Simultaneous activation of p38 MAPK and p42/44 MAPK by ATP stimulates the K+ current ITREK in cardiomyocytes

Correlation of Insulin-Like Growth Factor 1 With Cognitive Functions in Mild Traumatic Brain Injury Patients

Mild traumatic brain injury (mTBI) is a prevalent health concern with variable recovery trajectories, necessitating reliable prognostic markers. Insulin-like growth factor 1 (IGF-1) emerges as a potential candidate because of its role in cellular growth, repair, and neuroprotection. However, limited studies investigate IGF-1 as a prognostic marker in mTBI patients. This study aimed to explore the correlation of IGF-1 with cognitive functions assessed using the Wisconsin Card Sorting Test (WCST) in mTBI patients. We analyzed data from 295 mTBI and 200 healthy control participants, assessing demographic characteristics, injury causes, and IGF-1 levels. Cognitive functions were evaluated using the WCST. Correlation analyses and regression models were used to investigate the associations between IGF-1 levels, demographic factors, and WCST scores. Significant differences were observed between mTBI and control groups in the proportion of females and average education years. Falls and traffic accidents were identified as the primary causes of mTBI. The mTBI group demonstrated worse cognitive outcomes on the WCST, except for the "Learning to Learn" index. Correlation analyses revealed significant relationships between IGF-1 levels, demographic factors, and specific WCST scores. Regression models demonstrated that IGF-1, age, and education years significantly influenced various WCST scores, suggesting their roles as potential prognostic markers for cognitive outcomes in mTBI patients. We provide valuable insights into the potential correlation of IGF-1 with cognitive functions in mTBI patients, particularly in tasks requiring cognitive flexibility and problem solving.

TREK channels in Mechanotransduction: a Focus on the Cardiovascular System

Mechano-electric feedback is one of the most important subsystems operating in the cardiovascular system, but the underlying molecular mechanism remains rather unknown. Several proteins have been proposed to explain the molecular mechanism of mechano-transduction. Transient receptor potential (TRP) and Piezo channels appear to be the most important candidates to constitute the molecular mechanism behind of the inward current in response to a mechanical stimulus. However, the inhibitory/regulatory processes involving potassium channels that operate on the cardiac system are less well known. TWIK-Related potassium (TREK) channels have emerged as strong candidates due to their capacity for the regulation of the flow of potassium in response to mechanical stimuli. Current data strongly suggest that TREK channels play a role as mechano-transducers in different components of the cardiovascular system, not only at central (heart) but also at peripheral (vascular) level. In this context, this review summarizes and highlights the main existing evidence connecting this important subfamily of potassium channels with the cardiac mechano-transduction process, discussing molecular and biophysical aspects of such a connection.

Identification of the molecular mechanism of insulin-like growth factor-1 (IGF-1): a promising therapeutic target for neurodegenerative diseases associated with metabolic syndrome

Neurodegenerative disorders are accompanied by neuronal degeneration and glial dysfunction, resulting in cognitive, psychomotor, and behavioral impairment. Multiple factors including genetic, environmental, metabolic, and oxidant overload contribute to disease progression. Recent evidences suggest that metabolic syndrome is linked to various neurodegenerative diseases. Metabolic syndrome (MetS) is known to be accompanied by symptoms such as hyperglycemia, abdominal obesity, hypertriglyceridemia, and hypertension. Despite advances in knowledge about the pathogenesis of neurodegenerative disorders, effective treatments to combat neurodegenerative disorders caused by MetS have not been developed to date. Insulin growth factor-1 (IGF-1) deficiency has been associated with MetS-related pathologies both in-vivo and in-vitro. IGF-1 is essential for embryonic and adult neurogenesis, neuronal plasticity, neurotropism, angiogenesis, metabolic function, and protein clearance in the brain. Here, we review the evidence for the potential therapeutic effects of IGF-1 in the neurodegeneration related to metabolic syndrome. We elucidate how IGF-1 may be involved in molecular signaling defects that occurs in MetS-related neurodegenerative disorders and highlight the importance of IGF-1 as a potential therapeutic target in MetS-related neurological diseases.

TREK-1 in the heart: Potential physiological and pathophysiological roles

The TREK-1 channel belongs to the TREK subfamily of two-pore domains channels that are activated by stretch and polyunsaturated fatty acids and inactivated by Protein Kinase A phosphorylation. The activation of this potassium channel must induce a hyperpolarization of the resting membrane potential and a shortening of the action potential duration in neurons and cardiac cells, two phenomena being beneficial for these tissues in pathological situations like ischemia-reperfusion. Surprisingly, the physiological role of TREK-1 in cardiac function has never been thoroughly investigated, very likely because of the lack of a specific inhibitor. However, possible roles have been unraveled in pathological situations such as atrial fibrillation worsened by heart failure, right ventricular outflow tract tachycardia or pulmonary arterial hypertension. The inhomogeneous distribution of TREK-1 channel within the heart reinforces the idea that this stretch-activated potassium channel might play a role in cardiac areas where the mechanical constraints are important and need a particular protection afforded by TREK-1. Consequently, the main purpose of this mini review is to discuss the possible role played by TREK -1 in physiological and pathophysiological conditions and its potential role in mechano-electrical feedback. Improved understanding of the role of TREK-1 in the heart may help the development of promising treatments for challenging cardiac diseases.

Two-Pore-Domain Potassium (K2P-) Channels: Cardiac Expression Patterns and Disease-Specific Remodelling Processes

Two-pore-domain potassium (K_2P-) channels conduct outward K⁺ currents that maintain the resting membrane potential and modulate action potential repolarization. Members of the K_2P channel family are widely expressed among different human cell types and organs where they were shown to regulate important physiological processes. Their functional activity is controlled by a broad variety of different stimuli, like pH level, temperature, and mechanical stress but also by the presence of lipids or pharmacological agents. In patients suffering from cardiovascular diseases, alterations in K_2P-channel expression and function have been observed, suggesting functional significance and a potential therapeutic role of these ion channels. For example, upregulation of atrial specific K_2P3.1 (TASK-1) currents in atrial fibrillation (AF) patients was shown to contribute to atrial action potential duration shortening, a key feature of AF-associated atrial electrical remodelling. Therefore, targeting K_2P3.1 (TASK-1) channels might constitute an intriguing strategy for AF treatment. Further, mechanoactive K_2P2.1 (TREK-1) currents have been implicated in the development of cardiac hypertrophy, cardiac fibrosis and heart failure. Cardiovascular expression of other K_2P channels has been described, functional evidence in cardiac tissue however remains sparse. In the present review, expression, function, and regulation of cardiovascular K_2P channels are summarized and compared among different species. Remodelling patterns, observed in disease models are discussed and compared to findings from clinical patients to assess the therapeutic potential of K_2P channels.

Contribution of K2P Potassium Channels to Cardiac Physiology and Pathophysiology

Years before the first two-pore domain potassium channel (K2P) was cloned, certain ion channels had already been demonstrated to be present in the heart with characteristics and properties usually attributed to the TREK channels (a subfamily of K2P channels). K2P channels were later detected in cardiac tissue by RT-PCR, although the distribution of the different K2P subfamilies in the heart seems to depend on the species analyzed. In order to collect relevant information in this regard, we focus here on the TWIK, TASK and TREK cardiac channels, their putative roles in cardiac physiology and their implication in coronary pathologies. Most of the RNA expression data and electrophysiological recordings available to date support the presence of these different K2P subfamilies in distinct cardiac cells. Likewise, we show how these channels may be involved in certain pathologies, such as atrial fibrillation, long QT syndrome and Brugada syndrome.

Mechanosensitive TREK-1 two-pore-domain potassium (K2P) channels in the cardiovascular system

TWIK-related K⁺ channel (TREK-1) two-pore-domain potassium (K_2P) channels mediate background potassium currents and regulate cellular excitability in many different types of cells. Their functional activity is controlled by a broad variety of different physiological stimuli, such as temperature, extracellular or intracellular pH, lipids and mechanical stress. By linking cellular excitability to mechanical stress, TREK-1 currents might be important to mediate parts of the mechanoelectrical feedback described in the heart. Furthermore, TREK-1 currents might contribute to the dysregulation of excitability in the heart in pathophysiological situations, such as those caused by abnormal stretch or ischaemia-associated cell swelling, thereby contributing to arrhythmogenesis. In this review, we focus on the functional role of TREK-1 in the heart and its putative contribution to cardiac mechanoelectrical coupling. Its cardiac expression among different species is discussed, alongside with functional evidence for TREK-1 currents in cardiomyocytes. In addition, evidence for the involvement of TREK-1 currents in different cardiac arrhythmias, such as atrial fibrillation or ventricular tachycardia, is summarized. Furthermore, the role of TREK-1 and its interaction partners in the regulation of the cardiac heart rate is reviewed. Finally, we focus on the significance of TREK-1 in the development of cardiac hypertrophy, cardiac fibrosis and heart failure.

Cardiac Mechano-Electric Coupling: Acute Effects of Mechanical Stimulation on Heart Rate and Rhythm

The heart is vital for biological function in almost all chordates, including humans. It beats continually throughout our life, supplying the body with oxygen and nutrients while removing waste products. If it stops, so does life. The heartbeat involves precise coordination of the activity of billions of individual cells, as well as their swift and well-coordinated adaption to changes in physiological demand. Much of the vital control of cardiac function occurs at the level of individual cardiac muscle cells, including acute beat-by-beat feedback from the local mechanical environment to electrical activity (as opposed to longer term changes in gene expression and functional or structural remodeling). This process is known as mechano-electric coupling (MEC). In the current review, we present evidence for, and implications of, MEC in health and disease in human; summarize our understanding of MEC effects gained from whole animal, organ, tissue, and cell studies; identify potential molecular mediators of MEC responses; and demonstrate the power of computational modeling in developing a more comprehensive understanding of ‟what makes the heart tick.ˮ.

A regulatory domain in the K2P2.1 (TREK-1) carboxyl-terminal allows for channel activation by monoterpenes

Potassium K_2P ('leak') channels conduct current across the entire physiological voltage range and carry leak or 'background' currents that are, in part, time- and voltage-independent. K_2P2.1 channels (i.e., TREK-1, KCNK2) are highly expressed in excitable tissues, where they play a key role in the cellular mechanisms of neuroprotection, anesthesia, pain perception, and depression. Here, we report for the first time that human K_2P2.1 channel activity is regulated by monoterpenes (MTs). We found that cyclic, aromatic monoterpenes containing a phenol moiety, such as carvacrol, thymol and 4-IPP had the most profound effect on current flowing through the channel (up to a 6-fold increase). By performing sequential truncation of the carboxyl-terminal domain of the channel and testing the activity of several channel regulators, we identified two distinct regulatory domains within this portion of the protein. One domain, as previously reported, was needed for regulation by arachidonic acid, anionic phospholipids, and temperature changes. Within a second domain, a triple arginine residue motif (R344-346), an apparent PIP₂-binding site, was found to be essential for regulation by holding potential changes and important for regulation by monoterpenes.

TREK-1 protects the heart against ischemia-reperfusion-induced injury and from adverse remodeling after myocardial infarction

The TWIK-related K+ channel (TREK-1) is a two-pore-domain potassium channel that produces background leaky potassium currents. TREK-1 has a protective role against ischemia-induced neuronal damage. TREK-1 is also expressed in the heart, but its role in myocardial ischemia-reperfusion (IR)-induced injury has not been examined. In the current study, we used a TREK-1 knockout (KO) mouse model to show that TREK-1 has a critical role in the cardiac I/R-induced injury and during remodeling after myocardial infarction (MI). At baseline, TREK-1 KO mice had similar blood pressure and heart rate as the wild-type (WT) mice. However, the lack of TREK-1 was associated with increased susceptibility to ischemic injury and compromised functional recovery following ex vivo I/R-induced injury. TREK-1 deficiency increased infarct size following permanent coronary artery ligation, resulting in greater systolic dysfunction than the WT counterpart. Electrocardiographic (ECG) analysis revealed QT interval prolongation in TREK-1 KO mice, but normal heart rate (HR). Acutely isolated TREK-1 KO cardiomyocytes exhibited prolonged Ca²⁺ transient duration associated with action potential duration (APD) prolongation. Our data suggest that TREK-1 has a protective effect against I/R-induced injury and influences the post-MI remodeling processes by regulating membrane potential and maintaining intracellular Ca²⁺ homeostasis. These data suggest that TREK-1 activation could be an effective strategy to provide cardioprotection against ischemia-induced damage.

Inhibitory effects of TGP on KGF‑induced hyperproliferation of HaCaT cells via suppression of the p38 MAPK/NF‑κB p65 pathway

Psoriasis is a chronic inflammatory skin disease, primarily caused by overgrowth and abnormal differentiation of epidermal keratinocytes. Studies have suggested that keratinocyte growth factor (KGF) may be involved in the regulation of differentiation and development of keratinocytes. Total glucosides of peony (TGP) have been widely used for the treatment of psoriasis. The present study aimed to determine whether the therapeutic effect of TGP on psoriasis is mediated by modulation of the p38 mitogen‑activated protein kinase (p38 MAPK)/nuclear factor (NF)‑κB p65 signaling pathways. Cell proliferation was evaluated by CCK‑8 and cell cycle was assessed by flow cytometry assay. Protein and mRNA expression of genes were determined by western blot and reverse transcription‑quantitative polymerase chain reaction, respectively. The results of the present study demonstrated that KGF can promote proliferation of HaCaT cells in a dose‑dependent manner. In addition, it was demonstrated that TGP may suppress the hyperproliferation of HaCaT cells stimulated by KGF by inducing arrest of the cell cycle at the G1 phase. The expression levels of the proinflammatory cytokines interleukin (IL)‑22 and vascular endothelial growth factor (VEGF) were markedly elevated in cells treated with KGF, whereas they were downregulated in cells treated with TGP. Furthermore, combination treatments with p38 MAPK inhibitor SB203580 and KGF, or TGP and KGF suppressed the mRNA and protein expression levels of IL‑22 and VEGF, compared with cells treated with KGF alone. Furthermore, the expression profiles of phosphorylated‑p38 MAPK and NF‑κB p65 were similar to those of IL‑22 and VEGF. The results of the present study suggested that the therapeutic effect of TGP on psoriasis may be mediated by modulation of the p38 MAPK/NF‑κB p65 signaling pathway. The results of the present study contribute to the understanding of the role of TGP in the treatment of psoriasis. The present study provides insights suggesting that p38 MAPK may be a novel regulatory signaling pathway for the treatment of psoriasis.

Stretch-activated two-pore-domain (K2P) potassium channels in the heart: Focus on atrial fibrillation and heart failure

Two-pore-domain potassium (K_2P) channels modulate cellular excitability. The significance of stretch-activated cardiac K_2P channels (K_2P2.1, TREK-1, KCNK2; K_2P4.1, TRAAK, KCNK4; K_2P10.1, TREK-2, KCNK10) in heart disease has not been elucidated in detail. The aim of this work was to assess expression and remodeling of mechanosensitive K_2P channels in atrial fibrillation (AF) and heart failure (HF) patients in comparison to murine models. Cardiac K_2P channel levels were quantified in atrial (A) and ventricular (V) tissue obtained from patients undergoing open heart surgery. In addition, control mice and mouse models of AF (cAMP-response element modulator (CREM)-IbΔC-X transgenic animals) or HF (cardiac dysfunction induced by transverse aortic constriction, TAC) were employed. Human and murine KCNK2 displayed highest mRNA abundance among mechanosensitive members of the K_2P channel family (V > A). Disease-associated K_2P2.1 remodeling was studied in detail. In patients with impaired left ventricular function, atrial KCNK2 (K_2P2.1) mRNA and protein expression was significantly reduced. In AF subjects, downregulation of atrial and ventricular KCNK2 (K_2P2.1) mRNA and protein levels was observed. AF-associated suppression of atrial Kcnk2 (K_2P2.1) mRNA and protein was recapitulated in CREM-transgenic mice. Ventricular Kcnk2 expression was not significantly altered in mouse models of disease. In conclusion, mechanosensitive K_2P2.1 and K_2P10.1 K⁺ channels are expressed throughout the heart. HF- and AF-associated downregulation of KCNK2 (K_2P2.1) mRNA and protein levels suggest a mechanistic contribution to cardiac arrhythmogenesis.

Stretch-activated potassium currents in the heart: Focus on TREK-1 and arrhythmias

This review focuses on the role and the molecular candidates of the cardiac stretch-activated potassium current (SAK). The functional properties of the two-pore domain potassium (K_2P) channel TREK-1, a major candidate for the cardiac SAK, are analyzed and the molecular mechanism of stretch-activation in K_2P potassium channels is discussed. Furthermore, the functional modulation of TREK-1 by different cardiac interaction partners, as well as evidence for the functional role of the stretch-dependent TREK-1 and its putative subunits in the heart is reviewed. In addition, we summarize the recent evidence that TREK-1 is involved in the pathogenesis of human cardiac arrhythmias.

Therapeutic targeting of two-pore-domain potassium (K(2P)) channels in the cardiovascular system

The improvement of treatment strategies in cardiovascular medicine is an ongoing process that requires constant optimization. The ability of a therapeutic intervention to prevent cardiovascular pathology largely depends on its capacity to suppress the underlying mechanisms. Attenuation or reversal of disease-specific pathways has emerged as a promising paradigm, providing a mechanistic rationale for patient-tailored therapy. Two-pore-domain K(+) (K(2P)) channels conduct outward K(+) currents that stabilize the resting membrane potential and facilitate action potential repolarization. K(2P) expression in the cardiovascular system and polymodal K2P current regulation suggest functional significance and potential therapeutic roles of the channels. Recent work has focused primarily on K(2P)1.1 [tandem of pore domains in a weak inwardly rectifying K(+) channel (TWIK)-1], K(2P)2.1 [TWIK-related K(+) channel (TREK)-1], and K(2P)3.1 [TWIK-related acid-sensitive K(+) channel (TASK)-1] channels and their role in heart and vessels. K(2P) currents have been implicated in atrial and ventricular arrhythmogenesis and in setting the vascular tone. Furthermore, the association of genetic alterations in K(2P)3.1 channels with atrial fibrillation, cardiac conduction disorders and pulmonary arterial hypertension demonstrates the relevance of the channels in cardiovascular disease. The function, regulation and clinical significance of cardiovascular K(2P) channels are summarized in the present review, and therapeutic options are emphasized.

Cardiac Mechano-Gated Ion Channels and Arrhythmias

Mechanical forces will have been omnipresent since the origin of life, and living organisms have evolved mechanisms to sense, interpret, and respond to mechanical stimuli. The cardiovascular system in general, and the heart in particular, is exposed to constantly changing mechanical signals, including stretch, compression, bending, and shear. The heart adjusts its performance to the mechanical environment, modifying electrical, mechanical, metabolic, and structural properties over a range of time scales. Many of the underlying regulatory processes are encoded intracardially and are, thus, maintained even in heart transplant recipients. Although mechanosensitivity of heart rhythm has been described in the medical literature for over a century, its molecular mechanisms are incompletely understood. Thanks to modern biophysical and molecular technologies, the roles of mechanical forces in cardiac biology are being explored in more detail, and detailed mechanisms of mechanotransduction have started to emerge. Mechano-gated ion channels are cardiac mechanoreceptors. They give rise to mechano-electric feedback, thought to contribute to normal function, disease development, and, potentially, therapeutic interventions. In this review, we focus on acute mechanical effects on cardiac electrophysiology, explore molecular candidates underlying observed responses, and discuss their pharmaceutical regulation. From this, we identify open research questions and highlight emerging technologies that may help in addressing them.

Two-Pore K+ Channel TREK-1 Regulates Sinoatrial Node Membrane Excitability

Background

Two‐pore K⁺ channels have emerged as potential targets to selectively regulate cardiac cell membrane excitability; however, lack of specific inhibitors and relevant animal models has impeded the effort to understand the role of 2‐pore K⁺ channels in the heart and their potential as a therapeutic target. The objective of this study was to determine the role of mechanosensitive 2‐pore K⁺ channel family member TREK‐1 in control of cardiac excitability.

Methods and Results

Cardiac‐specific TREK‐1–deficient mice (αMHC‐Kcnk^f/f) were generated and found to have a prevalent sinoatrial phenotype characterized by bradycardia with frequent episodes of sinus pause following stress. Action potential measurements from isolated αMHC‐Kcnk2^f/f sinoatrial node cells demonstrated decreased background K⁺ current and abnormal sinoatrial cell membrane excitability. To identify novel pathways for regulating TREK‐1 activity and sinoatrial node excitability, mice expressing a truncated allele of the TREK‐1–associated cytoskeletal protein β_IV‐spectrin (qv^4J mice) were analyzed and found to display defects in cell electrophysiology as well as loss of normal TREK‐1 membrane localization. Finally, the β_IV‐spectrin/TREK‐1 complex was found to be downregulated in the right atrium from a canine model of sinoatrial node dysfunction and in human cardiac disease.

Conclusions

These findings identify a TREK‐1–dependent pathway essential for normal sinoatrial node cell excitability that serves as a potential target for selectively regulating sinoatrial node cell function.

Phospholipase A2 regulation of lipid droplet formation

The classical regard of lipid droplets as mere static energy-storage organelles has evolved dramatically. Nowadays these organelles are known to participate in key processes of cell homeostasis, and their abnormal regulation is linked to several disorders including metabolic diseases (diabetes, obesity, atherosclerosis or hepatic steatosis), inflammatory responses in leukocytes, cancer development and neurodegenerative diseases. Hence, the importance of unraveling the cell mechanisms controlling lipid droplet biosynthesis, homeostasis and degradation seems evident Phospholipase A2s, a family of enzymes whose common feature is to hydrolyze the fatty acid present at the sn-2 position of phospholipids, play pivotal roles in cell signaling and inflammation. These enzymes have recently emerged as key regulators of lipid droplet homeostasis, regulating their formation at different levels. This review summarizes recent results on the roles that various phospholipase A2 forms play in the regulation of lipid droplet biogenesis under different conditions. These roles expand the already wide range of functions that these enzymes play in cell physiology and pathophysiology.

Functional study of TREK-1 potassium channels during rat heart development and cardiac ischemia using RNAi techniques

To explore the physiological and pathological significance of the 2-pore domain potassium channel TWIK-related K(+) (TREK)-1 in rat heart, its expression and role during heart development and cardiac ischemia were investigated. In the former study, the ventricles of Sprague Dawley rats were collected from embryo day 19 to postnatal 18 months and examined for mRNA and protein expression of TREK-1. It was found that both increased during development, reached a maximum at postnatal day 28, and remained higher at postnatal day 3 through to postnatal 18 months. In the latter study, protein expression of TREK-1 was examined after initiation of acute heart ischemia by ligation of the left anterior descending coronary artery. TREK-1 expression was found to be increased in the endocardium but unchanged in the epicardium. In primary cultured rat neonatal ventricular myocytes subjected to hypoxia (oxygen-glucose deprivation), TREK-1 expression was increased. In cultured neonatal cardiomyocytes, silencing of the TREK-1 gene by lentivirus delivery of the short-hairpin RNAs, L-sh-492 and L-sh-605, was found to promote their viability and number. In addition, both short-hairpin RNA provided protection against hypoxia-induced injury to cardiomyocytes in vitro. These results suggest that TREK-1 plays an important role in neonatal rat heart development and downregulation of TREK-1 may provide protection against ischemic injury. It seems that TREK-1 is a potential drug target for treatment of acute heart ischemia.

Inhibition of a TREK-like K+ channel current by noradrenaline requires both β1- and β2-adrenoceptors in rat atrial myocytes

AIMS

Noradrenaline plays an important role in the modulation of atrial electrophysiology. However, the identity of the modulated channels, their mechanisms of modulation, and their role in the action potential remain unclear. This study aimed to investigate the noradrenergic modulation of an atrial steady-state outward current (IKss).

METHODS AND RESULTS

Rat atrial myocyte whole-cell currents were recorded at 36°C. Noradrenaline potently inhibited IKss (IC50 = 0.90 nM, 42.1 ± 4.3% at 1 µM, n = 7) and potentiated the L-type Ca(2+) current (ICaL, EC50 = 136 nM, 205 ± 40% at 1 µM, n = 6). Noradrenaline-sensitive IKss was weakly voltage-dependent, time-independent, and potentiated by the arachidonic acid analogue, 5,8,11,14-eicosatetraynoic acid (EYTA; 10 µM), or by osmotically induced membrane stretch. Noise analysis revealed a unitary conductance of 8.4 ± 0.42 pS (n = 8). The biophysical/pharmacological properties of IKss indicate a TREK-like K(+) channel. The effect of noradrenaline on IKss was abolished by combined β1-/β2-adrenoceptor antagonism (1 µM propranolol or 10 µM β1-selective atenolol and 100 nM β2-selective ICI-118,551 in combination), but not by β1- or β2-antagonist alone. The action of noradrenaline could be mimicked by β2-agonists (zinterol and fenoterol) in the presence of β1-antagonist. The action of noradrenaline on IKss, but not on ICaL, was abolished by pertussis toxin (PTX) treatment. The action of noradrenaline on ICaL was mediated by β1-adrenoceptors via a PTX-insensitive pathway. Noradrenaline prolonged APD30 by 52 ± 19% (n = 5; P < 0.05), and this effect was abolished by combined β1-/β2-antagonism, but not by atenolol alone.

CONCLUSION

Noradrenaline inhibits a rat atrial TREK-like K(+) channel current via a PTX-sensitive mechanism involving co-operativity of β1-/β2-adrenoceptors that contributes to atrial APD prolongation.

Modulation of K2P 2.1 and K2P 10.1 K(+) channel sensitivity to carvedilol by alternative mRNA translation initiation

BACKGROUND AND PURPOSE

The β-receptor antagonist carvedilol blocks a range of ion channels. K2P 2.1 (TREK1) and K2P 10.1 (TREK2) channels are expressed in the heart and regulated by alternative translation initiation (ATI) of their mRNA, producing functionally distinct channel variants. The first objective was to investigate acute effects of carvedilol on human K2P 2.1 and K2P 10.1 channels. Second, we sought to study ATI-dependent modulation of K2P K(+) current sensitivity to carvedilol.

EXPERIMENTAL APPROACH

Using standard electrophysiological techniques, we recorded currents from wild-type and mutant K2P 2.1 and K2P 10.1 channels in Xenopus oocytes and HEK 293 cells.

KEY RESULTS

Carvedilol concentration-dependently inhibited K2P 2.1 channels (IC50 ,oocytes = 20.3 μM; IC50 , HEK = 1.6 μM) and this inhibition was frequency-independent. When K2P 2.1 isoforms generated by ATI were studied separately in oocytes, the IC50 value for carvedilol inhibition of full-length channels (16.5 μM) was almost 5-fold less than that for the truncated channel variant (IC50 = 79.0 μM). Similarly, the related K2P 10.1 channels were blocked by carvedilol (IC50 ,oocytes = 24.0 μM; IC50 , HEK = 7.6 μM) and subject to ATI-dependent modulation of drug sensitivity.

CONCLUSIONS AND IMPLICATIONS

Carvedilol targets K2P 2.1 and K2P 10.1 K(+) channels. This previously unrecognized mechanism supports a general role of cardiac K2P channels as antiarrhythmic drug targets. Furthermore, the work reveals that the sensitivity of the cardiac ion channels K2P 2.1 and K2P 10.1 to block was modulated by alternative mRNA translation initiation.

mTOR pathway is involved in ADP-evoked astrocyte activation and ATP release in the spinal dorsal horn in a rat neuropathic pain model

BACKGROUND

ATP/ADP-evoked spinal astrocyte activation plays a vital role in the development of neuropathic pain. We aim to investigate the role of mammalian target of rapamycin (mTOR) pathway on the spinal astrocyte activation in the neuropathic pain development in rats.

METHODS

Sprague Dawley (SD) rats were subjected to chronic constriction of the sciatic nerve (CCI). Rapamycin or ADP was intrathecally injected daily to explore their effects on spinal astrocyte activation and pain development. Expression of glial fibrillary acidic protein (GFAP) and mTOR in the spinal dorsal horn was assessed by immunohistochemistry. Von Frey hairs and Hargreaves paw withdrawal test were conducted to evaluate mechanical allodynia and thermal sensitivity, respectively. Firefly luciferase ATP assay was used to assess the change of ATP level in cerebrospinal fluid (CSF) and medium of cultured astrocytes.

RESULTS

GFAP expression was enhanced in the ipsilateral spinal dorsal horn from day 3 after surgery. GFAP and mTOR expression in the rat spinal dorsal horn on post-surgical day 14 was enhanced by daily intrathecal injection of ADP, which was inhibited by rapamycin. Rapamycin decreased lower mechanical pain threshold and the thermal withdrawal latency. Intrathecal injection of ADP enhanced the ATP release, which was partially inhibited by rapamycin. Study of cultured astrocytes indicated that ATP could be released from astrocytes.

CONCLUSION

Our data demonstrated that ADP enhanced neuropathic pain in CCI rats, which was inhibited by rapamycin. This study indicates that targeting mTOR pathway could serve as a novel therapeutic strategy in neuropathic pain management.

Molecular candidates for cardiac stretch-activated ion channels

The heart is a mechanically-active organ that dynamically senses its own mechanical environment. This environment is constantly changing, on a beat-by-beat basis, with additional modulation by respiratory activity and changes in posture or physical activity, and further overlaid with more slowly occurring physiological (e.g. pregnancy, endurance training) or pathological challenges (e.g. pressure or volume overload). Far from being a simple pump, the heart detects changes in mechanical demand and adjusts its performance accordingly, both via heart rate and stroke volume alteration. Many of the underlying regulatory processes are encoded intracardially, and are thus maintained even in heart transplant recipients. Over the last three decades, molecular substrates of cardiac mechanosensitivity have gained increasing recognition in the scientific and clinical communities. Nonetheless, the processes underlying this phenomenon are still poorly understood. Stretch-activated ion channels (SAC) have been identified as one contributor to mechanosensitive autoregulation of the heartbeat. They also appear to play important roles in the development of cardiac pathologies – most notably stretch-induced arrhythmias. As recently discovered, some established cardiac drugs act, in part at least, via mechanotransduction pathways suggesting SAC as potential therapeutic targets. Clearly, identification of the molecular substrate of cardiac SAC is of clinical importance and a number of candidate proteins have been identified. At the same time, experimental studies have revealed variable–and at times contrasting–results regarding their function. Further complication arises from the fact that many ion channels that are not classically defined as SAC, including voltage and ligand-gated ion channels, can respond to mechanical stimulation. Here, we summarise what is known about the molecular substrate of the main candidates for cardiac SAC, before identifying potential further developments in this area of translational research.

Enhancement of TWIK-related acid-sensitive potassium channel 3 (TASK3) two-pore domain potassium channel activity by tumor necrosis factor α

TASK3 two-pore domain potassium (K2P) channels are responsible for native leak K channels in many cell types which regulate cell resting membrane potential and excitability. In addition, TASK3 channels contribute to the regulation of cellular potassium homeostasis. Because TASK3 channels are important for cell viability, having putative roles in both neuronal apoptosis and oncogenesis, we sought to determine their behavior under inflammatory conditions by investigating the effect of TNFα on TASK3 channel current. TASK3 channels were expressed in tsA-201 cells, and the current through them was measured using whole cell voltage clamp recordings. We show that THP-1 human myeloid leukemia monocytes, co-cultured with hTASK3-transfected tsA-201 cells, can be activated by the specific Toll-like receptor 7/8 activator, R848, to release TNFα that subsequently enhances hTASK3 current. Both hTASK3 and mTASK3 channel activity is increased by incubation with recombinant TNFα (10 ng/ml for 2-15 h), but other K2P channels (hTASK1, hTASK2, hTREK1, and hTRESK) are unaffected. This enhancement by TNFα is not due to alterations in levels of channel expression at the membrane but rather to an alteration in channel gating. The enhancement by TNFα can be blocked by extracellular acidification but persists for mutated TASK3 (H98A) channels that are no longer acid-sensitive even in an acidic extracellular environment. TNFα action on TASK3 channels is mediated through the intracellular C terminus of the channel. Furthermore, it occurs through the ASK1 pathway and is JNK- and p38-dependent. In combination, TNFα activation and TASK3 channel activity can promote cellular apoptosis.

An increased TREK-1-like potassium current in ventricular myocytes during rat cardiac hypertrophy

To elucidate the expression and identify the functional changes of 2 pore domain potassium channel TREK-1 during cardiac hypertrophy in rats, left ventricular hypertrophy was induced by subcutaneous injection with isoproterenol. Western blot was used to detect the expression of TREK-1 channel protein, and inside-out and whole-cell recordings were used to record TREK-1 currents. The results showed that TREK-1 protein expression in endocardium was slightly higher than that in epicardium in control left ventricles. However, it was obviously upregulated by 89.8% during hypertrophy, 2.3-fold higher than in epicardium. Mechanical stretch, intracellular acidification, and arachidonic acid could activate a TREK-1-like current in cardiomyocytes. The slope conductances of cardiac TREK-1 and CHO/TREK-1 channels were 123 ± 7 and 113 ± 17 pS, respectively. The TREK-1 inhibitor L-3-n-butylphthalide (10 μM) reduced the currents in CHO/TREK-1 cells, normal cardiomyocytes, and hypertrophic cardiomyocytes by 48.5%, 54.3%, and 55.5%, respectively. The percentage of L-3-n-butylphthalide-inhibited outward whole-cell current in hypertrophic cardiomyocytes (23.7%) was larger than that in normal cardiomyocytes (14.2%). The percentage of chloroform-activated outward whole-cell current in hypertrophic cardiomyocytes (58.3%) was also larger than normal control (40.2%). Our results demonstrated that in hypertrophic rats, TREK-1 protein expression in endocardium was specifically increased and the ratio of TREK-1 channel current in cardiac outward currents was also enhanced. TREK-1 might balance potassium ion flow during hypertrophy and might be a potential drug target for heart protection.

Extracellular ATP inhibits Schwann cell dedifferentiation and proliferation in an ex vivo model of Wallerian degeneration

After nerve injury, Schwann cells proliferate and revert to a phenotype that supports nerve regeneration. This phenotype-changing process can be viewed as Schwann cell dedifferentiation. Here, we investigated the role of extracellular ATP in Schwann cell dedifferentiation and proliferation during Wallerian degeneration. Using several markers of Schwann cell dedifferentiation and proliferation in sciatic explants, we found that extracellular ATP inhibits Schwann cell dedifferentiation and proliferation during Wallerian degeneration. Furthermore, the blockage of lysosomal exocytosis in ATP-treated sciatic explants is sufficient to induce Schwann cell dedifferentiation. Together, these findings suggest that ATP-induced lysosomal exocytosis may be involved in Schwann cell dedifferentiation.

Simultaneous activation of p38 and JNK by arachidonic acid stimulates the cytosolic phospholipase A2-dependent synthesis of lipid droplets in human monocytes

Exposure of human peripheral blood monocytes to free arachidonic acid (AA) results in the rapid induction of lipid droplet (LD) formation by these cells. This effect appears specific for AA in that it is not mimicked by other fatty acids, whether saturated or unsaturated. LDs are formed by two different routes: (i) the direct entry of AA into triacylglycerol and (ii) activation of intracellular signaling, leading to increased triacylglycerol and cholesteryl ester formation utilizing fatty acids coming from the de novo biosynthetic route. Both routes can be dissociated by the arachidonyl-CoA synthetase inhibitor triacsin C, which prevents the former but not the latter. LD formation by AA-induced signaling predominates, accounting for 60-70% of total LD formation, and can be completely inhibited by selective inhibition of the group IVA cytosolic phospholipase A(2)α (cPLA(2)α), pointing out this enzyme as a key regulator of AA-induced signaling. LD formation in AA-treated monocytes can also be blocked by the combined inhibition of the mitogen-activated protein kinase family members p38 and JNK, which correlates with inhibition of cPLA(2)α activation by phosphorylation. Collectively, these results suggest that concomitant activation of p38 and JNK by AA cooperate to activate cPLA(2)α, which is in turn required for LD formation possibly by facilitating biogenesis of this organelle, not by regulating neutral lipid synthesis.

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.

Two-pore potassium ion channels are inhibited by both G(q/11)- and G(i)-coupled P2Y receptors

Two-pore potassium (K(2P)) ion channels and P2Y receptors modulate the activity of neurones and are targets for the treatment of neuronal disorders. Here we have characterised their interaction. In cells coexpressing the Galpha(i)-coupled hP2Y(12) receptor, ADP and ATP significantly inhibited hK(2P)2.1 currents. This was abolished by pertussis toxin (PTX), the hP2Y(12) antagonist AR-C69931MX, the hP2Y(1) antagonist MRS2179 and by mutating potential PKA/PKC phosphorylation sites in the channel C terminal. In cells coexpressing the Galpha(q/11)-coupled hP2Y(1) receptor, ADP and ATP also inhibited hK(2P)2.1 currents, which were abolished by MRS2179, but unaffected by AR-C69931MX and PTX. When both receptors were coexpressed with K(2P)2.1 channels, ADP-induced inhibition was antagonised by AR-C69913MX and MRS2179, but not PTX. Thus, both Galpha(q/11)- and Galpha(i)-coupled P2Y receptors inhibit K(2P) channels and the action of hP2Y(12) receptors appears to involve co-activation of endogenous hP2Y(1) receptors. This represents a novel mechanism by which P2Y receptors may modulate neuronal activity.

Molecular mechanisms underlying membrane-potential-mediated regulation of neuronal K2P2.1 channels

The activity of background K(2P) channels adjusts the resting membrane potential to enable plasticity of excitable cells. Here we have studied the regulation of neuronal K(2P)2.1 (KCNK2, TREK-1) channel activity by resting membrane potential. When heterologously expressed, K(2P)2.1 currents gradually increased at hyperpolarizing potentials and declined at depolarizing potentials, with a midpoint potential of -60 mV. As K(2P) channels are not equipped with an integral voltage sensor, we sought extrinsic cellular components that could convert changes in the membrane electrical field to cellular activity that would indirectly modify K(2P)2.1 currents. We propose that membrane depolarization activated the Gq protein-coupled receptor pathway, in the apparent absence of ligand, resulting in phosphatidylinositol-4,5-bisphosphate (PIP(2)) depletion through the action of phospholipase C. Our results suggest a novel mechanism in which an indirect pathway confers membrane potential regulation onto channels that are not intrinsically voltage sensitive to enhance regulation of neuronal excitability levels.

Pain-associated signals, acidosis and lysophosphatidic acid, modulate the neuronal K(2P)2.1 channel

Pain is a physiological state promoting protective responses to harmful episodes. However, pain can become pathophysiological and become a chronic disruptive condition, damaging quality of life. The mammalian K(2P)2.1 (KCNK2, TREK-1) channel, expressed in sensory neurons of the dorsal root ganglia, was previously identified as a polymodal molecular sensor involved in pain perception. Here, we report that two pain-associated signals, external acidosis and lysophosphatidic acid (LPA), known to rise during injury, inflammation and cancer, profoundly down-modulate human K(2P)2.1 activity. The pH regulatory effect was mediated by activation of proton-sensitive G-protein coupled receptors and phospholipase C. Physiological concentrations of LPA overcame the effects of known K(2P)2.1 activators, such as arachidonic acid, lysophosphatidylcholine and temperature, by activating cell-surface receptors stimulating the G(q) pathway. Furthermore, we identified three K(2P)2.1 carboxy-terminal residues that mediate both pH and LPA regulatory effects. Our results highlight the important role of K(2P)2.1 channels as receptors for mediators known to cause nociception.

Basal and IGF-I-dependent regulation of potassium channels by MAP kinases and PI3-kinase during eccentric cardiac hypertrophy

The potassium channels I(K) and I(K1), responsible for the action potential repolarization and resting potential respectively, are altered during cardiac hypertrophy. The activation of insulin-like growth factor-I (IGF-I) during hypertrophy may affect channel activity. The aim was to examine the modulatory effects of IGF-I on I(K) and I(K1) through mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K) pathways during hypertrophy. With the use of specific inhibitors for ERK1/2 (PD98059), p38 MAPK (SB203580) and PI3K/Akt (LY294002), Western blot and whole cell patch-clamp were conducted on sham and aorto-caval shunt-induced hypertrophy adult rat myocytes. Basal activation levels of MAPKs and Akt were increased during hypertrophy. Acute IGF-I (10(-8) M) enhanced basal activation levels of these kinases in normal hearts but only those of Akt in hypertrophied ones. I(K) and I(K1) activities were lowered by IGF-I. Inhibition of ERK1/2, p38 MAPK, or Akt reduced basal I(K) activity by 70, 32, or 50%, respectively, in normal cardiomyocytes vs. 53, 34, or 52% in hypertrophied ones. However, basal activity of I(K1) was reduced by 45, 48, or 45% in the former vs. 63, 43, or 24% in the latter. The inhibition of either MAPKs or Akt alleviated IGF-I effects on I(K) and I(K1). We conclude that basal I(K) and I(K1) are positively maintained by steady-state Akt and ERK activities. K+ channels seem to be regulated in a dichotomic manner by acutely stimulated MAPKs and Akt. Eccentric cardiac hypertrophy may be associated with a change in the regulation of the steady-state basal activities of K+ channels towards MAPKs, while that of the acute IGF-I-stimulated ones toward Akt.

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.

TNFR1 and TNFR2 Signaling Interplay in Cardiac Myocytes

Tumor necrosis factor alpha (TNFalpha) plays a major role in chronic heart failure, signaling through two different receptor subtypes, TNFR1 and TNFR2. Our aim was to further delineate the functional role and signaling pathways related to TNFR1 and TNFR2 in cardiac myocytes. In cardiac myocytes isolated from control rats, TNFalpha induced ROS production, exerted a dual positive and negative action on [Ca(2+)] transient and cell fractional shortening, and altered cell survival. Neutralizing anti-TNFR2 antibodies exacerbated TNFalpha responses on ROS production and cell death, arguing for a major protective role of the TNFR2 pathway. Treatment with either neutralizing anti-TNFR1 antibodies or the glutathione precursor, N-acetylcysteine (NAC), favored the emergence of TNFR2 signaling that mediated a positive effect of TNFalpha on [Ca(2+)] transient and cell fractional shortening. The positive effect of TNFalpha relied on TNFR2-dependent activation of the cPLA(2) activity, independently of serine 505 phosphorylation of the enzyme. Together with cPLA(2) redistribution and AA release, TNFalpha induced a time-dependent phosphorylation of ERK, MSK1, PKCzeta, CaMKII, and phospholamban on the threonine 17 residue. Taken together, our results characterized a TNFR2-dependent signaling and illustrated the close interplay between TNFR1 and TNFR2 pathways in cardiac myocytes. Although apparently predominant, TNFR1-dependent responses were under the yoke of TNFR2, acting as a critical limiting factor. In vivo NAC treatment proved to be a unique tool to selectively neutralize TNFR1-mediated effects of TNFalpha while releasing TNFR2 pathways.

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.

COX-2-dependent and potentially cardioprotective effects of negative inotropic substances released after ischemia

During reperfusion, cardiodepressive factors are released from isolated rat hearts after ischemia. The present study analyzes the mechanisms by which these substances mediate their cardiodepressive effect. After 10 min of global stop-flow ischemia, rat hearts were reperfused and coronary effluent was collected over a period of 30 s. We tested the effect of this postischemic effluent on systolic cell shortening and Ca(2+) metabolism by application of fluorescence microscopy of field-stimulated rat cardiomyocytes stained with fura-2 AM. Cells were preincubated with various inhibitors, e.g., the cyclooxygenase (COX) inhibitor indomethacin, the COX-2 inhibitors NS-398 and lumiracoxib, the COX-1 inhibitor SC-560, and the potassium (ATP) channel blocker glibenclamide. Lysates of cardiomyocytes and extracts from whole rat hearts were tested for expression of COX-2 with Western blot analysis. As a result, in contrast to nonischemic effluent (control), postischemic effluent induced a reduction of Ca(2+) transient and systolic cell shortening in the rat cardiomyocytes (P < 0.001 vs. control). After preincubation of cells with indomethacin, NS-398, and lumiracoxib, the negative inotropic effect was attenuated. SC-560 did not influence the effect of postischemic effluent. The inducibly expressed COX-2 was detected in cardiomyocytes prepared for fluorescence microscopy. The effect of postischemic effluent was eliminated with applications of glibenclamide. Furthermore, postischemic effluent significantly reduced the intracellular diastolic and systolic Ca(2+) increase (P < 0.01 vs. control). In conclusion, the cardiodepressive effect of postischemic effluent is COX-2 dependent and protective against Ca(2+) overload in the cells.

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.

Apoptosis is mediated by cytosolic phospholipase A2 during simulated ischaemia/reperfusion-induced injury in neonatal cardiac myocytes

It has become increasingly clear that apoptosis plays a major role in ischaemia/reperfusion (I/R)-induced cell death, but the molecular basis of this process remains to be elucidated. Therefore, the aim of this study was to investigate the role of cPLA(2) in MAPK phosphorylation and apoptosis in simulated ischaemia/reperfusion (SI/R)-induced injury in neonatal cardiomyocytes. Inhibition of cPLA(2) with AACOCF(3) significantly improved cell viability during SI/R (60.17+/-1.77 to 80.17+/-1.97%, p<0.05). The increase in cell viability was associated with a significant inhibition of p38 phosphorylation (135.3+/-4.47% to 87.94+/-10.71%, p<0.001) as well as with a significant decrease in caspase-3- (320.32+/-17.32% to 146.7+/-28.69%, p<0.01) and PARP-(263.9+/-8.15% to 154.7+/-2.24%, p<0.001) cleavage during SI/R. This study provides evidence for a role for cPLA(2) during SI/R-induced injury. It appears that p38 MAPK is a central role player in the signalling pathway involved.

Inhibition of MAPK stimulates the Ca2+ -dependent big-conductance K channels in cortical collecting duct

The kidney plays a key role in maintaining potassium (K) homeostasis. K excretion is determined by the balance between K secretion and absorption in distal tubule segments such as the connecting tubule and cortical collecting duct. K secretion takes place by K entering principal cells (PC) from blood side through Na+, K+ -ATPase and being secreted into the lumen via both ROMK-like small-conductance K (SK) channels and Ca2+ -activated big-conductance K (BK) channels. K reabsorption occurs by stimulation of apical K/H-ATPase and inhibition of K recycling across the apical membrane in intercalated cells (IC). The role of ROMK channels in K secretion is well documented. However, the importance of BK channels in mediating K secretion is incompletely understood. It has been shown that their activity increases with high tubule flow rate and augmented K intake. However, BK channels have a low open probability and are mainly located in IC, which lack appropriate transporters for effective K secretion. Here we demonstrate that inhibition of ERK and P38 MAPKs stimulates BK channels in both PC and IC in the cortical collecting duct and that changes in K intake modulate their activity. Under control conditions, BK channel activity in PC was low but increased significantly by inhibition of both ERK and P38. Blocking MAPKs also increased channel open probability of BK in IC and thereby it may affect K backflux and net K absorption Thus, modulation of ERK and P38 MAPK activity is involved in controlling net K secretion in the distal nephron.

Neuronal two-pore-domain potassium channels and their regulation by G protein-coupled receptors

Leak potassium currents in the nervous system are often carried through two-pore-domain potassium (K2P) channels. These channels are regulated by a number of different G protein-coupled receptor (GPCR) pathways. The TASK subfamily of K2P channels are inhibited following activation of the G protein Galpha(q). The mechanism(s) that transduce this inhibition have yet to be established but there is evidence to support a role of phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis products, depletion of PIP2 itself from the membrane, or a direct action of activated Galpha(q) on TASK channels. It seems possible that more than one pathway may act in parallel to transduce inhibition. By contrast, TRESK channels are stimulated following activation of Galpha(q). This is due to stimulation of the protein phosphatase, calcineurin, which dephosphorylates TRESK channels and enhances their activity. TREK channels are the most widely regulated of the K2P channel subfamilies being inhibited following activation of Galpha(q) and Galpha(s) but enhanced following activation of Galpha(i). The multiple pathways activated and the apparent promiscuous coupling of at least some K2P channel types to different G protein regulatory pathways suggests that the excitability of neurons that express K2P channels will be profoundly sensitive to variations in GPCR activity.

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.

Mitogen-activated protein kinases inhibit the ROMK (Kir 1.1)-like small conductance K channels in the cortical collecting duct

It was demonstrated previously that low dietary potassium (K) intake stimulates Src family protein tyrosine kinase (PTK) expression via a superoxide-dependent signaling. This study explored the role of mitogen-activated protein kinase (MAPK) in mediating the effect of superoxide anions on PTK expression and ROMK (Kir 1.1) channel activity. Western blot analysis demonstrated that low K intake significantly increased the phosphorylation of P38 MAPK (P38) and extracellular signal-regulated kinase (ERK) but had no effect on phosphorylation of c-JUN N-terminus kinase in renal cortex and outer medulla. The stimulatory effect of low K intake on P38 and ERK was abolished by treatment of rats with tempol. The possibility that increases in superoxide and related products that are induced by low K intake were responsible for stimulating phosphorylation of P38 and ERK also was supported by the finding that application of H(2)O(2) increased the phosphorylation of ERK and P38 in the cultured mouse collecting duct cells. Simultaneous blocking of ERK and P38 completely abolished the effect of H(2)O(2) on c-Src expression in mouse collecting duct cells. For determination of the role of P38 and ERK in the regulation of ROMK-like small-conductance K (SK) channels, the patch-clamp technique was used to study the effect of inhibiting P38 and ERK on SK channels in the cortical collecting duct from rats that were on a control K diet (1.1%) and on a K-deficient diet for 1 d. Inhibition of ERK, c-JUN N-terminus kinase, or P38 alone had no effect on SK channels. In contrast, simultaneous inhibition of P38 and ERK significantly increased channel activity. The effect of inhibiting MAPK on SK channels was not affected in the presence of herbimycin A, a PTK inhibitor, and was larger in rats that were on a K-deficient diet than in rats that were on a normal-K diet. However, the stimulatory effect of inhibiting ERK and P38 on SK was absent in the cortical collecting duct that was treated with colchicine. It is concluded that low K intake-induced increases in superoxide levels are responsible for stimulation of P38 and ERK and that MAPK inhibit the SK channels by stimulating PTK expression and via a PTK-independent mechanism.

GENE EXPRESSION OF STRETCH-ACTIVATED CHANNELS AND MECHANOELECTRIC FEEDBACK IN THE HEART

1. Mechanoelectric feedback (MEF) in the heart is the process by which mechanical forces on the myocardium can change its electrical properties. Mechanoelectric feedback has been demonstrated in many animal models, ranging from isolated cells, through isolated hearts to whole animals. In humans, MEF has been demonstrated directly in both the atria and the ventricles. It seems likely that MEF provides either the trigger or the substrate for some types of clinically important arrhythmias. 2. Mechanoelectric feedback may arise because of the presence of stretch-sensitive (or mechano-sensitive) ion channels in the cell membrane of the cardiac myocytes. Two types have been demonstrated: (i) a non-specific cation channel (stretch-activated channel (SAC); conductance of approximately 25 pS); and (ii) a potassium channel with a conductance of approximately 100 pS. The gene coding for the SAC has not yet been identified. The gene for the potassium channel is likely to be TREK, a member of the tandem pore potassium channel gene family. We have recorded stretch-sensitive potassium channels in rat isolated myocytes that have the properties of TREK channels expressed in heterologous systems. 3. It has been shown that TREK mRNA is expressed heterogeneously in the rat ventricular wall, with 17-fold more expression in endocardial compared with epicardial cells. This difference is reflected in the TREK currents recorded from endocardial and epicardial cells using whole-cell patch-clamp techniques, although the difference in current density was less pronounced (approximately threefold). Consistent with this, we show here that when the ventricle is stretched by inflation of an intraventricular balloon in a Langendorff perfused rat isolated heart, action potential shortening was more pronounced in the endocardium (30% shortening at 40 mmHg) compared with that in the epicardium (10% shortening at the same pressure). 4. Computer models of the mechanics of the (pig) heart show pronounced spatial variations in strain in the myocardium with large transmural differences (in the left ventricle in particular) and also large differences between the base and apex of the ventricle. 5. The importance of MEF and the non-homogeneous gene expression and strain distribution for arrhythmias is discussed.

Sphingosine-1-phosphate via activation of a G-protein-coupled receptor(s) enhances the excitability of rat sensory neurons

Sphingosine-1-phosphate (S1P) is released by immune cells and is thought to play a key role in chemotaxis and the onset of the inflammatory response. The question remains whether this lipid mediator also contributes to the enhanced sensitivity of nociceptive neurons that is associated with inflammation. Therefore we examined whether S1P alters the excitability of small diameter, capsaicin-sensitive sensory neurons by measuring action potential (AP) firing and two of the membrane currents critical in regulating the properties of the AP. External application of S1P augments the number of APs evoked by a depolarizing current ramp. The enhanced firing is associated with a decrease in the rheobase and an increase in the resistance at firing threshold although neither the firing threshold nor the resting membrane potential are changed. Treatment with S1P enhanced the tetrodotoxin-resistant sodium current and decreased the total outward potassium current (IK). When sensory neurons were internally perfused with GDP-beta-S, a blocker of G protein activation, the S1P-induced increase in APs was completely blocked and suggests the excitatory actions of S1P are mediated through G-protein-coupled receptors called endothelial differentiation gene or S1PR. In contrast, internal perfusion with GDP-beta-S and S1P increased the number of APs evoked by the current ramp. These results and our finding that the mRNAs for S1PRs are expressed in both the intact dorsal root ganglion and cultures of adult sensory neurons supports the notion that S1P acts on S1PRs linked to G proteins. Together these findings demonstrate that S1P can regulate the excitability of small diameter sensory neurons by acting as an external paracrine-type ligand through activation of G-protein-coupled receptors and thus may contribute to the hypersensitivity during inflammation.

ATP stimulates Na+-glucose cotransporter activity via cAMP and p38 MAPK in renal proximal tubule cells

Extracellular ATP plays an important role in the regulation of renal function. However, the effect of ATP on the Na(+)-glucose cotransporters (SGLTs) has not been elucidated in proximal tubule cells (PTCs). Therefore, this study was performed to examine the action of ATP on SGLTs and their related signal pathways in primary cultured rabbit renal PTCs. ATP increased [(14)C]-alpha-methyl-d-glucopyranoside (alpha-MG) uptake in a time-dependent (>1 h) and dose-dependent (>10(-6) M) manner. ATP stimulated alpha-MG uptake by increasing in V(max) without affecting K(m). ATP-induced increase of alpha-MG uptake was correlated with the increase in both SGLT1 and SGLT2 protein expression levels. ATP-induced stimulation of alpha-MG uptake was blocked by suramin (nonspecific P2 receptor antagonist), RB-2 (P2Y receptor antagonist), and MRS-2179 (P2Y(1) receptor antagonist), suggesting a role for the P2Y receptor. ATP-induced stimulation of alpha-MG uptake was blocked by pertussis toxin (PTX, a G(i) protein inhibitor), SQ-22536 (an adenylate cyclase inhibitor), and PKA inhibitor amide 14-22 (PKI). ATP also increased cAMP formation, which was blocked by PTX and RB-2. However, pretreatment of adenosine deaminase did not block ATP-induced cAMP formation. In addition, ATP-induced stimulation of alpha-MG uptake was blocked by SB-203580 (p38 MAPK inhibitor), but not by PD-98059 (p44/42 MAPK inhibitor) or SP-600125 (JNK inhibitor). Indeed, ATP induced phosphorylation of p38 MAPK. In conclusion, ATP increases alpha-MG uptake via cAMP and p38 MAPK in renal PTCs.