A TREK-1-like potassium channel in atrial cells inhibited by beta-adrenergic stimulation and activated by volatile anesthetics

Regional differential expression of TREK-1 at left ventricle in myocardial infarction

BACKGROUND

Altered membrane electrophysiology contributes to arrhythmias after myocardial infarction (MI). TREK-1 channel is essential in various physiological and pathological conditions through its regulation on resting membrane potential and voltage-dependent action potential duration.

OBJECTIVES

The aim of this study was to investigate changes in gene expression and electrophysiology of TREK-1 in the left ventricle in a MI model.

METHODS

Fifty-five rats were divided into 5 groups: sham-operated group, 6 hours, 24 hours, 3 days, and 7 days post MI group (n=11 per group). TREK-1 messenger RNA (mRNA) expression level in the infarct region (IR) and infarct border region (IBR) were quantified by real-time polymerase chain reaction (PCR), and TREK-1 current density at the IBR was recorded with whole-cell patch-clamp technique.

RESULTS

TREK-1 mRNA expression decreased significantly in both endocardial and epicardial cells in the infarct region after MI. Conversely, TREK-1 increased significantly in endocardial and epicardial cells from the IBR (P<0.01). Current density of TREK-1 at IBR increased significantly in both epicardial and endocardial cells after MI (P<0.01).

CONCLUSIONS

TREK-1 demonstrates specific changes in expression and electrophysiological function in left ventricle post MI. These results suggest that TREK-1 may participate in pathophysiologic alteration and electrical remodelling of left ventricular myocardium after MI, which may eventually lead to post-MI ventricular arrhythmias.

TREK-1 K(+) channels in the cardiovascular system: their significance and potential as a therapeutic target

Potassium (K(+) ) channels are important in cardiovascular disease both as drug targets and as a cause of underlying pathology. Voltage-dependent K(+) (K(V) ) channels are inhibited by the class III antiarrhythmic agents. Certain vasodilators work by opening K(+) channels in vascular smooth muscle cells (VSMCs), and K(+) channel activation may also be a route to improving endothelial function. The two-pore domain K(+) (K(2P) ) channels form a group of 15 known channels with an expanding list of functions in the cardiovascular system. One of these K(2P) channels, TREK-1, is the focus of this review. TREK-1 channel activity is tightly regulated by intracellular and extracellular pH, membrane stretch, polyunsaturated fatty acids (PUFAs), temperature, and receptor-coupled second messenger systems. TREK-1 channels are also activated by volatile anesthetics and some neuroprotectant agents, and they are inhibited by selective serotonin reuptake inhibitors (SSRIs) as well as amide local anesthetics. Some of the clinical cardiovascular effects and side effects of these drugs may be through their actions on TREK-1 channels. It has recently been suggested that TREK-1 channels have a role in mechano-electrical coupling in the heart. They also seem important in the vascular responses to PUFAs, and this may underlie some of the beneficial cardiovascular effects of the essential dietary fatty acids. Development of selective TREK-1 openers and inhibitors may provide promising routes for intervention in cardiovascular diseases.

Molecular background of leak K+ currents: two-pore domain potassium channels

Two-pore domain K(+) (K(2P)) channels give rise to leak (also called background) K(+) currents. The well-known role of background K(+) currents is to stabilize the negative resting membrane potential and counterbalance depolarization. However, it has become apparent in the past decade (during the detailed examination of the cloned and corresponding native K(2P) channel types) that this primary hyperpolarizing action is not performed passively. The K(2P) channels are regulated by a wide variety of voltage-independent factors. Basic physicochemical parameters (e.g., pH, temperature, membrane stretch) and also several intracellular signaling pathways substantially and specifically modulate the different members of the six K(2P) channel subfamilies (TWIK, TREK, TASK, TALK, THIK, and TRESK). The deep implication in diverse physiological processes, the circumscribed expression pattern of the different channels, and the interesting pharmacological profile brought the K(2P) channel family into the spotlight. In this review, we focus on the physiological roles of K(2P) channels in the most extensively investigated cell types, with special emphasis on the molecular mechanisms of channel regulation.

Specificities of atrial electrophysiology: Clues to a better understanding of cardiac function and the mechanisms of arrhythmias

The electrical properties of the atria and ventricles differ in several aspects reflecting the distinct role of the atria in cardiac physiology. The study of atrial electrophysiology had greatly contributed to the understanding of the mechanisms of atrial fibrillation (AF). Only the atrial L-type calcium current is regulated by serotonine or, under basal condition, by phosphodiesterases. These distinct regulations can contribute to I(Ca) down-regulation observed during AF, which is an important determinant of action potential refractory period shortening. The voltage-gated potassium current, I(Kur), has a prominent role in the repolarization of the atrial but not ventricular AP. In many species, this current is based on the functional expression of K(V)1.5 channels, which might represent a specific therapeutic target for AF. Mechanisms regulating the trafficking of K(V)1.5 channels to the plasma membrane are being actively investigated. The resting potential of atrial myocytes is maintained by various inward rectifier currents which differ with ventricle currents by a reduced density of I(K1), the presence of a constitutively active I(KACh) and distinct regulation of I(KATP). Stretch-sensitive or mechanosensitive ion channels are particularly active in atrial myocytes and are involved in the secretion of the natriuretic peptide. Integration of knowledge on electrical properties of atrial myocytes in comprehensive schemas is now necessary for a better understanding of the physiology of atria and the mechanisms of AF.

Transcriptional analysis of the mammalian heart with special reference to its endocrine function

Background

Pharmacological and gene ablation studies have demonstrated the crucial role of the endocrine function of the heart as mediated by the polypeptide hormones ANF and BNP in the maintenance of cardiovascular homeostasis. The importance of these studies lies on the fact that hypertension and chronic congestive heart failure are clinical entities that may be regarded as states of relative deficiency of ANF and BNP. These hormones are produced by the atrial muscle cells (cardiocytes), which display a dual secretory/muscle phenotype. In contrast, ventricular cardiocytes display mainly a muscle phenotype. Comparatively little information is available regarding the genetic background for this important phenotypic difference with particular reference to the endocrine function of the heart. We postulated that comparison of gene expression profiles between atrial and ventricular muscles would help identify gene transcripts that underlie the phenotypic differences associated with the endocrine function of the heart.

Results

Comparison of gene expression profiles in the rat heart revealed a total of 1415 differentially expressed genes between the atria and ventricles based on a 1.8 fold cut-off. The identification of numerous chamber specific transcripts, such as ANF for the atria and Irx4 for the ventricles among several others, support the soundness of the GeneChip data and demonstrates that the differences in gene expression profiles observed between the atrial and ventricular tissues were not spurious in nature. Pathway analysis revealed unique expression profiles in the atria for G protein signaling that included Gα_o1, Gγ₂ and Gγ₃, AGS1, RGS2, and RGS6 and the related K⁺ channels GIRK1 and GIRK4. Transcripts involved in vesicle trafficking, hormone secretion as well as mechanosensors (e.g. the potassium channel TREK-1) were identified in relationship to the synthesis, storage and secretion of hormones.

Conclusion

The data developed in this investigation describes for the first time data on gene expression particularly centred on the secretory function of the heart. This provides for a rational approach in the investigation of determinants of the endocrine of the heart in health and disease.

Stretch-activated potassium channels in hypotonically induced blebs of atrial myocytes

Stress in the lipids of the cell membrane may be responsible for activating stretch-activated channels (SACs) in nonspecialized sensory cells such as cardiac myocytes, where they are likely to play a role in cardiac mechanoelectric feedback. We examined the influence of the mechanical microenvironment on the gating of stretch-activated potassium channels (SAKCs) in rat atrial myocytes. The goal was to examine the role of the cytoskeleton in the gating process. We recorded from blebs that have minimal cytoskeleton and cells treated with cytochalasin B (cyto-B) to disrupt filamentous actin. Histochemical and electron microscopic techniques confirmed that the bleb membrane was largely free of F-actin. Channel currents showed mechanosensitivity and potassium selectivity and were activated by low pH and arachidonic acid, similar to properties of TREK-1. Some patches showed a time-dependent decrease in current that may be adaptation or inactivation, and since this decrease appeared in control cells and blebs, it is probably not the result of adaptation in the cytoskeleton. Cyto-B treatment and blebbing caused an increase in background channel activity, suggesting a transfer of stress from actin to bilayer and then to the channel. The slope sensitivity of gating before and after cyto-B treatment was similar to that of blebs, implying the characteristic change of dimensions associated with channel gating was the same in the three mechanical environments. The mechanosensitivity of SAKCs appears to be the result of interaction with membrane lipids and not of direct involvement of the cytoskeleton.

Modeling of arrhythmogenic automaticity induced by stretch in rat atrial myocytes

Since first discovered in chick skeletal muscles, stretch-activated channels (SACs) have been proposed as a probable mechano-transducer of the mechanical stimulus at the cellular level. Channel properties have been studied in both the single-channel and the whole-cell level. There is growing evidence to indicate that major stretch-induced changes in electrical activity are mediated by activation of these channels. We aimed to investigate the mechanism of stretch-induced automaticity by exploiting a recent mathematical model of rat atrial myocytes which had been established to reproduce cellular activities such as the action potential, Ca(2+) transients, and contractile force. The incorporation of SACs into the mathematical model, based on experimental results, successfully reproduced the repetitive firing of spontaneous action potentials by stretch. The induced automaticity was composed of two phases. The early phase was driven by increased background conductance of voltage-gated Na(+) channel, whereas the later phase was driven by the reverse-mode operation of Na(+)/Ca(2+) exchange current secondary to the accumulation of Na(+) and Ca(2+) through SACs. These results of simulation successfully demonstrate how the SACs can induce automaticity in a single atrial myocyte which may act as a focus to initiate and maintain atrial fibrillation in concert with other arrhythmogenic changes in the heart.

Emerging roles for two-pore-domain potassium channels and their potential therapeutic impact

A distinct gene family of widely distributed and well-modulated two-pore-domain background potassium (K(2P)) channels establish resting membrane potential and cell excitability. By using new mouse models in which K(2P)-channel genes are deleted, the contributions of these channels to important physiological functions are now being revealed. Here, we highlight results of recent studies using mice deleted for K(2P)-channel subunits that uncover physiological functions of these channels, mostly those of the TASK and TREK subgroup. Consistent with activation of these K(2P) channels by volatile anesthetics, TASK-1, TASK-3 and TREK-1 contribute to anesthetic-induced hypnosis and immobilization. The acid-sensitive TASK channels are not required for brainstem control of breathing by CO(2) or pH, despite widespread expression in respiratory-related neurons. TASK channels are necessary, however, for homeostatic regulation of adrenal aldosterone secretion. The heat-, stretch- and lipid-activated TREK-1 channels contribute to temperature and mechanical pain sensation, neuroprotection by polyunsaturated fatty acids and, unexpectedly, mood regulation. The alkaline-activated TASK-2 channel is necessary for HCO(3)(-) reabsorption and osmotic volume regulation in kidney proximal tubule cells. Development of compounds that selectively modulate K(2P) channels is crucial for verifying these results and assessing the efficacy of therapies targeting these interesting channels.

Distribution of two-pore-domain potassium channels in the adult rat vestibular periphery

Constitutively active background or "leak" two-pore-domain potassium (K(+)) channels (Kcnk family), as defined by lack of voltage and time dependency are central to electrical excitability of cells by controlling resting membrane potential and membrane resistance. Inhibition of these channels by several neurotransmitters, e.g. glutamate, or acetylcholine, induces membrane depolarization and subsequent action potential firing as well as increases membrane resistance amplifying responses to synaptic inputs. In contrast, their opening contributes to hyperpolarization. Because of their central role in determining cellular excitability and response to synaptic stimulation, these channels likely play a role in the differential effects of vestibular efferent neurons on afferent discharge. Microarray data from previous experiments showed Kcnk 1, 2, 3, 6, 12 and 1 5 mRNA in Scarpa's ganglia. Real-time RT-PCR showed Kcnk 1, 2, 3, 6, 12 and 15 mRNA expression in Scarpa's ganglia and Kcnk 1, 2, 3, 6, 12 but not 15 mRNA expression in the crista ampullaris. We studied the distribution of two-pore-domain potassium channels K(2P)1.1, 2.1, 3.1 and 6.1 like immunoreactivity (corresponding to Kcnk genes 1, 2, 3 and 6) in the vestibular periphery. K(2P)1.1 (TWIK 1) immunoreactivity was detected along nerve terminals, supporting cells and blood vessels of the crista ampullaris and in the cytoplasm of neurons of the Scarpa's ganglia. K(2P)2.1 (TREK 1) immunoreactivity was detected in nerve terminals and transitional cells of the crista ampullaris, in the vestibular dark cells and in neuronal fibers and somata of neurons of Scarpa's ganglia. K(2P)3.1 (TASK 1) immunoreactivity was detected in supporting cells and transitional cells of the crista ampullaris, in vestibular dark cells and in neuron cytoplasm within Scarpa's ganglia. K(2P)6.1 (TWIK 2) immunoreactivity was detected in nerve terminals, blood vessels hair cells and transitional cells of the crista ampullaris and in the somata and neuron fibers of Scarpa's ganglia.

Temperature-sensitive TREK currents contribute to setting the resting membrane potential in embryonic atrial myocytes

TREK channels belong to the superfamily of two-pore-domain K(+) channels and are activated by membrane stretch, arachidonic acid, volatile anaesthetics and heat. TREK-1 is highly expressed in the atrium of the adult heart. In this study, we investigated the role of TREK-1 and TREK-2 channels in regulating the resting membrane potential (RMP) of isolated chicken embryonic cardiac myocytes. At room temperature, the average RMP of embryonic day (ED) 11 atrial myocytes was -22 +/- 2 mV. Raising the temperature to 35 degrees C hyperpolarized the membrane to -69 +/- 2 mV and activated a large outwardly rectifying K(+) current that was relatively insensitive to conventional K(+) channel inhibitors (TEA, 4-AP and Ba(2+)) but completely inhibited by tetracaine (200 microM), an inhibitor of TREK channels. The heat-induced hyperpolarization was mimicked by 10 microM arachidonic acid, an agonist of TREK channels. There was little or no inwardly rectifying K(+) current (I(K1)) in the ED11 atrial cells. In marked contrast, ED11 ventricular myocytes exhibited a normal RMP (-86.1 +/- 3.4 mV) and substantial I(K1), but no temperature- or tetracaine-sensitive K(+) currents. Both RT-PCR and real-time PCR further demonstrated that TREK-1 and TREK-2 are highly and almost equally expressed in ED11 atrium but much less expressed in ED11 ventricle. In addition, immunofluorescence demonstrated TREK-1 protein in the membrane of atrial myocytes. These data indicate the presence and function of TREK-1 and TREK-2 in the embryonic atrium. Moreover, we demonstrate that TREK-like currents have an essential role in determining membrane potential in embryonic atrial myocytes, where I(K1) is absent.

PPARalpha-mediated remodeling of repolarizing voltage-gated K+ (Kv) channels in a mouse model of metabolic cardiomyopathy

Diabetes is associated with increased risk of diastolic dysfunction, heart failure, QT prolongation and rhythm disturbances independent of age, hypertension or coronary artery disease. Although these observations suggest electrical remodeling in the heart with diabetes, the relationship between the metabolic and the functional derangements is poorly understood. Exploiting a mouse model (MHC-PPARalpha) with cardiac-specific overexpression of the peroxisome proliferator-activated receptor alpha (PPARalpha), a key driver of diabetes-related lipid metabolic dysregulation, the experiments here were aimed at examining directly the link(s) between alterations in cardiac fatty acid metabolism and the functioning of repolarizing, voltage-gated K(+) (Kv) channels. Electrophysiological experiments on left (LV) and right (RV) ventricular myocytes isolated from young (5-6 week) MHC-PPARalpha mice revealed marked K(+) current remodeling: I(to,f) densities are significantly (P<0.01) lower, whereas I(ss) densities are significantly (P<0.001) higher in MHC-PPARalpha, compared with age-matched wild type (WT), LV and RV myocytes. Consistent with the observed reductions in I(to,f) density, expression of the KCND2 (Kv4.2) transcript is significantly (P<0.001) lower in MHC-PPARalpha, compared with WT, ventricles. Western blot analyses revealed that expression of the Kv accessory protein, KChIP2, is also reduced in MHC-PPARalpha ventricles in parallel with the decrease in Kv4.2. Although the properties of the endogenous and the "augmented" I(ss) suggest a role(s) for two pore domain K(+) channel (K2P) pore-forming subunits, the expression levels of KCNK2 (TREK1), KCNK3 (TASK1) and KCNK5 (TASK2) in MHC-PPARalpha and WT ventricles are not significantly different. The molecular mechanisms underlying I(to,f) and I(ss) remodeling in MHC-PPARalpha ventricular myocytes, therefore, are distinct.

Regulation of stretch-activated ANP secretion by chloride channels

This study was aimed to define roles of stretch-activated ion channels (SACs), especially Cl(-) channels, in regulation of atrial natriuretic peptide (ANP) secretion using isolated perfused beating atria. The volume load was achieved by elevating height of outflow catheter connected to isolated rat atria and the pressure load was achieved by decreasing diameter of outflow catheter. Both methods increased atrial contractility similarly although volume load was different (736microl for volume load vs. 129microl for pressure load). Atrial stretch by volume load markedly increased ECF translocation and ANP secretion but the pressure load slightly increased. The ANP secretion was positively correlated to workload generated by volume or pressure load. Treatment of atria with gadolinium, a blocker for SACs, attenuated the ECF translocation and the ANP secretion induced by volume load. A blocker for Ca2+-activated Cl(-) channel, niflumic acid (NFA), accentuated the ANP secretion induced by volume load whereas a blocker for swelling-activated Cl(-) channel, diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS), attenuated the ANP secretion. The ANP secretion of hypertrophied atria by volume load was markedly reduced and the augmented effect of NFA on volume load-induced ANP secretion was not observed. These results indicate that Cl(-) channels may differently regulate stretch-activated ANP secretion.

The role of stretch-activated channels in atrial fibrillation and the impact of intracellular acidosis

The incidence of atrial fibrillation correlates with increasing atrial size. The electrical consequences of atrial stretch contribute to both the initiation and maintenance of atrial fibrillation. It is suggested that altered calcium handling and stretch-activated channel activity could explain the experimental findings of stretch-induced depolarisation, shortened refractoriness, slowed conduction and increased heterogeneity of refractoriness and conduction. Stretch-activated channel blocking agents protect against these pro-arrhythmic effects. Gadolinium, GsMTx-4 toxin and streptomycin prevent the stretch-related vulnerability to atrial fibrillation without altering the drop in refractory period associated with stretch. Changes the activity of two-pore K+ channels, which are sensitive to stretch and pH but not gadolinium, could underlie the drop in refractoriness. Intracellular acidosis induced with propionate amplified the change in refractoriness with stretch in the isolated rabbit heart model in keeping with the clinical observation of increased propensity to atrial fibrillation with acidosis. We propose that activation of non-specific cation stretch-activated channels provides the triggers for acute atrial fibrillation with high atrial pressure while activation of atrial two-pore K+ channels shortens atrial refractory period and increases heterogeneity of refractoriness, providing the substrate for atrial fibrillation to be sustained. Stretch-activated channel blockade represents an exciting target for future antiarrhythmic drugs.

Modulation of TRPC5 cation channels by halothane, chloroform and propofol

BACKGROUND AND PURPOSE

TRPC5 is a mammalian homologue of the Drosophila Transient Receptor Potential (TRP) channel and has expression and functions in the cardiovascular and nervous systems. It forms a calcium-permeable cation channel that can be activated by a variety of signals including carbachol (acting at muscarinic receptors), lanthanides (e.g. Gd3+) and phospholipids (e.g. lysophosphatidylcholine: LPC). Here we report the effects of inhalational (halothane and chloroform) and intravenous (propofol) general anaesthetics upon TRPC5.

EXPERIMENTAL APPROACH

Human TRPC5 channels were expressed in HEK 293 cells and studied using fura-2 and patch-clamp recording to measure intracellular calcium and membrane currents respectively at room temperature. Human TRPM2 channels were studied for comparison.

KEY RESULTS

TRPC5 activation by carbachol, Gd3+ or LPC was inhibited by halothane and chloroform at > or =0.1 and 0.2 mM respectively. Neither agent inhibited TRPM2. Propofol had an initial stimulatory effect on TRPC5 (evident in patch-clamp recordings only) and an inhibitory effect at > or =10 microM. TRPM2 was not affected by propofol. Propofol inhibited activation of TRPC5 by Gd3+ but not LPC, suggesting the effect was not directly on the channel. Propofol's anti-oxidant property was not necessary for its inhibitory effect because di-isopropyl benzene, a propofol analogue that lacks the hydroxyl group, also inhibited TRPC5.

CONCLUSIONS AND IMPLICATIONS

The data show the sensitivity of TRPC5 channel to general anaesthetics and suggest that some of the effects could have clinical relevance. The effects may be explained in part by the sensitivity of the channel to biophysical properties of the lipid bilayer.

Biophysical, pharmacological, and functional characteristics of cloned and native mammalian two-pore domain K+ channels

The mammalian family of two-pore domain K+ (K2P) channel proteins are encoded by 15 KCNK genes and subdivided into six subfamilies on the basis of sequence similarities: TWIK, TREK, TASK, TALK, THIK, and TRESK. K2P channels are expressed in cells throughout the body and have been implicated in diverse cellular functions including maintenance of the resting potential and regulation of excitability, sensory transduction, ion transport, and cell volume regulation, as well as metabolic regulation and apoptosis. In recent years K2P channel isoforms have been identified as important targets of several widely employed drugs, including: general anesthetics, local anesthetics, neuroprotectants, and anti-depressants. An important goal of future studies will be to identify the basis of drug actions and channel isoform selectivity. This goal will be facilitated by characterization of native K2P channel isoforms, their pharmacological properties and tissue-specific expression patterns. To this end the present review examines the biophysical, pharmacological, and functional characteristics of cloned mammalian K2P channels and compares this information with the limited data available for native K2P channels in order to determine criteria which may be useful in identifying ionic currents mediated by native channel isoforms and investigating their pharmacological and functional characteristics.

Potent Inhibition of Native TREK-1 K+Channels by Selected Dihydropyridine Ca2+Channel Antagonists

Bovine adrenal zona fasciculata (AZF) cells express bTREK-1 background K+ channels that set the resting membrane potential. Whole-cell and single-channel patch-clamp recording were used to compare five Ca2+ channel antagonists with respect to their potency as inhibitors of native bTREK-1 K+ channels. The dihydropyridine (DHP) Ca2+ channel antagonists amlodipine and niguldipine potently and specifically inhibited bTREK-1 with IC50 values of 0.43 and 0.75 microM, respectively. The other Ca2+ channel antagonists, including the DHP nifedipine, the diphenyldiperazine flunarizine, and the cannabinoid anandamide were less potent, with IC50 values of 8.18, 2.48, and 5.07 microM, respectively. Additional studies with the highly prescribed antihypertensive amlodipine showed that inhibition of bTREK-1 by this agent was voltage-independent and specific. At concentrations that produced near complete block of bTREK-1, amlodipine inhibited voltage-gated Kv1.4 K+ and T-type Ca2+ currents in AZF cells by less than 10%. At the single-channel level, amlodipine reduced bTREK-1 open probability without altering the unitary conductance. The results demonstrate that selected DHP L-type Ca2+ channel antagonists potently inhibit native bTREK-1 K+ channels, whereas other Ca2+ channel antagonists also inhibit bTREK-1 at higher concentrations. Collectively, organic Ca2+ channel antagonists make up the most potent class of TREK-1 inhibitors yet described. Because TREK-1 K+ channels are widely expressed in the central nervous and cardiovascular systems, it is possible that some of the therapeutic or toxic effects of frequently prescribed drugs such as amlodipine may be due to their interaction with TREK-1 K+ rather L-type Ca2+ channels.

Disulfide trapping the mechanosensitive channel MscL into a gating-transition state

The mechanosensitive channel of large conductance, MscL, serves as a biological emergency release valve protecting bacteria from acute osmotic downshock, and is to date the best characterized mechanosensitive channel. The N-terminal region of the protein has been shown to be critical for function by random, site-directed, and deletion mutagenesis, yet is structurally poorly understood. One model proposes that the extreme N-termini form a cluster of amphipathic helices that serves as a cytoplasmic second gate, separated from the pore-forming transmembrane domain by a "linker". Here, we have utilized cysteine trapping of single-cysteine mutated channels to determine the proximity, within the homopentameric complex, of residues within and just peripheral to this proposed linker. Our results indicate that all residues in this region can form disulfide bridges, and that the percentage of dimers increases when the channel is gated in vivo. Functional studies suggest that oxidation traps one of these mutated channels, N15C, into a gating-transition state that retains the capacity to obtain both fully open and closed states. The data are not easily explained by current models for the smooth transition from closed-to-open states, but predict that an asymmetric movement of one or more of the subunits commonly occurs upon gating.

AKAP150, a switch to convert mechano-, pH- and arachidonic acid-sensitive TREK K(+) channels into open leak channels

TREK channels are unique among two-pore-domain K(+) channels. They are activated by polyunsaturated fatty acids (PUFAs) including arachidonic acid (AA), phospholipids, mechanical stretch and intracellular acidification. They are inhibited by neurotransmitters and hormones. TREK-1 knockout mice have impaired PUFA-mediated neuroprotection to ischemia, reduced sensitivity to volatile anesthetics and altered perception of pain. Here, we show that the A-kinase-anchoring protein AKAP150 is a constituent of native TREK-1 channels. Its binding to a key regulatory domain of TREK-1 transforms low-activity outwardly rectifying currents into robust leak conductances insensitive to AA, stretch and acidification. Inhibition of the TREK-1/AKAP150 complex by Gs-coupled receptors such as serotonin 5HT4sR and noradrenaline beta2AR is as extensive as for TREK-1 alone, but is faster. Inhibition of TREK-1/AKAP150 by Gq-coupled receptors such as serotonin 5HT2bR and glutamate mGluR5 is much reduced when compared to TREK-1 alone. The association of AKAP150 with TREK channels integrates them into a postsynaptic scaffold where both G-protein-coupled membrane receptors (as demonstrated here for beta2AR) and TREK-1 dock simultaneously.

VOCCs and TREK-1 ion channel expression in human tenocytes

Mechanosensitive and voltage-gated ion channels are known to perform important roles in mechanotransduction in a number of connective tissues, including bone and muscle. It is hypothesized that voltage-gated and mechanosensitive ion channels also may play a key role in some or all initial responses of human tenocytes to mechanical stimulation. However, to date there has been no direct investigation of ion channel expression by human tenocytes. Human tenocytes were cultured from patellar tendon samples harvested from five patients undergoing routine total knee replacement surgery (mean age: 66 yr; range: 63-73 yr). RT-PCR, Western blotting, and whole cell electrophysiological studies were performed to investigate the expression of different classes of ion channels within tenocytes. Human tenocytes expressed mRNA and protein encoding voltage-operated calcium channel (VOCC) subunits (Ca alpha(1A), Ca alpha(1C), Ca alpha(1D), Ca alpha(2)delta(1)) and the mechanosensitive tandem pore domain potassium channel (2PK(+)) TREK-1. They exhibit whole cell currents consistent with the functional expression of these channels. In addition, other ionic currents were detected within tenocytes consistent with the expression of a diverse array of other ion channels. VOCCs and TREK channels have been implicated in mechanotransduction signaling pathways in numerous connective tissue cell types. These mechanisms may be present in human tenocytes. In addition, human tenocytes may express other channel currents. Ion channels may represent potential targets for the pharmacological management of chronic tendinopathies.

Desensitization of mechano-gated K2P channels

The neuronal mechano-gated K2P channels TREK-1 and TRAAK show pronounced desensitization within 100 ms of membrane stretch. Desensitization persists in the presence of cytoskeleton disrupting agents, upon patch excision, and when channels are expressed in membrane blebs. Mechanosensitive currents evoked with a variety of complex stimulus protocols were globally fit to a four-state cyclic kinetic model in detailed balance, without the need to introduce adaptation of the stimulus. However, we show that patch stress can be a complex function of time and stimulation history. The kinetic model couples desensitization to activation, so that gentle conditioning stimuli do not cause desensitization. Prestressing the channels with pressure, amphipaths, intracellular acidosis, or the E306A mutation reduces the peak-to-steady-state ratio by changing the preexponential terms of the rate constants, increasing the steady-state current amplitude. The mechanical responsivity can be accounted for by a change of in-plane area of approximately 2 nm2 between the closed and open conformations. Desensitization and its regulation by chemical messengers is predicted to condition the physiological role of K2P channels.

Two-pore-domain potassium channels in smooth muscles: new components of myogenic regulation

Gastrointestinal (GI) smooth muscles are influenced by many levels of regulation, including those provided by enteric motor neurones, hormones and paracrine substances. The integrated contractile responses to these regulatory mechanisms depend heavily on the state of excitability of smooth muscle cells. Resting ionic conductances and myogenic responses to agonists and physical parameters, such as stretch, are important in establishing basal excitability. This review discusses the role of 2-pore-domain K+ channels in contributing to background conductances and in mediating responses of GI muscles to enteric inhibitory nerve stimulation and stretch. Murine GI muscles express TREK-1 channels and display a stretch-dependent K+ (SDK) conductance that is also activated by nitric oxide via a cGMP-dependent mechanism. Cloning and expression of mTREK-1 produced an SDK conductance that was activated by cGMP-dependent phosphorylation at serine-351. GI muscle cells also express TASK-1 and TASK-2 channels that are inhibited by lidocaine and external acidification. These conductances appear to provide significant background K+ permeability that contributes to the negative resting potentials of GI muscles.

Expression of TASK and TREK, two-pore domain K+ channels, in human myometrium

Two-pore domain K+ channels are an emerging family of K+ channels that may contribute to setting membrane potential in both electrically excitable and non-excitable cells and, as such, influence cellular function. The human uteroplacental unit contains both excitable (e.g. myometrial) and non-excitable cells, whose function depends upon the activity of K+ channels. We have therefore investigated the expression of two members of this family, TWIK (two-pore domain weak inward rectifying K+ channel)-related acid-sensitive K+ channel (TASK) and TWIK-related K+ channel (TREK) in human myometrium. Using RT-PCR the mRNA expression of TASK and TREK isoforms was examined in myometrial tissue from pregnant women. mRNAs encoding TASK1, 4 and 5 and TREK1 were detected whereas weak or no signals were observed for TASK2, TASK3 and TREK2. Western blotting for TASK1 gave two bands of approximately 44 and 65 kDa, whereas TREK1 gave bands of approximately 59 and 90 kDa in myometrium from pregnant women. TASK1 and TREK1 immunofluorescence was prominent in intracellular and plasmalemmal locations within myometrial cells. Therefore, we conclude that the human myometrium is a site of expression for the two-pore domain K+ channel proteins TASK1 and TREK1.

Stretch-activated channels: a mini-review. Are stretch-activated channels an ocular barometer?

All cells are subject to physical forces by virtue of their position in a dynamically changing environment. This review outlines the various putative 'mechanosensors', or sensors of pressure cells possess, and discusses in particular the role stretch-activated membrane channels play in pressure recognition and transduction. The widespread occurrence of these channels is discussed and these 'mechanosensors' are related to pressure-related diseases, in particular, glaucoma.

Thermosensitivity of the two-pore domain K+ channels TREK-2 and TRAAK

TREK-1, TREK-2 and TRAAK are members of the two-pore domain K+ (K2P) channel family and are activated by membrane stretch and free fatty acids. TREK-1 has been shown to be sensitive to temperature in expression systems. We studied the temperature-sensitivity of TREK-2 and TRAAK in COS-7 cells and in neuronal cells. In transfected COS-7 cells, TREK-2 and TRAAK whole-cell currents increased approximately 20-fold as the bath temperature was raised from 24 degrees C to 42 degrees C. Similarly, in cell-attached patches of COS-7 cells, channel activity was very low, but increased progressively as the bath temperature was raised from 24 degrees C to 42 degrees C. The thresholds for activation of TREK-2 and TRAAK were approximately 25 degrees C and approximately 31 degrees C, respectively. Other K2P channels such as TASK-3 and TRESK-2 were not significantly affected by an increase in temperature from 24 degrees C to 37 degrees C. When the C-terminus of TREK-2 was replaced with that of TASK-3, its sensitivity to free fatty acids and protons was abolished, but the mutant could still be activated by heat. At 37 degrees C, TREK-1, TREK-2 and TRAAK were sensitive to arachidonic acid, pH and membrane stretch in both cell-attached and inside-out patches. In cerebellar granule and dorsal root ganglion neurones, TREK-1, TREK-2 and TRAAK were generally inactive in the cell-attached state at 24 degrees C, but became very active at 37 degrees C. In cell-attached patches of ventricular myocytes, TREK-1 was also normally closed at 24 degrees C, but was active at 37 degrees C. These results show that TREK-2 and TRAAK are also temperature-sensitive channels, are active at physiological body temperature, and therefore would contribute to the background K+ conductance and regulate cell excitability in response to various physical and chemical stimuli.

Expression of the mechanosensitive 2PK+ channel TREK-1 in human osteoblasts

TREK-1 is a mechanosensitive member of the two-pore domain potassium channel family (2PK+) that is also sensitive to lipids, free fatty acids (including arachidonic acid), temperature, intracellular pH, and a range of clinically relevant compounds including volatile anaesthetics. TREK-1 is known to be expressed at high levels in excitable tissues, such as the nervous system, the heart and smooth muscle, where it is believed to play a prominent role in controlling resting cell membrane potential and electrical excitability. In this report, we use RT-PCR, Western blotting and immunohistochemistry to confirm that human derived osteoblasts and MG63 cells express TREK-1 mRNA and protein. In addition, we show gene expression of TREK2c and TRAAK channels. Furthermore, whole cell patch clamp electrophysiology demonstrates that these cells express a spontaneously active, outwardly rectifying potassium "background leak" current that shares many similarities to TREK-1. The outward current is largely insensitive to TEA and Ba2+, and is sensitive to application of lysophosphatidylcholine (LPC). In addition, blocking TREK-1 channel activity is shown to upregulate bone cell proliferation. It is concluded that human osteoblasts functionally express TREK-1 and that these channels contribute, at least in part, to the resting membrane potential of human osteoblast cells. We hypothesise a possible role for TREK-1 in mechanotransduction, leading to bone remodelling.

Mechanosensitive ion channels: molecules of mechanotransduction

Cells respond to a wide variety of mechanical stimuli, ranging from thermal molecular agitation to potentially destructive cell swelling caused by osmotic pressure gradients. The cell membrane presents a major target of the external mechanical forces that act upon a cell, and mechanosensitive (MS) ion channels play a crucial role in the physiology of mechanotransduction. These detect and transduce external mechanical forces into electrical and/or chemical intracellular signals. Recent work has increased our understanding of their gating mechanism, physiological functions and evolutionary origins. In particular, there has been major progress in research on microbial MS channels. Moreover, cloning and sequencing of MS channels from several species has provided insights into their evolution, their physiological functions in prokaryotes and eukaryotes, and their potential roles in the pathology of disease.

Functional expression of TREK-2 in insulin-secreting MIN6 cells

Insulin secretion from pancreatic beta cells is partly regulated by cell membrane potential. Background K+ channels that stabilize the resting membrane potential would suppress excitability and insulin secretion. Recent studies show that members of the two-pore domain K+ (K2P) channel family behave as background K+ channels in many excitable cells. Therefore, the expression of K2P channels was studied in insulin-secreting MIN6 cells. Reverse transcriptase PCR showed that, among nine K2P channels tested, TASK-1, TASK-2, TASK-3, TREK-2, and TRESK-2 were expressed in MIN6 cells. Cell-attached recordings on MIN6 cells revealed five types of K+ channels that were open at rest. Two were ATP-sensitive and Ca2+-activated K+ channels, as judged by their sensitivity to ATP and Ca2+, respectively, and single-channel conductance. Among five K2P channels, only TREK-2 could be clearly identified in MIN6 cells. The molecular identity of two other K+ channels is not yet known. TREK-2 in MIN6 cells was activated by arachidonic acid, membrane stretch, and low pH solution (pH 5.8). Arachidonic acid increased Ba2+-sensitive whole-cell current in MIN6 cell. These results suggest that TREK-2 contributes to the background K+ conductance in MIN6 cells, and may regulate depolarization-induced secretion of insulin.

Evidence for a Novel K+Channel Modulated by α1A-Adrenoceptors in Cardiac Myocytes

Accumulating evidence suggests that steady-state K(+) currents modulate excitability and action potential duration, particularly in cardiac cell types with relatively abbreviated action potential plateau phases. Despite representing potential drug targets, at present these currents and their modulation are comparatively poorly characterized. Therefore, we investigated the effects of phenylephrine [PE; an alpha(1)-adrenoceptor (alpha(1)-AR) agonist] on a sustained outward K(+) current in rat ventricular myocytes. Under K(+) current-selective conditions at 35 degrees C and whole-cell patch clamp, membrane depolarization elicited transient (I(t)) and steady-state (I(ss)) outward current components. PE (10 microM) significantly decreased I(ss) amplitude, without significant effect on I(t). Preferential modulation of I(ss) by PE was confirmed by intracellular application of the voltage-gated K(+) channel blocker tetraethylammonium, which largely inhibited I(t) without affecting the PE-sensitive current (I(ss,PE)). I(ss,PE) had the properties of an outwardly rectifying steady-state K(+)-selective conductance. Acidification of the external solution or externally applied BaCl(2) or quinidine strongly inhibited I(ss,PE). However, I(ss,PE) was not abolished by anandamide, ruthenium red, or zinc, inhibitors of TASK acid-sensitive background K(+) channels. Furthermore, the PE-sensitive current was partially inhibited by external administration of high concentrations of tetraethylammonium and 4-aminopyridine, which are voltage-gated K(+) channel-blockers. Power spectrum analysis of I(ss,PE) yielded a large unitary conductance of 78 pS. I(ss,PE) resulted from PE activation of the alpha(1A)-AR subtype, involved a pertussis toxin-insensitive G-protein, and was independent of cytosolic Ca(2+). These results collectively demonstrate that alpha(1A)-AR activation results in the inhibition of an outwardly rectifying steady-state K(+) current with properties distinct from previously characterized cardiac K(+) channels.

Modulation of native TREK-1 and Kv1.4 K+ channels by polyunsaturated fatty acids and lysophospholipids

The modulation of TREK-1 leak and Kv1.4 voltage-gated K+ channels by fatty acids and lysophospholipids was studied in bovine adrenal zona fasciculata (AZF) cells. In whole-cell patch-clamp recordings, arachidonic acid (AA) (1-20 microM) dramatically and reversibly increased the activity of bTREK-1, while inhibiting bKv1.4 current by mechanisms that occurred with distinctly different kinetics. bTREK-1 was also activated by the polyunsaturated cis fatty acid linoleic acid but not by the trans polyunsaturated fatty acid linolelaidic acid or saturated fatty acids. Eicosatetraynoic acid (ETYA), which blocks formation of active AA metabolites, failed to inhibit AA activation of bTREK-1, indicating that AA acts directly. Compared to activation of bTREK-1, inhibition of bKv1.4 by AA was rapid and accompanied by a pronounced acceleration of inactivation kinetics. Cis polyunsaturated fatty acids were much more effective than trans or saturated fatty acids at inhibiting bKv1.4. ETYA also effectively inhibited bKv1.4, but less potently than AA. bTREK-1 current was markedly increased by lysophospholipids including lysophosphatidyl choline (LPC) and lysophosphatidyl inositol (LPI). At concentrations from 1-5 microM, LPC produced a rapid, transient increase in bTREK-1 that peaked within one minute and then rapidly desensitized. The transient lysophospholipid-induced increases in bTREK-1 did not require the presence of ATP or GTP in the pipette solution. These results indicate that the activity of native leak and voltage-gated K+ channels are directly modulated in reciprocal fashion by AA and other cis unsaturated fatty acids. They also show that lysophospholipids enhance bTREK-1, but with a strikingly different temporal pattern. The modulation of native K+ channels by these agents differs from their effects on the same channels expressed in heterologous cells, highlighting the critical importance of auxiliary subunits and signaling. Finally, these results reveal that AZF cells express thousands of bTREK-1 K+ channels that lie dormant until activated by metabolites including phospholipase A2 (PLA2)-generated fatty acids and lysophospholipids. These metabolites may alter the electrical and secretory properties of AZF cells by modulating bTREK-1 and bKv1.4 K+ channels.

Acid-sensitive two-pore domain potassium (K2P) channels in mouse taste buds

Sour (acid) taste is postulated to result from intracellular acidification that modulates one or more acid-sensitive ion channels in taste receptor cells. The identity of such channel(s) remains uncertain. Potassium channels, by regulating the excitability of taste cells, are candidates for acid transducers. Several 2-pore domain potassium leak conductance channels (K(2)P family) are sensitive to intracellular acidification. We examined their expression in mouse vallate and foliate taste buds using RT-PCR, and detected TWIK-1 and -2, TREK-1 and -2, and TASK-1. Of these, TWIK-1 and TASK-1 were preferentially expressed in taste cells relative to surrounding nonsensory epithelium. The related TRESK channel was not detected, whereas the acid-insensitive TASK-2 was. Using confocal imaging with pH-, Ca(2+)-, and voltage-sensitive dyes, we tested pharmacological agents that are diagnostic for these channels. Riluzole (500 microM), selective for TREK-1 and -2 channels, enhanced acid taste responses. In contrast, halothane (< or = approximately 17 mM), which acts on TREK-1 and TASK-1 channels, blocked acid taste responses. Agents diagnostic for other 2-pore domain and voltage-gated potassium channels (anandamide, 10 microM; Gd(3+), 1 mM; arachidonic acid, 100 microM; quinidine, 200 microM; quinine, 100 mM; 4-AP, 10 mM; and TEA, 1 mM) did not affect acid responses. The expression of 2-pore domain channels and our pharmacological characterization suggest that a matrix of ion channels, including one or more acid-sensitive 2-pore domain K channels, could play a role in sour taste transduction. However, our results do not unambiguously identify any one channel as the acid taste transducer.

Effects of volatile anesthetics on cardiac ion channels

The focus of the present review is on how interference with various ion channels in the heart may be the molecular basis for cardiac side-effects of gaseous anesthetics. Electrophysiological studies in isolated animal and human cardiomyocytes have identified the L-type Ca(2+) channel as a prominent target of anesthetics. Since this ion channel is of fundamental importance for the plateau phase of the cardiac action potential as well as for Ca(2+)-mediated electromechanical coupling, its inhibition may facilitate arrhythmias by shortening the refractory period and may decrease the contractile force. Effective inhibition of this ion channel has been shown for clinically used concentrations of halothane and, to a lesser extent, of isoflurane and sevoflurane, whereas xenon was without effect. Anesthetics furthermore inhibit several types of voltage-gated K(+) channels. Thereby, they may disturb the repolarization and bear a considerable risk for the induction of ventricular tachycardia in predisposed patients. In future, an advanced understanding of cardiac side-effects of anesthetics will derive from more detailed analyses of how and which channels are affected as well as from a better comprehension of how altered channel function influences heart function.

Li+ enhances GABAergic inputs to granule cells in the rat hippocampal dentate gyrus

Defects in GABAergic interneurons are thought to be involved in the pathophysiology of bipolar disorder, and Li+ has been used as a primary therapeutic agent in the treatment. We used the patch clamp technique to investigate whether Li+ affects on spontaneous GABAergic synaptic inputs to granule cells (GCs) in hippocampal dentate gyrus. Extracellularly applied Li+ (25 mM) markedly increased the frequency and amplitude of spontaneous inhibitory postsynaptic currents (sIPSCs), an effect completely blocked by picrotoxin or bicuculline. Li+ increased sIPSCs frequency in the presence of tetrodotoxin (TTX), but to a lesser extent than its absence. Li+ caused no change in the cumulative amplitude distribution of miniature IPSCs, indicating that a presynaptic mechanism is involved. When TTX was added in the presence of Li+, large-amplitude sIPSCs (>30 pA) were abolished specifically with no effect on small-amplitude sIPSCs (<20 pA). Intracellular Li+ (6 mM) applied via the patch pipette depolarized the resting membrane potential in fast-spiking interneurons, resulting in an increase in spontaneous action potential (AP) firing. This change, however, was not observed in GCs. These results suggest that Li(+)-induced spontaneous AP firing in GABAergic interneurons contributes to the increase in GABAergic synaptic inputs to GCs.

Differential expression of the mechanosensitive potassium channel TREK-1 in epicardial and endocardial myocytes in rat ventricle

Mechanoelectric feedback (MEF) is the process by which mechanical forces on the myocardium induce electrical responses. It is thought that MEF is important in controlling the beat to beat force of contraction in the ventricle, in response to fluctuations in load, and it may also play a role in controlling the dispersion of repolarization. The transduction mechanism for MEF is via stretch sensitive ion channels in the surface membrane of myocytes. Two types of stretch sensitive channels have been described; a non-selective cation channel, and a potassium selective channel. TREK-1 is a member of the recently cloned tandem pore potassium channels that has been shown to be mechanosensitive and to be expressed in rat heart. Here we report that the gene expression level of TREK-1, quantified using real-time RT-PCR against glyceraldehyde phosphate dehydrogenase (GAPDH) as a comparator gene, was found to be 0.34 +/- 0.14 in endocardial cells compared to 0.02 +/- 0.02 in epicardial cells (P < 0.05). To confirm that this is reflected in a different current density, whole cell TREK-1 currents, activated by chloroform, were recorded with patch clamp techniques in epicardial and endocardial cells. TREK-1 current density in epicardial and endocardial cells was 0.21 +/- 0.06 pA/pF and 0.8 +/- 0.27 pA/pF, respectively (P</= 0.05). We discuss the implications of this differential expression of TREK-1 for controlling action potential repolarization when the myocardium is stretched. We hypothesize that the gene expression of TREK-1 is controlled by the different amounts of stretch experienced by muscle cells across the ventricular wall.

Comparison of time- and voltage-dependent K+ currents in myocytes from left and right atria of adult mice

Consistent differences in K+ currents in left and right atria of adult mouse hearts have been identified by the application of current- and voltage-clamp protocols to isolated single myocytes. Left atrial myocytes had a significantly (P < 0.05) larger peak outward K+ current density than myocytes from the right atrium. Detailed analysis revealed that this difference was due to the rapidly activating sustained K+ current, which is inhibited by 100 muM 4-aminopyridine (4-AP); this current was almost three times larger in the left atrium than in the right atrium. Accordingly, 100 muM 4-AP caused a significantly (P < 0.05) larger increase in action potential duration in left than in right atrial myocytes. Inward rectifier K+ current density was also significantly (P < 0.05) larger in left atrial myocytes. There was no difference in the voltage-dependent L-type Ca2+ current between left and right atria. As expected from this voltage-clamp data, the duration of action potentials recorded from single myocytes was significantly (P < 0.05) shorter in myocytes from left atria, and left atrial tissue was found to have a significantly (P < 0.05) shorter effective refractory period than right atrial tissue. These results reveal similarities between mice and other mammalian species where the left atrium repolarizes more quickly than the right, and provide new insight into cellular electrophysiological mechanisms responsible for this difference. These findings, and previous results, suggest that the atria of adult mice may be a suitable model for detailed studies of atrial electrophysiology and pharmacology under control conditions and in the context of induced atrial rhythm disturbances.

Amide local anesthetics potently inhibit the human tandem pore domain background K+ channel TASK-2 (KCNK5)

Blockade of voltage-gated sodium (Na+) channels by local anesthetics represents the main mechanism for inhibition of impulse propagation. Local anesthetic-induced potassium (K+) channel inhibition is also known to influence transmission of sensory impulses and to potentiate inhibition. K+ channels involved in this mechanism may belong to the emerging family of background tandem pore domain K+ channels (2P K+ channels). To determine more precisely the effects of local anesthetics on members of this ion channel family, we heterologously expressed the 2P K+ channels TASK-2 (KCNK5), TASK-1 (KCNK3), and chimeric TASK-1/TASK-2 channels in oocytes of Xenopus laevis. TASK-2 cDNA-transfected HEK 293 cells were used for single-channel recordings. Local anesthetic inhibition of TASK-2 was dose-dependent, agent-specific, and stereoselective. The IC50 values for R-(+)-bupivacaine and S-(-)-bupivacaine were 17 and 43 micro M and for R-(+)-ropivacaine and S-(-)-ropivacaine, 85 and 236 micro M. Lidocaine (1 mM) inhibited TASK-2 currents by 55 +/- 4%, whereas its quaternary positively charged analog N-ethyl lidocaine (QX314) had no effect. Bupivacaine (100 micro M) decreased channel open probability from 20.8 +/- 1.6% to 5.6 +/- 2.2%. Local anesthetics [300 micro M R-(+)-bupivacaine] caused significantly greater depolarization of the resting membrane potential of TASK-2-expressing oocytes compared with water-injected control oocytes (15.8 +/- 2.5 mV versus 0.1 +/- 0.05 mV; p < 0.001). Chimeric TASK-1/TASK-2 2P K+ channel subunits that retained pH sensitivity demonstrated that the carboxy domain of TASK-2 mediates the greater local anesthetic sensitivity of TASK-2. These results show that clinically achievable concentrations of local anesthetics inhibit background K+ channel function and may thereby enhance conduction blockade.

Corticotropin induces the expression of TREK-1 mRNA and K+ current in adrenocortical cells

Bovine adrenal zona fasciculata (AZF) cells express a two-pore/four-transmembrane segment bTREK-1 K+ channel that sets the resting potential and couples hormonal signals to depolarization-dependent Ca2+ entry and cortisol secretion. It was discovered that corticotropin (1-2000 pM) enhances the expression of bTREK-1 mRNA and membrane current in cultured AZF cells. Forskolin and 8-pcpt-cAMP mimicked corticotropin induction of bTREK-1 mRNA, but angiotensin II (AII) was ineffective. The induction of bTREK-1 mRNA by corticotropin was partially blocked by the A-kinase antagonist H-89. 8-(4-Chloro-phenylthio)-2-O-methyladenosine-3'-5'-cyclic monophosphate, a cAMP analog that activates cAMP-regulated guanine nucleotide exchange factors (Epac), failed to increase bTREK-1 mRNA. Corticotropin-stimulated increases in bTREK-1 mRNA were eliminated by inhibitors of protein synthesis or gene transcription. bTREK-1 current disappeared after 24 h in serum-supplemented medium, but in the presence of corticotropin, bTREK-1 expression was maintained for at least 48 h. The enhancement of bTREK-1 mRNA and ionic current contrasts with the corticotropin-induced down-regulation of the Kv1.4 voltage-gated K+ current and associated mRNA in AZF cells. These results demonstrate that corticotropin rapidly and potently induces the expression of bTREK-1 in AZF cells at the pretranslational level by a cAMP-dependent mechanism that is partially dependent on A-kinase but independent of Epac and Ca2+. They further indicate that prolonged stimulation of AZF cells by corticotropin, as occurs during long-term stress or disease, may produce pronounced changes in the expression of genes encoding ion channels, thereby reshaping the electrical properties of these cells to enhance or limit cortisol secretion.

Dynamic properties of stretch-activated K+ channels in adult rat atrial myocytes

The effect of mechanical stress on the heart's electrical activity has been termed mechanoelectric feedback. The response to stretch depends upon the magnitude and the waveform of the stimulus, and upon the timing relative to the cardiac cycle. Stretch-activated ion channels (SACs) have been regarded as the most likely candidates for serving as the primary transducers of mechanical stress. We explored the steady state and dynamic responses of single channels in adult rat atrial cells using the patch clamp with a pressure clamp. Surprisingly, we only observed K(+)-selective SACs, probably of the 2P domain family. The channels were weakly outward rectifying with flickery bursts. In cell attached mode, the mean conductance was 74+/-14 and 65+/-16 pS for +60 and -60 mV, respectively (140 mM [K(+)](out), 2mM [Mg(2+)](out) and 0mM [Ca(2+)](out)). The latency of the response to pressure steps was 50-100 ms and the time to peak approximately 400 ms. About half of the channels in cell-attached patches showed adaptation/inactivation where channel activity declined to a plateau of 20-30% of peak in approximately 1s. The time dependent behavior of these SACs is generally consistent with whole-cell currents observed in chick and rat ventricular cells, although the net current was outward rather than inward.

The p42/44mitogen-activated protein kinase inhibitor PD 98059, but not U 0126, increases a K+ current in cardiomyocytes

1. The effects of the mitogen-activated protein kinase (MAPK) inhibitors PD 98059 and U 0126, useful tools to investigate MAPK involvement in intracellular signal transduction pathways, were assessed on cardiomyocytes. 2. In rat freshly isolated ventricular myocytes, under current-clamp conditions, PD 98059 (40 micro mol/L) shortened the action potential. Under whole-cell patch-clamp, this compound slowly induced a fast activating sustained outward K+ current that was sensitive to 1 mmol/L Ba2+, 100 micro mol/L Gd3+, 3 mmol/L 4-aminopyridine and 100 micro mol/L tetracain. The PD 98059-induced current was prevented by 40 micro mol/L AACOCF3, a cytosolic phospholipase A2 inhibitor. 3. U 0126 (1 micro mol/L), a recently developed highly potent p42/44 MAPK inhibitor, did not alter K+ currents. 4. PD 98059, but not U 0126, increased arachidonic acid content, probably as a consequence of its reported cyclo-oxygenase inhibitory effect. 5. These observations indicate that PD 98059 activates a TREK-1 like current. Thus, this MAPK inhibitor has to be used with caution because alterations in cell metabolism can be secondary to changes in electrophysiological behaviour.

Antiarrhythmic drugs: new agents and evolving concepts

Properties of several new antiarrhythmic drugs are summarised in this review article. Recent concepts concerning their safety and efficacy of antiarrhythmics are discussed. A brief perspective on possible future strategies for pharmacotherapy of arrhythmias is provided.

An ACTH- and ATP-regulated background K+ channel in adrenocortical cells is TREK-1

Bovine adrenal zona fasciculata (AZF) cells express a background K(+) channel (I(AC)) that sets the resting potential and acts pivotally in ACTH-stimulated cortisol secretion. We have cloned a bTREK-1 (KCNK2) tandem-pore K(+) channel cDNA from AZF cells with properties that identify it as the native I(AC). The bTREK-1 cDNA is expressed robustly in AZF cells and includes transcripts of 4.9, 3.6, and 2.8 kb. In patch clamp recordings made from transiently transfected cells, bTREK-1 displayed distinctive properties of I(AC) in AZF cells. Specifically, bTREK-1 currents were outwardly rectifying with a large instantaneous and smaller time-dependent component. Similar to I(AC), bTREK-1 increased spontaneously in amplitude over many minutes of whole cell recording and was blocked potently by Ca(2+) antagonists including penfluridol and mibefradil and by 8-(4-chlorophenylthio)-cAMP. Unitary TREK-1 and I(AC) currents were nearly identical in amplitude. The native I(AC) current, in turn, displayed properties that together are specific to TREK-1 K(+) channels. These include activation by intracellular acidification, enhancement by the neuroprotective agent riluzole, and outward rectification. bTREK-1 current differed from native K(+) current only in its lack of ATP dependence. In contrast to I(AC), the current density of bTREK-1 in human embryonic kidney-293 cells was not increased by raising pipette ATP from 0.1 to 5 mm. Further, the enhancement of I(AC) current in AZF cells by low pH and riluzole was facilitated by, and dependent on, ATP at millimolar concentrations in the pipette solution. Overall, these results establish the identity of I(AC) K(+) channels, demonstrate the expression of bTREK-1 in a specific endocrine cell, identify potent new TREK-1 antagonists, and assign a pivotal role for these tandem-pore channels in the physiology of cortisol secretion. The activation of I(AC) by ATP indicates that native bTREK-1 channels may function as sensors that couple the metabolic state of the cell to membrane potential, perhaps through an associated ATP-binding protein.