Oxidative stress impairs IKCa- and SKCa-mediated vasodilatation in mesenteric arteries from diabetic rats

OBJECTIVE

To investigate the role of oxidative stress in impaired intermediate-conductance Ca(2+)-activated potassium channels (IKCa)- and small-conductance Ca(2+)-activated potassium channels (SKCa)-mediated relaxation in diabetic resistance arteries.

METHODS

Rat diabetic model was induced by a high fat and high glucose diet and streptozotocin (STZ) injection. Endothelial function of mesenteric arteries was assessed with the use of wire myography. The expression levels of IKCa and SKCa in cultured human umbilical vein endothelial cells (HUVECs) treated with H2O2 and/or antioxidant alpha lipoic acid (ALA) were measured using Western blotting.

RESULTS

IKCa- and SKCa-mediated vasodilatation in response to acetylcholine was impaired in the third-order mesenteric arterioles of diabetic rats. In cultured HUVECs, H2O2 significantly decreased the protein expression of IKCa and SKCa. ALA alleviated the impairment of both vasodilatation function of the mesenteric arterioles ex vivo and enhanced the expression of IKCa and SKCa challenged with H2O2 in cultured HUVECs.

CONCLUSION

Our data demonstrated for the first time that impaired IKCa- and SKCa-mediated vasodilatation in diabetes was induced, at least in part, by oxidative stress via down-regulation of IKCa and SKCa channels.

Mammalian target of rapamycin-independent S6K1 and 4E-BP1 phosphorylation during contraction in rat skeletal muscle

Muscle protein synthesis rates decrease during contraction/exercise, but rapidly increase post-exercise. Previous studies mainly focused on signaling pathways that control protein synthesis during post-exercise recovery, such as mTOR and its downstream targets S6K1 and 4E-BP1. In this study, we investigated the effect of high-frequency electrical stimulation on the phosphorylation state of signaling components controlling protein synthesis in rat skeletal muscle. Electrical stimulation increased S6K1 Thr389 phosphorylation, which was unaffected by Torin1, a selective mTOR inhibitor, suggesting that S6K1 phosphorylation by contraction was mTOR-independent. Phosphorylation of eIF4B Ser422 was also increased during electrical stimulation, which was abrogated by inhibition of MEK/ERK/RSK1 activation. Moreover, although phosphorylation of conventional mTOR sites in 4E-BP1 decreased during contraction, mTOR-independent phosphorylation was also apparent, which was associated with the release of 4E-BP1 from eIF4E. The results indicate mTOR-independent phosphorylation of S6K1 and 4E-BP1 and suggest MEK/ERK/RSK1-dependent phosphorylation of eIF4B during skeletal muscle contraction. These phosphorylation events would keep the translation initiation machinery "primed" in an active state so that protein synthesis could quickly resume post-exercise.

Two distinct channels mediated by m2mAChR and α9nAChR co-exist in type II vestibular hair cells of guinea pig

Acetylcholine (ACh) is the principal vestibular efferent neurotransmitter among mammalians. Pharmacologic studies prove that ACh activates a small conductance Ca2+-activated K+ channels (KCa) current (SK2), mediated by α9-containing nicotinic ACh receptor (α9nAChR) in mammalian type II vestibular hair cells (VHCs II). However, our studies demonstrate that the m2 muscarinic ACh receptor (m2mAChR) mediates a big conductance KCa current (BK) in VHCs II. To better elucidate the correlation between these two distinct channels in VHCs II of guinea pig, this study was designed to verify whether these two channels and their corresponding AChR subtypes co-exist in the same VHCs II by whole-cell patch clamp recordings. We found that m2mAChR sensitive BK currents were activated in VHCs II isolated by collagenase IA, while α9nAChR sensitive SK2 currents were activated in VHCs II isolated by trypsin. Interestingly, after exposing the patched cells isolated by trypsin to collagenase IA for 3 min, the α9nAChR sensitive SK2 current was abolished, while m2mAChR-sensitive BK current was activated. Therefore, our findings provide evidence that the two distinct channels and their corresponding AChR subtypes may co-exist in the same VHCs II, and the alternative presence of these two ACh receptors-sensitive currents depended on isolating preparation with different enzymes.

Mitochondrial small conductance SK2 channels prevent glutamate-induced oxytosis and mitochondrial dysfunction

Small conductance calcium-activated potassium (SK2/K(Ca)2.2) channels are known to be located in the neuronal plasma membrane where they provide feedback control of NMDA receptor activity. Here, we provide evidence that SK2 channels are also located in the inner mitochondrial membrane of neuronal mitochondria. Patch clamp recordings in isolated mitoplasts suggest insertion into the inner mitochondrial membrane with the C and N termini facing the intermembrane space. Activation of SK channels increased mitochondrial K(+) currents, whereas channel inhibition attenuated these currents. In a model of glutamate toxicity, activation of SK2 channels attenuated the loss of the mitochondrial transmembrane potential, blocked mitochondrial fission, prevented the release of proapoptotic mitochondrial proteins, and reduced cell death. Neuroprotection was blocked by specific SK2 inhibitory peptides and siRNA targeting SK2 channels. Activation of mitochondrial SK2 channels may therefore represent promising targets for neuroprotective strategies in conditions of mitochondrial dysfunction.

Small-conductance Ca2+-activated K+ channels modulate action potential-induced Ca2+ transients in hippocampal neurons

In hippocampal pyramidal neurons, voltage-gated Ca(2+) channels open in response to action potentials. This results in elevations in the intracellular concentration of Ca(2+) that are maximal in the proximal apical dendrites and decrease rapidly with distance from the soma. The control of these action potential-evoked Ca(2+) elevations is critical for the regulation of hippocampal neuronal activity. As part of Ca(2+) signaling microdomains, small-conductance Ca(2+)-activated K(+) (SK) channels have been shown to modulate the amplitude and duration of intracellular Ca(2+) signals by feedback regulation of synaptically activated Ca(2+) sources in small distal dendrites and dendritic spines, thus affecting synaptic plasticity in the hippocampus. In this study, we investigated the effect of the activation of SK channels on Ca(2+) transients specifically induced by action potentials in the proximal processes of hippocampal pyramidal neurons. Our results, obtained by using selective SK channel blockers and enhancers, show that SK channels act in a feedback loop, in which their activation by Ca(2+) entering mainly through L-type voltage-gated Ca(2+) channels leads to a reduction in the subsequent dendritic influx of Ca(2+). This underscores a new role of SK channels in the proximal apical dendrite of hippocampal pyramidal neurons.

Functional upregulation of Ca(2+)-activated K(+) channels in the development of substantia nigra dopamine neurons

Many connections in the basal ganglia are made around birth when animals are exposed to a host of new affective, cognitive, and sensori-motor stimuli. It is thought that dopamine modulates cortico-striatal synapses that result in the strengthening of those connections that lead to desired outcomes. We propose that there must be a time before which stimuli cannot be processed into functional connections, otherwise it would imply an effective link between stimulus, response, and reward in uterus. Consistent with these ideas, we present evidence that early in development dopamine neurons are electrically immature and do not produce high-frequency firing in response to salient stimuli. We ask first, what makes dopamine neurons immature? and second, what are the implications of this immaturity for the basal ganglia? As an answer to the first question, we find that at birth the outward current is small (3nS-V), insensitive to Ca(2+), TEA, BK, and SK blockers. Rapidly after birth, the outward current increases to 15nS-V and becomes sensitive to Ca(2+), TEA, BK, and SK blockers. We make a detailed analysis of the kinetics of the components of the outward currents and produce a model for BK and SK channels that we use to reproduce the outward current, and to infer the geometrical arrangement of BK and Ca(2+) channels in clusters. In the first cluster, T-type Ca(2+) and BK channels are coupled within distances of ~20 nm (200 Å). The second cluster consists of L-type Ca(2+) and BK channels that are spread over distances of at least 60 nm. As for the second question, we propose that early in development, the mechanism of action selection is in a "locked-in" state that would prevent dopamine neurons from reinforcing cortico-striatal synapses that do not have a functional experiential-based value.

Glucose-sensitivity of the afterhyperpolarization potential: role of SK1 channel in insulin-secreting cells

The role of the small-conductance, calcium-activated SK potassium channel in regulating pancreatic β cell function remains controversial with conflicting pharmacological results. In this study, we used current clamp recordings to further characterize the function of SK channels in INS-1 cell line. We compared afterhyperpolarization potential (AHP) responses of SK1-downregulated cells with those of control INS-1 cells. They were tested with and without the presence of glucose. We found that cells in which SK1 channel subunit expression had been downregulated exhibited AHPs in the presence of 20mM glucose while control INS-1 cells had AHPs only in the absence of glucose. Our findings show that the glucose-dependence of the AHP in the rat INS-1 cell line depends only on SK1 channel subunit expression.

Ca2+-activated K+ channels of small and intermediate conductance control eNOS activation through NAD(P)H oxidase

Ca(2+)-activated K(+) channels (K(Ca)) and NO play a central role in the endothelium-dependent control of vasomotor tone. We evaluated the interaction of K(Ca) with NO production in isolated arterial mesenteric beds of the rat. In phenylephrine-contracted mesenteries, acetylcholine (ACh)-induced vasodilation was reduced by NO synthase (NOS) inhibition with N(ω)-nitro-L-arginine (L-NA), but in the presence of tetraethylammonium, L-NA did not further affect the response. In KCl-contracted mesenteries, the relaxation elicited by 100 nM ACh or 1 μM ionomycin was abolished by L-NA, tetraethylammonium, or simultaneous blockade of small-conductance K(Ca) (SK(Ca)) channels with apamin and intermediate-conductance K(Ca) (IK(Ca)) channels with triarylmethane-34 (TRAM-34). Apamin-TRAM-34 treatment also abolished 100 nM ACh-activated NO production, which was associated with an increase in superoxide formation. Endothelial cell Ca(2+) buffering with BAPTA elicited a similar increment in superoxide. Apamin-TRAM-34 treatment increased endothelial NOS phosphorylation at threonine 495 (P-eNOS(Thr495)). Blockade of NAD(P)H oxidase with apocynin or superoxide dismutation with PEG-SOD prevented the increment in superoxide and changes in P-eNOS(Thr495) observed during apamin and TRAM-34 application. Our results indicate that blockade of SK(Ca) and IK(Ca) activates NAD(P)H oxidase-dependent superoxide formation, which leads to inhibition of NO release through P-eNOS(Thr495). These findings disclose a novel mechanism involved in the control of NO production.

Postnatal expression of an apamin-sensitive k(ca) current in vestibular calyx terminals

Afferent innervation patterns in the vestibular periphery are complex, and vestibular afferents show a large variation in their regularity of firing. Calyx fibers terminate on type I vestibular hair cells and have firing characteristics distinct from the bouton fibers that innervate type II hair cells. Whole-cell patch clamp was used to investigate ionic currents that could influence firing patterns in calyx terminals. Underlying K(Ca) conductances have been described in vestibular ganglion cells, but their presence in afferent terminals has not been investigated previously. Apamin, a selective blocker of SK-type calcium-activated K(+) channels, was tested on calyx afferent terminals isolated from gerbil semicircular canals during postnatal days 1-50. Lowering extracellular calcium or application of apamin (20-500 nM) reduced slowly activating outward currents in voltage clamp. Apamin also reduced the action potential afterhyperpolarization (AHP) in whole-cell current clamp, but only after the first two postnatal weeks. K(+) channel expression increased during the first postnatal month, and SK channels were found to contribute to the AHP, which may in turn influence discharge regularity in calyx vestibular afferents.

Chlorzoxazone, an SK-type potassium channel activator used in humans, reduces excessive alcohol intake in rats

BACKGROUND

Alcoholism imposes a tremendous social and economic burden. There are relatively few pharmacological treatments for alcoholism, with only moderate efficacy, and there is considerable interest in identifying additional therapeutic options. Alcohol exposure alters SK-type potassium channel (SK) function in limbic brain regions. Thus, positive SK modulators such as chlorzoxazone (CZX), a US Food and Drug Administration-approved centrally acting myorelaxant, might enhance SK function and decrease neuronal activity, resulting in reduced alcohol intake.

METHODS

We examined whether CZX reduced alcohol consumption under two-bottle choice (20% alcohol and water) in rats with intermittent access to alcohol (IAA) or continuous access to alcohol (CAA). In addition, we used ex vivo electrophysiology to determine whether SK inhibition and activation can alter firing of nucleus accumbens (NAcb) core medium spiny neurons.

RESULTS

Chlorzoxazone significantly and dose-dependently decreased alcohol but not water intake in IAA rats, with no effects in CAA rats. Chlorzoxazone also reduced alcohol preference in IAA but not CAA rats and reduced the tendency for rapid initial alcohol consumption in IAA rats. Chlorzoxazone reduction of IAA drinking was not explained by locomotor effects. Finally, NAcb core neurons ex vivo showed enhanced firing, reduced SK regulation of firing, and greater CZX inhibition of firing in IAA versus CAA rats.

CONCLUSIONS

The potent CZX-induced reduction of excessive IAA alcohol intake, with no effect on the more moderate intake in CAA rats, might reflect the greater CZX reduction in IAA NAcb core firing observed ex vivo. Thus, CZX could represent a novel and immediately accessible pharmacotherapeutic intervention for human alcoholism.

An SK3 channel/nWASP/Abi-1 complex is involved in early neurogenesis

Background

The stabilization or regulated reorganization of the actin cytoskeleton is essential for cellular structure and function. Recently, we could show that the activation of the SK3-channel that represents the predominant SK-channel in neural stem cells, leads to a rapid local outgrowth of long filopodial processes. This observation indicates that the rearrangement of the actin based cytoskeleton via membrane bound SK3-channels might selectively be controlled in defined micro compartments of the cell.

Principal Findings

We found two important proteins for cytoskeletal rearrangement, the Abelson interacting protein 1, Abi-1 and the neural Wiskott Aldrich Syndrome Protein, nWASP, to be in complex with SK3- channels in neural stem cells (NSCs). Moreover, this interaction is also found in spines and postsynaptic compartments of developing primary hippocampal neurons and regulates neurite outgrowth during early phases of differentiation. Overexpression of the proteins or pharmacological activation of SK3 channels induces obvious structural changes in NSCs and hippocampal neurons. In both neuronal cell systems SK3 channels and nWASP act synergistic by strongly inducing filopodial outgrowth while Abi-1 behaves antagonistic to its interaction partners.

Conclusions

Our results give good evidence for a functional interplay of a trimeric complex that transforms incoming signals via SK3-channel activation into the local rearrangement of the cytoskeleton in early steps of neuronal differentiation involving nWASP and Abi-1 actin binding proteins.

[Expression and functional role of small conductance Ca(2+)-activated K(+) channels in human atrial myocytes]

OBJECTIVE

To investigate the expression and functional role of the small conductance Ca(2+)-activated K(+) channels in human atrial myocytes.

METHODS

We collected the right atrial appendage tissues from 8 patients with congenital heart defect with sinus rhythm undergoing open-heart surgery. Immunohistochemistry was performed to identify the expression of 3 isoforms of SK channel (SK1, SK2 and SK3). Using the classical two-step enzymatic isolation method, perforated patch clamp and conventional voltage-clamp techniques were performed to record the action potentials (APs) and the whole-cell Ca(2+)-activated K(+) current (I(K, Ca)) in the single atrial myocyte. We compared the changes in action potential duration (APD) before and after the application of a specific SK channels blocker apamin (100 nmol/L).

RESULTS

Human atrial myocytes showed positivity for all the SK1, SK2 and SK3 isoform channels. Patch-clamp recording confirmed the presence of I(K,Ca), and apamin significantly prolonged APD at 90% repolarization (APD(90)), but produced no obvious effect on APD(50).

CONCLUSION

The three isoforms of SK channels are all expressed in human atrial myocytes. SK channels play a prominent role in the late phase of repolarization in human atrial myocytes, which is distinct from their functional roles in neurons where they mediate the process of afterhyperpolarization following APs.

[Increased small conductance calcium-activated potassium channel (SK2 channel) current in atrial myocytes of patients with persistent atrial fibrillation]

OBJECTIVE

To compare the amplitude of the SK2 current (small conductance calcium-activated potassium channel) in human atrial myocytes with or without persistent atrial fibrillation (AF).

METHODS

Right atrial appendage was obtained from 15 patients with sinus rate (SR) and 7 patients with AF underwent surgical valve replacement. Single myocyte was isolated by enzymatic dissociation method and the SK2 channel current density was recorded using whole-cell patch clamp techniques to detect the changes. Immunofluorescence was used to observe SK2 channel protein distribution on right atrial appendage.

RESULTS

Using the whole cell patch-clamp recording techniques, an inward rectifier K(+) mix currents could be obtained from both SR (n = 15) and AF (n = 7) samples, I(K1) mix currents density in single myocyte of AF group was significantly increased than in SR group [(-16.42 ± 5.32) pA/pF vs (-6.59 ± 2.24) pA/pF, P < 0.01], which could be partially inhibited by apamin (100 nmol/L). The apamin-sensitive current was obtained by subtraction of the currents before and after treatment with apamin. SK2 current density was significantly increased in AF group than that of SR group [(-9.81 ± 2.54) pA/pF vs (-3.67 ± 0.37) pA/pF, P < 0.01]. SK2 channel protein was evidenced with immunofluorescence method in right atrial appendage from AF group and SR group.

CONCLUSION

SK2 channel protein and current were present in atrial myocytes. The SK2 current density was significantly increased in AF group than in SR group suggesting that the increase of SK2 current might contribute to the electrical remodeling in AF patients.

The SK3/K(Ca)2.3 potassium channel is a new cellular target for edelfosine

BACKGROUND AND PURPOSE

The 1-O-octadecyl-2-O-methyl-sn-glycero-3-phosphocholine (edelfosine) is an ether-linked phospholipid with promising anti-cancer properties but some side effects that preclude its full clinical therapeutic exploitation. We hypothesized that this lipid could interact with plasma membrane ion channels and modulate their function.

EXPERIMENTAL APPROACH

Using cell migration-proliferation assays, patch clamp, spectrofluorimetry and ¹²⁵I-Apamin binding experiments, we studied the effects of edelfosine on the migration of breast cancer MDA-MB-435s cells, mediated by the small conductance Ca²(+) -activated K(+) channel, SK3/K(Ca)2.3.

KEY RESULTS

Edelfosine (1 µM) caused plasma membrane depolarization by substantially inhibiting activity of SK3/K(Ca)2.3 channels, which we had previously demonstrated to play an important role in cancer cell migration. Edelfosine did not inhibit ¹²⁵I-Apamin binding to this SK(Ca) channel; rather, it reduced the calcium sensitivity of SK3/K(Ca)2.3 channel and dramatically decreased intracellular Ca²(+) concentration, probably by insertion in the plasma membrane, as suggested by proteinase K experiments. Edelfosine reduced cell migration to the same extent as known SK(Ca) channel blockers. In contrast, K+ channel openers prevented edelfosine-induced anti-migratory effects. SK3 protein knockdown decreased cell migration and totally abolished the effect of edelfosine on MDA-MB-435s cell migration. In contrast, transient expression of SK3/K(Ca)2.3 protein in a SK3/K(Ca)2.3-deficient cell line increased cell migration and made these cells responsive to edelfosine.

CONCLUSIONS AND IMPLICATIONS

Our data clearly establish edelfosine as an inhibitor of cancer cell migration by acting on SK3/K(Ca)2.3 channels and provide insights into the future development of a new class of migration-targeted, anti-cancer agents.

Facilitation of long-term potentiation by muscarinic M(1) receptors is mediated by inhibition of SK channels

Muscarinic receptor activation facilitates the induction of synaptic plasticity and enhances cognitive function. However, the specific muscarinic receptor subtype involved and the critical intracellular signaling pathways engaged have remained controversial. Here, we show that the recently discovered highly selective allosteric M(1) receptor agonist 77-LH-28-1 facilitates long-term potentiation (LTP) induced by theta burst stimulation at Schaffer collateral synapses in the hippocampus. Similarly, release of acetylcholine by stimulation of cholinergic fibers facilitates LTP via activation of M(1) receptors. N-methyl-D-aspartate receptor (NMDAR) opening during theta burst stimulation was enhanced by M(1) receptor activation, indicating this is the mechanism for LTP facilitation. M(1) receptors were found to enhance NMDAR activation by inhibiting SK channels that otherwise act to hyperpolarize postsynaptic spines and inhibit NMDAR opening. Thus, we describe a mechanism where M(1) receptor activation inhibits SK channels, allowing enhanced NMDAR activity and leading to a facilitation of LTP induction in the hippocampus.

M1 muscarinic receptors boost synaptic potentials and calcium influx in dendritic spines by inhibiting postsynaptic SK channels

Acetylcholine release and activation of muscarinic cholinergic receptors (mAChRs) enhance synaptic plasticity in vitro and cognition and memory in vivo. Within the hippocampus, mAChRs promote NMDA-type glutamate receptor-dependent forms of long-term potentiation. Here, we use calcium (Ca) imaging combined with two-photon laser glutamate uncaging at apical spines of CA1 pyramidal neurons to examine postsynaptic mechanisms of muscarinic modulation of glutamatergic transmission. Uncaging-evoked excitatory postsynaptic potentials and Ca transients are increased by muscarinic stimulation; however, this is not due to direct modulation of glutamate receptors. Instead, mAChRs modulate a negative feedback loop in spines that normally suppresses synaptic signals. mAChR activation reduces the Ca sensitivity of small conductance Ca-activated potassium (SK) channels that are found in the spine, resulting in increased synaptic potentials and Ca transients. These effects are mediated by M1-type muscarinic receptors and occur in a casein kinase-2-dependent manner. Thus, muscarinic modulation regulates synaptic transmission by tuning the activity of nonglutamatergic postsynaptic ion channels.

Ataxias and cerebellar dysfunction: involvement of synaptic plasticity deficits?

Adaptive processes within cerebellar circuits, such as long-term depression and long-term potentiation at parallel fiber-Purkinje cell synapses, have long been seen as important to cerebellar motor learning, and yet little attention has been given to any possible significance of these processes for cerebellar dysfunction and disease. Several forms of ataxia are caused by mutations in genes encoding for ion channels located at key junctures in pathways that lead to the induction of synaptic plasticity, suggesting that there might be an association between deficits in plasticity and the ataxic phenotype. Herein we explore this possibility and examine the available evidence linking the two together, highlighting specifically the role of P/Q-type calcium channels and their downstream effector small-conductance calcium-sensitive (SK2) potassium channels in the regulation of synaptic gain and intrinsic excitability, and reviewing their connections to ataxia.

NS309 restores EDHF-type relaxation in mesenteric small arteries from type 2 diabetic ZDF rats

BACKGROUND AND PURPOSE

The endothelium-derived hyperpolarizing factor (EDHF)-type relaxation in mesenteric small arteries from 21 week old Zucker lean (ZL) and Zucker diabetic fatty (ZDF) rats was investigated using (6,7-dichloro-1H-indole-2,3-dione 3-oxime) (NS309), a potent activator of small-conductance, calcium-activated potassium channel (SK(Ca)) and intermediate-conductance, calcium-activated potassium channel (IK(Ca)).

EXPERIMENTAL APPROACH

In the presence of inhibitors of cyclooxygenase and nitric oxide synthase [indomethacin and N(omega)-nitro-L-arginine methyl ester (l-NAME), respectively], acetylcholine (ACh)-induced hyperpolarization and EDHF-type relaxation were investigated under isometric conditions in the wire myograph using 0.5 and 1 microM NS309 and/or selective blockers of SK(Ca) and IK(Ca) channels. Membrane potential was recorded with glass microelectrodes, and changes in the intracellular calcium concentration of endothelial cells were visualized by confocal microscopy. SK(Ca) expression was assessed by Western blotting.

KEY RESULTS

In arteries from ZDF rats, ACh-induced relaxation and membrane hyperpolarization were attenuated and, compared with arteries from ZL rats, NS309 was less potent at causing relaxation. Incubation with 0.5 microM NS309 did not increase ACh-induced relaxation in arteries from ZDF rats significantly. However, 1 microM NS309 restored it (both in the absence and in the presence of indomethacin and l-NAME) without changing endothelial intracellular calcium concentration. The restored EDHF-type relaxation was more sensitive to TRAM-34 (1-[(2-chlorophenyl) diphenylmethyl]-1H-pyrazole) (1 microM) than to apamin. Expression of the SK(Ca) channel was unaltered.

CONCLUSIONS AND IMPLICATIONS

The attenuated EDHF-type relaxation in mesenteric small arteries from ZDF rats can be restored by NS309 without changes in the intracellular calcium concentration of endothelial cells. These results may have clinical implications for the treatment of endothelial dysfunction in overweight type 2 diabetic patients.

Endothelium as target for large-conductance calcium-activated potassium channel openers

The endothelium is a highly active organ responsible for vasculatory tone and structure, angiogenesis, as well as hemodynamic, humoral, and inflammatory responses. The endothelium is constantly exposed to blood flow, sheer stress and tension. Endothelial cells are present as a vasculature in every tissue of the body and react to and control its microenvironment. A variety of ion channels are present in the plasma membranes of endothelial cells. These include potassium channels such as inwardly rectifying potassium (K(ir)) channels, voltage-dependent (K(v)) channels, ATP-regulated potassium (K(ATP)) channels and three types of calcium-activated potassium channels (K(Ca)), the large (BK(Ca)), intermediate (IK(Ca)), and small (SK(Ca)) -conductance potassium channels. Potassium current plays a critical role in action potentials in excitable cells, in setting the resting membrane potential, and in regulating neurotransmitter release. Mitochondrial isoforms of potassium channel contribute to the cytoprotection of endothelial cells. Prominent among potassium channels are families of calcium-activated potassium channels, and especially large-conductance calcium-activated potassium channels. The modulation of BK(Ca) channels, which are voltage- and calcium-dependent, has been intensively studied. The BK(Ca) channels show large expression dynamics in endothelial cells and tissue-specific expression of large numbers of alternatively spliced isoforms. In this review, a few examples of the modulatory mechanisms and physiological consequences of the expression of BK(Ca) channels are discussed in relation to potential targets for pharmacological intervention.

Molecular cloning and tissue expression patterns of a small conductance calcium-activated potassium channel gene in turbot (Scophthalmus maximus L.)

Calcium-activated potassium channels on plasma membrane enable potassium influx into the cell with ensuing changes in plasma membrane potential and consequent effects on cellular metabolic functions. Recently, this potassium channel was reported to regulate the cellular responses of mammalian immune cells. We have postulated the presence of such a channel in fish immune cells and its potential role in immunoregulation in fish. Employing specific primers and RNA template, we cloned a segment of a novel gene from turbot blood sample and subsequently obtained a full cDNA sequence using RACE approaches. Bioinformatic analysis revealed structural and phylogenetic characteristics of a novel small conductance calcium-activated potassium channel gene, we called TSKCa, which exhibits homologous domains to other species particularly in the transmembrane regions. Full-length TSKCa cDNA is 1698 bp with a 1632 bp open reading frame encoding a protein of 544 amino acids. TSKCa gene is expressed in majority of the tested organs and tissues of turbot. To assess the postulated immune function of TSKCa, we infected turbot with the pathogen Vibrio anguillarum. Here, semi-quantitative RT-PCR analysis demonstrated increased mRNA expression of TSKCa in head kidney, spleen and blood, indicating an important role of TSKCa in these organ tissues that mediate the immune defense response of turbot. In contrast, there was less change in expression in the turbot intestines and liver which were less implicated in the immune response in present study.

Protein expression of small conductance calcium-activated potassium channels is altered in inferior colliculus neurons of the genetically epilepsy-prone rat

The genetically epilepsy-prone rat (GEPR) exhibits inherited predisposition to sound stimuli-induced generalized tonic-clonic seizures (audiogenic reflex seizures) and is a valid model to study the physiopathology of epilepsy. In this model, the inferior colliculus (IC) exhibits enhanced neuronal firing that is critical in the initiation of reflex audiogenic seizures. The mechanisms underlying IC neuronal hyperexcitability that leads to seizure susceptibility are not as yet fully understood. The present report shows that the levels of protein expression of SK1 and SK3 subtypes of the small conductance Ca2+-activated K+ channels were significantly decreased, while SK2 channel proteins were increased in IC neurons of seizure-naive GEPR-3s (SN-GEPR-3), as compared to control Sprague-Dawley rats. No significant change was found in the expression of BK channel proteins in IC neurons of SN-GEPR-3s. Single episode of reflex audiogenic seizures in the GEPR-3s did not significantly alter the protein expression of SK1-3 and BK channels in IC neurons compared to SN-GEPR-3s. Thus, downregulation of SK1 and SK3 channels and upregulation of SK2 channels provide direct evidence that these Ca2+-activated K+ channels play important roles in IC neuronal hyperexcitability that leads to inherited seizure susceptibility in the GEPR.

Ca2+-activated K+ channels in gonadotropin-releasing hormone-stimulated mouse gonadotrophs

GnRH receptor activation elicits release of intracellular Ca(2+), which leads to secretion and also activates Ca(2+)-activated ion channels underlying membrane voltage changes. The predominant Ca(2+)-activated ion channels in rat and mouse gonadotrophs are Ca(2+)-activated K(+) channels. To establish the temporal relationship between GnRH-induced changes in intracellular [Ca(2+)] ([Ca(2+)](i)) and membrane current (I(m)), and to identify specific Ca(2+)-activated K(+) channels linking GnRH-induced increase in [Ca(2+)](i) to changes in plasma membrane electrical activity, we used single female mouse gonadotrophs in the perforated patch configuration of the patch-clamp technique, which preserves signaling pathways. Simultaneous measurement of [Ca(2+)](i) and I(m) in voltage-clamped gonadotrophs revealed that GnRH stimulates an increase in [Ca(2+)](i) that precedes outward I(m), and that activates two kinetically distinct currents identified, using specific toxin inhibitors, as small conductance Ca(2+)-activated K(+) (SK) current (I(SK)) and large (big) conductance voltage- and Ca(2+)-activated K(+) (BK) current (I(BK)). We show that the apamin-sensitive current has an IC(50) of 69 pM, consistent with the SK2 channel subtype and confirmed by immunocytochemistry. The magnitude of the SK current response to GnRH was attenuated by 17beta-estradiol (E(2)) pretreatment. Iberiotoxin, an inhibitor of BK channels, completely blocked the residual apamin-insensitive outward I(m), substantiating that I(BK) is a component of the GnRH-induced outward I(m). In contrast to its suppression of I(SK), E(2) pretreatment augmented peak I(BK). SK or BK channel inhibition modulated GnRH-stimulated LH secretion, implicating a role for these channels in gonadotroph function. In summary, in mouse gonadotrophs the GnRH-stimulated increase in [Ca(2+)](i) activates I(SK) and I(BK), which are differentially regulated by E(2) and which may be targets for E(2) positive feedback in LH secretion.

Identification and characterization of Ca2+-activated K+ channels in granulosa cells of the human ovary

BACKGROUND

Granulosa cells (GCs) represent a major endocrine compartment of the ovary producing sex steroid hormones. Recently, we identified in human GCs a Ca2+-activated K+ channel (K(Ca)) of big conductance (BK(Ca)), which is involved in steroidogenesis. This channel is activated by intraovarian signalling molecules (e.g. acetylcholine) via raised intracellular Ca2+ levels. In this study, we aimed at characterizing 1. expression and functions of K(Ca) channels (including BK(Ca) beta-subunits), and 2. biophysical properties of BK(Ca) channels.

METHODS

GCs were obtained from in vitro-fertilization patients and cultured. Expression of mRNA was determined by standard RT-PCR and protein expression in human ovarian slices was detected by immunohistochemistry. Progesterone production was measured in cell culture supernatants using ELISAs. Single channels were recorded in the inside-out configuration of the patch-clamp technique.

RESULTS

We identified two K(Ca) types in human GCs, the intermediate- (IK) and the small-conductance K(Ca) (SK). Their functionality was concluded from attenuation of human chorionic gonadotropin-stimulated progesterone production by K(Ca) blockers (TRAM-34, apamin). Functional IK channels were also demonstrated by electrophysiological recording of single K(Ca) channels with distinctive features. Both, IK and BK(Ca) channels were found to be simultaneously active in individual GCs. In agreement with functional data, we identified mRNAs encoding IK, SK1, SK2 and SK3 in human GCs and proteins of IK and SK2 in corresponding human ovarian cells. Molecular characterization of the BK(Ca) channel revealed the presence of mRNAs encoding several BK(Ca) beta-subunits (beta2, beta3, beta4) in human GCs. The multitude of beta-subunits detected might contribute to variations in Ca2+ dependence of individual BK(Ca) channels which we observed in electrophysiological recordings.

CONCLUSION

Functional and molecular studies indicate the presence of active IK and SK channels in human GCs. Considering the already described BK(Ca), they express all three K(Ca) types known. We suggest that the plurality and co-expression of different K(Ca) channels and BK(Ca) beta-subunits might allow differentiated responses to Ca2+ signals over a wide range caused by various intraovarian signalling molecules (e.g. acetylcholine, ATP, dopamine). The knowledge of ovarian K(Ca) channel properties and functions should help to understand the link between endocrine and paracrine/autocrine control in the human ovary.

Analysis of Single Nucleotide Polymorphisms (SNPs) of the small-conductance calcium activated potassium channel (SK3) gene as genetic modifier of the cardiac phenotype in myotonic dystrophy type 1 patients

Myotonic dystrophy type 1 (DM1) is the most frequently inherited neuromuscular disease in adults. It is a multisystemic disorder with major cardiac involvement most commonly represented by first-degree atrioventricular heart block (AVB), followed by different degrees of bundle-branch and intraventricular blocks In search for candidate genes, modifiers of the AVB phenotype in DM1, the expression of the small-conductance calcium activated potassium channel (SK3) gene was analysed in muscle biopsies from DM1 patients. The association between SK3 polymorphisms and the AVB phenotype was then studied analyzing 40 DM1 patients with AVB and 40 age-matched DM1 affected individuals with no ECG abnormalities. [CTG]n repeat length and cardiac clinical picture were also assessed for correlation. QRT-PCR experiments showed an over-expression of the SK3 transcript in DM1 muscle biopsies compared to healthy controls. However, no statistical association between the AVB phenotype and either the [CTG]n expansion length or the presence of specific SNPs in the SK3 gene were detected. These findings suggest that modifier genes, other than SK3, should be identified in order to explain the cardiac phenotypic variability among DM1 patients.