue in mouse extensor digitorum longus (EDL) and soleus muscle. For this, we measured the effects of 100 microM of pinacidil, a channel opener, and of 10 microM of glibenclamide, a channel blocker, in denervated muscles and compared the data to those observed in innervated muscles from the study of Matar et al. (Matar W, Nosek TM, Wong D, and Renaud JM. Pinacidil suppresses contractility and preserves energy but glibenclamide has no effect during fatigue in skeletal muscle. Am J Physiol Cell Physiol 278: C404-C416, 2000). Pinacidil increased the (86)Rb(+) fractional loss during fatigue, and this effect was 2.6- to 3.4-fold greater in denervated than innervated muscle. Pinacidil also increased the rate of fatigue; for EDL the effect was 2.5-fold greater in denervated than innervated muscle, whereas for soleus the difference was 8.6-fold. A major effect of glibenclamide was an increase in resting tension during fatigue, which was for the EDL and soleus muscle 2.7- and 1.9-fold greater, respectively, in denervated than innervated muscle. A second major effect of glibenclamide was a reduced capacity to recover force after fatigue, an effect observed only in denervated muscle. We therefore suggest that the physiological effects of the K(ATP) channel are enhanced after denervation.

Effect on insulin release of compounds structurally related to the potassium-channel opener 7-chloro-3-isopropylamino-4H-1,2,4-benzothiadiazine 1,1-dioxide (BPDZ 73): introduction of heteroatoms on the 3-alkylamino side chain of the benzothiadiazine 1,1-dioxide ring

7-Chloro-3-pyridyl(alkyl)amino-4H-1,2,4-benzothiadiazine 1,1-dioxides and 3-alkylamino-7-chloro-4H-1,2,4-benzothiadiazine 1,1-dioxides containing one or more heteroatoms on the side chain in the 3 position have been synthesized in an attempt to discover new potent KATP-channel openers. The compounds were tested as putative pancreatic B-cells KATP channel openers by measuring their inhibitory activity on the insulin releasing process. The influence on the biological activity of the nature of the side chain in the 3 position is discussed.

Nicorandil accelerates recovery of neuromuscular block caused by vecuronium

PURPOSE

To examine the effect of nicorandil, a K ATP channel agonist, on neuromuscular block caused by vecuronium in patients anesthetized with nitrous oxide, oxygen, isoflurane, and fentanyl.

METHODS

Sixty adult patients were allocated to four groups of 15: nicorandil-post-tetanic count (N-PTC), nicorandil-train-of-four (N-TOF), control-post-tetanic count (C-PTC) or control-train-of-four (C-TOF) group. In the N-PTC and N-TOF groups, 0.1 mg kg nicorandil was given as a bolus followed by an infusion at 1 microg x kg(-1) x min(-1). Two minutes after the bolus, 0.1 mg x kg(-1) vecuronium was administered. In the C-PTC or C-TOF group normal saline was given instead of nicorandil. PTC and TOF responses were measured mechanically using a force displacement transducer.

RESULTS

Time from the administration of vecuronium to the onset of neuromuscular block in the N-PTC or N-TOF group did not differ from that in the C-PTC or C-TOF group (241 +/- 33 vs 225 +/- 32 sec, mean +/- SD). Times from vecuronium injection to the return of PTC in the N-PTC and C-PTC groups, and those of T1, T2, T3, and T4 (first, second, third, and fourth stimulation of TOF) in the N-TOF and C-TOF groups did not differ. Recoveries of PTC in the N-PTC and C-PTC groups followed similar time course. T1/control twitch height and TOF ratio (T4/T1) in the N-TOF group were higher than those in the C-TOF group 80-120 min and 100-120 min after administration of vecuronium, respectively.

CONCLUSION

Nicorandil accelerates recovery of neuromuscular block caused by vecuronium.

Pinacidil suppresses contractility and preserves energy but glibenclamide has no effect during muscle fatigue

The effects of 10 microM glibenclamide, an ATP-sensitive K(+) (K(ATP)) channel blocker, and 100 microM pinacidil, a channel opener, were studied to determine how the K(ATP) channel affects mouse extensor digitorum longus (EDL) and soleus muscle during fatigue. Fatigue was elicited with 200-ms-long tetanic contractions every second. Glibenclamide did not affect rate and extent of fatigue, force recovery, or (86)Rb(+) fractional loss. The only effects of glibenclamide during fatigue were: an increase in resting tension (EDL and soleus), a depolarization of the cell membrane, a prolongation of the repolarization phase of action potential, and a greater ATP depletion in soleus. Pinacidil, on the other hand, increased the rate but not the extent of fatigue, abolished the normal increase in resting tension during fatigue, enhanced force recovery, and increased (86)Rb(+) fractional loss in both the EDL and soleus. During fatigue, the decreases in ATP and phosphocreatine of soleus muscle were less in the presence of pinacidil. The glibenclamide effects suggest that fatigue, elicited with intermittent contractions, activates few K(ATP) channels that affect resting tension and membrane potentials but not tetanic force, whereas opening the channel with pinacidil causes a faster decrease in tetanic force, improves force recovery, and helps in preserving energy.

Molecular aspects of ATP-sensitive K+ channels in the cardiovascular system and K+ channel openers

ATP-sensitive K+ (K(ATP)) channels are inhibited by intracellular ATP (ATPi) and activated by intracellular nucleoside diphosphates and thus, provide a link between cellular metabolism and excitability. K(ATP) channels are widely distributed in various tissues and may be associated with diverse cellular functions. In the heart, the K(ATP) channel appears to be activated during ischemic or hypoxic conditions, and may be responsible for the increase of K+ efflux and shortening of the action potential duration. Therefore, opening of this channel may result in cardioprotective, as well as proarrhythmic, effects. These channels are clearly heterogeneous. The cardiac K(ATP) channel is the prototype of K(ATP) channels possessing approximately 80 pS of single-channel conductance in the presence of approximately 150 mM extracellular K+ and opens spontaneously in the absence of ATPi. A vascular K(ATP) channel called a nucleoside diphosphate-dependent K+ (K(NDP)) channel exhibits properties significantly different from those of the cardiac K(ATP) channel. The K(NDP) channel has the single-channel conductance of approximately 30-40 pS in the presence of approximately 150 mM extracellular K+, is closed in the absence of ATPi, and requires intracellular nucleoside di- or triphosphates, including ATPi to open. Nevertheless, K(ATP) and K(NDP) channels are both activated by K+ channel openers, including pinacidil and nicorandil, and inhibited by sulfonylurea derivatives such as glibenclamide. It recently was found that the cardiac K(ATP) channel is composed of a sulfonylurea receptor (SUR)2A and a two-transmembrane-type K+ channel subunit Kir6.2, while the vascular K(NDP) channel may be the complex of SUR2B and Kir6.1. By precisely comparing the functional properties of the SUR2A/Kir6.2 and the SUR2B/Kir6.1 channels, we shall show that the single-channel characteristics and pharmacological properties of SUR/Kir6.0 channels are determined by Kir and SUR subunits, respectively, while responses to intracellular nucleotides are determined by both SUR and Kir subunits.

Molecular biology of adenosine triphosphate-sensitive potassium channels

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

ATP-sensitive K+ channel blocker glibenclamide and diaphragm fatigue during normoxia and hypoxia

The role of ATP-sensitive K+ channels in skeletal muscle contractile performance is controversial: blockers of these channels have been found to not alter, accelerate, or attenuate fatigue. The present study reexamined whether glibenclamide affects contractile performance during repetitive contraction. Experiments systematically assessed the effects of stimulation paradigm, temperature, and presence of hypoxia and in addition compared intertrain with intratrain fatigue. Adult rat diaphragm muscle strips were studied in vitro. At 37 degrees C and normoxia, glibenclamide did not significantly affect any measure of fatigue during continuous 5- or 100-Hz or intermittent 20-Hz stimulation but progressively prolonged relaxation time during 20-Hz stimulation. At 20 degrees C and normoxia, neither force nor relaxation rate was affected significantly by glibenclamide during 20-Hz stimulation. At 37 degrees C and hypoxia, glibenclamide did not significantly affect fatigue at 5-Hz or intertrain fatigue during 20-Hz stimulation but reduced intratrain fatigue and prolonged relaxation time during 20-Hz stimulation. These findings indicate that, although ATP-sensitive K+ channels may be activated during repetitive contraction, their activation has only a modest effect on the rate of fatigue development.

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.

Nucleotide regulation and characteristics of potassium channel opener binding to skeletal muscle membranes

[3H]P1075 binding to membrane preparations of rabbit skeletal muscle were observed in the presence of nucleotide triphosphates or diphosphates but not AMP, cAMP, adenosine, tripolyphosphate, or pyrophosphate. Nonhydrolyzable or poorly hydrolyzable ATP analogs inhibited MgATP-supported binding. The EC50 value for MgATP-supported binding (0.4 mM) was decreased approximately 10-fold in the presence of an ATP-regenerating system, and significant metabolism by membrane nucleotidases was confirmed by high performance liquid chromatographic analysis. [3H]P1075 bound to skeletal muscle with a Kd value of 37 +/- 3 nM and a Bmax value of 280 +/- 14 fmol/mg of protein. [3H]P1075 binding to subcellular fractions was highest in membranes enriched in T tubules. Specific binding was reversible, trypsin-sensitive, maximal at pH 8, and stereoselective for the (3S,4R)-enantiomer of cromakalim. Potassium channel openers exhibited a rank order of potency of P1075 > pinacidil > levcromakalim = BMS-180448 > nicorandil > diazoxide = BRL 38226. Fluorescein analogs (ethyleosin, phloxine B, and rose bengal) were relatively potent inhibitors of binding (Ki = 200-300 nM). The potassium channel openers cromakalim and BMS-180448 were competitive inhibitors of [3H]P1075 binding. In contrast, rose bengal and the ATP-regulated potassium channel antagonist glyburide increased the rate of [3H]P1075 dissociation in a manner consistent with noncompetitive interaction.

The effects of pharmacological modulation of KATP on the guinea-pig isolated diaphragm

The purpose of the present study was to investigate the functional consequences of KATP modulation in the normal and the metabolically inhibited guinea-pig isolated diaphragm using the K+ channel openers cromakalim, pinacidil, RP49356 (N-methyl-2-(3-pyridil)-tetrahydrothiopyran-2-carbothiami de-1-oxide) and ZM260384 (2-(2,2-bis(difluoromethyl)-6-nitro-3,4-dihydro-2H-1,4-benzoxazine -4-yl)pyridine-N-oxide) and the K+ channel inhibitors glibenclamide, phentolamine and ciclazindol. All K+ channel openers accelerated the decline in function induced by intermittent tetanic contractions following metabolic inhibition and delayed the development of contracture. Cromakalim also improved the recovery of twitch tension following 10 min intermittent tetanic stimulation in the hypoxic guinea-pig diaphragm preparation. Of the K+ channel inhibitors tested, only ciclazindol, at the highest concentration tested (10 microM), significantly delayed the decline in tetanic tension following metabolic inhibition in the guinea-pig isolated diaphragm. None of the inhibitors significantly accelerated the development of contracture. All inhibitors however, antagonised the actions of the K+ channel opener, cromakalim. The results indicate that opening of KATP can accelerate the decline in function following metabolic inhibition in the guinea-pig isolated diaphragm. In the absence of K+ channel openers however, KATP does not appear to contribute to this decline under the conditions of the present study.

Dose-dependent activation and block by bisG10, a K+ channel blocker, of mouse and frog skeletal muscle KATP channels

The effects of a K+ channel blocker, bisG10, were examined on ATP-sensitive K+ (KATP) channels in membrane patches excised from mammalian and amphibian skeletal muscle fibres using the patch-clamp technique. At micromolar concentrations, bisG10, added on the intracellular side, induced a strong, reversible, flickery block of KATP channels. BisG10, added on the extracellular side, was about 100-fold less potent at inhibiting channel activity. At 10 nM, intracellular bisG10 increased KATP channel activity. This activation was independent of the presence of internal ATP or Mg2+. The inhibitory effect of bisG10 most likely arose from open-channel block whereas activation could result from more complex, indirect interactions.

Coexistence of two classes of glibenclamide-inhibitable ATP-regulated K+ channels in avian skeletal muscle

Avian skeletal muscle expresses two types of ATP-sensitive K+ channels which have a unitary conductance of 15pS. These K+ channels can be distinguished pharmacologically by their high or low sensitivity to the antidiabetic sulphonylurea blocker glibenclamide. Both channels are activated by the K+ channel opener cromakalim. Chick skeletal muscle expresses high-affinity binding sites for [3H]glibenclamide (Kd = 0.6nM) which presumably correspond to the ATP-sensitive K+ channels with the greatest sensitivity to glibenclamide. The density of these high-affinity binding sites varies during muscle development. The maximum density (500fmol/mg protein) appears at 16 days in ovo, i.e. at a period when myoblasts have differentiated into myotubes and when innervation of myotubes has started. After this maximum, the level of [3H]glibenclamide-binding sites decreases to a plateau value of 100fmol/mg protein at 2-5 days post-natal. When muscle cells are put in cultures, the high-affinity binding sites disappear rapidly. Neither glibenclamide nor cromakalim have any effect on normal physiological chick muscle contraction. They have no effect on contracture and/or 86Rb+ efflux produced by metabolic poisoning.

Inhibition of ATP-sensitive K+ channels of mouse skeletal muscle by disopyramide

Single ATP-sensitive K+ channels (KATP channels) were studied in inside-out membrane patches excised from mouse skeletal muscle. The class Ia antiarrhythmic, disopyramide (5-100 microM), applied to the cytoplasmic membrane surface inhibited KATP channels at -40 and +40 mV. Channel inhibition by disopyramide started slowly and reached an almost stationary level within 1 min. Recovery from channel inhibition by disopyramide was incomplete. At pH 7.4, the disopyramide concentrations producing 50% channel inhibition were 8.1 microM at -40 mV and 7.1 microM at +40 mV. The Hill coefficients of the concentration-response curves were close to unity at both potentials. Raising the internal pH from 7.4 to 8.0 had no significant effect on the actions of disopyramide, but lowering the pH to 6.5 greatly potentiated the inhibition of KATP channels by the antiarrhythmic. Thus the open probabilities of KATP channels at -40 mV and in the presence of disopyramide (20 microM) were smaller by a factor of 18 at pH 6.5 than at pH 7.4. The results suggest that disopyramide interacts with KATP channels through the lipid phase of the membrane and that lowering the intracellular pH increases the affinity of KATP channels to disopyramide. Thus disopyramide at therapeutic concentrations (6-15 microM) affects muscular KATP channels, in particular at reduced intracellular pH values that occur under ischaemic conditions and during fatiguing exercise.

Activation of ATP-dependent K+ channels by metabolic poisoning in adult mouse skeletal muscle: role of intracellular Mg(2+) and pH

1. The effects of metabolic poisoning, intracellular Mg(2+) and pH on ATP-dependent K+ (K+ATP) channels were examined in adult mouse isolated skeletal muscle fibres using the patch clamp technique. 2. In cell-attached membrane patches, while openings of one kind of channel could only rarely be detected under control conditions, cell poisoning with fluorodinitrobenzene (FDNB), dinitrophenol (DNP) and cyanide (CN) induced a strong and partially reversible increase in channel activity. 3. Slope conductance and glibenclamide sensitivity of this outward current indicated that the channel activated during poisoning was the K+ATP channel. 4. Single channel current amplitude was reduced during poisoning, but remained unchanged when activation of the K+ATP channel was induced by cromakalim. 5. In inside-out membrane patches, in the absence of intracellular ATP, intracellular application of Mg2+ decreased channel activity and single channel current amplitude. Inhibition of K+ATP channels by ATP was also reduced. 6. In the absence of intracellular ATP, a decrease in intracellular pH induced a reduction in channel activity and single channel current amplitude. Inhibition of K+ATP channels by ATP was also reduced. 7. The reduction of single channel current amplitude during poisoning was attributed to an increase in intracellular Mg2+ concentration caused by a fall in intracellular ATP concentration. These results also show that metabolic poisoning causes direct activation of K+ATP channels in skeletal muscle, and that is activation is at least partially mediated through an increase in intracellular Mg(2+) concentration and a decrease in intracellular pH.

Time-dependent fading of the activation of KATP channels, induced by aprikalim and nucleotides, in excised membrane patches from cardiac myocytes

1. The effects of the potassium channel opener (KCO) aprikalim (RP 52891) on the nucleotide-induced modulation of ATP-sensitive K+ (KATP) channels in freshly dissociated ventricular myocytes of guinea-pig heart, were studied by use of the inside-out patch-clamp technique. The internal surface of the excised membrane patch was initially bathed with a standard solution (Mg(2+)-free with EDTA), then sequentially superfused with solutions containing nucleoside diphosphates (NDPs: 200 microM ADP and 50 microM GDP) and NDPs plus 1 mM MgCl2 (with EGTA; referred to as Mg-NDP solution). 2. The normalized concentration-response (channel closing) relationship to ATP was shifted to the right when the standard solution was replaced by the Mg-NDP solution. Hence, the internal concentration of ATP ([ATP]i) inhibiting the channel activity by half (Ki) increased from 56 microM to 180 microM, with an apparently constant slope factor (s = 2.37). NDPs in the absence of Mg2+ did not decrease the sensitivity of the channels to ATP. 3. In standard solution, aprikalim (100 microM) activated KATP channels in the presence of a maximally inhibitory [ATP]i (500 microM). This effect was strongly enhanced when aprikalim was applied to patches exposed to Mg-NDP solution, as demonstrated by the 9 fold increase in Ki for [ATP]i (from 180 microM to 1.5 mM and s = 2.37). 4. The ability of aprikalim to overcome the channel closing effects of ATP in Mg-NDP solution waned rapidly. Similarly, the NDP-induced activation of ATP-blocked channels was also time-dependent. Both activation processes disappeared before the channel run-down phenomenon appeared in ATP-free conditions. 5. In conclusion, aprikalim is much more potent in opening KATP channels in membrane patches bathed in Mg-NDP solution than in standard solution. However, under the former experimental conditions, the effect of aprikalim waned rapidly. It is proposed that the waning phenomenon results from changes in the intrinsic enzymatic activity of the KATP channel protein (possibly linked to the experimental conditions) which lead to the channel closure.

Biophysical, pharmacological and developmental properties of ATP-sensitive K+ channels in cultured myotomal muscle cells from Xenopus embryos

Unlike mammalian muscle cells in culture, cultured myotomal muscle cells of Xenopus embryos express ATP-sensitive K+ (KATP) channels. The KATP channels are blocked by internal ATP (half-maximal inhibition K0.5 = 16 microM) and to a lesser extent by internal ADP, are voltage independent, have an inward rectification at positive potentials and are inhibited by glibenclamide (K0.5 = 2 microM). Surprisingly, these KATP channels are not sensitive to K+ channel openers such as cromakalim. Opening of these KATP channels does not occur under normal physiological conditions. It is elicited by metabolic exhaustion of the muscle cell and it precedes the development of an irreversible rigor state. Neither intracellular acidosis nor an increase of intracellular Ca2+ are involved in KATP channel opening. Different types of K+ channels are successively expressed after plating of myotomal muscle cells: (1) sustained delayed-rectifier K+ channels; (2) KATP channels; (3) inward-rectifier K+ channels; (4) transient delayed-rectifier K+ channels. The current density associated with KATP channels far exceeds that of voltage-dependent K+ channels. Innervation controls the expression of these KATP channels. Co-culture of muscle cells with neurons from the neural tube decreases the number of active KATP channels per patch. Similarly, in situ innervated submaxillaris muscle of tadpoles at stage 50-55 has a very low density of KATP channels.

Glibenclamide selectively blocks ATP-sensitive K+ channels reconstituted from skeletal muscle

Glibenclamide, a blocker of ATP-sensitive K+ (KATP) channels, was tested on three different types of rat skeletal K+ channels incorporated into bilayers. Glibenclamide (10 microM) blocked a class of KATP channels (unitary conductance of 57 pS in symmetric 150 mM KCl), which were inhibited by ATP. High concentrations of glibenclamide (100 microM) had no effect on either voltage-gated K+ channels (37 pS), or Ca(2+)-activated K+ channels (210 pS). Our results show that glibenclamide, even at high concentrations (100 microM) that may be required for quick action in whole muscle experiments, is a selective and specific blocker of skeletal KATP channels.