Arachidonic acid activation of BKCa (Slo1) channels associated to the β1-subunit in human vascular smooth muscle cells

Direct Inhibition of BK Channels by Cannabidiol, One of the Principal Therapeutic Cannabinoids Derived from Cannabis sativa

Cannabidiol (CBD), one of the main Cannabis sativa bioactive compounds, is utilized in the treatment of major epileptic syndromes. Its efficacy can be attributed to a multimodal mechanism of action that includes, as potential targets, several types of ion channels. In the brain, CBD reduces the firing frequency in rat hippocampal neurons, partly prolonging the duration of action potentials, suggesting a potential blockade of voltage-operated K+ channels. We postulate that this effect might involve the inhibition of the large-conductance voltage- and Ca2+-operated K+ channel (BK channel), which plays a role in the neuronal action potential's repolarization. Thus, we assessed the impact of CBD on the BK channel activity, heterologously expressed in HEK293 cells. Our findings, using the patch-clamp technique, revealed that CBD inhibits BK channel currents in a concentration-dependent manner with an IC50 of 280 nM. The inhibition is through a direct interaction, reducing both the unitary conductance and voltage-dependent activation of the channel. Additionally, the cannabinoid significantly delays channel activation kinetics, indicating stabilization of the closed state. These effects could explain the changes induced by CBD in action potential shape and duration, and they may contribute to the observed anticonvulsant activity of this cannabinoid.

A Novel Role of Uricosuric Agent Benzbromarone in BK Channel Activation and Reduction of Airway Smooth Muscle Contraction

The uricosuric drug benzbromarone, widely used for treatment of gout, hyperpolarizes the membrane potential of airway smooth muscle cells, but how it works remains unknown. Here we show a novel role of benzbromarone in activation of large conductance calcium-activated K⁺ channels. Benzbromarone results in dose-dependent activation of macroscopic big potassium (BK) currents about 1.7- to 14.5-fold with an EC₅₀ of 111 μM and shifts the voltage-dependent channel activation to a more hyperpolarizing direction about 10 to 54 mV in whole-cell patch clamp recordings. In single-channel recordings, benzbromarone decreases single BKα channel closed dwell time and increases the channel open probability. Coexpressing β1 subunit also enhances BK activation by benzbromarone with an EC₅₀ of 67 μM and a leftward shift of conductance-voltage (G-V) curve about 44 to 138 mV. Site-directed mutagenesis reveals that a motif of three amino acids ³²⁹RKK³³¹ in the cytoplasmic linker between S6 and C-terminal regulator of potassium conductance (RCK) gating ring mediates the pharmacological activation of BK channels by benzbromarone. Further ex vivo assay shows that benzbromarone causes reduction of tracheal strip contraction. Taken together, our findings demonstrate that uricosuric benzbromarone activates BK channels through molecular mechanism of action involving the channel RKK motif of S6-RCK linker. Pharmacological activation of BK channel by benzbromarone causes reduction of tracheal strip contraction, holding a repurposing potential for asthma and pulmonary arterial hypertension or BK channelopathies. SIGNIFICANCE STATEMENT: We describe a novel role of uricosuric agent benzbromarone in big potassium (BK) channel activation and relaxation of airway smooth muscle contraction. In this study, we find that benzbromarone is an activator of the large-conductance Ca²⁺- and voltage-activated K⁺ channel (BK channel), which serves numerous cellular functions, including control of smooth muscle contraction. Pharmacological activation of BK channel by the FDA-approved drug benzbromarone may hold repurposing potential for treatment of asthma and pulmonary arterial hypertension or BK channelopathies.

Mapping the functional expression of auxiliary subunits of KCa1.1 in glioblastoma

Glioblastoma (GBM) is the most aggressive glial tumor, where ion channels, including K_Ca1.1, are candidates for new therapeutic options. Since the auxiliary subunits linked to K_Ca1.1 in GBM are largely unknown we used electrophysiology combined with pharmacology and gene silencing to address the functional expression of K_Ca1.1/β subunits complexes in both primary tumor cells and in the glioblastoma cell line U-87 MG. The pattern of the sensitivity (activation/inhibition) of the whole-cell currents to paxilline, lithocholic acid, arachidonic acid, and iberiotoxin; the presence of inactivation of the whole-cell current along with the loss of the outward rectification upon exposure to the reducing agent DTT collectively argue that K_Ca1.1/β3 complex is expressed in U-87 MG. Similar results were found using human primary glioblastoma cells isolated from patient samples. Silencing the β3 subunit expression inhibited carbachol-induced Ca²⁺ transients in U-87 MG thereby indicating the role of the K_Ca1.1/β3 in the Ca²⁺ signaling of glioblastoma cells. Functional expression of the K_Ca1.1/β3 complex, on the other hand, lacks cell cycle dependence. We suggest that the K_Ca1.1/β3 complex may have diagnostic and therapeutic potential in glioblastoma in the future.

Perspectives on Potential Fatty Acid Modulations of Motility Associated Human Sperm Ion Channels

Human spermatozoan ion channels are specifically distributed in the spermatozoan membrane, contribute to sperm motility, and are associated with male reproductive abnormalities. Calcium, potassium, protons, sodium, and chloride are the main ions that are regulated across this membrane, and their intracellular concentrations are crucial for sperm motility. Fatty acids (FAs) affect sperm quality parameters, reproductive pathologies, male fertility, and regulate ion channel functions in other cells. However, to date the literature is insufficient to draw any conclusions regarding the effects of FAs on human spermatozoan ion channels. Here, we aimed to discern the possible effects of FAs on spermatozoan ion channels and direct guidance for future research. After investigating the effects of FAs on characteristics related to human spermatozoan motility, reproductive pathologies, and the modulation of similar ion channels in other cells by FAs, we extrapolated polyunsaturated FAs (PUFAs) to have the highest potency in modulating sperm ion channels to increase sperm motility. Of the PUFAs, the ω-3 unsaturated fatty acids have the greatest effect. We speculate that saturated and monounsaturated FAs will have little to no effect on sperm ion channel activity, though the possible effects could be opposite to those of the PUFAs, considering the differences between FA structure and behavior.

The Large-Conductance, Calcium-Activated Potassium Channel: A Big Key Regulator of Cell Physiology

Large-conductance Ca²⁺-activated K⁺ channels facilitate the efflux of K⁺ ions from a variety of cells and tissues following channel activation. It is now recognized that BK channels undergo a wide range of pre- and post-translational modifications that can dramatically alter their properties and function. This has downstream consequences in affecting cell and tissue excitability, and therefore, function. While finding the “silver bullet” in terms of clinical therapy has remained elusive, ongoing research is providing an impressive range of viable candidate proteins and mechanisms that associate with and modulate BK channel activity, respectively. Here, we provide the hallmarks of BK channel structure and function generally, and discuss important milestones in the efforts to further elucidate the diverse properties of BK channels in its many forms.

Coronary Large Conductance Ca2+-Activated K+ Channel Dysfunction in Diabetes Mellitus

Diabetes mellitus (DM) is an independent risk of macrovascular and microvascular complications, while cardiovascular diseases remain a leading cause of death in both men and women with diabetes. Large conductance Ca2+-activated K+ (BK) channels are abundantly expressed in arteries and are the key ionic determinant of vascular tone and organ perfusion. It is well established that the downregulation of vascular BK channel function with reduced BK channel protein expression and altered intrinsic BK channel biophysical properties is associated with diabetic vasculopathy. Recent efforts also showed that diabetes-associated changes in signaling pathways and transcriptional factors contribute to the downregulation of BK channel expression. This manuscript will review our current understandings on the molecular, physiological, and biophysical mechanisms that underlie coronary BK channelopathy in diabetes mellitus.

Arachidonic acid effect on the allosteric gating mechanism of BK (Slo1) channels associated with the β1 subunit

Arachidonic acid (AA) is a fatty acid involved in the modulation of several ion channels. Previously, we reported that AA activates the high conductance Ca²⁺- and voltage-dependent K⁺ channel (BK) in vascular smooth muscle depending on the expression of the auxiliary β1 subunit. Here, using the patch-clamp technique on BK channel co-expressed with β1 subunit in a heterologous cell expression system, we analyzed whether AA modifies the three functional modules involved in the channel gating: the voltage sensor domain (VSD), the pore domain (PD), and the intracellular calcium sensor domain (CSD). We present evidence that AA activates BK channel in a direct way, inducing VSD stabilization on its active configuration observed as a significant left shift in the Q-V curve obtained from gating currents recordings. Moreover, AA facilitates the channel opening transitions when VSD are at rest, and the CSD are unoccupied. Furthermore, the activation was independent of the intracellular Ca²⁺ concentration and reduced when the BK channel was co-expressed with the Y74A mutant of the β1 subunit. These results allow us to present new insigths in the mechanism by which AA modulates BK channels co-expressed with its auxiliary β1 subunit.

Transcriptomic analysis reveals different responses to ammonia stress and subsequent recovery between Coilia nasus larvae and juveniles

Excessive ammonia triggered negative effects on aquatic animals' health, growth, and mass death, especially at different developmental periods. However, the underlying responses to ammonia stress in fish larvae and juveniles were much less explored. Transcriptomic analysis of Coilia nasus larvae and juveniles treated with ammonia stress and subsequent recovery in freshwater were performed. Total 958,213,132 clean reads were obtained. A total of 234,830 unigenes with an average length of 1397 bp and N50 value 2521 bp were assembled. 831 and 952 DEGs were identified in C. nasus larvae and juveniles, respectively. Transcriptomic analysis revealed that genes associated with purine metabolism, immune, inflammation, epigenetic modification, and nerve conduction presented different expression trends between C. nasus larvae and juveniles. Other genes related to purine metabolism (XDH) and epigenetic modifications (DNMT1, DNMT3A, and DNMT3B) detected by RT-qPCR also displayed different expression trends. These results indicated that ammonia detoxify strategies and gene regulation patterns were different in C. nasus larvae and juveniles. Higher TNF-α, ILF-2, and ILF-3 expression and reduced LZM, AKP, and ACP activities suggested that inflammation and declined immunity were triggered by ammonia stress. Additionally, nervous conduction was severely affected under ammonia stress in C. nasus juveniles. Furthermore, recovery in freshwater had positive effects on nervous conduction. However, it was worth noting that reduced immunity and inflammation were still existed after recovery in freshwater. In conclusion, our study would be beneficial to reveal the different responses to ammonia stress between larvae and juveniles.

The molecular nature of the 17β-Estradiol binding site in the voltage- and Ca2+-activated K+ (BK) channel β1 subunit

The accessory β1 subunit modulates the Ca²⁺- and voltage-activated K⁺ (BK) channel gating properties mainly by increasing its apparent Ca²⁺ sensitivity. β1 plays an important role in the modulation of arterial tone and blood pressure by vascular smooth muscle cells (SMCs). 17β-estradiol (E2) increases the BK channel open probability (P_o) in SMCs, through a β1 subunit-dependent modulatory effect. Here, using molecular modeling, bioinformatics, mutagenesis, and electrophysiology, we identify a cluster of hydrophobic residues in the second transmembrane domain of the β1 subunit, including the residues W163 and F166, as the binding site for E2. We further show that the increase in P_o induced by E2 is associated with a stabilization of the voltage sensor in its active configuration and an increase in the coupling between the voltage sensor activation and pore opening. Since β1 is a key molecular player in vasoregulation, the findings reported here are of importance in the design of novel drugs able to modulate BK channels.

Lipid metabolism and Calcium signaling in epithelial ovarian cancer

Epithelial Ovarian cancer (EOC) is the deadliest gynecologic malignancy and represents the fifth leading cause of all cancer-related deaths in women. The majority of patients are diagnosed at an advanced stage of the disease that has spread beyond the ovaries to the peritoneum or to distant organs (stage FIGO III-IV) with a 5-year overall survival of about 29%. Consequently, it is necessary to understand the pathogenesis of this disease. Among the factors that contribute to cancer development, lipids and ion channels have been described to be associated to cancerous diseases particularly in breast, colorectal and prostate cancers. Here, we reviewed the literature data to determine how lipids or lipid metabolites may influence EOC risk or progression. We also highlighted the role and the expression of the calcium (Ca²⁺) and calcium-activated potassium (KCa) channels in EOC and how lipids might regulate them. Although lipids and some subclasses of nutritional lipids may be associated to EOC risk, lipid metabolism of LPA (lysophosphatidic acid) and AA (arachidonic acid) emerges as an important signaling network in EOC. Clinical data showed that they are found at high concentrations in EOC patients and in vitro and in vivo studies referred to them as triggers of the Ca²⁺entry in the cancer cells inducing their proliferation, migration or drug resistance. The cross-talk between lipid mediators and Ca²⁺ and/or KCa channels needs to be elucidated in EOC in order to facilitate the understanding of its outcomes and potentially suggest novel therapeutic strategies including treatment and prevention.

Activation of human smooth muscle BK channels by hydrochlorothiazide requires cell integrity and the presence of BK β1 subunit

Thiazide-like diuretics are the most commonly used drugs to treat arterial hypertension, with their efficacy being linked to their chronic vasodilatory effect. Previous studies suggest that activation of the large conductance voltage- and Ca²⁺-dependent K⁺ (BK) channel (Slo 1, MaxiK channel) is responsible for the thiazide-induced vasodilatory effect. But the direct electrophysiological evidence supporting this claim is lacking. BK channels can be associated with one small accessory β-subunit (β₁-β₄) that confers specific biophysical and pharmacological characteristics to the current phenotype. The β1-subunit is primarily expressed in smooth muscle cells (SMCs). In this study we investigated the effect of hydrochlorothiazide (HCTZ) on BK channel activity in native SMCs from human umbilical artery (HUASMCs) and HEK293T cells expressing the BK channel (with and without the β1-subunit). Bath application of HCTZ (10 μmol/L) significantly augmented the BK current in HUASMCs when recorded using the whole-cell configurations, but it did not affect the unitary conductance and open probability of the BK channel in HUASMCs evaluated in the inside-out configuration, suggesting an indirect mechanism requiring cell integrity. In HEK293T cells expressing BK channels, HCTZ-augmented BK channel activity was only observed when the β₁-subunit was co-expressed, being concentration-dependent with an EC₅₀ of 28.4 μmol/L, whereas membrane potential did not influence the concentration relationship. Moreover, HCTZ did not affect the BK channel current in HEK293T cells evaluated in the inside-out configuration, but significantly increases the open probability in the cell-attached configuration. Our data demonstrate that a β₁-subunit-dependent mechanism that requires SMC integrity leads to HCTZ-induced BK channel activation.

Apelin/APJ system: A critical regulator of vascular smooth muscle cell

APJ, an orphan G protein-coupled receptor, is first identified through homology cloning in 1993. Apelin is endogenous ligand of APJ extracted from bovine stomach tissue in 1998. Apelin/APJ system is widely expressed in many kinds of cells such as endothelial cells, cardiomyocytes, especially vascular smooth muscle cell. Vascular smooth muscle cell (VSMC), an integral part of the vascular wall, takes part in many normal physiological processes. Our experiment firstly finds that apelin/APJ system enhances VSMC proliferation by ERK1/2-cyclin D1 signal pathway. Accumulating studies also show that apelin/APJ system plays a pivotal role in mediating the function of VSMC. In this paper, we review the exact role of apelin/APJ system in VSMC, including induction of proliferation and migration, enhance of contraction and relaxation, inhibition of calcification. Furthermore, we discuss the role of apelin/APJ system in vascular diseases, such as atherosclerosis, hypertension, and chronic kidney disease (CKD) from the point of VSMC. Above all, apelin/APJ system is a promising target for managing vascular disease.

Actions and Mechanisms of Polyunsaturated Fatty Acids on Voltage-Gated Ion Channels

Polyunsaturated fatty acids (PUFAs) act on most ion channels, thereby having significant physiological and pharmacological effects. In this review we summarize data from numerous PUFAs on voltage-gated ion channels containing one or several voltage-sensor domains, such as voltage-gated sodium (Na_V), potassium (K_V), calcium (Ca_V), and proton (H_V) channels, as well as calcium-activated potassium (K_Ca), and transient receptor potential (TRP) channels. Some effects of fatty acids appear to be channel specific, whereas others seem to be more general. Common features for the fatty acids to act on the ion channels are at least two double bonds in cis geometry and a charged carboxyl group. In total we identify and label five different sites for the PUFAs. PUFA site 1: The intracellular cavity. Binding of PUFA reduces the current, sometimes as a time-dependent block, inducing an apparent inactivation. PUFA site 2: The extracellular entrance to the pore. Binding leads to a block of the channel. PUFA site 3: The intracellular gate. Binding to this site can bend the gate open and increase the current. PUFA site 4: The interface between the extracellular leaflet of the lipid bilayer and the voltage-sensor domain. Binding to this site leads to an opening of the channel via an electrostatic attraction between the negatively charged PUFA and the positively charged voltage sensor. PUFA site 5: The interface between the extracellular leaflet of the lipid bilayer and the pore domain. Binding to this site affects slow inactivation. This mapping of functional PUFA sites can form the basis for physiological and pharmacological modifications of voltage-gated ion channels.

Obesogenic and Diabetogenic Effects of High-Calorie Nutrition Require Adipocyte BK Channels

Elevated adipose tissue expression of the Ca²⁺- and voltage-activated K⁺ (BK) channel was identified in morbidly obese men carrying a BK gene variant, supporting the hypothesis that K⁺ channels affect the metabolic responses of fat cells to nutrients. To establish the role of endogenous BKs in fat cell maturation, storage of excess dietary fat, and body weight (BW) gain, we studied a gene-targeted mouse model with global ablation of the BK channel (BK^L1/L1) and adipocyte-specific BK-deficient (adipoqBK^L1/L2) mice. Global BK deficiency afforded protection from BW gain and excessive fat accumulation induced by a high-fat diet (HFD). Expansion of white adipose tissue-derived epididymal BK^L1/L1 preadipocytes and their differentiation to lipid-filled mature adipocytes in vitro, however, were improved. Moreover, BW gain and total fat masses of usually superobese ob/ob mice were significantly attenuated in the absence of BK, together supporting a central or peripheral role for BKs in the regulatory system that controls adipose tissue and weight. Accordingly, HFD-fed adipoqBK^L1/L2 mutant mice presented with a reduced total BW and overall body fat mass, smaller adipocytes, and reduced leptin levels. Protection from pathological weight gain in the absence of adipocyte BKs was beneficial for glucose handling and related to an increase in body core temperature as a result of higher levels of uncoupling protein 1 and a low abundance of the proinflammatory interleukin-6, a common risk factor for diabetes and metabolic abnormalities. This suggests that adipocyte BK activity is at least partially responsible for excessive BW gain under high-calorie conditions, suggesting that BK channels are promising drug targets for pharmacotherapy of metabolic disorders and obesity.

BK Channels in the Vascular System

Autoregulation of blood flow is essential for the preservation of organ function to ensure continuous supply of oxygen and essential nutrients and removal of metabolic waste. This is achieved by controlling the diameter of muscular arteries and arterioles that exhibit a myogenic response to changes in arterial blood pressure, nerve activity and tissue metabolism. Large-conductance voltage and Ca(2+)-dependent K(+) channels (BK channels), expressed exclusively in smooth muscle cells (SMCs) in the vascular wall of healthy arteries, play a critical role in regulating the myogenic response. Activation of BK channels by intracellular, local, and transient ryanodine receptor-mediated "Ca(2+) sparks," provides a hyperpolarizing influence on the SMC membrane potential thereby decreasing the activity of voltage-dependent Ca(2+) channels and limiting Ca(2+) influx to promote SMC relaxation and vasodilation. The BK channel α subunit, a large tetrameric protein with each monomer consisting of seven-transmembrane domains, a long intracellular C-terminal tail and an extracellular N-terminus, associates with the β1 and γ subunits in vascular SMCs. The BK channel is regulated by factors originating within the SMC or from the endothelium, perivascular nerves and circulating blood, that significantly alter channel gating properties, Ca(2+) sensitivity and expression of the α and/or β1 subunit. The BK channel thus serves as a central receiving dock that relays the effects of the changes in several such concomitant autocrine and paracrine factors and influences cardiovascular health. This chapter describes the primary mechanism of regulation of myogenic response by BK channels and the alterations to this mechanism wrought by different vasoactive mediators.

Molecular mechanism underlying β1 regulation in voltage- and calcium-activated potassium (BK) channels

Being activated by depolarizing voltages and increases in cytoplasmic Ca(2+), voltage- and calcium-activated potassium (BK) channels and their modulatory β-subunits are able to dampen or stop excitatory stimuli in a wide range of cellular types, including both neuronal and nonneuronal tissues. Minimal alterations in BK channel function may contribute to the pathophysiology of several diseases, including hypertension, asthma, cancer, epilepsy, and diabetes. Several gating processes, allosterically coupled to each other, control BK channel activity and are potential targets for regulation by auxiliary β-subunits that are expressed together with the α (BK)-subunit in almost every tissue type where they are found. By measuring gating currents in BK channels coexpressed with chimeras between β1 and β3 or β2 auxiliary subunits, we were able to identify that the cytoplasmic regions of β1 are responsible for the modulation of the voltage sensors. In addition, we narrowed down the structural determinants to the N terminus of β1, which contains two lysine residues (i.e., K3 and K4), which upon substitution virtually abolished the effects of β1 on charge movement. The mechanism by which K3 and K4 stabilize the voltage sensor is not electrostatic but specific, and the α (BK)-residues involved remain to be identified. This is the first report, to our knowledge, where the regulatory effects of the β1-subunit have been clearly assigned to a particular segment, with two pivotal amino acids being responsible for this modulation.

Fundamental molecular mechanism for the cellular uptake of guanidinium-rich molecules

Guanidinium-rich molecules, such as cell-penetrating peptides, efficiently enter living cells in a non-endocytic energy-independent manner and transport a wide range of cargos, including drugs and biomarkers. The mechanism by which these highly cationic molecules efficiently cross the hydrophobic barrier imposed by the plasma membrane remains a fundamental open question. Here, a combination of computational results and in vitro and live-cell experimental evidence reveals an efficient energy-independent translocation mechanism for arginine-rich molecules. This mechanism unveils the essential role of guanidinium groups and two universal cell components: fatty acids and the cell membrane pH gradient. Deprotonated fatty acids in contact with the cell exterior interact with guanidinium groups, leading to a transient membrane channel that facilitates the transport of arginine-rich peptides toward the cell interior. On the cytosolic side, the fatty acids become protonated, releasing the peptides and resealing the channel. This fundamental mechanism appears to be universal across cells from different species and kingdoms.

Activation of calcium- and voltage-gated potassium channels of large conductance by leukotriene B4

Calcium/voltage-gated, large conductance potassium (BK) channels control numerous physiological processes, including myogenic tone. BK channel regulation by direct interaction between lipid and channel protein sites has received increasing attention. Leukotrienes (LTA4, LTB4, LTC4, LTD4, and LTE4) are inflammatory lipid mediators. We performed patch clamp studies in Xenopus oocytes that co-expressed BK channel-forming (cbv1) and accessory β1 subunits cloned from rat cerebral artery myocytes. Leukotrienes were applied at 0.1 nm-10 μm to either leaflet of cell-free membranes at a wide range of [Ca(2+)]i and voltages. Only LTB4 reversibly increased BK steady-state activity (EC50 = 1 nm; Emax reached at 10 nm), with physiological [Ca(2+)]i and voltages favoring this activation. Homomeric cbv1 or cbv1-β2 channels were LTB4-resistant. Computational modeling predicted that LTB4 docked onto the cholane steroid-sensing site in the BK β1 transmembrane domain 2 (TM2). Co-application of LTB4 and cholane steroid did not further increase LTB4-induced activation. LTB4 failed to activate β1 subunit-containing channels when β1 carried T169A, A176S, or K179I within the docking site. Co-application of LTB4 with LTA4, LTC4, LTD4, or LTE4 suppressed LTB4-induced activation. Inactive leukotrienes docked onto a portion of the site, probably preventing tight docking of LTB4. In summary, we document the ability of two endogenous lipids from different chemical families to share their site of action on a channel accessory subunit. Thus, cross-talk between leukotrienes and cholane steroids might converge on regulation of smooth muscle contractility via BK β1. Moreover, the identification of LTB4 as a highly potent ligand for BK channels is critical for the future development of β1-specific BK channel activators.

Pharmacological consequences of the coexpression of BK channel α and auxiliary β subunits

Coded by a single gene (Slo1, KCM) and activated by depolarizing potentials and by a rise in intracellular Ca(2+) concentration, the large conductance voltage- and Ca(2+)-activated K(+) channel (BK) is unique among the superfamily of K(+) channels. BK channels are tetramers characterized by a pore-forming α subunit containing seven transmembrane segments (instead of the six found in voltage-dependent K(+) channels) and a large C terminus composed of two regulators of K(+) conductance domains (RCK domains), where the Ca(2+)-binding sites reside. BK channels can be associated with accessory β subunits and, although different BK modulatory mechanisms have been described, greater interest has recently been placed on the role that the β subunits may play in the modulation of BK channel gating due to its physiological importance. Four β subunits have currently been identified (i.e., β1, β2, β3, and β4) and despite the fact that they all share the same topology, it has been shown that every β subunit has a specific tissue distribution and that they modify channel kinetics as well as their pharmacological properties and the apparent Ca(2+) sensitivity of the α subunit in different ways. Additionally, different studies have shown that natural, endogenous, and synthetic compounds can modulate BK channels through β subunits. Considering the importance of these channels in different pathological conditions, such as hypertension and neurological disorders, this review focuses on the mechanisms by which these compounds modulate the biophysical properties of BK channels through the regulation of β subunits, as well as their potential therapeutic uses for diseases such as those mentioned above.

Lipid regulation of BK channel function

This mini-review focuses on lipid modulation of BK (MaxiK, BK_Ca) current by a direct interaction between lipid and the BK subunits and/or their immediate lipid environment. Direct lipid-BK protein interactions have been proposed for fatty and epoxyeicosatrienoic acids, phosphoinositides and cholesterol, evidence for such action being less clear for other lipids. BK α (slo1) subunits are sufficient to support current perturbation by fatty and epoxyeicosatrienoic acids, glycerophospholipids and cholesterol, while distinct BK β subunits seem necessary for current modulation by most steroids. Subunit domains or amino acids that participate in lipid action have been identified in a few cases: hslo1 Y318, cerebral artery smooth muscle (cbv1) R334,K335,K336, cbv1 seven cytosolic CRAC domains, slo1 STREX and β1 T169,L172,L173 for docosahexaenoic acid, PIP₂, cholesterol, sulfatides, and cholane steroids, respectively. Whether these protein motifs directly bind lipids or rather transmit the energy of lipid binding to other areas and trigger protein conformation change remains unresolved. The impact of direct lipid-BK interaction on physiology is briefly discussed.