Inhalation of the BK(Ca)-opener NS1619 attenuates right ventricular pressure and improves oxygenation in the rat monocrotaline model of pulmonary hypertension

Pulmonary Circulation Under Pressure: Pathophysiological and Therapeutic Implications of BK Channel

The large-conductance Ca2+-activated K+ (BK) channel is widely expressed in the pulmonary blood vessels and plays a significant role in regulating pulmonary vascular tonus. It opens under membrane depolarization, increased intracellular Ca+2 concentration, and chronic hypoxia, resulting in massive K+ efflux, membrane hyperpolarization, decreased L-type Ca+2 channel opening, and smooth muscle relaxation. Several reports have demonstrated an association between BK channel dysfunction and pulmonary hypertension (PH) development. Decreased BK channel subunit expression and impaired regulation by paracrine hormones result in decreased BK channel opening, increased pulmonary vascular resistance, and pulmonary arterial pressure being the cornerstone of PH. The resulting right ventricular pressure overload ultimately leads to ventricular remodeling and failure. Therefore, it is unsurprising that the BK channel has arisen as a potential target for treating PH. Recently, a series of selective, synthetic BK channel agonists have proven effective in attenuating the pathophysiological progression of PH without adverse effects in animal models.

Pathophysiology and pathogenic mechanisms of pulmonary hypertension: role of membrane receptors, ion channels, and Ca2+ signaling

The pulmonary circulation is a low-resistance, low-pressure, and high-compliance system that allows the lungs to receive the entire cardiac output. Pulmonary arterial pressure is a function of cardiac output and pulmonary vascular resistance, and pulmonary vascular resistance is inversely proportional to the fourth power of the intraluminal radius of the pulmonary artery. Therefore, a very small decrease of the pulmonary vascular lumen diameter results in a significant increase in pulmonary vascular resistance and pulmonary arterial pressure. Pulmonary arterial hypertension is a fatal and progressive disease with poor prognosis. Regardless of the initial pathogenic triggers, sustained pulmonary vasoconstriction, concentric vascular remodeling, occlusive intimal lesions, in situ thrombosis, and vascular wall stiffening are the major and direct causes for elevated pulmonary vascular resistance in patients with pulmonary arterial hypertension and other forms of precapillary pulmonary hypertension. In this review, we aim to discuss the basic principles and physiological mechanisms involved in the regulation of lung vascular hemodynamics and pulmonary vascular function, the changes in the pulmonary vasculature that contribute to the increased vascular resistance and arterial pressure, and the pathogenic mechanisms involved in the development and progression of pulmonary hypertension. We focus on reviewing the pathogenic roles of membrane receptors, ion channels, and intracellular Ca2+ signaling in pulmonary vascular smooth muscle cells in the development and progression of pulmonary hypertension.

Cinaciguat (BAY-582667) Modifies Cardiopulmonary and Systemic Circulation in Chronically Hypoxic and Pulmonary Hypertensive Neonatal Lambs in the Alto Andino

Neonatal pulmonary hypertension (NPHT) is produced by sustained pulmonary vasoconstriction and increased vascular remodeling. Soluble guanylyl cyclase (sGC) participates in signaling pathways that induce vascular vasodilation and reduce vascular remodeling. However, when sGC is oxidized and/or loses its heme group, it does not respond to nitric oxide (NO), losing its vasodilating effects. sGC protein expression and function is reduced in hypertensive neonatal lambs. Currently, NPHT is treated with NO inhalation therapy; however, new treatments are needed for improved outcomes. We used Cinaciguat (BAY-582667), which activates oxidized and/or without heme group sGC in pulmonary hypertensive lambs studied at 3,600 m. Our study included 6 Cinaciguat-treated (35 ug kg−1 day−1x 7 days) and 6 Control neonates. We measured acute and chronic basal cardiovascular variables in pulmonary and systemic circulation, cardiovascular variables during a superimposed episode of acute hypoxia, remodeling of pulmonary arteries and changes in the right ventricle weight, vasoactive functions in small pulmonary arteries, and expression of NO-sGC-cGMP signaling pathway proteins involved in vasodilation. We observed a decrease in pulmonary arterial pressure and vascular resistance during the acute treatment. In contrast, the pulmonary pressure did not change in the chronic study due to increased cardiac output, resulting in lower pulmonary vascular resistance in the last 2 days of chronic study. The latter may have had a role in decreasing right ventricular hypertrophy, although the direct effect of Cinaciguat on the heart should also be considered. During acute hypoxia, the pulmonary vascular resistance remained low compared to the Control lambs. We observed a higher lung artery density, accompanied by reduced smooth muscle and adventitia layers in the pulmonary arteries. Additionally, vasodilator function was increased, and vasoconstrictor function was decreased, with modifications in the expression of proteins linked to pulmonary vasodilation, consistent with low pulmonary vascular resistance. In summary, Cinaciguat, an activator of sGC, induces cardiopulmonary modifications in chronically hypoxic and pulmonary hypertensive newborn lambs. Therefore, Cinaciguat is a potential therapeutic tool for reducing pulmonary vascular remodeling and/or right ventricular hypertrophy in pulmonary arterial hypertension syndrome.

The zinc transporter ZIP12 regulates monocrotaline-induced proliferation and migration of pulmonary arterial smooth muscle cells via the AKT/ERK signaling pathways

Background

The zinc transporter ZIP12 is a membrane-spanning protein that transports zinc ions into the cytoplasm from the extracellular space. Recent studies demonstrated that upregulation of ZIP12 is involved in elevation of cytosolic free zinc and excessive proliferation of pulmonary arterial smooth muscle cells (PASMCs) induced by hypoxia. However, the expression of ZIP12 and its role in pulmonary arterial hypertension (PAH) induced by monocrotaline (MCT) in rats have not been evaluated previously. The aim of this study was to investigate the effect of ZIP12 on the proliferation and migration of PASMCs and its underlying mechanisms in MCT-induced PAH.

Methods

A PAH rat model was generated by intraperitoneal injection of 20 mg/kg MCT twice at one-week intervals. PASMCs were isolated from the pulmonary arteries of rats with MCT-induced PAH or control rats. The expression of ZIP12 and related molecules was detected in the lung tissues and cells. A ZIP12 knockdown lentivirus and an overexpressing lentivirus were constructed and transfected into PASMCs derived from PAH and control rats, respectively. EdU assays, wound healing assays and Western blotting were carried out to explore the function of ZIP12 in PASMCs.

Results

Increased ZIP12 expression was observed in PASMCs derived from MCT-induced PAH rats. The proliferation and migration of PASMCs from PAH rats were significantly increased compared with those from control rats. These results were corroborated by Western blot analysis of PCNA and cyclin D1. All these effects were significantly reversed by silencing ZIP12. Comparatively, ZIP12 overexpression resulted in the opposite effects as shown in PASMCs from control rats. Furthermore, selective inhibition of AKT phosphorylation by LY294002 abolished the effect of ZIP12 overexpression on enhancing cell proliferation and migration and partially suppressed the increase in ERK1/2 phosphorylation induced by ZIP12 overexpression. However, inhibition of ERK activity by U0126 resulted in partial reversal of this effect and did not influence an increase in AKT phosphorylation induced by ZIP12 overexpression.

Conclusions

ZIP12 is involved in MCT-induced pulmonary vascular remodeling and enhances the proliferation and migration of PASMCs. The mechanism of these effects was partially mediated by enhancing the AKT/ERK signaling pathways.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12890-022-01905-3.

Pretreatment with the active fraction of Rhodiola tangutica (Maxim.) S.H. Fu rescues hypoxia-induced potassium channel inhibition in rat pulmonary artery smooth muscle cells

ETHNOPHARMACOLOGICAL RELEVANCE

Previous studies have shown that the active fraction of Rhodiola tangutica (Maxim.) S.H. Fu (ACRT) dilates pulmonary arteries and thwarts pulmonary artery remodelling. The dilatation effect of ACRT on pulmonary artery vascular rings could be reduced by potassium (K⁺) channel blockers. However the exact mechanisms of ACRT on ion channels are still unclear.

AIM OF THE STUDY

This study aimed to investigate whether the effect of ACRT on K⁺ channels inhibits cell proliferation after pulmonary artery smooth muscle cells (PASMCs) are exposed to hypoxia.

MATERIALS AND METHODS

The whole-cell patch-clamp method was used to clarify the effect of ACRT on the K⁺ current (I_K) of rat PASMCs exposed to hypoxia. The mRNA and protein expression levels were detected using real-time quantitative polymerase chain reaction (RT-qPCR) and western blotting, respectively. The intracellular calcium (Ca²⁺) concentration ([Ca²⁺]_i) values in rat PASMCs were detected by laser scanning confocal microscopy. The cell cycle and cell proliferation were assessed using flow cytometry analysis and CCK-8 and EdU assays.

RESULTS

ACRT pretreatment alleviated the inhibition of I_K induced by hypoxia in rat PASMCs. Compared with hypoxia, ACRT upregulated voltage-dependent K⁺ channel (K_v) 1.5 and big-conductance calcium-activated K⁺ channel (BK_Ca) mRNA and protein expression and downregulated voltage-dependent Ca²⁺ channel (Ca_v) 1.2 mRNA and protein expression. ACRT decreased [Ca²⁺]_i, inhibited the promotion of cyclin D1 and proliferating cell nuclear antigen (PCNA) expression, and prevented the proliferation of rat PASMCs exposed to hypoxia.

CONCLUSION

In conclusion, the present study demonstrated that ACRT plays a key role in restoring ion channel function and then inhibiting the proliferation of PASMCs under hypoxia, ACRT has preventive and therapeutic potential in hypoxic pulmonary hypertension.

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.

Revisiting the Large-Conductance Calcium-Activated Potassium (BKCa) Channels in the Pulmonary Circulation

Potassium ion concentrations, controlled by ion pumps and potassium channels, predominantly govern a cell's membrane potential and the tone in the vessels. Calcium-activated potassium channels respond to two different stimuli-changes in voltage and/or changes in intracellular free calcium. Large conductance calcium-activated potassium (BKCa) channels assemble from pore forming and various modulatory and auxiliary subunits. They are of vital significance due to their very high unitary conductance and hence their ability to rapidly cause extreme changes in the membrane potential. The pathophysiology of lung diseases in general and pulmonary hypertension, in particular, show the implication of either decreased expression and partial inactivation of BKCa channel and its subunits or mutations in the genes encoding different subunits of the channel. Signaling molecules, circulating humoral molecules, vasorelaxant agents, etc., have an influence on the open probability of the channel in pulmonary arterial vascular cells. BKCa channel is a possible therapeutic target, aimed to cause vasodilation in constricted or chronically stiffened vessels, as shown in various animal models. This review is a comprehensive collation of studies on BKCa channels in the pulmonary circulation under hypoxia (hypoxic pulmonary vasoconstriction; HPV), lung pathology, and fetal to neonatal transition, emphasising pharmacological interventions as viable therapeutic options.

BKCa Channel Activation Attenuates the Pathophysiological Progression of Monocrotaline-Induced Pulmonary Arterial Hypertension in Wistar Rats

PURPOSE

In the present study, the therapeutic efficacy of a selective BK_Ca channel opener (compound X) in the treatment of monocrotaline (MCT)-induced pulmonary arterial hypertension (PAH) was investigated.

METHODS

PAH was induced in male Wistar rats by a single injection of MCT. After two weeks, the MCT-treated group was divided into two groups that were either treated with compound X or vehicle. Compound X was administered daily at 28 mg/kg. Electrocardiographic, echocardiographic, and haemodynamic analyses were performed; ex vivo evaluations of pulmonary artery reactivity, right ventricle (RV) and lung histology as well as expression levels of α and β myosin heavy chain, brain natriuretic peptide, and cytokines (TNFα and IL10) in heart tissue were performed.

RESULTS

Pulmonary artery rings of the PAH group showed a lower vasodilatation response to acetylcholine, suggesting endothelial dysfunction. Compound X promoted strong vasodilation in pulmonary artery rings of both control and MCT-induced PAH rats. The untreated hypertensive rats presented remodelling of pulmonary arterioles associated with increased resistance to pulmonary flow; increased systolic pressure, hypertrophy and fibrosis of the RV; prolongation of the QT and T_peak-T_end intervals (evaluated during electrocardiogram); increased lung and liver weights; and autonomic imbalance with predominance of sympathetic activity. On the other hand, treatment with compound X reduced pulmonary vascular remodelling, pulmonary flow resistance and RV hypertrophy and afterload.

CONCLUSION

The use of a selective and potent opener to activate the BK_Ca channels promoted improvement of haemodynamic parameters and consequent prevention of RV maladaptive remodelling in rats with MCT-induced PAH.

Potassium (K+) channels in the pulmonary vasculature: Implications in pulmonary hypertension Physiological, pathophysiological and pharmacological regulation

The large K⁺ channel functional diversity in the pulmonary vasculature results from the multitude of genes expressed encoding K⁺ channels, alternative RNA splicing, the post-transcriptional modifications, the presence of homomeric or heteromeric assemblies of the pore-forming α-subunits and the existence of accessory β-subunits modulating the functional properties of the channel. K⁺ channels can also be regulated at multiple levels by different factors controlling channel activity, trafficking, recycling and degradation. The activity of these channels is the primary determinant of membrane potential (Em) in pulmonary artery smooth muscle cells (PASMC), providing an essential regulatory mechanism to dilate or contract pulmonary arteries (PA). K⁺ channels are also expressed in pulmonary artery endothelial cells (PAEC) where they control resting Em, Ca²⁺ entry and the production of different vasoactive factors. The activity of K⁺ channels is also important in regulating the population and phenotype of PASMC in the pulmonary vasculature, since they are involved in cell apoptosis, survival and proliferation. Notably, K⁺ channels play a major role in the development of pulmonary hypertension (PH). Impaired K⁺ channel activity in PH results from: 1) loss of function mutations, 2) downregulation of its expression, which involves transcription factors and microRNAs, or 3) decreased channel current as a result of increased vasoactive factors (e.g., hypoxia, 5-HT, endothelin-1 or thromboxane), exposure to drugs with channel-blocking properties, or by a reduction in factors that positively regulate K⁺ channel activity (e.g., NO and prostacyclin). Restoring K⁺ channel expression, its intracellular trafficking and the channel activity is an attractive therapeutic strategy in PH.

Vasoconstrictor Mechanisms in Chronic Hypoxia-Induced Pulmonary Hypertension: Role of Oxidant Signaling

Elevated resistance of pulmonary circulation after chronic hypoxia exposure leads to pulmonary hypertension. Contributing to this pathological process is enhanced pulmonary vasoconstriction through both calcium-dependent and calcium sensitization mechanisms. Reactive oxygen species (ROS), as a result of increased enzymatic production and/or decreased scavenging, participate in augmentation of pulmonary arterial constriction by potentiating calcium influx as well as activation of myofilament sensitization, therefore mediating the development of pulmonary hypertension. Here, we review the effects of chronic hypoxia on sources of ROS within the pulmonary vasculature including NADPH oxidases, mitochondria, uncoupled endothelial nitric oxide synthase, xanthine oxidase, monoamine oxidases and dysfunctional superoxide dismutases. We also summarize the ROS-induced functional alterations of various Ca2+ and K+ channels involved in regulating Ca2+ influx, and of Rho kinase that is responsible for myofilament Ca2+ sensitivity. A variety of antioxidants have been shown to have beneficial therapeutic effects in animal models of pulmonary hypertension, supporting the role of ROS in the development of pulmonary hypertension. A better understanding of the mechanisms by which ROS enhance vasoconstriction will be useful in evaluating the efficacy of antioxidants for the treatment of pulmonary hypertension.

MicroRNA-mediated downregulation of K+ channels in pulmonary arterial hypertension

Downregulated expression of K+ channels and decreased K+ currents in pulmonary artery smooth muscle cells (PASMC) have been implicated in the development of sustained pulmonary vasoconstriction and vascular remodeling in patients with idiopathic pulmonary arterial hypertension (IPAH). However, it is unclear exactly how K+ channels are downregulated in IPAH-PASMC. MicroRNAs (miRNAs) are small non-coding RNAs that are capable of posttranscriptionally regulating gene expression by binding to the 3'-untranslated regions of their targeted mRNAs. Here, we report that specific miRNAs are responsible for the decreased K+ channel expression and function in IPAH-PASMC. We identified 3 miRNAs (miR-29b, miR-138, and miR-222) that were highly expressed in IPAH-PASMC in comparison to normal PASMC (>2.5-fold difference). Selectively upregulated miRNAs are correlated with the decreased expression and attenuated activity of K+ channels. Overexpression of miR-29b, miR-138, or miR-222 in normal PASMC significantly decreased whole cell K+ currents and downregulated voltage-gated K+ channel 1.5 (KV1.5/KCNA5) in normal PASMC. Inhibition of miR-29b in IPAH-PASMC completely recovered K+ channel function and KV1.5 expression, while miR-138 and miR-222 had a partial or no effect. Luciferase assays further revealed that KV1.5 is a direct target of miR-29b. Additionally, overexpression of miR-29b in normal PASMC decreased large-conductance Ca2+-activated K+ (BKCa) channel currents and downregulated BKCa channel β1 subunit (BKCaβ1 or KCNMB1) expression, while inhibition of miR-29b in IPAH-PASMC increased BKCa channel activity and BKCaβ1 levels. These data indicate upregulated miR-29b contributes at least partially to the attenuated function and expression of KV and BKCa channels in PASMC from patients with IPAH.

Redox Regulation of Ion Channels and Receptors in Pulmonary Hypertension

Significance: Pulmonary hypertension (PH) is characterized by elevated vascular resistance due to vasoconstriction and remodeling of the normally low-pressure pulmonary vasculature. Redox stress contributes to the pathophysiology of this disease by altering the regulation and activity of membrane receptors, K+ channels, and intracellular Ca2+ homeostasis. Recent Advances: Antioxidant therapies have had limited success in treating PH, leading to a growing appreciation that reductive stress, in addition to oxidative stress, plays a role in metabolic and cell signaling dysfunction in pulmonary vascular cells. Reactive oxygen species generation from mitochondria and NADPH oxidases has substantial effects on K+ conductance and membrane potential, and both receptor-operated and store-operated Ca2+ entry. Critical Issues: Some specific redox changes resulting from oxidation, S-nitrosylation, and S-glutathionylation are known to modulate membrane receptor and ion channel activity in PH. However, many sites of regulation that have been elucidated in nonpulmonary cell types have not been tested in the pulmonary vasculature, and context-specific molecular mechanisms are lacking. Future Directions: Here, we review what is known about redox regulation of membrane receptors and ion channels in PH. Further investigation of the mechanisms involved is needed to better understand the etiology of PH and develop better targeted treatment strategies.

Tsantan Sumtang Alleviates Chronic Hypoxia-Induced Pulmonary Hypertension by Inhibiting Proliferation of Pulmonary Vascular Cells

Hypoxia-induced pulmonary hypertension (HPH) is a severe condition associated with significant morbidity and mortality in people living at high altitude. Tsantan Sumtang, a traditional Tibetan medicine, has been routinely used for the treatment of cardiopyretic disease, as well as stenocardia. Interestingly, our previous research found that Tsantan Sumtang improved HPH in rats maintaining in a hypobaric chamber. We performed a series of experiments to test the indexes of vasoconstriction and vascular remodeling, the key pathophysiological characteristics of HPH. Our results showed that Tsantan Sumtang relaxed noradrenaline (NE)-precontracted rat pulmonary artery rings in a concentration-dependent manner in vitro. The PGI2-cAMP (prostaglandin I2-cyclic adenosine monophosphate) pathway, NO-cGMP (nitric oxide-cyclic guanosine monophosphate) pathway, and the opening of K⁺ channels (inward rectifier K⁺ channels, large conductance Ca²⁺-activated K⁺ channels, and voltage-dependent K⁺ channels) might play major roles in the vasorelaxation effect. In vivo, the administration of Tsantan Sumtang resulted in a substantial decrease in the rat mean pulmonary artery pressure (mPAP) and the right ventricular hypertrophy index (RVHI). The reduction of thickness of small pulmonary arterial wall and the WT% (the ratio of the vascular wall thickness to the vascular diameter) were observed. The smooth muscle muscularization of the arterials was alleviated by Tsantan Sumtang treatment at the same time. Tsantan Sumtang also reduced remodeling of pulmonary arterioles by suppressing the expression of proliferating cell nuclear antigen (PCNA), α-smooth muscle actin (α-SMA), cyclin D1, and cyclin-dependent kinase 4 (CDK4) through inhibition of p27Kip1 degradation. Therefore, Tsantan Sumtang could be applied as a preventative medication for HPH, which would be a new use for this traditional medicine.

Trapidil improves hemodynamic, echocardiographic and redox state parameters of right ventricle in monocrotaline-induced pulmonary arterial hypertension model

BACKGROUND

Pulmonary arterial hypertension is a disease characterized by increased pulmonary vascular resistance and redox imbalance, leading to failure of right ventricle. Trapidil has been described to improve the redox balance and cardiac conditions.

HYPOTHESIS

Trapidil can improve the redox balance and contribute to functional improvements of the RV in PAH.

METHODS AND RESULTS

Male, 5week-old Wistar rats were divided into four groups: Control, Control + Trapidil, Monocrotaline and Monocrotaline + Trapidil. PAH was induced by an intraperitoneal injection of monocrotaline 60 mg/kg at day 0. Treatment started at day 7 (5 or 8 mg/kg/day) until day 14, when animals were euthanized after echocardiography and catheterism. Right ventricular systolic pressure and pressure/time derivatives were increased in monocrotaline animals. The increased right ventricular diameters in monocrotaline groups were reduced with trapidil. Monocrotaline groups showed higher lipid peroxidation and glutathione peroxidase activity. Trapidil reduced NADPH oxidases activities and increased the reduced glutathiones/total glutathiones ratio. Protein expression of phospholamban in RV was diminished in monocrotaline groups, whereas expression of RyR and SERCA was enhanced in the groups treated with trapidil.

CONCLUSION

Our data suggest that trapidil induces an improvement in RV remodeling in PAH model, mitigating the progression of the disease.

Molecular structure and function of big calcium-activated potassium channels in skeletal muscle: pharmacological perspectives

The large-conductance Ca2+-activated K+ (BK) channel is broadly expressed in various mammalian cells and tissues such as neurons, skeletal muscles (sarco-BK), and smooth muscles. These channels are activated by changes in membrane electrical potential and by increases in the concentration of intracellular calcium ion (Ca2+). The BK channel is subjected to many mechanisms that add diversity to the BK channel α-subunit gene. These channels are indeed subject to alternative splicing, auxiliary subunits modulation, posttranslational modifications, and protein-protein interactions. BK channels can be modulated by diverse molecules that may induce either an increase or decrease in channel activity. The linkage of these channels to many intracellular metabolites and pathways, as well as their modulation by extracellular natural agents, have been found to be relevant in many physiological processes. BK channel diversity is obtained by means of alternative splicing and modulatory β- and γ-subunits. The association of the α-subunit with β- or with γ-subunits can change the BK channel phenotype, functional diversity, and pharmacological properties in different tissues. In the case of the skeletal muscle BK channel (sarco-BK channel), we established that the main mechanism regulating BK channel diversity is the alternative splicing of the KCNMA1/slo1 gene encoding for the α-subunit generating different splicing isoform in the muscle phenotypes. This finding helps to design molecules selectively targeting the skeletal muscle subtypes. The use of drugs selectively targeting the skeletal muscle BK channels is a promising strategy in the treatment of familial disorders affecting muscular skeletal apparatus including hyperkalemia and hypokalemia periodic paralysis.

BK channels in rat and human pulmonary smooth muscle cells are BKα-β1 functional complexes lacking the oxygen-sensitive stress axis regulated exon insert

A loss of K⁺ efflux in pulmonary arterial smooth muscle cells (PASMCs) contributes to abnormal vasoconstriction and PASMC proliferation during pulmonary hypertension (PH). Activation of high-conductance Ca²⁺-activated (BK) channels represents a therapeutic strategy to restore K⁺ efflux to the affected PASMCs. However, the properties of BK channels in PASMCs-including sensitivity to BK channel openers (BKCOs)-are poorly defined. The goal of this study was to compare the properties of BK channels between PASMCs of normoxic (N) and chronic hypoxic (CH) rats and then explore key findings in human PASMCs. Polymerase chain reaction results revealed that 94.3% of transcripts encoding BKα pore proteins in PASMCs from N rats represent splice variants lacking the stress axis regulated exon insert, which confers oxygen sensitivity. Subsequent patch-clamp recordings from inside-out (I-O) patches confirmed a dense population of BK channels insensitive to hypoxia. The BK channels were highly activated by intracellular Ca²⁺ and the BKCO lithocholate; these responses require BKα-β₁ subunit coupling. PASMCs of CH rats with established PH exhibited a profound overabundance of the dominant oxygen-insensitive BKα variant. Importantly, human BK (hBK) channels in PASMCs from human donor lungs also represented the oxygen-insensitive BKα variant activated by BKCOs. The hBK channels showed significantly enhanced Ca²⁺ sensitivity compared with rat BK channels. We conclude that rat BK and hBK channels in PASMCs are oxygen-insensitive BKα-β₁ complexes highly sensitive to Ca²⁺ and the BKCO lithocholate. BK channels are overexpressed in PASMCs of a rat model of PH and may provide an abundant target for BKCOs designed to restore K⁺ efflux.

Molecular Determinants of BK Channel Functional Diversity and Functioning

Large-conductance Ca2+- and voltage-activated K+ (BK) channels play many physiological roles ranging from the maintenance of smooth muscle tone to hearing and neurosecretion. BK channels are tetramers in which the pore-forming α subunit is coded by a single gene ( Slowpoke, KCNMA1). In this review, we first highlight the physiological importance of this ubiquitous channel, emphasizing the role that BK channels play in different channelopathies. We next discuss the modular nature of BK channel-forming protein, in which the different modules (the voltage sensor and the Ca2+ binding sites) communicate with the pore gates allosterically. In this regard, we review in detail the allosteric models proposed to explain channel activation and how the models are related to channel structure. Considering their extremely large conductance and unique selectivity to K+, we also offer an account of how these two apparently paradoxical characteristics can be understood consistently in unison, and what we have learned about the conduction system and the activation gates using ions, blockers, and toxins. Attention is paid here to the molecular nature of the voltage sensor and the Ca2+ binding sites that are located in a gating ring of known crystal structure and constituted by four COOH termini. Despite the fact that BK channels are coded by a single gene, diversity is obtained by means of alternative splicing and modulatory β and γ subunits. We finish this review by describing how the association of the α subunit with β or with γ subunits can change the BK channel phenotype and pharmacology.

BK channel activators and their therapeutic perspectives

The large conductance calcium- and voltage-activated K⁺ channel (KCa1.1, BK, MaxiK) is ubiquitously expressed in the body, and holds the ability to integrate changes in intracellular calcium and membrane potential. This makes the BK channel an important negative feedback system linking increases in intracellular calcium to outward hyperpolarizing potassium currents. Consequently, the channel has many important physiological roles including regulation of smooth muscle tone, neurotransmitter release and neuronal excitability. Additionally, cardioprotective roles have been revealed in recent years. After a short introduction to the structure, function and regulation of BK channels, we review the small organic molecules activating BK channels and how these tool compounds have helped delineate the roles of BK channels in health and disease.