ATP-sensitive Potassium Current Research Articles

Phase 2 Re-Entry Without Ito: Role of Sodium Channel Kinetics in Brugada Syndrome Arrhythmias

BackgroundIn Brugada syndrome (BrS), phase 2 re-excitation/re-entry (P2R) induced by the transient outward potassium current (Ito) is a proposed arrhythmia mechanism; yet, the most common genetic defects are loss-of-function sodium channel mutations. ObjectivesThe authors used computer simulations to investigate how sodium channel dysfunction affects P2R-mediated arrhythmogenesis in the presence and absence of Ito. MethodsComputer simulations were carried out in 1-dimensional cables and 2-dimensional tissue using guinea pig and human ventricular action potential models. ResultsIn the presence of Ito sufficient to generate robust P2R, reducing sodium current (INa) peak amplitude alone only slightly potentiated P2R. When INa inactivation kinetics were also altered to simulate reported effects of BrS mutations and sodium channel blockers, however, P2R occurred even in the absence of Ito. These effects could be potentiated by delaying L-type calcium channel activation or increasing ATP-sensitive potassium current, consistent with experimental and clinical findings. INa-mediated P2R also accounted for sex-related, day and night–related, and fever-related differences in arrhythmia risk in BrS patients. ConclusionsAltered INa kinetics synergize powerfully with reduced INa amplitude to promote P2R-induced arrhythmias in BrS in the absence of Ito, establishing a robust mechanistic link between altered INa kinetics and the P2R-mediated arrhythmia mechanism.

Protein kinase G signaling pathway is involved in sympathetically maintained pain by modulating ATP-sensitive potassium channels

BackgroundSympathetically maintained pain (SMP) involves an increased excitability of dorsal root ganglion (DRG) neurons to sympathetic nerve stimulation and circulating norepinephrine. The current treatment of SMP has limited efficacy, and...

Identification of an ATP-Sensitive Potassium Channel in the Inner Mitochondrial Membrane

ATP-sensitive potassium channels (KATP) are widely distributed ion channels acting as sensors of cellular metabolism. In the plasma membrane (pmKATP) they couple cell excitability with energy availability. They are also reported to be located to intracellular membranes, in particular in mitochondria, but in this context even their existence is still a matter of debate. Here, we unambiguously identify a mitochondria-localized protein complex that mediates ATP-sensitive potassium currents, referred to as mitoKATP. We show that similarly to its plasma membrane counterpart, mitoKATP is composed of a pore-forming subunit (MITOK) and an ATP-binding subunit (MITOSUR). In vitro reconstitution of MITOK together with MITOSUR recapitulates the main electrophysiological properties and pharmacological profile of mitoKATP. Overexpression of MITOK alone causes loss of mitochondrial membrane potential (▵Ψm), decrease of mitochondrial Ca2+ uptake, organelle fragmentation and disruption of cristae, in line with an increased cation permeability of the inner mitochondrial membrane. However, the concomitant overexpression of the MITOSUR subunit restores the correct channel gating and rescues organelle injury. Conversely, MITOK ablation causes mitochondrial dysfunction characterized by instability of mitochondrial membrane potential, widening of intracristal space and decreased performance of oxidative phosphorylation. Most importantly, loss of MITOK in mice suppresses cardioprotection elicited by diazoxide-induced pharmacological preconditioning. Overall, our data indicate that the hereby-identified novel components forming mitoKATP control organelle volume, thereby representing key players in shaping mitochondrial physiology with a potential impact on several pathological processes.

Mechanism underlying impaired cardiac pacemaking rhythm during ischemia: A simulation study.

Ischemia in the heart impairs function of the cardiac pacemaker, the sinoatrial node (SAN). However, the ionic mechanisms underlying the ischemia-induced dysfunction of the SAN remain elusive. In order to investigate the ionic mechanisms by which ischemia causes SAN dysfunction, action potential models of rabbit SAN and atrial cells were modified to incorporate extant experimental data of ischemia-induced changes to membrane ion channels and intracellular ion homeostasis. The cell models were incorporated into an anatomically detailed 2D model of the intact SAN-atrium. Using the multi-scale models, the functional impact of ischemia-induced electrical alterations on cardiac pacemaking action potentials (APs) and their conduction was investigated. The effects of vagal tone activity on the regulation of cardiac pacemaker activity in control and ischemic conditions were also investigated. The simulation results showed that at the cellular level ischemia slowed the SAN pacemaking rate, which was mainly attributable to the altered Na+-Ca2+ exchange current and the ATP-sensitive potassium current. In the 2D SAN-atrium tissue model, ischemia slowed down both the pacemaking rate and the conduction velocity of APs into the surrounding atrial tissue. Simulated vagal nerve activity, including the actions of acetylcholine in the model, amplified the effects of ischemia, leading to possible SAN arrest and/or conduction exit block, which are major features of the sick sinus syndrome. In conclusion, this study provides novel insights into understanding the mechanisms by which ischemia alters SAN function, identifying specific conductances as contributors to bradycardia and conduction block.

Arrhythmia Vulnerability in Diabetic Cardiac Tissue is Species-Dependent: Effects of I KATP, Uncoupling, and Connexin Lateralization.

Amongst the complications of diabetes is arrhythmia, the risk of which depends on multiple factors. This study was designed to investigate several factors, including the effects of ATP-sensitive potassium current, lateralized connexins, and gap junction uncoupling. ATP-sensitive potassium channel (I KATP) opening is caused by ischemia, which can occur in diabetic or non-diabetic hearts. I KATP opening was simulated in this work to determine if the risk of ischemia-induced arrhythmias is affected by diabetes. Simulations were performed using healthy and diabetic models of rat and rabbit ventricle. Results showed that the diabetic rat model is less vulnerable to reentrant arrhythmia than the healthy rat model. The diabetic rabbit model was more vulnerable to reentrant arrhythmia than the healthy rabbit model. In both rabbit models, the vulnerability increased as the gap junctional coupling decreased. Opening of I KATP resulted in larger window of vulnerability. Conduction reserve was simulated based on 1D simulations for both rat and rabbit models. There was no difference between rat and rabbit conduction reserve. Our results showed that the simulation results are model-dependent, i.e., results from the rabbit model are similar to human clinical data, while the results from the rat model contradict human clinical observations, suggesting a significant species-dependence in arrhythmia vulnerability in the diabetic heart.

2-Deoxy-d-glucose enhances tonic inhibition through the neurosteroid-mediated activation of extrasynaptic GABAA receptors.

The inhibition of glycolysis exerts potent antiseizure effects, as demonstrated by the efficacy of ketogenic and low-glucose/nonketogenic diets in the treatment of drug-resistant epilepsy. ATP-sensitive potassium (KATP ) channels have been initially identified as the main determinant of the reduction of neuronal hyperexcitability. However, a plethora of other mechanisms have been proposed. Herein, we report the ability of 2-deoxy-d-glucose (2-DG), a glucose analog that inhibits glycolytic enzymes, of potentiating γ-aminobutyric acid (GABA)ergic tonic inhibition via neurosteroid-mediated activation of extrasynaptic GABAA receptors. Acute effects of 2-DG on the ATP-sensitive potassium currents, GABAergic tonic inhibition, firing activity, and interictal events were assessed in hippocampal slices by whole-cell patch-clamp and local field potential recordings of dentate gyrus granule cells. Acute application of 2-DG activates two distinct outward conductances: a KATP channel-mediated current and a bicuculline-sensitive tonic current. The effect of 2-DG on such GABAergic tonic currents was fully prevented by either finasteride or PK11195, which are specific inhibitors of the neurosteroidogenesis pathway acting via different mechanisms. Moreover, the oxidized form of vitamin C, dehydroascorbic acid, known for its ability to induce neurosteroidogenesis, also activated a bicuculline-sensitive tonic current in a manner indistinguishable from that of 2-DG. Finally, we found that the enhancement of KATP current by 2-DG primarily regulates intrinsic firing rate of granule cells, whereas the increase of the GABAergic tonic current plays a key role in reducing the frequency of interictal events evoked by treatment of hippocampal slices with the convulsive agent 4-aminopyridine. We demonstrated, for the first time, that 2-DG potentiates the extrasynaptic tonic GABAergic current through activation of neurosteroidogenesis. Such tonic inhibition represents the main conductance responsible for the antiseizure action of this glycolytic inhibitor.

Early afterdepolarizations promote transmural reentry in ischemic human ventricles with reduced repolarization reserve

AimsAcute ischemia is a major cause of sudden arrhythmic death, further promoted by potassium current blockers. Macro-reentry around the ischemic region and early afterdepolarizations (EADs) caused by electrotonic current have been suggested as potential mechanisms in animal and isolated cell studies. However, ventricular and human-specific arrhythmia mechanisms and their modulation by repolarization reserve remain unclear. The goal of this paper is to unravel multiscale mechanisms underlying the modulation of arrhythmic risk by potassium current (IKr) block in human ventricles with acute regional ischemia. Methods and resultsA human ventricular biophysically-detailed model, with acute regional ischemia is constructed by integrating experimental knowledge on the electrophysiological ionic alterations caused by coronary occlusion. Arrhythmic risk is evaluated by determining the vulnerable window (VW) for reentry following ectopy at the ischemic border zone. Macro-reentry around the ischemic region is the main reentrant mechanism in the ischemic human ventricle with increased repolarization reserve due to the ATP-sensitive potassium current (IK(ATP)) activation. Prolongation of refractoriness by 4% caused by 30% IKr reduction counteracts the establishment of macro-reentry and reduces the VW for reentry (by 23.5%). However, a further decrease in repolarization reserve (50% IKr reduction) is less anti-arrhythmic despite further prolongation of refractoriness. This is due to the establishment of transmural reentry enabled by electrotonically-triggered EADs in the ischemic border zone. EADs are produced by L-type calcium current (ICaL) reactivation due to prolonged low amplitude electrotonic current injected during the repolarization phase. ConclusionsElectrotonically-triggered EADs are identified as a potential mechanism facilitating intramural reentry in a regionally-ischemic human ventricles model with reduced repolarization reserve.

Docetaxel Modulates the Delayed Rectifier Potassium Current (I K) and ATP-Sensitive Potassium Current (I KATP) in Human Breast Cancer Cells

Ion channel expression and activity may be affected during tumor development and cancer growth. Activation of potassium (K(+)) channels in human breast cancer cells is reported to be involved in cell cycle progression. In this study, we investigated the effects of docetaxel on the delayed rectifier potassium current (I K) and the ATP-sensitive potassium current (I KATP) in two human breast cancer cell lines, MCF-7 and MDA-MB-435S, using the whole-cell patch-clamp technique. Our results show that docetaxel inhibited the I K and I KATP in both cell lines in a dose-dependent manner. Compared with the control at a potential of +60 mV, treatment with docetaxel at doses of 0.1, 1, 5, and 10 µM significantly decreased the I K in MCF-7 cells by 16.1 ± 3.5, 30.2 ± 5.2, 42.5 ± 4.3, and 46.4 ± 9% (n = 5, P < 0.05), respectively and also decreased the I KATP at +50 mV. Similar results were observed in MDA-MB-435S cells. The G-V curves showed no significant changes after treatment of either MCF-7 or MDA-MB-435S cells with 10 μM docetaxel. The datas indicate that the possible mechanisms of I K and I KATP inhibition by docetaxel may be responsible for its effect on the proliferation of human breast cancer cells.

Electrophysiology of human cardiac atrial and ventricular telocytes

Telocytes (TCs) with exceptionally long cellular processes of telopodes have been described in human epicardium to act as structural supporting cells in the heart. We examined myocardial chamber-specific TCs identified in atrial and ventricular fibroblast culture using immunocytochemistry and studied their electrophysiological property by whole-cell patch clamp. Atrial and ventricular TCs with extended telopodes and alternating podoms and podomers that expressed CD34, c-Kit and PDGFR-β were identified. These cells expressed large conductance Ca2+-activated K+ current (BKCa) and inwardly rectifying K+ current (IKir), but not transient outward K+ current (Ito) and ATP-sensitive potassium current (KATP). The active channels were functionally competent with demonstrated modulatory response to H2S and transforming growth factor (TGF)-β1 whereby H2S significantly inhibited the stimulatory effect of TGF-β1 on current density of both BKCa and IKir. Furthermore, H2S attenuated TGF-β1-stimulated KCa1.1/Kv1.1 (encode BKCa) and Kir2.1 (encode IKir) expression in TCs. Our results show that functionally competent K+ channels are present in human atrial and ventricular TCs and their modulation may have significant implications in myocardial physiopathology.

ATP-sensitive potassium currents from channels formed by Kir6 and a modified cardiac mitochondrial SUR2 variant

Cardiac ATP-sensitive potassium channels (KATP) are found in both the sarcoplasmic reticulum (sarcKATP) and the inner membrane of mitochondria (mitoKATP). SarcKATP are composed of a pore containing subunit Kir6.2 and a regulatory sulfonylurea receptor subunit (SUR2), but the composition of mitoKATP remains unclear. An unusual intra-exonic splice variant of SUR2 (SUR2A-55) was previously identified in mitochondria of mammalian heart and brain, and by analogy with sarcKATP we proposed SUR2A-55 as a candidate regulatory subunit of mitoKATP. Although SUR2A-55 lacks the first nucleotide binding domain (NBD) and 2 transmembrane domains (TMD), it has a hybrid TMD and retains the second NBD. It resembles a hemi-ABC transporter suggesting it could multimerize to function as a regulatory subunit. A putative mitochondrial targeting signal in the N-terminal domain of SUR2A-55 was removed by truncation and when co-expressed with Kir6.1 and Kir6.2 it targeted to the plasma membrane and yielded KATP currents. Single channel conductance, mean open time, and burst open time of SUR2A-55 based KATP was similar to the full-length SUR2A based KATP. However, the SUR2A-55 KATP were 70-fold less sensitive to block by ATP, and twice as resistant to intracellular Ca2+ inhibition compared with the SUR2A KATP, and were markedly insensitive to KATP drugs, pinacidil, diazoxide, and glybenclamide. These results suggest that the SUR2A-55 based channels would tend to be open under physiological conditions and in ischemia, and could account for cardiac and mitochondrial phenotypes protective for ischemia.

GW9508 inhibits insulin secretion by activating ATP-sensitive potassium channels in rat pancreatic β-cells

GW9508 is an agonist of G protein-coupled receptor 40 (GPR40) that is expressed in pancreatic β-cells and is reported to regulate insulin secretion. However, the effects of GW9508 on pancreatic β-cells in primary culture have not been well investigated. This study measured the acute effects of GW9508 on insulin secretion from rat pancreatic islets in primary culture, and the insulin secretion-related events such as the changes in membrane potential, ATP-sensitive potassium currents (KATP currents), and intracellular Ca(2+) concentrations ([Ca(2+)]i) of rat islet β-cells were also recorded. GW9508 (10-40 μM) did not influence basal insulin levels at 2 mM glucose, but it (above 20 μM) significantly inhibited 5 and 15 mM glucose-stimulated insulin secretion (GSIS). GW9508 did not inhibit insulin secretion stimulated by tolbutamide, the closer of KATP channels. GW9508 activated KATP channels and blocked the membrane depolarization and the increase in [Ca(2+)]i that were stimulated by glucose. GW9508 itself stimulated a transient increase in [Ca(2+)]i, which was fully blocked by depletion of intracellular Ca(2+) stores with thapsigargin or by inhibition of phospholipase C (PLC) activity with U73122. GW9508-induced activation of KATP channels was only partly inhibited by U73122 treatment. In conclusion, although it stimulates a transient release of Ca(2+) from intracellular Ca(2+) stores via activation of PLC, GW9508 inhibits GSIS by activating KATP channels probably in a distal step to GPR40 activation in rat β-cells.

Acute regional left atrial ischemia causes acceleration of atrial drivers during atrial fibrillation.

The mechanisms by which acute left atrial ischemia (LAI) leads to atrial fibrillation (AF) initiation and perpetuation remain unclear. To investigate the electrophysiological mechanisms of AF perpetuation in the presence of regional atrial ischemia. LAI (90-minute ischemia) was obtained in isolated sheep hearts by selectively perfusing microspheres into the left anterior atrial artery. Two charge-coupled device cameras and several bipolar electrodes enabled recording from multiple atrial locations: with a dual-camera setup (protocol 1, n = 10, and protocol 1', n = 4, for biatrial or atrioventricular camera setups, respectively), in the presence of propranolol/atropine (1 μM) added to the perfusate after LAI (protocol 2, n = 3) and after a pretreatment with glibenclamide (10 μM; protocol 3, n = 4). Spontaneous AF occurred in 41.2% (7 of 17) of the hearts that were in sinus rhythm before LAI. LAI caused action potential duration shortening in both the ischemic (IZ) and nonischemic (NIZ) zones by 21% ± 8% and 34% ± 13%, respectively (pacing, 5 Hz; P<.05 compared to baseline). Apparent impulse velocity was significantly reduced in the IZ but not in the NIZ (-65% ± 19% and 9% ± 18%; P = .001 and NS, respectively). During LAI-related AF, a significant NIZ maximal dominant frequency increase from 7.4 ± 2.5 to 14.0 ± 5.5 Hz (P<.05) was observed. Glibenclamide, an ATP-sensitive potassium current (IKATP) channel blocker, averted LAI-related maximal dominant frequency increase (NIZ: LAI vs glibenclamide 14.0 ± 5.5 Hz vs 5.9 ± 1.3 Hz; P<.05). An interplay between spontaneous focal discharges and rotors, locating at the IZ-NIZ border zone, maintained LAI-related AF. LAI leads to an IKATP conductance-dependent action potential duration shortening and spontaneous AF maintained by both spontaneous focal discharges and reentrant circuits locating at the IZ border zone.

Role of KATP channel in electrical depression and asystole during long-duration ventricular fibrillation in ex vivo canine heart.

Long-duration ventricular fibrillation (LDVF) in the globally ischemic heart is characterized by transmurally heterogeneous decline in ventricular fibrillation rate (VFR), emergence of inexcitable regions, and eventual global asystole. Rapid loss of both local and global excitability is detrimental to successful defibrillation and resuscitation during cardiac arrest. We sought to assess the role of the ATP-sensitive potassium current (I(KATP)) in the timing and spatial pattern of electrical depression during LDVF in a structurally normal canine heart. We analyzed endo-, mid-, and epicardial unipolar electrograms and epicardial optical recordings in the left ventricle of isolated canine hearts during 10 min of LDVF in the absence (control) and presence of an I(KATP) blocker glybenclamide (60 μM). In all myocardial layers, average VFR was the same or higher in glybenclamide-treated than in control hearts. The difference increased with time of LDVF and was overall significant in all layers (P < 0.05). However, glybenclamide did not significantly affect the transmural VFR gradient. In epicardial optical recordings, glybenclamide shortened diastolic intervals, prolonged action potential duration, and decreased the percentage of inexcitable area (all differences P < 0.001). During 10 min of LDVF, asystole occurred in 55.6% of control and none of glybenclamide-treated hearts (P < 0.05). In three hearts paced after the onset of asystole, there was no response to LV epicardial or atrial pacing. In structurally normal canine hearts, I(KATP) opening during LDVF is a major factor in the onset of local and global inexcitability, whereas it has a limited role in overall deceleration of VFR and the transmural VFR gradient.

Effects of anandamide on potassium channels in rat ventricular myocytes: a suppression of Ito and augmentation of KATP channels

Anandamide is an endocannabinoid that has antiarrhythmic effects through inhibition of L-type Ca(2+) channels in cardiomyocytes. In this study, we investigated the electrophysiological effects of anandamide on K(+) channels in rat ventricular myocytes. Whole cell patch-clamp technique was used to record K(+) currents, including transient outward potassium current (I(to)), steady-state outward potassium current (I(ss)), inward rectifier potassium current (I(K1)), and ATP-sensitive potassium current (I(KATP)) in isolated rat cardiac ventricular myocytes. Anandamide decreased I(to) while increasing I(KATP) in a concentration-dependent manner but had no effect on I(ss) and I(K1) in isolated ventricular myocytes. Furthermore, anandamide shifted steady-state inactivation curve of I(to) to the left and shifted the recovery curve of I(to) to the right. However, neither cannabinoid 1 (CB(1)) receptor antagonist AM251 nor CB(2) receptor antagonist AM630 eliminated the inhibitory effect of anandamide on I(to). In addition, blockade of CB(2) receptors, but not CB(1) receptors, eliminated the augmentation effect of anandamide on I(KATP). These data suggest that anandamide suppresses I(to) through a non-CB(1) and non-CB(2) receptor-mediated pathway while augmenting I(KATP) through CB(2) receptors in ventricular myocytes.

ATP-sensitive potassium currents in rat primary afferent neurons: biophysical, pharmacological properties, and alterations by painful nerve injury

ATP-sensitive potassium (K ATP) channels may be linked to mechanisms of pain after nerve injury, but remain under-investigated in primary afferents so far. We therefore characterized these channels in dorsal root ganglion (DRG) neurons, and tested whether they contribute to hyperalgesia after spinal nerve ligation (SNL). We compared K ATP channel properties between DRG somata classified by diameter into small or large, and by injury status into neurons from rats that either did or did not become hyperalgesic after SNL, or neurons from control animals. In cell-attached patches, we recorded basal K ATP channel opening in all neuronal subpopulations. However, higher open probabilities and longer open times were observed in large compared to small neurons. Following SNL, this channel activity was suppressed only in large neurons from hyperalgesic rats, but not from animals that did not develop hyperalgesia. In contrast, no alterations of channel activity developed in small neurons after axotomy. On the other hand, cell-free recordings showed similar ATP sensitivity, inward rectification and unitary conductance (70–80 pS) between neurons classified by size or injury status. Likewise, pharmacological sensitivity to the K ATP channel opener diazoxide, and to the selective blockers glibenclamide and tolbutamide, did not differ between groups. In large neurons, selective inhibition of whole-cell ATP-sensitive potassium channel current (I K(ATP)) by glibenclamide depolarized resting membrane potential (RMP). The contribution of this current to RMP was also attenuated after painful axotomy. Using specific antibodies, we identified SUR1, SUR2, and Kir6.2 but not Kir6.1 subunits in DRGs. These findings indicate that functional K ATP channels are present in normal DRG neurons, wherein they regulate RMP. Alterations of these channels may be involved in the pathogenesis of neuropathic pain following peripheral nerve injury. Their biophysical and pharmacological properties are preserved even after axotomy, suggesting that K ATP channels in primary afferents remain available for therapeutic targeting against established neuropathic pain.

Acute ischemia of canine interventricular septum produces asymmetric suppression of conduction

Acute ischemia depresses tissue excitability more rapidly in the epicardium than in the endocardium of the canine left ventricular (LV) free wall. However, the effects of acute ischemia on conduction in the interventricular septum (IVS), which is composed of right ventricular (RV) and LV endocardium and midmyocardium without epicardium, are less known. The purpose of this study was to evaluate the hypothesis that the IVS exhibits transseptal differences in local tissue response to acute ischemia. Isolated canine IVS preparations were perfused through the septal branch of the anterior descending coronary artery, and conduction on the cut-exposed transseptal surfaces was optically mapped before and after two sequential episodes of 8 minutes of global ischemia (separated by >60 minutes of reperfusion). The preparations were paced alternately between the RV endocardium and LV endocardium at cycle lengths of 250, 300, and 1,500 ms. Prior to ischemia, transseptal conduction was radial and symmetric during either RV endocardial or LV endocardial pacing at all cycle lengths. Eight minutes of ischemia depressed conduction velocity more in the RV half than in the LV half of the IVS and caused local conduction block in the sub-RV endocardium, especially during rapid pacing. The K(ATP) channel blocker glibenclamide (10 micromol/L) prevented development of this transseptal asymmetry and conduction block during ischemia. Acute global ischemia increased transseptal heterogeneity and induced sub-RV endocardial block of conduction via activation of the ATP-sensitive potassium current. Such changes could contribute to initiation of arrhythmia in patients with septal infarction.

Axotomy alters sodium and ATP-sensitive potassium currents elicited by action potential waveform voltage commands in mammalian primary afferent neurons

Axotomy alters sodium and ATP-sensitive potassium currents elicited by action potential waveform voltage commands in mammalian primary afferent neurons

ATP-sensitive potassium currents reduce the PGE 2-mediated enhancement of excitability in adult rat sensory neurons

Behavioral studies have shown that the hyperalgesia arising from inflammatory agents, such as prostaglandin E 2 (PGE 2), can be antagonized by activators of the ATP-sensitive potassium current (K ATP). This observation raises questions as to whether this suppression results from a direct action on sensory neurons and what are the cellular mechanisms giving rise to this inhibition. We found that small to medium diameter sensory neurons isolated from the L4–6 DRGs expressed the mRNAs for Kir6.1, Kir6.2, and SUR1. In perforated-patch clamp recordings from acutely dissociated sensory neurons from the young adult rat, exposure to 300 μM diazoxide, a K ATP channel agonist, significantly hyperpolarized the resting membrane potential, reduced the number of action potentials evoked by a ramp of depolarizing current, and increased the amplitude of inward K ATP currents evoked by the voltage ramp. Similar results were obtained with the protonophore FCCP, which is known to reduce the levels of intracellular ATP and lead to the activation of K ATP. Only a subpopulation of sensory neurons was sensitive to diazoxide whereas other neurons were unaffected. Treatment with 1 μM PGE 2 significantly enhanced the excitability of these small to medium diameter capsaicin-sensitive sensory neurons; this enhancement was reversed by subsequent exposure to diazoxide in a subpopulation of neurons. Similar to diazoxide, exposure to 8-Br-cyclic GMP antagonized the PGE 2-induced increase in excitability. The effects of 8-Br-cyclic GMP could be reversed by exposure to glibenclamide, an antagonist of K ATP channels. As with diazoxide, only a subpopulation of sensory neurons were affected by 8-Br-cyclic GMP. These results demonstrate that activation of K ATP can reverse the sensitization produced by PGE 2 and may be an important means to modulate the enhanced excitability that results from inflammatory or injury conditions.

Effects of urocortin via ion mechanisms or CRF receptors?

Urocortin (UCN), a newly isolated peptide related to hypothalamic corticotrophin releasing factor (CRF) family, had been reported to play biologically diverse roles in several systems such as cardiovascular, reproductive, appetite, stress, and inflammatory responses, etc. It was thought previously to be an endogenous agonist, producing the several actions previously attributed to CRF. But, recently, it was shown to directly reduce L-type calcium currents of acute isolated cardiac myocytes and T-type calcium currents in mouse spermatogenic cells via inhibiting calcium channel instead of binding first to its CRF-R 2 receptors. UCN could also reduce the intracellular calcium in vascular smooth muscle cells via inhibiting calcium channel directly. Furthermore, UCN could increase the gene expression of ATP-sensitive potassium channels (K ATP) and activate sarcolemmal ATP-sensitive potassium current during normal or hypoxia, which could be inhibited by glibenclamide, a specific K ATP blocker. This review will highlight the current novel findings on the ionic mechanisms by which UCN may exert its several actions.

Inhibition of G protein-coupled and ATP-sensitive potassium currents by 2-methyl-3-(3,5-diiodo-4-carboxymethoxybenzyl)benzofuran (KB130015), an amiodarone derivative.

2-Methyl-3- (3,5-diiodo-4-carboxymethoxybenzyl) benzofuran (KB130015; KB), a novel compound derived from amiodarone, has been proposed to have antiarrhythmic properties. Its effect on the G protein-coupled inward rectifying K+ current [IK(ACh) or IK(Ado)], ATP-sensitive K+ current [IK(ATP)], and background inward rectifying current (I(K1)) were studied in guinea pig atrial and ventricular myocytes by the whole-cell voltage-clamp technique. Receptor-activated IK(ACh/Ado), induced in atrial myocytes by the stimulation of either muscarinic or Ado receptors was concentration dependently (IC50 value of approximately 0.6-0.8 microM) inhibited by KB. Receptor-independent guanosine 5'-O-(3-thio)triphosphate-induced and background IK(ACh), which contributes to the resting conductance of atrial myocytes, were equally sensitive to KB (IC50 value of approximately 0.9 microM). IK(ATP) induced in atrial myocytes during metabolic inhibition with 2,4-dinitrophenol (DNP) was also suppressed by KB, whereas IK1 measured in ventricular myocytes was insensitive to the drug (KB < or =50 microM). Although being effective when applied from the outside, intracellular application of KB via the patch pipette affected neither IK(ACh) nor IK(ATP). 3,3',5-triodo-L-thyronin, which shares structural groups with KB, did not have an effect on the K+ currents. Consistent with the effects on single myocytes, KB did not depolarize the resting potential but antagonized the shortening of action potential duration by carbamylcholine-chloride or by DNP in multicellular preparations and antagonized the shortening of action potential duration by acetylcholine in single myocytes. It is concluded that KB inhibits IK(ACh) and IK(ATP) by direct drug-channel interaction at a site more easily accessible from extracellular side of the membrane.