| | Reduced expression of Kir6.2/SUR2A subunits explains KATP deficiency in K+-depleted ratsReceived 18 January 2007; received in revised form 17 May 2007; accepted 25 July 2007. Abstract We investigated on the mechanism responsible for the reduced ATP-sensitive K+(KATP) channel activity recorded from skeletal muscle of K+-depleted rats. Patch-clamp and gene expression measurements of KATP channel subunits were performed. A down-regulation of the KATP channel subunits Kir6.2(−70%) and SUR2A(−46%) in skeletal muscles of K+-depleted rats but no changes in the expression of Kir6.1, SUR1 and SUR2B subunits were observed. A reduced KATP channel currents of −69.5% in K+-depleted rats was observed. The Kir6.2/SUR2A-B agonist cromakalim showed similar potency in activating the KATP channels of normokalaemic and K+-depleted rats but reduced efficacy in K+-depleted rats. The Kir6.2/SUR1-2B agonist diazoxide activated KATP channels in normokalaemic and K+-depleted rats with equal potency and efficacy. The down-regulation of the Kir6.2 explains the reduced KATP channel activity in K+-depleted rats. The lower expression of SUR2A explains the reduced efficacy of cromakalim; preserved SUR1 expression accounts for the efficacy of diazoxide. Kir6.2/SUR2A deficiency is associated with impaired muscle function in K+-depleted rats and in hypoPP. 1. Introduction  ATP-sensitive K(KATP) channels are involved in pathophysiological conditions of various tissues [1], [2], [3], [4], [5], [6], [7]. In skeletal muscle, recent evidence supports the role of KATP channels in the physiological performance of different muscle fibre types in response to their metabolic needs [8], [9]. This is related to a muscle-dependent molecular composition and properties of KATP channels that for instance may determine a muscle-specific regulation of the extracellular K+ concentration with local vasodilation, modulation of glucose uptake and drug responses [9]. A reduced activity of sarcolemmal KATP channels has been observed in the primary and secondary forms of hypokalaemic periodic paralysis (hypoPP), neuromuscular disorders associated with abnormal insulin response leading to fibre depolarization, transient weakness and hypokalaemia [10], [11], [12], [13]. In primary hypoPP, the phenotype is characterized by episodic attacks of flaccid muscle paralysis or weakness associated with a transient decrease in the blood K+ concentration. Attacks are precipitated by carbohydrate-rich meals, rest after exercise, sudden exposure to heat or cold, glucose or insulin infusion and acute stress [14], [15]. The episodes of muscle weakness observed in hypoPP are accompanied by a silent electromyogram and a lack of muscle action potentials, even upon electrical stimulation. Isolated muscle fibres also exhibit membrane depolarization on acute “in vitro” exposure to low extracellular K+ concentrations. Familial hypoPP is caused by loss-of-function mutations in at least two different genes: CACNL1A3 and SCN4A encoding for the α1 subunits of the skeletal muscle L-type Ca2+ channel and voltage-dependent Na+ channels, respectively [15], [16]. Muscle biopsies from hypoPP patients carrying the CACNL1A3-R528H mutation showed also an abnormal sarcolemmal KATP channel [13]. This channel exhibited reduced single-channel conductance due to the appearance of subconductance states and an impaired activation by nucleotide diphosphates. The abnormal KATP channel from hypoPP type-1 patients also exhibited smaller macroscopic KATP currents; it is therefore likely that this abnormality plays a role in the symptoms of hypoPP. The secondary, or non familial, forms of hypoPP share a phenotype similar to that of the familial form, but are caused by K+ deficiency secondary to diuretic abuse, toxins and other factors. The K+-depleted rats indeed show permanent muscle weakness, paralysis induced by insulin/glucose injection and plasma K+ concentration constantly below 3.2 mEq/l in contrast with interictal periods in familial hypoPP where the plasma K+ concentration is normal [10], [11], [17], [18]. As observed in humans hypoPP type-1, skeletal muscle fibres isolated from K+-depleted rats possess a KATP channel with a reduced single-channel conductance that is insensitive to insulin stimulation [10], [11]. Also, muscles from K+-depleted rats exhibited smaller macroscopic KATP currents, however the molecular mechanism responsible for this abnormality is not known. To address the mechanisms responsible for reduced macroscopic KATP currents in K+-depleted rats, we combined electrophysiological recordings of sarcolemmal KATP channel activity in fast-twitch muscle with quantification of mRNA expression levels of KATP channel subunits: the inwardly rectifying K-channels (Kir6.1, Kir6.2) and sulfonylurea receptors (SUR1, SUR2A-B) in muscle samples from K+-depleted and normokalaemic rats. The responses of muscle KATP channels of K+-depleted and normokalaemic rats to cromakalim, a Kir6.2/SUR2A-B agonist and diazoxide, a Kir6.2/SUR1-2B agonist, were also compared. 2. Materials and methods 2.2. Muscle biopsies The flexor digitorum brevis (FDB), tibialis anterior (TA) and extensor digitorum longus (EDL) muscles were dissected under urethane anaesthesia (1.2g/kg). After dissection, the animals were rapidly sacrificed with an overdose of urethane. Single muscle fibres were prepared from FDB muscles by enzymatic dissociation for patch-clamp experiments, while the contralateral FDB muscles removed from the same rats were rapidly frozen in situ with liquid nitrogen and used for mRNA analysis. TA and EDL muscles were also collected from the same rats and used for patch-clamp and mRNA analyses. The normal Ringer solution used during muscle biopsy and for preparation of isolated fibres contained (mM): 145 NaCl, 5 KCl, 1 MgCl2, 0.5 CaCl2, 5 glucose, 10 3-(N-morpholino)-propanesulfonic (MOPS) sodium salt and was adjusted to pH 7.2 with MOPS acid. 2.3. Real time quantitative PCR For each muscle sample, total RNA was isolated using Trizol reagent and treated with DNase I (4U, 37°C, 1h). RNA was quantified using a spectrophotometer (Beckman DU 530) and 3μg was used for reverse transcription. Synthesis of cDNA was performed using random hexamers (annealed 10min, 25°C) and Superscript II reverse transcriptase (Invitrogen-Life Technologies, Carlsbad, CA, USA) incubated at 42°C for 50min. We used the available rat sequences for Kir6.1 (GenBank Accession No. NM_017099), Kir6.2 (NM_031358), SUR1 (NM_013039), SUR2A (D83598), SUR2B (AF019628) and β-actin (NM_031144). Fluorescently labeled TaqMan (Applied Biosystems, Foster City, CA, USA) probes for Kir6.1, SUR1, SUR2A, SUR2B and β-actin were designed using PrimerExpress (Applied Biosystems) to amplify 124–141bp products encompassing each probe annealing site. Specific primer and probe sequences for each gene have been reported previously [9]. To achieve a high level of specificity and to avoid detection of genomic DNA, we designed probes to span exon–exon junctions for each gene except for Kir6.2, which is intronless. For Kir6.2, a TaqMan probe was designed within a region having substantial sequence divergence with other genes and control amplifications of RNA without reverse transcription were performed to exclude genomic DNA contamination. None of the SUR and Kir primer and probe sets cross-reacted with non specific SUR or Kir sequences after 40 cycles of PCR, and no amplification was observed after 45 cycles of PCR in control reactions containing no DNA template. Triplicate reactions were carried out in parallel for each individual muscle sample. The results were compared to a gene specific standard curve and normalized to expression of the housekeeping gene, β-actin, in the same sample. Template used for determining standard curves consisted of plasmid DNA containing the expected target sequence quantified by Pico Green fluorescence (Molecular Probes, Eugene, OR, USA). 2.4. Electrophysiology Experiments were performed with the inside-out configuration of the patch-clamp technique. Current recordings were performed during voltage steps from the holding potential (0mV) to test potentials ranging from −70mV to +70mV immediately after excision, at 20–22°C. Current was recorded at 1kHz (filter=0.2kHz) using an Axopatch-1D amplifier equipped with a CV-4 headstage (Axon Ins. Foster City, CA, USA). Pipettes having an average tip area of 8.1±0.8μm2 (n=300 patches) were used to measure KATP currents and the pharmacological responses of the channels. The pipette area was measured by scanning electron microscopy as previously described [10]. The patch pipette solution contained (mM) 150 KCl, 2 CaCl2, 1 MOPS, pH 7.2. The bath solution contained (mM) 150 KCl, 5 EGTA, 1 MOPS, pH 7.2. Stock solutions of cromakalim (50mM) and diazoxide (50mM) (SIGMA, Co Milano) were prepared by dissolving the drugs in dimethylsulphoxide (DMSO). Microliter amounts of these stock solutions were then added to the bath solutions in the presence of ATPMg. DMSO applied at the maximal concentration tested (0.1%) did not affect KATP channel currents in the absence or in the presence of ATPMg. Current amplitude was measured using the Clampfit program (Axon Ins. Foster City, CA, USA). No correction for liquid junction potentials were made as these were estimated to be <2mV under our experimental conditions. Concentration-response relationships experiments were performed by applying increasing concentrations of diazoxide (10−12–1.5×10−4M) or cromakalim (10−10–2×10−4M) to inside-out macropatches in the presence of internal ATPMg (0.1mM). 3. Results K+-depleted rats displayed low serum K+ levels (2.4±0.12mEq/l; n=7 rats), a delay in the righting reflex ranging between 2 and 25s, and general weakness (evaluated by observing the movement of the animal in its cage). In contrast, rats fed a normal diet showed normal serum K+ levels (4.6±0.04; n=7 rats) and a righting reflex of less than 1s. 3.1. KATP channel activity in K+-depleted and normokalaemic rat muscles We used fast-twitch muscles for our studies because secondary and primary hypoPP affects this muscle type. Differences in the biophysical and pharmacological properties, and in molecular composition of KATP channels in slow-twitch and fast-twitch muscles, have recently been found [9]. For example, fast-twitch muscle has a higher KATP channel activity per unit area than slow-twitch muscle. Patch excision from FDB muscle fibres into ATP-free solution produced a dramatic increase of inward currents in 75% of macropatches from normokalaemic rats but only in 19% of patches from K+-depleted rats. Mean inward current recorded immediately after excision, which was calculated by averaging conductive and non conductive patches, was −248.4±25pA (n=69 patches; n=7 rats) for normokalaemic rats and −78.09±11pA (n=231 patches; n=7 rats) for K+-depleted rats. Exposure of macropatches from both K+-depleted and normokalaemic rats to intracellular ATPMg (5mM) reduced the current amplitude indicating that the current flowed through KATP channels (Fig. 1a). The ATP-sensitive current was −76.1±7pA (n=31 patches) in K+-depleted rats and −250.2±23pA (n=42 patches) in normokalaemic rats, respectively. There was no significant difference in the ATP-insensitive current, which was −62±10pA and −74±11pA in the normokalaemic and K+-depleted rats, respectively. Thus, the lower inward current observed on patch excision in K+-depleted rats results from a specific reduction in the KATP current, of approximately 69.5%. A similar reduction of the KATP current was also observed in other muscles (e.g. EDL ) of K+-depleted rats (data not shown). ATPMg applied on the intracellular side of the patches caused a dose-dependent reduction of KATP currents in FDB muscles of normokalaemic and K+-depleted rats. However, there was a small but significant difference in potency. The IC50 values for ATPMg were 15±6μM (n=6 patches) and 6.3±1μM (slope=1) (n=5 patches) for FDB muscles of normokalaemic and K+-depleted rats, respectively. 3.2. Expression of KATP channel subunits Quantitative real time RT-PCR measurements demonstrated a significant difference in the expression levels of Kir6.2 mRNA between normokalaemic and K+-depleted rats of FDB muscles (Fig. 1b). Expression of Kir6.2 was indeed 68% lower in K+-depleted rats. As previously reported [9], the relative expression of Kir6.2 in normokalaemic rats was significantly higher than that of the Kir6.1 (which was negligible). Expression of Kir6.1 was unaltered in K+-depleted rats. Similar data were obtained from other muscles from K+-depleted rats (e.g. EDL and TA; data not shown). We also found that the expression of the SUR2A subunit was significantly reduced in FDB muscles of the K+-depleted rats and it was 46% lower as compared with that of the normokalaemic rats. By contrast, no significant differences in the expression levels of SUR1 and SUR2B subunits between the K+-depleted and normokalaemic rats were observed (Fig. 1c). 3.3. Effects of cromakalim and diazoxide Cromakalim and diazoxide, in the presence of 0.1mM internal ATPMg, caused a dose-dependent activation of KATP channels in patches excised from muscles of both K+-depleted and normokalaemic rats. The cromakalim concentration-response data were fitted with one stimulatory site function for both K+-depleted and normokalaemic rats (Fig. 2a). In the concentration range from 10−8M to 2×10−4M, cromakalim produced 88% activation of KATP channels in normokalaemic rat muscle, but the efficacy of the drug in activating KATP channels in muscles of K+-depleted rats was markedly reduced (Fig. 2a and b). No difference in the cromakalim response between normokalaemic and K+-depleted rats was observed at drug concentrations <10−8M (Fig. 2a and b). The parameters of the concentration-response relationships calculated by the fitting routine for the normokalaemic rat data were: DE50=1.1±0.5×10−9M, Emax=80±4, n=1; and for the K+-depleted rat data were: DE50=2.1±0.4×10−9M, Emax=63±5, n=0.9. In the concentration range from 10−9M to 10−4M, the diazoxide data were fitted with a single stimulatory site function in both normokalaemic and K+-depleted rats (Fig. 3a). At saturating concentrations, diazoxide produced similar activation of KATP channels of both normokalaemic and K+-depleted rats (Fig. 3a and b). The parameters of the concentration-response relationships calculated by the fitting routine for the normokalaemic rat data were: DE50=8±0.9×10−9M, Emax=57±9, n=0.7; and for the K+-depleted rat data were: DE50=9±1×10−9M, Emax=53±6, n=0.7. Therefore, no differences in the responses of KATP channels in muscles of K+-depleted rats and normokalaemic rats to diazoxide were observed in the range of concentrations tested (Fig. 3a and b). 4. Discussion In this study, we demonstrated that fast-twitch muscles of K+-depleted rats exhibited a 70% reduction in the expression of Kir6.2, which was paralleled by a 69.5% reduction in KATP channel current. These findings indicate that reduced expression of Kir6.2 is responsible for the lower KATP channel currents in the muscles of K+-depleted rats. It is well established that both Kir6.2 and SURs subunits are required to form a functional KATP channel and that neither subunit is trafficked to the surface membrane in the absence of the other [19]. Thus, down-regulation of Kir6.2 is expected to result in a lower KATP channel current of skeletal muscle as we observed. One point of interest is that members of the Kir1.x (ROMK) family of inward rectifiers, which control the renal excretion of K+ ions, are also down-regulated by hypokalaemia. For example, K+restriction leads to down-regulation of both Kir1.1 in the rat kidney and Kir7.1 in the basolateral membrane of the distal nephron and collecting duct [20], [21], [22]. This suggests that there may be a common K+-dependent mechanism that regulates transcription of Kir genes in both muscle and kidney. Down-regulation of Kir genes in response to hypokalaemia may have a protective role, sparing K+ excretion and helping to conserve plasma K+ levels. However, in skeletal muscle this mechanism can be also a precipitating factor for hypoPP, by impeding K+ efflux and impairing fibre repolarization. In addition to this mechanism which is associated with chronic K+-depletion, the hypokalaemia may directly affects the KATP channels. A brief hypokalaemic period indeed causes a direct block of the KATP and of other Kir channels; shut-off of the inward rectifier K+ channels occurs when they are acutely exposed to low external concentrations of K+ ions below 2mEq/l. This is the basis of the known bistable behaviour of the membrane potential in response to the lowering of external K+ concentrations [23]. Therefore the down-regulation of the Kir6.2 message, that also reduces the number of functional channels in the membrane, and the direct block of the channel pore due to the hypokalaemia are the mechanisms responsible for the generation of the subconductance states and low Popen characterizing the abnormal KATP channel found in K+-depleted rats and human hypoPP patients[10], [13]. A significant reduction in expression of SUR2A (46%) was also found in the muscles of K+-depleted rats. The SUR2 gene is alternatively spliced, but the reduction in SUR2A expression was not paralleled by a similar down-regulation of SUR2B mRNA. This suggests that a transcriptional splicing mechanism is also involved in the impaired KATP channel function of K+-depleted rat muscles. The data on the expression of SUR mRNAs complements our electrophysiological and pharmacological findings. For example, the reduced expression of SUR2A found in the K+-depleted rats explains why their KATP channels were less responsive to cromakalim, a well known Kir6.2/SUR2A-B agonist. The binding sites for diazoxide, possibly SUR1 and/or SUR2B subunits, were not affected by hypokalaemia as suggested by fact that the dose-response relationships of the KATP currents vs diazoxide concentrations of normokalaemic and K+-depleted rats were not different. The finding that KATP channels of K+-depleted rats were more sensitive to ATP inhibition than those of normokalaemic rats may be related to the subunit composition of muscle KATP channels. We have previously shown that KATP channels in normal fast-twitch muscles are mostly composed of Kir6.2/SUR2A and Kir6.1/SUR1, with possibly a small contribution from Kir6.2/SUR2B complexes. In contrast, slow-twitch muscle lacks Kir6.2/SUR1. KATP channels containing SUR1 are more sensitive to ATP inhibition than those containing SUR2, and they are sensitive to diazoxide but not to cromakalim [24]. Down-regulation of SUR2A in K+-depleted rats will leads to an increased fraction of Kir6.2/SUR1 channels which are more sensitive to ATP and are activated by diazoxide. We studied the K+-depleted rat model which is not a genetic animal model of hypoPP. However, our data may help to explain the insulin/glucose-dependent hypokalaemia, muscle weakness and paralysis which are commonly observed in both the secondary and primary forms of hypoPP. For example, in normal muscle insulin promotes a cascade of events that includes activation of 3Na+/2K+ATP-ase with uptake of K+ ions into the fibres and transient hypokalaemia which is balanced by the activation of the KATP channels that extrude K+ ions from the fibres. In human hypoPP and in K+-depleted rats a persistent hypokalaemia is observed following insulin and/or adrenergic stimulation because the abnormally reduced KATP channel activity is unable to counteract the low serum K+ ions levels. The lack of KATP currents that we found and possibly of inward rectifier K+ currents as observed by others [12], reduces the repolarizing components and may unmasks depolarizing currents activated by insulin or by other signals with fibre depolarization. In turn, depolarization is expected to cause inactivation of voltage-gated Na+ channels and reduces muscle excitability. KATP channel deficiency other than associated to hypokalaemia and fibre depolarization may also impairs the functionality of skeletal muscle. Kir6.2 deficiency indeed affects contractility of skeletal muscle as demonstrated by the fact that Kir6.2 knock-out (−/−) mice show reduced resting tension and impaired adaptation to exercise-induced physical stress [7]. Moreover, the SUR2A deficiency may leads to a KATP channel unresponsive to metabolic stimulus with impairment in the capability of the skeletal muscle to recover upon stress. Mutations in the SUR2A gene have been recently associated with some forms of dilated cardiomyopathy in humans and are responsible for the impaired performance of cardiac muscle in mice carrying those mutations [4], [6], [7]. While the reduced KATP activity found in the K+-depleted rats is clearly related to the K+-depletion, however the link between the gene mutations found in the familial hypoPP and the reduction of the sarcolemmal KATP channel activity found in the primary form is still an open question. One possible mechanism involves changes in the concentrations of intracellular Na+ ions (or Ca2+), as a consequence of mutations in SCN4A or CACNL1A3, that lead to down-regulation of KATP channel genes. The fact that human hypoPP patients show elevated intracellular Na+ ions in skeletal muscle corroborates this idea [12], [14], [16]. The elevated intracellular Na+ ions may stimulates the activity of the Na+/Ca2+ exchanger or other Na+-dependent pumps and transporters with effects on the intracellular levels of Ca2+ ions in turn affecting the KATP activity. Additional mechanisms involving protein–protein interactions, changes of membrane voltage and/or intracellular factors that regulate the activity of ion channels cannot be excluded [25]. Indeed, there is evidence that the L-type Ca2+channel is functionally coupled to the KATP channel in ventricular cells, since genetic deletion of Kir6.2 results in up-regulation of L-type Ca2+channels [4], [26]. In conclusion, we have demonstrated that the symptoms observed in K+-depleted rats can be explained by down-regulation of Kir6.2 and SUR2A and a consequent reduction in the muscle KATP channel activity. Because a reduction in KATPcurrent is also observed in muscle fibres isolated from human hypoPP patients type-1, it is possible that a similar reduction in Kir6.2 and SUR2A expression may occur in the human disease. Our experiments also suggest that diazoxide may be an effective treatment for hypoPP. Current drug therapy is based on the use of acetazolamide and dichlorphenamide which act by targeting another class of K+ channels, the muscle BK channel [15], [27]. In our experiments diazoxide was capable of directly activating muscle KATP channels by binding to SUR1 (and SUR2B subunits) which are expressed in K+-depleted rats and contribute to functional KATP channels in fast-twitch skeletal muscle. This provides a mechanistic basis for the potential effectiveness of this drug and its structural analogs in this disorder [14], [16]. Acknowledgement This work was supported by Telethon-Conte Grant GGP04140. References [1]. [1]Ashcroft FM. From molecule to malady. Nature. 2006;440:440–447.
CrossRef
[2]. [2]Seino S, Miki T. Physiological and pathophysiological roles of ATP-sensitive K(+) channels. Prog Biophys Mol Biol. 2003;81:133–176. MEDLINE |
CrossRef
[3]. [3]Ashcroft FM, Gribble FM. ATP-sensitive K+ channels and insulin secretion: their role in health and disease. Diabetologia. 1999;42:903–919.
CrossRef
[4]. [4]Nichols CG. KATP channels as molecular sensors of cellular metabolism. Nature. 2006;440:470–476.
CrossRef
[5]. [5]Babenko AP, Polak M, Cave H, et al. Activating mutations in the ABCC8 gene in neonatal diabetes mellitus. N Engl J Med. 2006;355:456–466.
CrossRef
[6]. [6]Alekseev AE, Hodgson DM, Karger AB, et al. ATP-sensitive K+ channel channel/enzyme multimer: metabolic gating in the heart. J Mol Cell Cardiol. 2005;38:895–905. Abstract | Full Text |
Full-Text PDF (287 KB)
|
CrossRef
[7]. [7]Kane GC, Liu X, Yamada S, et al. Cardiac KATP channels in health and disease. J Mol Cardiol. 2005;38:937–943. [8]. [8]Rodrigo GC, Standen NB. ATP-sensitive potassium channels. Curr Pharm Des. 2005;11:1915–1940. MEDLINE |
CrossRef
[9]. [9]Tricarico D, Mele A, Lundquist AL, et al. Hybrid assemblies of ATP-sensitive K+ channels determine their muscle-type-dependent biophysical and pharmacological properties. Proc Nat Acad Sci USA. 2006;103:1118–1123. MEDLINE |
CrossRef
[10]. [10]Tricarico D, Pierno S, Mallamaci R, et al. The biophysical and pharmacological characteristics of skeletal muscle KATP channels are modified in K+ depleted rat, an animal model of hypokalemic periodic paralysis. Mol Pharmacol. 1998;54:197–206. [11]. [11]Tricarico D, Capriulo R, Conte Camerino D. Insulin modulation of ATP-sensitive K+channel of rat skeletal muscle is impaired in the hypokalemic state. Pflugers Archiv. 1998;437:235–240. [12]. [12]Ruff RL. Insulin acts in hypokalemic periodic paralysis by reducing inward rectifier K+ current. Neurology. 1999;53:1556–1563. MEDLINE [13]. [13]Tricarico D, Servidei S, Tonali P, et al. Impairment of skeletal muscle adenosine triphosphate-sensitive K+ channels in patients with hypokalemic periodic paralysis. J Clin Inv. 1999;103:675–682. [14]. [14]Lehmann-Horn F, Jurkat-Rott K, Rüdel R. Periodic paralysis. Curr Neurol Neurosci Rep. 2002;2:61–69. MEDLINE |
CrossRef
[15]. [15]Venance SL, Cannon SC, Fialho D, et al. The primary periodic paralyses: diagnosis, pathogenesis and treatment. Brain. 2006;129:8–17.
CrossRef
[16]. [16]Cannon SC. Pathomechanisms in channelopathies of skeletal muscle and brain. Ann Rev Neurosci. 2006;29:387–415.
CrossRef
[17]. [17]Kao I, Gordon AM. Mechanism of insulin-induced paralysis of muscle from potassium depleted rats. Science. 1975;188:740–741. MEDLINE [18]. [18]Bond EF, Gordon AM. Insulin-induced membrane changes in K+ -depleted rat skeletal muscle. Am J Physiol. 1993;265:C257–C265. MEDLINE [19]. [19]Zerangue N, Schwappach B, Jan YN, et al. A new ER trafficking signal regulates the subunit stoichiometry of plasma membrane KATP channels. Neuron. 1999;22:537–548. MEDLINE |
CrossRef
[20]. [20]Wald H, Garty H, Palmer LG, et al. Differential regulation of ROMK expression in kidney cortex and medulla by aldosterone and potassium. Am J Physiol (Renal Physiol 44). 1998;275:F239–F245. [21]. [21]Mennit PA, Frindt G, Silver RB, et al. Potassium restriction downregulates ROMK expression in rat kidney. Am J Physiol (Renal Physiol). 2000;278:F916–F924. MEDLINE [22]. [22]Ookata K, Tojo A, Suzuki Y, et al. Localization of inward rectifier potassium channel Kir7.1 in the basolateral membrane of distal nephron and collecting duct. J Am Soc Nephrol. 2000;11:1987–1994. MEDLINE [23]. [23]Geukes Foppen RJ, vanMill HGJ, Siegenbeek van Heukelon J. Effects of chloride transport on bistable behaviour of the membrane potential in mouse skeletal muscle. J Physiol. 2001;542(1):181–191. MEDLINE |
CrossRef
[24]. [24]Song DK, Ashcroft FM. ATP modulation of ATP-sensitive potassium channel ATP sensitivity varies with the type of SUR subunit. J Biol Chem. 2001;276:7143–7149. MEDLINE |
CrossRef
[25]. [25]Michailova A, Saucerman J, Belik ME, et al. Modelling regulation of cardiac KATP and L-type Ca2+ currents by ATP, ADP, and Mg2+. Biophys J. 2005;88:2234–2249. MEDLINE |
CrossRef
[26]. [26]Flagg TP, Nichols CG. Sarcolemmal K(ATP) channels: what do we really know?. J Mol Cell Cardiol. 2005;39:61–70. Abstract | Full Text |
Full-Text PDF (450 KB)
|
CrossRef
[27]. [27]Tricarico D, Barbieri M, Mele A, et al. Carbonic anhydrase inhibitors are specific openers of skeletal muscle BK channel of K+-deficient rats. The FASEB J. 2004;18:760–761. a Department of Pharmacobiology, Faculty of Pharmacy, University of Bari, via Orabona n° 4, 70120 Bari, Italy b Division of Genetic Medicine, Department of Medicine, Vanderbilt University, 529 Light Hall, Nashville, TN 37232-0275, USA c University Laboratory of Physiology, Parks Road, Oxford OX1 3PT, UK d Department of General Physiology, Molecular Neurophysiology, University of Ulm, Albert Einsteinalle 11, 89081 Ulm, Germany  Corresponding author. Tel.: +39 0805442802; fax:+39 0805442801.
PII: S0960-8966(07)00684-0 doi:10.1016/j.nmd.2007.07.009 © 2007 Elsevier B.V. All rights reserved. | |
|
FULL TEXT00684-0/journalimage?img=PIIS0960896607006840.gr1.lrg.gif&fig=fig1&kwhquery=&issn=0960-8966&ishighres=false&allhighres=false&free=yes','journalimage');)
00684-0/journalimage?img=PIIS0960896607006840.gr2.lrg.gif&fig=fig2&kwhquery=&issn=0960-8966&ishighres=false&allhighres=false&free=yes','journalimage');)
00684-0/journalimage?img=PIIS0960896607006840.gr3.lrg.gif&fig=fig3&kwhquery=&issn=0960-8966&ishighres=false&allhighres=false&free=yes','journalimage');)