Molecular Mechanisms for Large Conductance Ca2+-Activated K+ Channel Activation by a Novel Opener, 12,14-Dichlorodehydroabietic Acid

Pimaric acid reduces vasoconstriction via BKCa channel activation and VDCC inhibition in rat pulmonary arterial smooth muscles

Pulmonary vessels play a pivotal role in oxygen circulation. We previously demonstrated that pimaric acid (PiMA) activated large-conductance Ca2+-activated K+ (BKCa) channels and inhibited voltage-dependent Ca2+ channels (VDCCs). In the present study, PiMA attenuated vasoconstriction induced by high K+ or endothelin-1 in rat pulmonary arterial smooth muscles (PASMs). PiMA also reduced high K+-induced cytosolic [Ca2+] increase in PASM cells. PiMA increased BKCa currents and decreased VDCC currents. BKCa channels and VDCCs were formed by the α/β1 and α1C/α1D/β2/β3 subunits, respectively. These results indicate that PiMA induces vasorelaxation through the dual effects of BKCa channel activation and VDCC inhibition in PASMs.

Peripheral Voltage-Gated Cation Channels in Neuropathic Pain and Their Potential as Therapeutic Targets

The persistence of increased excitability and spontaneous activity in injured peripheral neurons is imperative for the development and persistence of many forms of neuropathic pain. This aberrant activity involves increased activity and/or expression of voltage-gated Na⁺ and Ca²⁺ channels and hyperpolarization activated cyclic nucleotide gated (HCN) channels as well as decreased function of K⁺ channels. Because they display limited central side effects, peripherally restricted Na⁺ and Ca²⁺ channel blockers and K⁺ channel activators offer potential therapeutic approaches to pain management. This review outlines the current status and future therapeutic promise of peripherally acting channel modulators. Selective blockers of Na_v1.3, Na_v1.7, Na_v1.8, Ca_v3.2, and HCN2 and activators of K_v7.2 abrogate signs of neuropathic pain in animal models. Unfortunately, their performance in the clinic has been disappointing; some substances fail to meet therapeutic end points whereas others produce dose-limiting side effects. Despite this, peripheral voltage-gated cation channels retain their promise as therapeutic targets. The way forward may include (i) further structural refinement of K⁺ channel activators such as retigabine and ASP0819 to improve selectivity and limit toxicity; use or modification of Na⁺ channel blockers such as vixotrigine, PF-05089771, A803467, PF-01247324, VX-150 or arachnid toxins such as Tap1a; the use of Ca²⁺ channel blockers such as TTA-P2, TTA-A2, Z 944, ACT709478, and CNCB-2; (ii) improving methods for assessing “pain” as opposed to nociception in rodent models; (iii) recognizing sex differences in pain etiology; (iv) tailoring of therapeutic approaches to meet the symptoms and etiology of pain in individual patients via quantitative sensory testing and other personalized medicine approaches; (v) targeting genetic and biochemical mechanisms controlling channel expression using anti-NGF antibodies such as tanezumab or re-purposed drugs such as vorinostat, a histone methyltransferase inhibitor used in the management of T-cell lymphoma, or cercosporamide a MNK 1/2 inhibitor used in treatment of rheumatoid arthritis; (vi) combination therapy using drugs that are selective for different channel types or regulatory processes; (vii) directing preclinical validation work toward the use of human or human-derived tissue samples; and (viii) application of molecular biological approaches such as clustered regularly interspaced short palindromic repeats (CRISPR) technology.

Alternative Targets for Modulators of Mitochondrial Potassium Channels

Mitochondrial potassium channels control potassium influx into the mitochondrial matrix and thus regulate mitochondrial membrane potential, volume, respiration, and synthesis of reactive oxygen species (ROS). It has been found that pharmacological activation of mitochondrial potassium channels during ischemia/reperfusion (I/R) injury activates cytoprotective mechanisms resulting in increased cell survival. In cancer cells, the inhibition of these channels leads to increased cell death. Therefore, mitochondrial potassium channels are intriguing targets for the development of new pharmacological strategies. In most cases, however, the substances that modulate the mitochondrial potassium channels have a few alternative targets in the cell. This may result in unexpected or unwanted effects induced by these compounds. In our review, we briefly present the various classes of mitochondrial potassium (mitoK) channels and describe the chemical compounds that modulate their activity. We also describe examples of the multidirectional activity of the activators and inhibitors of mitochondrial potassium channels.

Resin-acid derivatives bind to multiple sites on the voltage-sensor domain of the Shaker potassium channel

Voltage-gated potassium (KV) channels can be opened by negatively charged resin acids and their derivatives. These resin acids have been proposed to attract the positively charged voltage-sensor helix (S4) toward the extracellular side of the membrane by binding to a pocket located between the lipid-facing extracellular ends of the transmembrane segments S3 and S4. By contrast to this proposed mechanism, neutralization of the top gating charge of the Shaker KV channel increased resin-acid-induced opening, suggesting other mechanisms and sites of action. Here, we explore the binding of two resin-acid derivatives, Wu50 and Wu161, to the activated/open state of the Shaker KV channel by a combination of in silico docking, molecular dynamics simulations, and electrophysiology of mutated channels. We identified three potential resin-acid-binding sites around S4: (1) the S3/S4 site previously suggested, in which positively charged residues introduced at the top of S4 are critical to keep the compound bound, (2) a site in the cleft between S4 and the pore domain (S4/pore site), in which a tryptophan at the top of S6 and the top gating charge of S4 keeps the compound bound, and (3) a site located on the extracellular side of the voltage-sensor domain, in a cleft formed by S1-S4 (the top-VSD site). The multiple binding sites around S4 and the anticipated helical-screw motion of the helix during activation make the effect of resin-acid derivatives on channel function intricate. The propensity of a specific resin acid to activate and open a voltage-gated channel likely depends on its exact binding dynamics and the types of interactions it can form with the protein in a state-specific manner.

Synthesis, antitumor and DNA cleavage activities of a novel class of dehydroabietylamine derivatives

Previous studies have reported higher biological activity of dehydrorosinamine derivatives. In order to further synthesize novel compounds with higher biological activity, a series of novel compounds containing benzo-azepine structures were synthesized from dehydroabietylamine in good yields in this study. The structures of synthesized compounds were identified by infra red (IR), ¹H-NMR, ¹³C-NMR, and mass spectra (MS) analysis. The antitumor activities of the target compounds against L02 and HepG2 cells were studied. Furthermore, the dehydroabietylamine derivatives were studied on plasmid DNA cleavage activities. The results showed that the synthesized target compound exhibit antitumor and DNA cleavage activities against plasmid DNA (Escherichia coli). Our results further demonstrate the relationship between the chemical structure and biological function of the synthesized compounds.

Involvement of sex hormonal regulation of K+ channels in electrophysiological and contractile functions of muscle tissues

Sex hormones, such as testosterone, progesterone, and 17β-estradiol, control various physiological functions. This review focuses on the sex hormonal regulation of K⁺ channels and the effects of such regulation on electrophysiological and contractile functions of muscles. In the cardiac tissue, testosterone and progesterone shorten action potential, and estrogen lengthens QT interval, a marker of increased risk of ventricular tachyarrhythmias. We have shown that testosterone and progesterone in physiological concentration activate KCNQ1 channels via membrane-delimited sex hormone receptor/eNOS pathways to shorten the action potential duration. Mitochondrial K⁺ channels are also involved in the protection of cardiac muscle. Testosterone and 17β-estradiol directly activate mitochondrial inner membrane K⁺ channels (Ca²⁺ activated K⁺ channel (K_Ca channel) and ATP-sensitive K⁺ channel (K_ATP channel)) that are involved in ischemic preconditioning and cardiac protection. During pregnancy, uterine blood flow increases to support fetal growth and development. It has been reported that 17β-estradiol directly activates large-conductance Ca²⁺-activated K⁺ channel (BK_Ca channel) attenuating arterial contraction. Furthermore, 17β-estradiol increases expression of BK_Ca channel β1 subunit which enhances BK_Ca channel activity by DNA demethylation. These findings are useful for understanding the mechanisms of sex or generation-dependent differences in the physiological and pathological functions of muscles, and the mechanisms of drug actions.

Modulation of Ca2+ oscillation and melatonin secretion by BKCa channel activity in rat pinealocytes

The pineal glands regulate circadian rhythm through the synthesis and secretion of melatonin. The stimulation of nicotinic acetylcholine receptor due to parasympathetic nerve activity causes an increase in intracellular Ca(2+) concentration and eventually downregulates melatonin production. Our previous report shows that rat pinealocytes have spontaneous and nicotine-induced Ca(2+) oscillations that are evoked by membrane depolarization followed by Ca(2+) influx through voltage-dependent Ca(2+) channels (VDCCs). These Ca(2+) oscillations are supposed to contribute to the inhibitory mechanism of melatonin secretion. Here we examined the involvement of large-conductance Ca(2+)-activated K(+) (BKCa) channel conductance on the regulation of Ca(2+) oscillation and melatonin production in rat pinealocytes. Spontaneous Ca(2+) oscillations were markedly enhanced by BKCa channel blockers (1 μM paxilline or 100 nM iberiotoxin). Nicotine (100 μM)-induced Ca(2+) oscillations were also augmented by paxilline. In contrast, spontaneous Ca(2+) oscillations were abolished by BKCa channel opener [3 μM 12,14-dichlorodehydroabietic acid (diCl-DHAA)]. Under whole cell voltage-clamp configurations, depolarization-elicited outward currents were significantly activated by diCl-DHAA and blocked by paxilline. Expression analyses revealed that the α and β3 subunits of BKCa channel were highly expressed in rat pinealocytes. Importantly, the activity of BKCa channels modulated melatonin secretion from whole pineal gland of the rat. Taken together, BKCa channel activation attenuates these Ca(2+) oscillations due to depolarization-synchronized Ca(2+) influx through VDCCs and results in a recovery of reduced melatonin secretion during parasympathetic nerve activity. BKCa channels may play a physiological role for melatonin production via a negative-feedback mechanism.

Effects of ketamine and its metabolites on ion currents in differentiated hippocampal H19-7 neuronal cells and in HEK293T cells transfected with α-hslo subunit

Ketamine (KT), a dissociative anesthetic, is known to induce schizophrenia-like psychosis. The percentage of KT abuse has recently grown fast despite KT being a controlled drug. The mechanism of KT actions is related to the inhibition of NMDA receptors. Whether KT produces other effects on ion currents in hippocampal neurons remains unclear. In this study, we attempted to evaluate the possible effects of KT and other related compounds on ion currents in hippocampal neuron-derived H19-7 cells. This drug exerted an inhibitory effect on Ca(2+)-activated K(+) current (IK(Ca)) in these cells with an IC(50) value of 274 μM. Pimaric acid (30 μM) or abietic acid (30 μM), known to stimulate large-conductance Ca(2+)-activated K(+) channels, reversed KT-induced inhibition of I(K)(Ca). In HEK293T cells expressing a-humans low poke, KT-induced inhibition of I(K)(Ca) still existed. Dehydronorketamine (300 μM) had little or no effect on the IK(Ca) amplitude, while norketamine (300 μM) slightly but significantly suppressed it. In inside–out configuration, KT applied to the intracellular face of the membrane did not alter single channel conductance of large-conductance Ca(2+)-activated K(+) (BKCa) channels; however, it did significantly reduce the probability of channel openings. Addition of KT was effective in depressing the peak amplitude of voltage-gated Na(+) current. Moreover, the presence of KT was noted to enhance the amplitude of membrane electroporation-induced inward currents (IMEP) in differentiated H19-7 cells. KT-stimulated IMEP was reversed by further application of LaCl(3) (100 μM), but not by NMDA (30 μM). The modulations by this compound of ion channels may contribute to the underlying mechanisms through which KT and its metabolites influence the electrical behavior of hippocampal neurons if similar findings occur in vivo.

Screening quality for Ca2+-activated potassium channel in IonWorks Quattro is greatly improved by using BAPTA-AM and ionomycin

INTRODUCTION

IonWorks automated patch clamp systems are being widely used for ion channel drug discovery, but the perforated patch mode of these systems makes it difficult to obtain a steady intracellular Ca(2+) concentration ([Ca(2+)](i)). This difficulty prevents obtaining high-quality data regarding Ca(2+)-activated channels such as BK and SK channels. We examined the methods for stabilizing [Ca(2+)](i) in the IonWorks Quattro automated patch clamp system to evaluate BK channels.

METHODS

Electrophysiological recordings were performed using the single-hole or population patch clamp mode of IonWorks Quattro. To increase [Ca(2+)](i), ionomycin was used. The variation in the BK current and the effect of BK channel modulators were examined in the presence and absence of an intracellular Ca(2+) chelator, BAPTA-AM (20μM).

RESULTS

BK current activated by step pulses to +100mV in the presence of ionomycin exhibited large variation (ranging from 0.086 to 11nA). In individual cells, oscillation of the current amplitude was observed when five repetitive pulses were applied at 0.1Hz. Approximately 30% of cells exhibited current variation exceeding 20% when the variation was calculated using the first and third pulses. However, BAPTA-AM treatment before current measurement decreased the number of cells displaying large variation (>20%) to 5%. In the presence of BAPTA-AM, the BK channel modulators NS1619 and 12,14-dichlorodehydroabietic acid increased the BK current at concentrations of 10μM or more showing clear concentration dependency, whereas in its absence, the effect of both compounds was detected only at 30μM.

DISCUSSION

The main finding of this study is that the [Ca(2+)](i) variation in the basal condition is very large and hinders the accurate evaluation of compounds in Ca(2+)-activated ion channels. The application of BAPTA-AM and ionomycin greatly improved the precision of BK channel screening, and this method should be applicable to other Ca(2+)-activated ion channels such as SK channels.

Methylglyoxal enhances sodium nitroprusside-induced relaxation in rat aorta

The concentration of methylglyoxal (MGO), a metabolite of glucose, increases in plasma of type II diabetic patients as well as in tissues of hypertensive rats. We have previously shown that MGO inhibited noradrenaline (NA)-induced smooth muscle contraction in rat aorta. However, the effect of MGO on relaxing responses in isolated blood vessel remains to be clarified. Thus, we examined if MGO affects acetylcholine (ACh)- or sodium nitroprusside (SNP)-induced vasodilation on NA (100 nM)-induced pre-contraction in rat thoracic aorta. Treatment of endothelium-intact aorta with MGO (420 microM, 30 min) did not change ACh (1 nM - 3 microM)-induced endothelium-dependent relaxation. In contrast, treatment of endothelium-denuded aorta with MGO shifted the concentration-response curve for SNP (0.1 - 300 nM) to the left. MGO increased reactive oxygen species (ROS) production in smooth muscle on analysis of protein carbonylation. Anti-oxidant agents such as tempol (10 microM), catalase (5000 U/mL), and nitric oxide synthase inhibitor, N(G)-nitro-L-arginine methylester (100 microM) had no effect on MGO-induced enhancement of SNP-induced relaxation. However, iberiotoxin (100 nM), a large-conductance Ca(2+)-activated K(+) (BK(Ca))-channel inhibitor, significantly prevented the effect. The present study revealed that MGO enhanced SNP-induced relaxation in a ROS-independent manner via in part opening smooth muscle BK(Ca) channels.

BK channel activation by NS11021 decreases excitability and contractility of urinary bladder smooth muscle

Large-conductance Ca(2+)-activated potassium (BK) channels play an important role in regulating the function and activity of urinary bladder smooth muscle (UBSM), and the loss of BK channel function has been shown to increase UBSM excitability and contractility. However, it is not known whether activation of BK channels has the converse effect of reducing UBSM excitability and contractility. Here, we have sought to investigate this possibility by using the novel BK channel opener NS11021. NS11021 (3 microM) caused an approximately threefold increase in both single BK channel open probability (P(o)) and whole cell BK channel currents. The frequency of spontaneous action potentials in UBSM strips was reduced by NS11021 from a control value of 20.9 + or - 5.9 to 10.9 + or - 3.7 per minute. NS11021 also reduced the force of UBSM spontaneous phasic contractions by approximately 50%, and this force reduction was blocked by pretreatment with the BK channel blocker iberiotoxin. NS11021 (3 microM) had no effect on contractions evoked by nerve stimulation. These findings indicate that activating BK channels reduces the force of UBSM spontaneous phasic contractions, principally through decreasing the frequency of spontaneous action potentials.

Synthesis and characterisation of NS13558: a new important tool for addressing KCa1.1 channel function ex vivo

Pharmacological activation of the large-conductance Ca(2+)-activated K(+) channel (KCa1.1) in the cardiac inner mitochondrial membrane has been found to protect the heart against ischemia reperfusion injuries. However, there are concerns about the selectivity of the pharmacological tools used to modulate the channel. Here, we address this issue by synthesising a methylated analogue of the tool KCa1.1 channel activator NS11021. The compound (NS13558) is designed as a structurally closely related and biologically inactive analogue of NS11021. NS13558 did not elicit any significant opening of cloned human KCa1.1 channels, but maintained comparable biological activity towards other cardiac ion channels as compared to NS11021. In isolated perfused rat hearts subjected to ischemia-reperfusion, infarct size was reduced from 29% in control to 7% in NS11021 treated hearts. In comparison, the inactive derivate of NS11021, i.e., NS13558, did not confer any cardioprotection, demonstrated by an infarct size identical to control hearts. This suggests that NS11021 exerts its primary effect through KCa1.1 channels, which indicates an important role of these channels in protection against ischemia-reperfusion injuries. Furthermore, the study demonstrates a novel way of combining an activator of the KCa1.1 channel (NS11021) and its structurally closely related inactive analogue NS13558 to address the functional role of KCa1.1 channels, and we believe these novel tools may constitute a valuable addition to understanding the functional role of KCa1.1 channels under physiological and pathophysiological conditions.

Novel channels of the inner mitochondrial membrane

Along with a large number of carriers, exchangers and "pumps", the inner mitochondrial membrane contains ion-conducting channels which endow it with controlled permeability to small ions. Some have been shown to be the mitochondrial counterpart of channels present also in other cellular membranes. The manuscript summarizes the current state of knowledge on the major inner mitochondrial membrane channels, properties, identity and proposed functions. Considerable attention is currently being devoted to two K(+)-selective channels, mtK(ATP) and mtBK(Ca). Their activation in "preconditioning" is considered by many to underlie the protection of myocytes and other cells against subsequent ischemic damage. We have recently shown that in apoptotic lymphocytes inner membrane mtK(V)1.3 interacts with the pro-apoptotic protein Bax after the latter has inserted into the outer mitochondrial membrane. Whether the just-discovered mtIK(Ca) has similar cellular role(s) remains to be seen. The Ca(2+) "uniporter" has been characterized electrophysiologically, but still awaits a molecular identity. Chloride-selective channels are represented by the 107 pS channel, the first mitochondrial channel to be observed by patch-clamp, and by a approximately 400 pS pore we have recently been able to fully characterize in the inner membrane of mitochondria isolated from a colon tumour cell line. This we propose to represent a component of the Permeability Transition Pore. The available data exclude the previous tentative identification with porin, and indicate that it coincides instead with the still molecularly unidentified "maxi" chloride channel.

Novel oxime and oxime ether derivatives of 12,14-dichlorodehydroabietic acid: design, synthesis, and BK channel-opening activity

Oxime ether derivatives of the benzylic ketone of 12,14-dichlorodehydroabietic acid (diCl-DHAA, 4b) were synthesised, and their BK channel-opening activity was evaluated in an assay system of CHO-K1 cells expressing hBKalpha channels. Oxime ether structure on the B ring of diCl-DHAA significantly increased the BK channel-opening activity.

Novel BK channel openers containing dehydroabietic acid skeleton: structure-activity relationship for peripheral substituents on ring C

A series of dehydroabietic acid (DHAA, 2) derivatives was synthesized and evaluated as BK channel openers in an assay system of CHO-K1 cells expressing hBKalpha channels. Systematic modifications of the peripheral functionality of ring C of DHAA showed that the introduction of a nitro or (thio)urea group in ring C greatly enhanced the BK channel-opening activity.

Design, synthesis and characterization of podocarpate derivatives as openers of BK channels

We found that the podocarpic acid structure provides a new scaffold for chemical modulators of large-conductance calcium-activated K(+) channels (BK channels). Structure-activity analysis indicates the importance of both the arrangement (i.e., location and orientation) of the carboxylic acid functionality of ring A and the hydrophobic region of ring C for expression of BK channel-opening activity.

Inhibition of martentoxin on neuronal BK channel subtype (alpha+beta4): implications for a novel interaction model

Martentoxin as a 37-residue peptide was capable of blocking large-conductance Ca(2+)-activated K(+) (BK) channels in adrenal medulla chromaffin cells. This study investigated the pharmacological discrimination of martentoxin on BK channel subtypes. The results showed that the iberiotoxin-insensitive neuronal BK channels (alpha+beta4) could be potently blocked by martentoxin (IC(50) = approximately 80 nM). In contrast, the iberiotoxin-sensitive BK channel consisting of only alpha-subunit was less sensitive to martentoxin. Distinctively, martentoxin inhibited neuronal BK channels (alpha+beta4) with a novel interaction mode. Two possible interaction sites of neuronal BK channels (alpha+beta4) might be responsible for the binding with martentoxin: one for trapping and the other located at the pore region for blocking. In addition, the inhibition of martentoxin on neuronal BK channels (alpha+beta4) depended on cytoplasmic Ca(2+) concentration. On the other hand, in vivo experiments from EEG recordings suggested that neuronal BK channels (alpha+beta4) were the primary target of martentoxin. Therefore, this research not only sheds light on a unique ligand for neuronal BK channels (alpha+beta4), but also highlights a novel model approach for the interaction between K(+) channels and specific-ligands.

Activation of large-conductance calcium-activated potassium channels by puerarin: the underlying mechanism of puerarin-mediated vasodilation

Puerarin is the main isoflavone found in Pueraria lobata (Willd) Ohwi, which has been used in therapy for various cardiovascular diseases. The present study examined the effects of puerarin on the large-conductance voltage- and Ca2+-activated potassium (BK(Ca)) channel and on rat thoracic aortas. BK(Ca) channels encoded with either alpha (BK-alpha) or alpha/beta subunits (BK-alpha+beta1) were heterologously expressed in Xenopus oocytes or human embryonic kidney 293 cells. The activities of BK(Ca) channels were measured using excised patch-clamp recordings. Puerarin activated BK-alpha+beta1 currents with a half-maximal concentration (EC50) of 0.8 nM and a Hill coefficient of 1.11 at 10 microM Ca2+ and with an EC50 of 12.6 nM and a Hill coefficient of 1.08 at 0 microM Ca2+. Puerarin (1 nM) induced a 16-mV leftward shift in the conductance-voltage curve for BK-alpha+beta1 currents at 10 microM Ca2+ and at 100 nM induced a 26-mV leftward shift at 0 microM Ca2+. Puerarin mainly increased the BK-alpha+beta1 channel open probability without changing the unitary conductance. Activation was also detected in the absence of the beta1 subunit. A deglycosylated analog of puerarin, daidzein, also activated BK(Ca) channels with weaker potency. In addition, puerarin (0.1 to 1000 microM) caused concentration-dependent relaxations of rat thoracic aortic rings contracted with 1 microM noradrenaline bitartrate (EC50 = 1.1 microM). These were significantly inhibited by 50 nM iberiotoxin, a specific blocker of BK(Ca) channels. This is the first study demonstrating that puerarin activates BK(Ca) channels, especially BK-alpha+beta1 channels. The activation of the BK(Ca) channel probably contributes to the puerarin-mediated vasodilation action.

The small molecule NS11021 is a potent and specific activator of Ca2+-activated big-conductance K+ channels

Large-conductance Ca(2+)- and voltage-activated K(+) channels (Kca1.1/BK/MaxiK) are widely expressed ion channels. They provide a Ca(2+)-dependent feedback mechanism for the regulation of various body functions such as blood flow, neurotransmitter release, uresis, and immunity. In addition, a mitochondrial K(+) channel with KCa1.1-resembling properties has been found in the heart, where it may be involved in regulation of energy consumption. In the present study, the effect of a novel NeuroSearch compound, 1-(3,5-bis-trifluoromethyl-phenyl)-3-[4-bromo-2-(1H-tetrazol-5-yl)-phenyl]-thiourea (NS11021), was investigated on cloned KCa1.1 expressed in Xenopus laevis oocytes and mammalian cells using electrophysiological methods. NS11021 at concentrations above 0.3 microM activated KCa1.1 in a concentration-dependent manner by parallel-shifting the channel activation curves to more negative potentials. Single-channel analysis revealed that NS11021 increased the open probability of the channel by altering gating kinetics without affecting the single-channel conductance. NS11021 (10 microM) influenced neither a number of cloned Kv channels nor endogenous Na(+) and Ca(2+) channels (L- and T-type) in guinea pig cardiac myocytes. In conclusion, NS11021 is a novel KCa1.1 channel activator with better specificity and a 10 times higher potency compared with the most broadly applied KCa1.1 opener, NS1619. Thus, NS11021 might be a valuable tool compound when addressing the physiological and pathophysiological roles of KCa1.1 channels.

Voltage-sensitive oxonol dyes are novel large-conductance Ca2+-activated K+ channel activators selective for beta1 and beta4 but not for beta2 subunits

The large-conductance Ca(2+)-activated K(+) (BK) channel is activated by both the increase of intracellular Ca(2+) concentration and membrane depolarization. The BK channel plays crucial roles as a key molecule in the negative feedback mechanism regulating membrane excitability and cellular Ca(2+) in various cell types. Here, we report that a widely used slow-response voltage-sensitive fluorescent dye, bis(1,3-dibutylbarbituric acid)trimethine oxonol [DiBAC(4)(3)], is a potent BK channel activator. The application of DiBAC(4)(3) at concentrations of 10 nM and higher significantly increased whole-cell BK channel currents in human embryonic kidney 293 cells expressing rat BK channel alpha and beta1 subunits (rBKalphabeta1). In the presence of 300 nM DiBAC(4)(3), the activation voltage of the BK channel current shifted to the negative direction by approximately 30 mV, but the single-channel conductance was not affected. DiBAC(4)(3) activated whole-cell rBKalphabeta1 and rBKalphabeta4 currents in the same concentration range but partially blocked rBKalphabeta2 currents. The BK channel alpha subunit alone and some other types of K(+) channels examined were not markedly affected by 1 microM DiBAC(4)(3). Structure-activity relationship analyses revealed that a set of oxo- and oxoanion-moieties in two 1,3-dialkylbarbituric acids, which are conjugated by oligomethine, is the novel skeleton for the beta-subunit-selective BK channel-opening property of DiBAC(4)(3) and related oxonol compounds. This conjugated structure may be located stereochemically in one plane. These findings provide a molecular and structural basis for understanding the regulatory mechanism of BK channel activity by an auxiliary beta subunit and will be fundamental to the development of beta-selective BK channel openers.

[Molecular pharmacological studies on potassium channels and their regulatory molecules]

K+ channels play important roles in the control of a large variety of physiological functions such as muscle contraction, neurotransmitter release, hormone secretion, and cell proliferation. Over 100 cloned K+ channel pore-forming alpha and accessory beta subunits have been identified so far. Here, we introduce a series of molecular pharmacological and physiological studies on some types of voltage-dependent K+ channels and Ca2+-activated K+ channels. We examined molecular cloning and functional characterization of novel, fast-inactivating, A-type K+ channel alpha (Kv4.3L) and beta (KChIP2S) subunits predominantly expressed in mammalian heart and found the sites in Kv4 channels for 1) the regulation of voltage dependency and 2) the CaMKII phosphorylation in the C-terminal cytoplasmic domain. Moreover, we found that delayed rectifier-type K+ channels (ERG1 and KCNQ) contribute to the resting membrane conductance in vascular and gastrointestinal smooth muscles. The large-conductance Ca2+-activated K+ (BK) channel is ubiquitously expressed and contributes to diverse physiological processes. Recent reports have shown that a BK-like channel (mitoKCa) is expressed in cardiac mitochondria, suggesting that BK channel openers protect mammalian hearts against ischemic injury. Our studies revealed that BKbeta1 interacts with cytochrome c oxidase I (Cco1) in cardiac mitochondria, and that the activation of BK channels by 17beta-estradiol results in a significant increase in the survival rate of ventricular myocytes. These findings suggest that BKbeta1 may play an important role in the regulation of cell respiration in cardiac myocytes and be a target for the modulation by female gonadal hormones.

Design, synthesis, and BK channel-opening activity of hexahydrodibenzazepinone derivatives

In order to explore new scaffolds for large-conductance Ca2+ -activated K+ channel (BK channel) openers, we carried out molecular design and synthesis on the basis of the following two concepts: (1) introduction of a heteroatom into the dehydroabietic acid (BK channel opener) skeleton would allow easier introduction of substituents. (2) Because of the fourfold symmetrical structure of BK channels, dimeric compounds in which two pharmacophores are linked through a tether are expected to have a greater binding probability to the channels, resulting in increased channel-opening activity. Herein, we explore the usefulness of the hexahydrodibenzazepinone structure as a new scaffold for BK channel openers. The synthesized monomer compounds of hexahydrodibenzazepinone derivatives, which can be derived from dehydroabietic acid, were subjected to electrophysiological patch-clamp studies, followed by Magnus contraction-relaxation assay using rabbit urinary bladder smooth muscle strips to assess overall activities. Dimeric compounds were designed by linking the monomeric hexahydrodibenzazepinone derivatives through a diacetylenebenzene tether, and their channel-opening activities were evaluated by electrophysiological methods. Finally, we concluded that the critical structure for BK channel-opening activity is the hexahydrodibenzazepinone monomer substituted with a phenyl-bearing alkynyl substituent on the lactam amide.