Stimulation of the calcium-sensing receptor induces relaxations of rat mesenteric arteries by endothelium-dependent and -independent pathways via BKCa and KATP channels

Stimulation of the calcium-sensing receptor (CaSR) induces both vasoconstrictions and vasorelaxations but underlying cellular processes remain unclear. This study investigates expression and effect of stimulating the CaSR by increasing external Ca2+ concentration ([Ca2+ ]o ) on contractility of rat mesenteric arteries. Immunofluorescence studies showed expression of the CaSR in perivascular nerves, vascular smooth muscle cells (VSMCs), and vascular endothelium cells. Using wire myography, increasing [Ca2+ ]o from 1 to 10 mM induced vasorelaxations which were inhibited by the calcilytic Calhex-231 and partially dependent on a functional endothelium. [Ca2+ ]o -induced vasorelaxations were reduced by endothelial NO synthase (eNOS, L-NAME) and large conductance Ca2+ -activated K+ channels (BKCa , iberiotoxin), with their inhibitory action requiring a functional endothelium. [Ca2+ ]o -induced vasorelaxations were also markedly inhibited by an ATP-dependent K+ channel (KATP ) blocker (PNU37883), which did not require a functional endothelium to produce its inhibitory action. Inhibitor studies also suggested contributory roles for inward rectifying K+ channels (Kir ), Kv7 channels, and small conductance Ca2+ -activated K+ channels (SKCa ) on [Ca2+ ]o -induced vasorelaxations. These findings indicate that stimulation of the CaSR mediates vasorelaxations involving multiple pathways, including an endothelium-dependent pathway involving NO production and activation of BKCa channels and an endothelium-independent pathway involving stimulation of KATP channels.

Functional and Histological Changes in Umbilical Artery and Myometrium Isolated from IUGR Complicated Pregnancies

Objective: To investigate the relaxation responses mediated by L-type Ca2+ channels and big-conductance Ca2+-activated K+ (BKCa) channels and histological changes in the human umbilical artery (HUA) and myometrium smooth muscle isolated from pregnancies complicated with intrauterine growth restriction (IUGR).Methods: The muscle reactivity and the histology of the smooth muscle of the HUA and myometrium retrieved from 14 women with IUGR and 14 controls were investigated by the isolated tissue bath and immunohistochemical method.Results: In HUA, the maximum relaxation responses and pD2 values of nifedipine and NS11021 (BKCa channel opener) were significantly increased and significant histopathological changes are observed in the IUGR group.Conclusions: The pathogenesis of IUGR might be associated with the impairment in the functional responses of L-type Ca2+ channels and BKCa channels in HUA smooth muscle. The increased staining of myometrium and UC with HIF-1α in IUGR may indicate apoptosis, histological damage, and impaired fetal growth.

Bidirectional TRP/L Type Ca2+ Channel/RyR/BKCa Molecular and Functional Signaloplex in Vascular Smooth Muscles

TRP channels are expressed both in vascular myocytes and endothelial cells, but knowledge of their operational mechanisms in vascular tissue is particularly limited. Here, we show for the first time the biphasic contractile reaction with relaxation followed by a contraction in response to TRPV4 agonist, GSK1016790A, in a rat pulmonary artery preconstricted with phenylephrine. Similar responses were observed both with and without endothelium, and these were abolished by the TRPV4 selective blocker, HC067047, confirming the specific role of TRPV4 in vascular myocytes. Using selective blockers of BK_Ca and L-type voltage-gated Ca²⁺ channels (Ca_L), we found that the relaxation phase was inducted by BK_Ca activation generating STOCs, while subsequent slowly developing TRPV4-mediated depolarisation activated Ca_L, producing the second contraction phase. These results are compared to TRPM8 activation using menthol in rat tail artery. Activation of both types of TRP channels produces highly similar changes in membrane potential, namely slow depolarisation with concurrent brief hyperpolarisations due to STOCs. We thus propose a general concept of bidirectional TRP-Ca_L-RyR-BK_Ca molecular and functional signaloplex in vascular smooth muscles. Accordingly, both TRPV4 and TRPM8 channels enhance local Ca²⁺ signals producing STOCs via TRP-RyR-BK_Ca coupling while simultaneously globally engaging BK_Ca and Ca_L channels by altering membrane potential.

Effects of KCa channels on biological behavior of trophoblasts

The Ca²⁺-activated potassium (KCa) channels are involved in many cellular functions, but their roles in trophoblasts are unclear. This study aimed to clarify the effects of KCa channels on the biological behavior of trophoblasts. The localization and expression of the three types of KCa channels, including large-conductance KCa channels (BKCa), intermediate-conductance KCa channels (IKCa), and small-conductance KCa channels (SKCa), were detected in human chorionic villi taken from pregnant women between 5 and 8 weeks of gestation (n = 15) and HTR-8/SVneo cells. The effects of KCa channels on proliferation, apoptosis, and migration of HTR-8/SVneo cells were examined by using the activators or inhibitors of KCa channels. Results showed that KCa channels were mainly localized on the membrane and in the cytoplasm of trophoblasts in human chorionic villi and HTR-8/SVneo cells. The proliferation and migration of HTR-8/SVneo cells were inhibited by activating KCa channels. Apoptosis of trophoblasts was promoted through activating BKCa channels but was not affected by neither activating nor inhibiting IKCa and SKCa channels. This study substantiated the abovementioned biological roles of KCa channels in trophoblast cells, which is fundamental to further research on whether dysfunction of KCa channels is involved in the pathogenesis of pregnancy-related complications.

Moderate Ethanol-Preconditioning Offers Ischemic Tolerance Against Focal Cerebral Ischemic/Reperfusion: Role of Large Conductance Calcium-Activated Potassium Channel

The mechanism underlying moderate ethanol (EtOH)-preconditioning (PC) against ischemic brain injury remains unclear. We evaluated the role of large conductance calcium-sensitive potassium (BK_Ca) channels in EtOH-PC. Almost one hundred and ninety normal adult SD rats (8 to 10 weeks, 320-350 g) were enrolled in this study. Ischemic/reperfusion (I/R) brain injury was induced in rats by middle cerebral artery occlusion for 2 h followed by reperfusion for 24 h. EtOH or the BK_Ca channel opener, NS11021, was administered 24 h before I/R with or without pre-treatment with the BK_Ca channel blocker, paxilline. Infarct volumes were measured by tissue staining and imaging, and neurological functions were assessed by a scoring system. The expression of BK_Ca channel subunit α was detected by Western blotting, and cell apoptosis was assessed using staining. Prior (24 h) administration of ethanol that produced a peak plasma concentration of ~ 45 mg/dl in rats would offer neuroprotection after cerebral I/R. In addition, the expression of BK_Ca channel α-subunit was significantly increased 24 h after EtOH-PC (n = 10; control: 2.00 ± 0.09, EtOH: 1.00 ± 0.06; P < 0.5). Compared to I/R, EtOH-PC enhanced the expression of BK_Ca channel α-subunit both in the penumbra (n = 10; 24 h: I/R: 1.25 ± 0.10, EtOH-PC + I/R: 1.99 ± 0.12; P < 0.01; 4 h: I/R: 1.03 ± 0.03, EtOH-PC + I/R: 1.49 ± 0.05; P < 0.001) and infarct core (n = 10; 4 h: I/R: 1.04 ± 0.04, EtOH-PC + I/R: 1.42 ± 0.05; P < 0.001), improved the neurological function (n = 10; I/R: 14.00 (12.75-15.00), EtOH-PC + I/R: 7.00 (4.75-8.25); P < 0.001), attenuated the apoptosis (n = 10; I/R: 26.80 ± 0.69, EtOH-PC + I/R: 8.46 ± 0.31; P < 0.001), and decreased the infarct volume (n = 10; I/R: 244.00 ± 26.24, EtOH-PC + I/R: 70.09 ± 14.69; P < 0.001) after experimental cerebral I/R. These changes were reversed by paxilline administration. The moderate EtOH-PC protects against I/R-induced brain damage dependent on the upregulation BK_Ca channels.

Endothelium-derived hydrogen sulfide acts as a hyperpolarizing factor and exerts neuroprotective effects via activation of large-conductance Ca2+ -activated K+ channels

BACKGROUND AND PURPOSE

Endothelium-derived hyperpolarizing factor (EDHF) has been suggested as a therapeutic target for vascular protection against ischaemic brain injury. However, the molecular entity of EDHF and its action on neurons remains unclear. This study was undertaken to demonstrate whether the hydrogen sulfide (H₂ S) acts as EDHF and exerts neuroprotective effect via large-conductance Ca²⁺ -activated K⁺ (BK_Ca /K_Ca 1.1) channels.

EXPERIMENTAL APPROACH

The whole-cell patch-clamp technology was used to record the changes of BK_Ca currents in rat neurons induced by EDHF. The cerebral ischaemia/reperfusion model of mice and oxygen-glucose deprivation/reoxygenation (OGD/R) model of neurons were used to explore the neuroprotection of EDHF by activating BK_Ca channels in these neurons.

KEY RESULTS

Increases of BK_Ca currents and membrane hyperpolarization in hippocampal neurons induced by EDHF could be markedly inhibited by BK_Ca channel inhibitor iberiotoxin or endothelial H₂ S synthase inhibitor propargylglycine. The H₂ S donor, NaHS-induced BK_Ca current and membrane hyperpolarization in neurons were also inhibited by iberiotoxin, suggesting that H₂ S acts as EDHF and activates the neuronal BK_Ca channels. Besides, we found that the protective effect of endothelium-derived H₂ S against mice cerebral ischaemia/reperfusion injury was disrupted by iberiotoxin. Importantly, the inhibitory effect of NaHS or BK_Ca channel opener on OGD/R-induced neuron injury and the increment of intracellular Ca²⁺ level could be inhibited by iberiotoxin but enhanced by co-application with L-type but not T-type calcium channel inhibitor.

CONCLUSION AND IMPLICATIONS

Endothelium-derived H₂ S acts as EDHF and exerts neuroprotective effects via activating the BK_Ca channels and then inhibiting the T-type calcium channels in hippocampal neurons.

Apelin Does Not Impair Coronary Artery Relaxation Mediated by Nitric Oxide-Induced Activation of BKCa Channels

Apelin-APJ receptor signaling regulates vascular tone in cerebral and peripheral arteries. We recently reported that apelin inhibits BK_Ca channel function in cerebral arteries, resulting in impaired endothelium-dependent relaxations. In contrast, apelin causes endothelium-dependent relaxation of coronary arteries. However, the effects of apelin on BK_Ca channel function in coronary arterial myocytes have not yet been explored. We hypothesized that apelin-APJ receptor signaling does not have an inhibitory effect on coronary arterial BK_Ca channels and hence does not alter nitric oxide (NO)-dependent relaxation of coronary arteries. Patch clamp recording was used to measure whole cell K⁺ currents in freshly isolated coronary smooth muscle cells. Apelin had no effect on the increases in current density in response to membrane depolarization or to NS1619 (a BK_Ca channel opener). Moreover, apelin did not inhibit NO/cGMP-dependent relaxations that required activation of BK_Ca channels in isolated coronary arteries. Apelin-APJ receptor signaling caused a marked increase in intracellular Ca²⁺ levels in coronary arterial smooth muscle cells, but failed to activate PI3-kinase to increase phosphorylation of Akt protein. Collectively, these data provide mechanistic evidence that apelin has no inhibitory effects on BK_Ca channel function in coronary arteries. The lack of inhibitory effect on BK_Ca channels makes it unlikely that activation of APJ receptors in coronary arteries would adversely affect coronary flow by creating a vasoconstrictive environment. It can be expected that apelin or other APJ receptor agonists in development will not interfere with the vasodilator effects of endogenous BK_Ca channel openers.

Glabridin Relaxes Vascular Smooth Muscles by Activating BKCa Channels and Inhibiting Phosphodiesterase in Human Saphenous Vein

The aim of the current study was to investigate the pharmacological activity of glabridin on the isolated human saphenous vein (SV) and explore the underlying mechanisms. Samples of patients' SVs were removed during bypass surgery, and 4-mm lengths of the vessels were placed in Krebs solution at +4°C and hung in an isolated organ bath to assess their contraction/relaxation responses. The contraction/relaxation responses were recorded to observe if the cyclic guanosine monophosphate (cGMP)/protein kinase G (PKG) pathway mediates the relaxant effect of glabridin after treatment with blockers like ODQ (a guanylate cyclase inhibitor), KT5823 (a PKG inhibitor), isobutylmethylxanthine [IBMX, a phosphodiesterase (PDE) inhibitor], and cantharidin [Cant, a myosin light-chain phosphatase (MLCP) inhibitor]. Moreover, nitric oxide (NO), cGMP, and PKG levels in SV tissues were determined by ELISA after incubation with glabridin, N(ω)-nitro-L-arginine methyl ester (L-Name, a NO synthetase inhibitor), phenylephrine (PE), ODQ, IBMX, and KT5823. The results showed that glabridin relaxed the vascular smooth muscle of human SV pretreated with PE in a dose-dependent manner, which was independent of the endothelium. The vasorelaxant effect of glabridin was only inhibited by iberiotoxin (IbTX), Cant, and KT5823. Glabridin increased cGMP and PKG levels in SV homogenates, whereas it did not alter the NO level. The enhancing effects of cGMP and PKG levels by glabridin were abolished by ODQ and KT5823. In conclusion, glabridin has a vasorelaxant effect, which is associated with the activation of BK_Ca channels and inhibition of PDE.

Extra Virgin Olive Oil Phenols Vasodilate Rat MesentericResistance Artery via Phospholipase C (PLC)-CalciumMicrodomains-Potassium Channels (BKCa) Signals

Recent evidence suggests that the reason Extra Virgin Olive Oil (EVOO) lowers blood pressure and reduces the risk of developing hypertension is partly due to minor components of EVOO, such as phenols. However, little is still known about the mechanism(s) through which EVOO phenols mediate anti-hypertensive effects. The aim of the present study was to investigate the mechanisms of action of EVOO phenols on mesenteric resistance arteries. A pressure myograph was used to test the effect of EVOO phenols on isolated mesenteric arteries in the presence of specific inhibitors of: 1) BKca channels (Paxillin, 10^-5 M); 2) L-type calcium channels (Verapamil, 10^-5 M); 3) Ryanodine receptor, RyR (Ryanodine, 10^-5 M); 4) inositol 1,4,5-triphosphate receptor, IP3R, (2-Aminoethyl diphenylborinate, 2-APB, 3 × 10^-3 M); 5) phospholipase C, PLC, (U73122, 10^-5 M), and 6) GPCR-G_αi signaling, (Pertussis Toxin, 10^-5 M). EVOO phenols induced vasodilation of mesenteric arteries in a dose-dependent manner, and this effect was reduced by pre-incubation with Paxillin, Verapamil, Ryanodine, 2-APB, U73122, and Pertussis Toxin. Our data suggest that EVOO phenol-mediated vasodilation requires activation of BKca channels potentially through a local increase of subcellular calcium microdomains, a pivotal mechanism on the base of artery vasodilation. These findings provide novel mechanistic insights for understanding the vasodilatory properties of EVOO phenols on resistance arteries.

BKCa channel participates in insulin-induced lipid deposition in adipocytes by increasing intracellular calcium

Storing energy in the form of triglyceride (TG) is one of the basic functions of adipose tissue. Large-conductance calcium-activated potassium channels (BK_Ca channels) are expressed in adipose tissue and adipocyte-specific BK_Ca deficiency resists obesity in mice, but the role of BK_Ca channels in lipid deposition and the underlying mechanisms have not been elucidated. In the present study, we generated BK_Ca knockout (KO) rats and performed a transcriptome analysis of adipose tissue. We found that the phosphoinositide 3-kinase (PI3K)-protein kinase B (Akt) signaling pathway, which is important for lipid deposition, exhibited the most notable reduction among various signaling pathways in BK_Ca KO rats compared to wild-type rats. Insulin-induced TG deposition, glucose uptake, and Akt (Ser473) phosphorylation were significantly reduced in cultured adipocytes differentiated from adipose-derived stem cells of BK_Ca KO rats. Furthermore, we found that the insulin-induced increase of intracellular calcium resulting from extracellular calcium influx was significantly impaired in BK_Ca KO adipocytes. Finally, insulin activated BK_Ca currents through PI3K, which was independent of Akt and intracellular calcium. The results of this study suggested that BK_Ca channels participate in the insulin signaling pathway and promote TG deposition by increasing extracellular calcium influx in adipocytes.

Hydrogen Sulfide Relaxes Human Uterine Artery via Activating Smooth Muscle BKCa Channels

Opening of large conductance calcium-activated and voltage-dependent potassium (BK_Ca) channels hyperpolarizes plasma membranes of smooth muscle (SM) to cause vasodilation, underling a key mechanism for mediating uterine artery (UA) dilation in pregnancy. Hydrogen sulfide (H₂S) has been recently identified as a new UA vasodilator, yet the mechanism underlying H₂S-induced UA dilation is unknown. Here, we tested whether H₂S activated BK_Ca channels in human UA smooth muscle cells (hUASMC) to mediate UA relaxation. Multiple BK_Ca subunits were found in human UA in vitro and hUASMC in vitro, and high β1 and γ1 proteins were localized in SM cells in human UA. Baseline outward currents, recorded by whole-cell and single-channel patch clamps, were significantly inhibited by specific BK_Ca blockers iberiotoxin (IBTX) or tetraethylammonium, showing specific BK_Ca activity in hUASMC. H₂S dose (NaHS, 1–1000 µM)-dependently potentiated BK_Ca currents and open probability. Co-incubation with a Ca²⁺ blocker nifedipine (5 µM) or a chelator (ethylene glycol-bis (β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid (EGTA), 5 mM) did not alter H₂S-potentiated BK_Ca currents and open probability. NaHS also dose-dependently relaxed phenylephrine pre-constricted freshly prepared human UA rings, which was inhibited by IBTX. Thus, H₂S stimulated human UA relaxation at least partially via activating SM BK_Ca channels independent of extracellular Ca²⁺.

BKCa channels regulate the immunomodulatory properties of WJ-MSCs by affecting the exosome protein profiles during the inflammatory response

Background

Wharton’s jelly-derived mesenchymal stem cells (WJ-MSCs) from the human umbilical cord have been studied extensively due to their immunomodulatory functions. Large-conductance Ca²⁺-activated K⁺ (BKCa channels) channels are involved in many inflammatory responses, but their involvement in the anti-inflammatory activity of WJ-MSCs is unknown. The underlying molecular mechanism, through which BKCa channels mediate the immunomodulation of WJ-MSC, which may include changes in exosomes proteomics, has not yet been clarified.

Methods

Alizarin staining, Oil Red O staining, and flow cytometry were used to identify WJ-MSCs, which were isolated from human umbilical cord Wharton’s jelly. BKCa channels were detected in WJ-MSCs using western blotting, real-time polymerase chain reaction (real-time PCR), and electrophysiology, and cytokine expression was examined using real-time PCR and enzyme-linked immunosorbent assays (ELISAs). Exosomes were characterized using transmission electron microscopy and nanoparticle tracking analysis. Proteomics analysis was performed to explore exosomal proteomic profiles.

Results

The cells derived from human umbilical cord Wharton’s jelly were identified as MSCs. BKCa channels were detected in the isolated WJ-MSCs, and the expression of these channels increased after lipopolysaccharide (LPS) stimulation. BKCa channels blockade in LPS-treated WJ-MSCs induced apoptosis and decreased interleukin-6 (IL-6) expression. Furthermore, THP-1 cells (human monocytic cell line) stimulated with LPS/interferon gamma (IFN-γ) produced more anti-inflammatory cytokines after treatment with exosomes derived from BKCa channel-knockdown WJ-MSCs (si-exo). We also observed altered expression of mitochondrial ATP synthase alpha subunit (ATP5A1), filamin B, and other proteins in si-exo, which might increase the anti-inflammatory activity of macrophages.

Conclusions

Our study described the functional expression of BKCa channels in WJ-MSCs, and BKCa channels regulated the immunomodulatory properties of WJ-MSCs by affecting the exosomal protein profiles during the inflammatory response.

BKCa Channels as Targets for Cardioprotection

The large-conductance calcium- and voltage-activated K⁺ channel (BK_Ca) are encoded by the Kcnma1 gene. They are ubiquitously expressed in neuronal, smooth muscle, astrocytes, and neuroendocrine cells where they are known to play an important role in physiological and pathological processes. They are usually localized to the plasma membrane of the majority of the cells with an exception of adult cardiomyocytes, where BK_Ca is known to localize to mitochondria. BK_Ca channels couple calcium and voltage responses in the cell, which places them as unique targets for a rapid physiological response. The expression and activity of BK_Ca have been linked to several cardiovascular, muscular, and neurological defects, making them a key therapeutic target. Specifically in the heart muscle, pharmacological and genetic activation of BK_Ca channels protect the heart from ischemia-reperfusion injury and also facilitate cardioprotection rendered by ischemic preconditioning. The mechanism involved in cardioprotection is assigned to the modulation of mitochondrial functions, such as regulation of mitochondrial calcium, reactive oxygen species, and membrane potential. Here, we review the progress made on BK_Ca channels and cardioprotection and explore their potential roles as therapeutic targets for preventing acute myocardial infarction.

BKCa (Slo) Channel Regulates Mitochondrial Function and Lifespan in Drosophila melanogaster

BK_Ca channels, originally discovered in Drosophila melanogaster as slowpoke (slo), are recognized for their roles in cellular and organ physiology. Pharmacological approaches implicated BK_Ca channels in cellular and organ protection possibly for their ability to modulate mitochondrial function. However, the direct role of BK_Ca channels in regulating mitochondrial structure and function is not deciphered. Here, we demonstrate that BK_Ca channels are present in fly mitochondria, and slo mutants show structural and functional defects in mitochondria. slo mutants display an increase in reactive oxygen species and the modulation of ROS affected their survival. We also found that the absence of BK_Ca channels reduced the lifespan of Drosophila, and overexpression of human BK_Ca channels in flies extends life span in males. Our study establishes the presence of BK_Ca channels in mitochondria of Drosophila and ascertains its novel physiological role in regulating mitochondrial structural and functional integrity, and lifespan.

The Cannabinoid-Like Compound, VSN16R, Acts on Large Conductance, Ca2+-Activated K+ Channels to Modulate Hippocampal CA1 Pyramidal Neuron Firing

Large conductance, Ca2+-activated K+ (BKCa) channels are widely expressed in the central nervous system, where they regulate action potential duration, firing frequency and consequential neurotransmitter release. Moreover, drug action on, mutations to, or changes in expression levels of BKCa can modulate neuronal hyperexcitability. Amongst other potential mechanisms of action, cannabinoid compounds have recently been reported to activate BKCa channels. Here, we examined the effects of the cannabinoid-like compound (R,Z)-3-(6-(dimethylamino)-6-oxohex-1-en-1-yl)-N-(1-hydroxypropan-2-yl) benzamide (VSN16R) at CA1 pyramidal neurons in hippocampal ex vivo brain slices using current clamp electrophysiology. We also investigated effects of the BKCa channel blockers iberiotoxin (IBTX) and the novel 7-pra-martentoxin (7-Pra-MarTx) on VSN16R action. VSN16R (100 μM) increased first and second fast after-hyperpolarization (fAHP) amplitude, decreased first and second inter spike interval (ISI) and shortened first action potential (AP) width under high frequency stimulation protocols in mouse hippocampal pyramidal neurons. IBTX (100 nM) decreased first fAHP amplitude, increased second ISI and broadened first and second AP width under high frequency stimulation protocols; IBTX also broadened first and second AP width under low frequency stimulation protocols. IBTX blocked effects of VSN16R on fAHP amplitude and ISI. 7-Pra-MarTx (100 nM) had no significant effects on fAHP amplitude and ISI but, unlike IBTX, shortened first and second AP width under high frequency stimulation protocols; 7-Pra-MarTx also shortened second AP width under low frequency stimulation protocols. However, in the presence of 7-Pra-MarTx, VSN16R retained some effects on AP waveform under high frequency stimulation protocols; moreover, VSN16R effects were revealed under low frequency stimulation protocols. These findings demonstrate that VSN16R has effects in native hippocampal neurons consistent with its causing an increase in initial firing frequency via activation of IBTX-sensitive BKCa channels. The differential pharmacological effects described suggest that VSN16R may differentially target BKCa channel subtypes.

Expression and Activation of BKCa Channels in Mice Protects Against Ischemia-Reperfusion Injury of Isolated Hearts by Modulating Mitochondrial Function

Aims: Activation and expression of large conductance calcium and voltage-activated potassium channel (BK_Ca) by pharmacological agents have been implicated in cardioprotection from ischemia-reperfusion (IR) injury possibly by regulating mitochondrial function. Given the non-specific effects of pharmacological agents, it is not clear whether activation of BK_Ca is critical to cardioprotection. In this study, we aimed to decipher the mechanistic role of BK_Ca in cardioprotection from IR injury by genetically activating BK_Ca channels. Methods and Results: Hearts from adult (3 months old) wild-type mice (C57/BL6) and mice expressing genetically activated BK_Ca (Tg-BK_Ca ^R207Q, referred as Tg-BK_Ca) along with wild-type BK_Ca were subjected to 20 min of ischemia and 30 min of reperfusion with or without ischemic preconditioning (IPC, 2 times for 2.5 min interval each). Left ventricular developed pressure (LVDP) was recorded using Millar's Mikrotip® catheter connected to ADInstrument data acquisition system. Myocardial infarction was quantified by 2,3,5-triphenyl tetrazolium chloride (TTC) staining. Our results demonstrated that Tg-BK_Ca mice are protected from IR injury, and BK_Ca also contributes to IPC-mediated cardioprotection. Cardiac function parameters were also measured by echocardiography and no differences were observed in left ventricular ejection fraction, fractional shortening and aortic velocities. Amplex Red® was used to assess reactive oxygen species (ROS) production in isolated mitochondria by spectrofluorometry. We found that genetic activation of BK_Ca reduces ROS after IR stress. Adult cardiomyocytes and mitochondria from Tg-BK_Ca mice were isolated and labeled with Anti-BK_Ca antibodies. Images acquired via confocal microscopy revealed localization of cardiac BK_Ca in the mitochondria. Conclusions: Activation of BK_Ca is essential for recovery of cardiac function after IR injury and is likely a factor in IPC mediated cardioprotection. Genetic activation of BK_Ca reduces ROS produced by complex I and complex II/III in Tg-BK_Ca mice after IR, and IPC further decreases it. These results implicate BK_Ca-mediated cardioprotection, in part, by reducing mitochondrial ROS production. Localization of Tg-BK_Ca in adult cardiomyocytes of transgenic mice was similar to BK_Ca in wild-type mice.

Differential targeting and signalling of voltage-gated T-type Cav 3.2 and L-type Cav 1.2 channels to ryanodine receptors in mesenteric arteries

KEY POINTS

In arterial smooth muscle, Ca2+ sparks are elementary Ca2+ -release events generated by ryanodine receptors (RyRs) to cause vasodilatation by opening maxi Ca2+ -sensitive K+ (BKCa ) channels. This study elucidated the contribution of T-type Cav 3.2 channels in caveolae and their functional interaction with L-type Cav 1.2 channels to trigger Ca2+ sparks in vascular smooth muscle cells (VSMCs). Our data demonstrate that L-type Cav 1.2 channels provide the predominant Ca2+ pathway for the generation of Ca2+ sparks in murine arterial VSMCs. T-type Cav 3.2 channels represent an additional source for generation of VSMC Ca2+ sparks. They are located in pit structures of caveolae to provide locally restricted, tight coupling between T-type Cav 3.2 channels and RyRs to ignite Ca2+ sparks.

ABSTRACT

Recent data suggest that T-type Cav 3.2 channels in arterial vascular smooth muscle cells (VSMCs) and pits structure of caveolae could contribute to elementary Ca2+ signalling (Ca2+ sparks) via ryanodine receptors (RyRs) to cause vasodilatation. While plausible, their precise involvement in igniting Ca2+ sparks remains largely unexplored. The goal of this study was to elucidate the contribution of caveolar Cav 3.2 channels and their functional interaction with Cav 1.2 channels to trigger Ca2+ sparks in VSMCs from mesenteric, tibial and cerebral arteries. We used tamoxifen-inducible smooth muscle-specific Cav 1.2-/- (SMAKO) mice and laser scanning confocal microscopy to assess Ca2+ spark generation in VSMCs. Ni2+ , Cd2+ and methyl-β-cyclodextrin were used to inhibit Cav 3.2 channels, Cav 1.2 channels and caveolae, respectively. Ni2+ (50 μmol L-1 ) and methyl-β-cyclodextrin (10 mmol L-1 ) decreased Ca2+ spark frequency by ∼20-30% in mesenteric VSMCs in a non-additive manner, but failed to inhibit Ca2+ sparks in tibial and cerebral artery VSMCs. Cd2+ (200 μmol L-1 ) suppressed Ca2+ sparks in mesenteric arteries by ∼70-80%. A similar suppression of Ca2+ sparks was seen in mesenteric artery VSMCs of SMAKO mice. The remaining Ca2+ sparks were fully abolished by Ni2+ or methyl-β-cyclodextrin. Our data demonstrate that Ca2+ influx through CaV 1.2 channels is the primary means of triggering Ca2+ sparks in murine arterial VSMCs. CaV 3.2 channels, localized to caveolae and tightly coupled to RyR, provide an additional Ca2+ source for Ca2+ spark generation in mesenteric, but not tibial and cerebral, arteries.

Inhibition of BKCa negatively alters cardiovascular function

Large conductance calcium and voltage-activated potassium channels (BKCa ) are transmembrane proteins, ubiquitously expressed in the majority of organs, and play an active role in regulating cellular physiology. In the heart, BKCa channels are known to play a role in regulating the heart rate and protect it from ischemia-reperfusion injury. In vascular smooth muscle cells, the opening of BKCa channels results in membrane hyperpolarization which eventually results in vasodilation mediated by a reduction in Ca2+ influx due to the closure of voltage-dependent Ca2+ channels. Ex vivo studies have shown that BKCa channels play an active role in the regulation of the function of the majority of blood vessels. However, in vivo role of BKCa channels in cardiovascular function is not completely deciphered. Here, we have evaluated the rapid in vivo role of BKCa channels in regulating the cardiovascular function by using two well-established, rapid-acting, potent blockers, paxilline and iberiotoxin. Our results show that BKCa channels are actively involved in regulating the heart rate, the function of the left and right heart as well as major vessels. We also found that the effect on BKCa channels by blockers is completely reversible, and hence, BKCa channels can be exploited as potential targets for clinical applications for modulating heart rate and cardiac contractility.

Melatonin mediates vasodilation through both direct and indirect activation of BKCa channels

Melatonin, synthesized primarily by the pineal gland, is a neuroendocrine hormone with high membrane permeability. The vascular effects of melatonin, including vasoconstriction and vasodilation, have been demonstrated in numerous studies. However, the mechanisms underlying these effects are not fully understood. Large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels are expressed broadly on smooth muscle cells and play an important role in vascular tone regulation. This study explored the mechanisms of myocyte BK_Ca channels and endothelial factors underlying the action of melatonin on the mesenteric arteries (MAs). Vascular contractility and patch-clamp studies were performed on myocytes of MAs from Wistar rats. Melatonin induced significant vasodilation on MAs. In the presence of N^ω-nitro-l-arginine methyl ester (l-NAME), a potent endothelial oxide synthase (eNOS) inhibitor, melatonin elicited concentration-dependent relaxation, with lowered pIC₅₀ The effect of melatonin was significantly attenuated in the presence of BK_Ca channel blocker iberiotoxin or MT1/MT2 receptor antagonist luzindole in both (+) l-NAME and (-) l-NAME groups. In the (+) l-NAME group, iberiotoxin caused a parallel rightward shift of the melatonin concentration-relaxation curve, with pIC₅₀ lower than that of luzindole. Both inside-out and cell-attached patch-clamp recordings showed that melatonin significantly increased the open probability, mean open time and voltage sensitivity of BK_Ca channels. In a cell-attached patch-clamp configuration, the melatonin-induced enhancement of BK_Ca channel activity was significantly suppressed by luzindole. These findings indicate that in addition to the activation of eNOS, melatonin-induced vasorelaxation of MAs is partially attributable to its direct (passing through the cell membrane) and indirect (via MT1/MT2 receptors) activation of the BK_Ca channels on mesenteric arterial myocytes.

Regulation of cerebral arterial BKCa channels by angiotensin II signaling in adult offspring exposed to prenatal high sucrose diets

Prenatal insults have been shown to affect vascular functions, leading to increased risks of cardiovascular diseases in offspring. The present study determined whether high sucrose (HS) intake in pregnancy affected central vascular functions in middle cerebral artery (MCA) of offspring. Sprague-Dawley rats were fed a standard food and tap water with normal or high (20%) sucrose content during pregnancy. Offspring were maintained with normal diets and tap water. Central vascular functions and related ion channels were assessed in male offspring at 5 months old. Compared with the control, angiotensin II (AII)-induced vasoconstrictions were significantly higher in the MCA of the offspring exposed to prenatal HS. In the MCA, large conductance Ca²⁺-activated K⁺ channels (BKCa) currents were decreased with a reduction of opening frequency, sensitivity to intracellular Ca²⁺/membrane voltage, and BKβ1 expression. mRNA levels of AT1α and AT2, as well as AT1/AT2 ratio, were significantly increased in the MCA of offspring following exposure to prenatal HS diets. The data suggested that prenatal HS diets could alter microvascular activities in the MCA, probably via changes of BKCa channels in the brain.

Expression and alteration of BKCa channels in the sphincter of Oddi's from rabbits with hypercholesterolemia

This study aimed to investigate the expression and function of BK_Ca channels in the Sphincter of Oddi (SO) in a rabbit model of hypercholesterolemia (HC). New Zealand white rabbits were randomly divided into 2 groups: the control group was fed standard chow (n = 18) whereas the high-cholesterol group was fed cholesterol-enriched chow containing 1.5% cholesterol (n = 18). The serum cholesterol level was significantly greater in the HC groups than in the control group, but there was no significant difference in body weight between the control and HC groups. Although the total protein expression of BK_Ca α- and β₁-subunit was not significantly different between the control and HC groups, the Tyr-phosphorylation of BK_Ca α-subunit was significantly decreased in the HC group than in the control group. In addition, hypercholesterolemia significantly increased Acetylcholine (ACh)-induced contraction of the SO rings. Pretreatment with 30 μM NS1619, a BK_Ca channel agonist, significantly reduced ACh-induced contraction of the SO rings in HC rabbits. Moreover, pretreatment with 100 μM Na₃OV₄, a protein tyrosine phosphatase inhibitor, significantly reduced ACh-induced contraction of the SO rings in HC rabbits, whereas it significantly increased upon pretreating with 10 μM Genistein, a tyrosine kinase inhibitor. Whole-cell patch clamp recordings showed that BK_Ca current density was significantly lower in SOSMCs from HC group than that from control group. Our findings suggest that hypercholesterolemia-induced downregulation of BK_Ca channel, and Tyr-phosphorylation of BK_Ca α-subunit may contribute to the hyperresponsiveness of the SO ring in HC rabbits.

Interplay among distinct Ca2+ conductances drives Ca2+ sparks/spontaneous transient outward currents in rat cerebral arteries

KEY POINTS

Distinct Ca²⁺ channels work in a coordinated manner to grade Ca²⁺ spark/spontaneous transient outward currents (STOCs) in rat cerebral arteries. The relative contribution of each Ca²⁺ channel to Ca²⁺ spark/STOC production depends upon their biophysical properties and the resting membrane potential of smooth muscle. Na⁺ /Ca²⁺ exchanger, but not TRP channels, can also facilitate STOC production.

ABSTRACT

Ca²⁺ sparks are generated in a voltage-dependent manner to initiate spontaneous transient outward currents (STOCs), events that moderate arterial constriction. In this study, we defined the mechanisms by which membrane depolarization increases Ca²⁺ sparks and subsequent STOC production. Using perforated patch clamp electrophysiology and rat cerebral arterial myocytes, we monitored STOCs in the presence and absence of agents that modulate Ca²⁺ entry. Beginning with Ca_V 3.2 channel inhibition, Ni²⁺ was shown to decrease STOC frequency in cells held at hyperpolarized (-40 mV) but not depolarized (-20 mV) voltages. In contrast, nifedipine, a Ca_V 1.2 inhibitor, markedly suppressed STOC frequency at -20 mV but not -40 mV. These findings aligned with the voltage-dependent profiles of L- and T-type Ca²⁺ channels. Furthermore, computational and experimental observations illustrated that Ca²⁺ spark production is intimately tied to the activity of both conductances. Intriguingly, this study observed residual STOC production at depolarized voltages that was independent of Ca_V 1.2 and Ca_V 3.2. This residual component was insensitive to TRPV4 channel modulation and was abolished by Na⁺ /Ca²⁺ exchanger blockade. In summary, our work highlights that the voltage-dependent triggering of Ca²⁺ sparks/STOCs is not tied to a single conductance but rather reflects an interplay among multiple Ca²⁺ permeable pores with distinct electrophysiological properties. This integrated orchestration enables smooth muscle to grade Ca²⁺ spark/STOC production and thus precisely tune negative electrical feedback.

Insulin Induces Relaxation and Decreases Hydrogen Peroxide-Induced Vasoconstriction in Human Placental Vascular Bed in a Mechanism Mediated by Calcium-Activated Potassium Channels and L-Arginine/Nitric Oxide Pathways

HIGHLIGHTS

Short-term incubation with insulin increases the L-arginine transport in HUVECs.

Short-term incubation with insulin increases the NO synthesis in HUVECs.

Insulin induces relaxation in human placental vascular bed.

Insulin attenuates the constriction induced by hydrogen peroxide in human placenta.

The relaxation induced by insulin is dependent on BKCa channels activity in human placenta.

Insulin induces relaxation in umbilical veins, increasing the expression of human amino acid transporter 1 (hCAT-1) and nitric oxide synthesis (NO) in human umbilical vein endothelial cells (HUVECs). Short-term effects of insulin on vasculature have been reported in healthy subjects and cell cultures; however, its mechanisms remain unknown. The aim of this study was to characterize the effect of acute incubation with insulin on the regulation of vascular tone of placental vasculature. HUVECs and chorionic vein rings were isolated from normal pregnancies. The effect of insulin on NO synthesis, L-arginine transport, and hCAT-1 abundance was measured in HUVECs. Isometric tension induced by U46619 (thromboxane A₂ analog) or hydrogen peroxide (H₂O₂) were measured in vessels previously incubated 30 min with insulin and/or the following pharmacological inhibitors: tetraethylammonium (KCa channels), iberiotoxin (BKCa channels), genistein (tyrosine kinases), and wortmannin (phosphatidylinositol 3-kinase). Insulin increases L-arginine transport and NO synthesis in HUVECs. In the placenta, this hormone caused relaxation of the chorionic vein, and reduced perfusion pressure in placental cotyledons. In vessels pre-incubated with insulin, the constriction evoked by H₂O₂ and U46619 was attenuated and the effect on H₂O₂-induced constriction was blocked with tetraethylammonium and iberiotoxin, but not with genistein, or wortmannin. Insulin rapidly dilates the placental vasculature through a mechanism involving activity of BKCa channels and L-arginine/NO pathway in endothelial cells. This phenomenon is related to quick increases of hCAT-1 abundance and higher capacity of endothelial cells to take up L-arginine and generate NO.

Different Effects of Hypertension and Age on the Function of Large Conductance Calcium- and Voltage-Activated Potassium Channels in Human Mesentery Artery Smooth Muscle Cells

Background

Large‐conductance calcium‐ and voltage‐activated potassium channels (BK_Ca channels) play important roles in the maintenance of vascular tone, and their dysregulation is associated with abnormal vascular relaxation and contraction. We tested the changes in BK_Ca channel properties in patients at different ages to assess the effects of hypertension and aging on the functional changes of BK_Ca channels.

Methods and Results

Patch clamp was performed to detect the activities of BK_Ca channels in freshly isolated human mesenteric artery smooth muscle cells from younger patients (aged ≤45 years) without hypertension, older patients (aged ≥65 years) without hypertension, and older patients with hypertension. The expression of mRNA and protein from BK_Ca channels was evaluated by reverse transcription polymerase chain reaction and Western blot analysis, respectively. Results showed that the whole‐cell current density, spontaneous transient outward current, and Ca²⁺ sensitivity of the artery smooth muscle cells were significantly decreased in the older patients with hypertension; the decreases were insignificant in the older patients without hypertension, although a clear tendency to have spontaneous transient outward current was detected in these patients. The expression of both mRNA and protein of BK_Ca subunits α and β1 was significantly decreased in the older patients with hypertension but not in the older patients without hypertension compared with the younger patients without hypertension.

Conclusions

Our findings demonstrate for the first time that hypertension is an important factor for the pathological alteration of the properties of BK_Ca channels in human mesenteric artery smooth muscle cells, and aging itself may also be a factor in these changes in the cells.

Stimulated release of a hyperpolarizing factor (ADHF) from mesenteric artery perivascular adipose tissue: involvement of myocyte BKCa channels and adiponectin

BACKGROUND AND PURPOSE

Perivascular adipose tissue (PVAT) releases adipocyte-derived hyperpolarizing factors (ADHFs) that may partly act by opening myocyte K(+) channels. The present study in rat and mouse mesenteric arteries aimed to identify the myocyte K(+) channel activated by PVAT and to determine whether adiponectin contributed to the hyperpolarizing effects of PVAT.

EXPERIMENTAL APPROACH

Myocyte membrane potential was recorded from de-endothelialized, non-contracted rat and mouse mesenteric arteries in the presence and absence of PVAT.

KEY RESULTS

The β3 -adrenoceptor agonist, CL-316,243 (10 μM), generated PVAT-dependent, iberiotoxin-sensitive myocyte hyperpolarizations resulting from BKCa channel opening and which were partially blocked by L-NMMA (100 μM). Adiponectin (5 μg·mL(-1) ) also produced iberiotoxin-sensitive hyperpolarizations in PVAT-denuded arterioles. Activation of myocyte AMP-activated protein kinase (AMPK) using 5 μM A-769662 also induced BKCa -mediated hyperpolarizations. Dorsomorphin abolished hyperpolarizations to CL-316,243, adiponectin and A-769662. In vessels from Adipo(-/-) mice, hyperpolarizations to CL-316,243 were absent whereas those to A-769662 and adiponectin were normal. In rat vessels, adipocyte-dependent hyperpolarizations were blocked by glibenclamide and clotrimazole but those to NS1619 (33 μM) were unaltered.

CONCLUSIONS AND IMPLICATIONS

Under basal, non-contracted conditions, β3 -adrenoceptor stimulation of PVAT releases an ADHF, which is probably adiponectin. This activates AMPK to open myocyte BKCa channels indirectly and additionally liberates NO, which also contributes to the observed PVAT-dependent myocyte hyperpolarizations. Clotrimazole and glibenclamide each reversed hyperpolarizations to adiponectin and A-769662, suggesting the involvement of myocyte TRPM4 channels in the ADHF-induced myocyte electrical changes mediated via the opening of BKCa channels.

Inhibition of BKCa channel currents in vascular smooth muscle cells contributes to HBOC-induced vasoconstriction

The exact mechanism of hemoglobin-based oxygen carrier (HBOC)-related vasoactivity is still unclear. This study measured the isometric tension of dog arteries and large conductance Ca(2+)-activated K(+) (BKCa) channel currents in vascular smooth muscle cells after exposure to HBOC with increasing concentrations. Data indicated that the net tensions of arteries were dramatically elevated and this elevation was more prominent in coronary artery. Moreover, HBOC exhibited inhibitory effect on BKCa channel, which is strongly correlated with changes in vascular tension. Collectively, HBOC-induced vasoconstriction in a dose-dependent manner and inhibition of BKCa channel is at least partially contributing to this effect.

Maternal nutrient restriction during pregnancy impairs an endothelium-derived hyperpolarizing factor-like pathway in sheep fetal coronary arteries

The mechanisms underlying developmental programming are poorly understood but may be associated with adaptations by the fetus in response to changes in the maternal environment during pregnancy. We hypothesized that maternal nutrient restriction during pregnancy alters vasodilator responses in fetal coronary arteries. Pregnant ewes were fed a control [100% U.S. National Research Council (NRC)] or nutrient-restricted (60% NRC) diet from days 50 to 130 of gestation (term = 145 days); fetal tissues were collected at day 130. In coronary arteries isolated from control fetal lambs, relaxation to bradykinin was unaffected by nitro-l-arginine (NLA). Iberiotoxin or contraction with KCl abolished the NLA-resistant response to bradykinin. In fetal coronary arteries from nutrient-restricted ewes, relaxation to bradykinin was fully suppressed by NLA. Large-conductance, calcium-activated potassium channel (BKCa) currents did not differ in coronary smooth muscle cells from control and nutrient-restricted animals. The BKCa openers, BMS 191011 and NS1619, and 14,15-epoxyeicosatrienoic acid [a putative endothelium-derived hyperpolarizing factor (EDHF)] each caused fetal coronary artery relaxation and BKCa current activation that was unaffected by maternal nutrient restriction. Expression of BKCa-channel subunits did not differ in fetal coronary arteries from control or undernourished ewes. The results indicate that maternal undernutrition during pregnancy results in loss of the EDHF-like pathway in fetal coronary arteries in response to bradykinin, an effect that cannot be explained by a decreased number or activity of BKCa channels or by decreased sensitivity to mediators that activate BKCa channels in vascular smooth muscle cells. Under these conditions, bradykinin-induced relaxation is completely dependent on nitric oxide, which may represent an adaptive response to compensate for the absence of the EDHF-like pathway.

Activation of lysophosphatidic Acid receptor is coupled to enhancement of ca(2+)-activated potassium channel currents

The calcium-activated K(+) (BKCa) channel is one of the potassium-selective ion channels that are present in the nervous and vascular systems. Ca(2+) is the main regulator of BKCa channel activation. The BKCa channel contains two high affinity Ca(2+) binding sites, namely, regulators of K(+) conductance, RCK1 and the Ca(2+) bowl. Lysophosphatidic acid (LPA, 1-radyl-2-hydroxy-sn-glycero-3-phosphate) is one of the neurolipids. LPA affects diverse cellular functions on many cell types through G protein-coupled LPA receptor subtypes. The activation of LPA receptors induces transient elevation of intracellular Ca(2+) levels through diverse G proteins such as Gαq/11, Gαi, Gα12/13, and Gαs and the related signal transduction pathway. In the present study, we examined LPA effects on BKCa channel activity expressed in Xenopus oocytes, which are known to endogenously express the LPA receptor. Treatment with LPA induced a large outward current in a reversible and concentration-dependent manner. However, repeated treatment with LPA induced a rapid desensitization, and the LPA receptor antagonist Ki16425 blocked LPA action. LPA-mediated BKCa channel activation was also attenuated by the PLC inhibitor U-73122, IP3 inhibitor 2-APB, Ca(2+) chelator BAPTA, or PKC inhibitor calphostin. In addition, mutations in RCK1 and RCK2 also attenuated LPA-mediated BKCa channel activation. The present study indicates that LPA-mediated activation of the BKCa channel is achieved through the PLC, IP3, Ca(2+), and PKC pathway and that LPA-mediated activation of the BKCa channel could be one of the biological effects of LPA in the nervous and vascular systems.