scholarly journals Hypoxic Regulation of the Large-Conductance, Calcium and Voltage-Activated Potassium Channel, BK

2021 ◽  
Vol 12 ◽  
Author(s):  
Sara V. Ochoa ◽  
Liliana Otero ◽  
Andres Felipe Aristizabal-Pachon ◽  
Fernando Hinostroza ◽  
Ingrid Carvacho ◽  
...  
Keyword(s):  
Potassium Channel ◽  
Bk Channels ◽  
Bk Channel ◽  
Mrna Levels ◽  
Oxygen Balance ◽  
Channel Activation ◽  
Hypoxic Response ◽  
Α Subunit ◽  
Subunit Mrna ◽  
Obstructive Sleep

Hypoxia is a condition characterized by a reduction of cellular oxygen levels derived from alterations in oxygen balance. Hypoxic events trigger changes in cell-signaling cascades, oxidative stress, activation of pro-inflammatory molecules, and growth factors, influencing the activity of various ion channel families and leading to diverse cardiovascular diseases such as myocardial infarction, ischemic stroke, and hypertension. The large-conductance, calcium and voltage-activated potassium channel (BK) has a central role in the mechanism of oxygen (O2) sensing and its activity has been related to the hypoxic response. BK channels are ubiquitously expressed, and they are composed by the pore-forming α subunit and the regulatory subunits β (β1–β4), γ (γ1–γ4), and LINGO1. The modification of biophysical properties of BK channels by β subunits underly a myriad of physiological function of these proteins. Hypoxia induces tissue-specific modifications of BK channel α and β subunits expression. Moreover, hypoxia modifies channel activation kinetics and voltage and/or calcium dependence. The reported effects on the BK channel properties are associated with events such as the increase of reactive oxygen species (ROS) production, increases of intracellular Calcium ([Ca2+]i), the regulation by Hypoxia-inducible factor 1α (HIF-1α), and the interaction with hemeproteins. Bronchial asthma, chronic obstructive pulmonary diseases (COPD), and obstructive sleep apnea (OSA), among others, can provoke hypoxia. Untreated OSA patients showed a decrease in BK-β1 subunit mRNA levels and high arterial tension. Treatment with continuous positive airway pressure (CPAP) upregulated β1 subunit mRNA level, decreased arterial pressures, and improved endothelial function coupled with a reduction in morbidity and mortality associated with OSA. These reports suggest that the BK channel has a role in the response involved in hypoxia-associated hypertension derived from OSA. Thus, this review aims to describe the mechanisms involved in the BK channel activation after a hypoxic stimulus and their relationship with disorders like OSA. A deep understanding of the molecular mechanism involved in hypoxic response may help in the therapeutic approaches to treat the pathological processes associated with diseases involving cellular hypoxia.

scholarly journals BK in Double-Membrane Organelles: A Biophysical, Pharmacological, and Functional Survey

2021 ◽  
Vol 12 ◽  
Author(s):  
Naileth González-Sanabria ◽  
Felipe Echeverría ◽  
Ignacio Segura ◽  
Rosangelina Alvarado-Sánchez ◽  
Ramon Latorre
Keyword(s):  
Skeletal Muscle ◽  
Plasma Membrane ◽  
Potassium Channel ◽  
Lipid Bilayers ◽  
Bk Channels ◽  
Bk Channel ◽  
Inner Mitochondrial Membrane ◽  
Modular Architecture ◽  
Open Probability ◽  
Α Subunit

In the 1970s, calcium-activated potassium currents were recorded for the first time. In 10years, this Ca2+-activated potassium channel was identified in rat skeletal muscle, chromaffin cells and characterized in skeletal muscle membranes reconstituted in lipid bilayers. This calcium- and voltage-activated potassium channel, dubbed BK for “Big K” due to its large ionic conductance between 130 and 300 pS in symmetric K+. The BK channel is a tetramer where the pore-forming α subunit contains seven transmembrane segments. It has a modular architecture containing a pore domain with a highly potassium-selective filter, a voltage-sensor domain and two intracellular Ca2+ binding sites in the C-terminus. BK is found in the plasma membrane of different cell types, the inner mitochondrial membrane (mitoBK) and the nuclear envelope’s outer membrane (nBK). Like BK channels in the plasma membrane (pmBK), the open probability of mitoBK and nBK channels are regulated by Ca2+ and voltage and modulated by auxiliary subunits. BK channels share common pharmacology to toxins such as iberiotoxin, charybdotoxin, paxilline, and agonists of the benzimidazole family. However, the precise role of mitoBK and nBK remains largely unknown. To date, mitoBK has been reported to play a role in protecting the heart from ischemic injury. At the same time, pharmacology suggests that nBK has a role in regulating nuclear Ca2+, membrane potential and expression of eNOS. Here, we will discuss at the biophysical level the properties and differences of mitoBK and nBK compared to those of pmBK and their pharmacology and function.

scholarly journals Differential Effects of β1 and β2 Subunits on BK Channel Activity

2005 ◽  
Vol 125 (4) ◽  
pp. 395-411 ◽  
Author(s):  
Patricio Orio ◽  
Ramon Latorre
Keyword(s):  
Steady State ◽  
Calcium Binding ◽  
Voltage Dependence ◽  
Calcium Sensitivity ◽  
Bk Channels ◽  
Bk Channel ◽  
Voltage Sensitivity ◽  
Channel Activation ◽  
Α Subunit ◽  
Voltage Sensors

High conductance, calcium- and voltage-activated potassium (BK) channels are widely expressed in mammals. In some tissues, the biophysical properties of BK channels are highly affected by coexpression of regulatory (β) subunits. β1 and β2 subunits increase apparent channel calcium sensitivity. The β1 subunit also decreases the voltage sensitivity of the channel and the β2 subunit produces an N-type inactivation of BK currents. We further characterized the effects of the β1 and β2 subunits on the calcium and voltage sensitivity of the channel, analyzing the data in the context of an allosteric model for BK channel activation by calcium and voltage (Horrigan and Aldrich, 2002). In this study, we used a β2 subunit without its N-type inactivation domain (β2IR). The results indicate that the β2IR subunit, like the β1 subunit, has a small effect on the calcium binding affinity of the channel. Unlike the β1 subunit, the β2IR subunit also has no effect on the voltage sensitivity of the channel. The limiting voltage dependence for steady-state channel activation, unrelated to voltage sensor movements, is unaffected by any of the studied β subunits. The same is observed for the limiting voltage dependence of the deactivation time constant. Thus, the β1 subunit must affect the voltage sensitivity by altering the function of the voltage sensors of the channel. Both β subunits reduce the intrinsic equilibrium constant for channel opening (L0). In the allosteric activation model, the reduction of the voltage dependence for the activation of the voltage sensors accounts for most of the macroscopic steady-state effects of the β1 subunit, including the increase of the apparent calcium sensitivity of the BK channel. All allosteric coupling factors need to be increased in order to explain the observed effects when the α subunit is coexpressed with the β2IR subunit.

scholarly journals The Membrane Cholesterol Modulates the Interaction Between 17-βEstradiol and the BK Channel

2021 ◽  
Vol 12 ◽  
Author(s):  
Sara T. Granados ◽  
Ramon Latorre ◽  
Yolima P. Torres
Keyword(s):  
Cholesterol Content ◽  
Bk Channels ◽  
Bk Channel ◽  
Cholesterol Concentration ◽  
Membrane Cholesterol ◽  
Cholesterol Depletion ◽  
Vascular Cells ◽  
Channel Activity ◽  
Channel Activation ◽  
Α Subunit

BK channels are composed by the pore forming α subunit and, in some tissues, is associated with different accessory β subunits. These proteins modify the biophysical properties of the channel, amplifying the range of BK channel activation according to the physiological context. In the vascular cells, the pore forming BKα subunit is expressed with the β1 subunit, where they play an essential role in the modulation of arterial tone and blood pressure. In eukaryotes, cholesterol is a structural lipid of the cellular membrane. Changes in the ratio of cholesterol content in the plasma membrane (PM) regulates the BK channel activation altering its open probability, and hence, vascular contraction. It has been shown that the estrogen 17β-Estradiol (E2) causes a vasodilator effect in vascular cells, inducing a leftward shift in the V0.5 of the GV curve. Here, we evaluate whether changes in the membrane cholesterol concentration modify the effect that E2 induces on the BKα/β1 channel activity. Using binding and electrophysiology assays after cholesterol depletion or enrichment, we show that the cholesterol enrichment significantly decreases the expression of the α subunit, while cholesterol depletion increased the expression of that α subunit. Additionally, we demonstrated that changes in the membrane cholesterol cause the loss of the modulatory effect of E2 on the BKα/β1 channel activity, without affecting the E2 binding to the complex. Our data suggest that changes in membrane cholesterol content could affect channel properties related to the E2 effect on BKα/β1 channel activity. Finally, the results suggest that an optimal membrane cholesterol content is essential for the activation of BK channels through the β1 subunit.

scholarly journals Knockout of the BK β2 subunit abolishes inactivation of BK currents in mouse adrenal chromaffin cells and results in slow-wave burst activity

2014 ◽  
Vol 144 (4) ◽  
pp. 275-295 ◽  
Author(s):  
Pedro L. Martinez-Espinosa ◽  
Chengtao Yang ◽  
Vivian Gonzalez-Perez ◽  
Xiao-Ming Xia ◽  
Christopher J. Lingle
Keyword(s):  
Chromaffin Cells ◽  
Bk Channels ◽  
Bk Channel ◽  
Burst Firing ◽  
Constant Current ◽  
Repetitive Firing ◽  
Gene Encoding ◽  
Channel Activation ◽  
Α Subunit ◽  
Adrenal Chromaffin

Rat and mouse adrenal medullary chromaffin cells (CCs) express an inactivating BK current. This inactivation is thought to arise from the assembly of up to four β2 auxiliary subunits (encoded by the kcnmb2 gene) with a tetramer of pore-forming Slo1 α subunits. Although the physiological consequences of inactivation remain unclear, differences in depolarization-evoked firing among CCs have been proposed to arise from the ability of β2 subunits to shift the range of BK channel activation. To investigate the role of BK channels containing β2 subunits, we generated mice in which the gene encoding β2 was deleted (β2 knockout [KO]). Comparison of proteins from wild-type (WT) and β2 KO mice allowed unambiguous demonstration of the presence of β2 subunit in various tissues and its coassembly with the Slo1 α subunit. We compared current properties and cell firing properties of WT and β2 KO CCs in slices and found that β2 KO abolished inactivation, slowed action potential (AP) repolarization, and, during constant current injection, decreased AP firing. These results support the idea that the β2-mediated shift of the BK channel activation range affects repetitive firing and AP properties. Unexpectedly, CCs from β2 KO mice show an increased tendency toward spontaneous burst firing, suggesting that the particular properties of BK channels in the absence of β2 subunits may predispose to burst firing.

Effect of experimental hypercholesterolaemia on K+ channel α-subunit mRNA levels in rabbit hearts

2007 ◽  
Vol 562 (1-2) ◽  
pp. 130-131 ◽  
Author(s):  
Angelika Varga ◽  
Péter Bagossi ◽  
József Tözsér ◽  
Barna Peitl ◽  
Zoltán Szilvássy
Keyword(s):  
K Channel ◽  
Mrna Levels ◽  
Α Subunit ◽  
Subunit Mrna

β-Subunit–Dependent Modulation of hSlo BK Current by Arachidonic Acid

2007 ◽  
Vol 97 (1) ◽  
pp. 62-69 ◽  
Author(s):  
X. Sun ◽  
D. Zhou ◽  
P. Zhang ◽  
E. G. Moczydlowski ◽  
G. G. Haddad
Keyword(s):  
Fatty Acids ◽  
Arachidonic Acid ◽  
Conformational Changes ◽  
Unsaturated Fatty Acids ◽  
Saturated Fatty Acids ◽  
Nordihydroguaiaretic Acid ◽  
Bk Channels ◽  
Bk Channel ◽  
Β Subunit ◽  
Α Subunit

In this study, we examined the effect of arachidonic acid (AA) on the BK α-subunit with or without β-subunits expressed in Xenopus oocytes. In excised patches, AA potentiated the hSlo-α current and slowed inactivation only when β2/3 subunit was co-expressed. The β2-subunit–dependent modulation by AA persisted in the presence of either superoxide dismutase or inhibitors of AA metabolism such as nordihydroguaiaretic acid and eicosatetraynoic acid, suggesting that AA acts directly rather than through its metabolites. Other cis unsaturated fatty acids (docosahexaenoic and oleic acid) also enhanced hSlo-α + β2 currents and slowed inactivation, whereas saturated fatty acids (palmitic, stearic, and caprylic acid) were without effect. Pretreatment with trypsin to remove the cytosolic inactivation domain largely occluded AA action. Intracellularly applied free synthetic β2-ball peptide induced inactivation of the hSlo-α current, and AA failed to enhance this current and slow the inactivation. These results suggest that AA removes inactivation by interacting, possibly through conformational changes, with β2 to prevent the inactivation ball from reaching its receptor. Our data reveal a novel mechanism of β-subunit–dependent modulation of BK channels by AA. In freshly dissociated mouse neocortical neurons, AA eliminated a transient component of whole cell K+ currents. BK channel inactivation may be a specific mechanism by which AA and other unsaturated fatty acids influence neuronal death/survival in neuropathological conditions.

Effects of the gonadotrophin-releasing hormone agonist 'Zoladex' upon pituitary and gonadal function in hypogonadal (hpg) male mice: a comparison with normal male and testicular feminized (tfm) mice

1992 ◽  
Vol 8 (3) ◽  
pp. 249-258 ◽  
Author(s):  
I. S. Scott ◽  
M. K. Bennett ◽  
A. E. Porter-Goff ◽  
C. J. Harrison ◽  
B. S. Cox ◽  
...  
Keyword(s):  
Seminal Vesicle ◽  
Mrna Levels ◽  
Gonadal Function ◽  
Glycoprotein Hormone ◽  
Α Subunit ◽  
Releasing Hormone ◽  
Subunit Mrna ◽  
Serum Lh ◽  
Gonadotrophin Releasing Hormone ◽  
The Common

ABSTRACT Hypogonadal (hpg) mutant mice, with a congenital deficiency of hypothalamic gonadotrophin-releasing hormone (GnRH), and testicular feminized (tfm) mice, which lack a functional androgen receptor, were used to study the effects of the potent GnRH agonist 'Zoladex' (ICI 118630; d-Ser (But)6, Azgly10-GnRH) on pituitary and gonadal function. Zoladex (0.5 mg) in a sustained-release lactide—glycolide copolymer depot was administered subcutaneously under anaesthesia and was left in place for 7 days, after which time the effects of the drug upon pituitary and serum gonadotrophin concentrations, glycoprotein hormone subunit mRNAs and testicular morphology were investigated. At the pituitary level, Zoladex treatment resulted in a substantial reduction in LH content in normal males, and LH content was depressed in hpg mice even below the basal levels normally found in these mutants. Pituitary LH content in the Zoladex-treated animals was depressed in the tfm groups, but not to the same levels as those found in the normal and castrated normal mice. Zoladex treatment at the time of castration prevented the post-operative elevation in serum LH associated with castration alone. In the androgen-deficient tfm mouse, Zoladex did not depress the normally elevated serum LH levels. Serum LH in the hpg animals was, in all cases, below the limit of detection of the assay. Pituitary FSH content was depressed into the hpg range in both the normal and castrated animals, but there was no further depression in the hpg mice. The pituitary content was reduced in the tfm mice, again the effects not being as dramatic as in the normal and castrated animals. Serum FSH content, as measured by radioimmunoassay, was depressed by 50% in normal mice; there was no reduction in the hpg mice, however. With regard to pituitary gonadotrophic hormone gene expression, Zoladex administration to normal mice caused a dramatic reduction in LHβ mRNA content, to a level approximating that found in untreated hpg mice. The drug also depressed LHβ mRNA in the castrated group to the hpg range when given at the time of castration, whereas in untreated castrated mice there was a significant increase in LHβ mRNA. In the tfm mouse, which can be considered as a model for long-term failure of androgen feedback, Zoladex again induced a fall in LHβ mRNA, but not to the same extent as in the normal and normal castrated group. Zoladex had no effect on the already low levels of LHβ mRNA found in hpg mice. Pituitary FSHβ mRNA levels were not significantly altered by Zoladex in any of the treatment groups, whereas the drug induced a substantial rise in the common α-subunit mRNA in normal and hpg mice, to a level equalling that found in castrated tfm mice. In the latter two groups, Zoladex treatment did not result in a further increase in α-subunit mRNA above that found after castration alone, or in the untreated tfm mutant. Treatment for 7 days with Zoladex resulted in a significant increase in testis weight, with spermatogenesis advancing beyond the first meiotic division with many round spermatids found within the seminiferous tubules. However, the interstitial cells remained atrophic and there was evidence of seminal vesicle growth. Nevertheless, there was a small but significant increase in testicular androgen content. Administration of the agonist to hypophysectomized hpg mice did not stimulate testicular or seminal vesicle growth, suggesting that the drug does not stimulate steroidogenesis via a direct action upon the testis. Overall, the pharmacological effects of the drug appear to have turned off the transcription of the LHβ gene, with a consequent reduction in LH synthesis and probably also secretion in the longer term. With FSHβ, gene transcription was apparently unchanged and, with a substantial increase in the common α-subunit message, it would appear that the pituitary gland of Zoladex-treated animals may be predominantly biased towards FSH secretion. Although the circulating FSH levels as measured by radioimmunoassay were unaltered by Zoladex, there are several reports that GnRH agonists increase serum levels of bioactive hormones, perhaps by altering glycosylation of the FSH dimer glycoprotein.

scholarly journals Chronic Prenatal Hypoxia Down-Regulated BK Channel Β1 Subunits in Mesenteric Artery Smooth Muscle Cells of the Offspring

2018 ◽  
Vol 45 (4) ◽  
pp. 1603-1616 ◽  
Author(s):  
Bailin Liu ◽  
Yanping Liu ◽  
Ruixiu Shi ◽  
Xueqin Feng ◽  
Xiang Li ◽  
...  
Keyword(s):  
Smooth Muscle ◽  
Smooth Muscle Cells ◽  
Single Channel ◽  
Muscle Cells ◽  
Bk Channels ◽  
Bk Channel ◽  
Mesenteric Arteries ◽  
Prenatal Hypoxia ◽  
Pregnant Rats ◽  
Α Subunit

Background/Aims: Chronic hypoxia in utero could impair vascular functions in the offspring, underlying mechanisms are unclear. This study investigated functional alteration in large-conductance Ca2+-activated K+ (BK) channels in offspring mesenteric arteries following prenatal hypoxia. Methods: Pregnant rats were exposed to normoxic control (21% O2, Con) or hypoxic (10.5% O2, Hy) conditions from gestational day 5 to 21, their 7-month-old adult male offspring were tested for blood pressure, vascular BK channel functions and expression using patch clamp and wire myograh technique, western blotting, and qRT-PCR. Results: Prenatal hypoxia increased pressor responses and vasoconstrictions to phenylephrine in the offspring. Whole-cell currents density of BK channels and amplitude of spontaneous transient outward currents (STOCs), not the frequency, were significantly reduced in Hy vascular myocytes. The sensitivity of BK channels to voltage, Ca2+, and tamoxifen were reduced in Hy myocytes, whereas the number of channels per patch and the single-channel conductance were unchanged. Prenatal hypoxia impaired NS1102- and tamoxifen-mediated relaxation in mesenteric arteries precontracted with phenylephrine in the presence of Nω-nitro-L-arginine methyl ester. The mRNA and protein expression of BK channel β1, not the α-subunit, was decreased in Hy mesenteric arteries. Conclusions: Impaired BK channel β1-subunits in vascular smooth muscle cells contributed to vascular dysfunction in the offspring exposed to prenatal hypoxia.

scholarly journals Effect of aldosterone on BK channel expression in mammalian cortical collecting duct

2008 ◽  
Vol 295 (3) ◽  
pp. F780-F788 ◽  
Author(s):  
Genevieve Estilo ◽  
Wen Liu ◽  
Nuria Pastor-Soler ◽  
Phillip Mitchell ◽  
Marcelo D. Carattino ◽  
...  
Keyword(s):  
Splice Variants ◽  
Collecting Duct ◽  
Bk Channels ◽  
Bk Channel ◽  
Mrna Abundance ◽  
Cortical Collecting Duct ◽  
Α Subunit ◽  
Channel Expression ◽  
Tubular Flow ◽  
Circulating Levels

Apical large-conductance Ca2+-activated K+ (BK) channels in the cortical collecting duct (CCD) mediate flow-stimulated K+ secretion. Dietary K+ loading for 10–14 days leads to an increase in BK channel mRNA abundance, enhanced flow-stimulated K+ secretion in microperfused CCDs, and a redistribution of immunodetectable channels from an intracellular pool to the apical membrane (Najjar F, Zhou H, Morimoto T, Bruns JB, Li HS, Liu W, Kleyman TR, Satlin LM. Am J Physiol Renal Physiol 289: F922–F932, 2005). To test whether this adaptation was mediated by a K+-induced increase in aldosterone, New Zealand White rabbits were fed a low-Na+ (LS) or high-Na+ (HS) diet for 7–10 days to alter circulating levels of aldosterone but not serum K+ concentration. Single CCDs were isolated for quantitation of BK channel subunit (total, α-splice variants, β-isoforms) mRNA abundance by real-time PCR and measurement of net transepithelial Na+ (JNa) and K+ (JK) transport by microperfusion; kidneys were processed for immunolocalization of BK α-subunit by immunofluorescence microscopy. At the time of death, LS rabbits excreted no urinary Na+ and had higher circulating levels of aldosterone than HS animals. The relative abundance of BK α-, β2-, and β4-subunit mRNA and localization of immunodetectable α-subunit were similar in CCDs from LS and HS animals. In response to an increase in tubular flow rate from ∼1 to 5 nl·min−1·mm−1, the increase in JNa was greater in LS vs. HS rabbits, yet the flow-stimulated increase in JK was similar in both groups. These data suggest that aldosterone does not contribute to the regulation of BK channel expression/activity in response to dietary K+ loading.

scholarly journals Detrusor overactivity is associated with downregulation of large-conductance calcium- and voltage-activated potassium channel protein

2010 ◽  
Vol 298 (6) ◽  
pp. F1416-F1423 ◽  
Author(s):  
Shaohua Chang ◽  
Cristiano Mendes Gomes ◽  
Joseph A. Hypolite ◽  
James Marx ◽  
Jaber Alanzi ◽  
...  
Keyword(s):  
Detrusor Overactivity ◽  
Bladder Outlet Obstruction ◽  
Rabbit Model ◽  
Outlet Obstruction ◽  
Bk Channels ◽  
Bk Channel ◽  
Β Subunit ◽  
Α Subunit ◽  
Translational Study ◽  
Channel Expression

Large-conductance voltage- and calcium-activated potassium (BK) channels have been shown to play a role in detrusor overactivity (DO). The goal of this study was to determine whether bladder outlet obstruction-induced DO is associated with downregulation of BK channels and whether BK channels affect myosin light chain 20 (MLC20) phosphorylation in detrusor smooth muscle (DSM). Partial bladder outlet obstruction (PBOO) was surgically induced in male New Zealand White rabbits. The rabbit PBOO model shows decreased voided volumes and increased voiding frequency. DSM from PBOO rabbits also show enhanced spontaneous contractions compared with control. Both BK channel α- and β-subunits were significantly decreased in DSM from PBOO rabbits. Immunostaining shows BKβ mainly expressed in DSM, and its expression is much less in PBOO DSM compared with control DSM. Furthermore, a translational study was performed to see whether the finding discovered in the animal model can be translated to human patients. The urodynamic study demonstrates several overactive DSM contractions during the urine-filling stage in benign prostatic hyperplasia (BPH) patients with DO, while DSM is very quiet in BPH patients without DO. DSM biopsies revealed significantly less BK channel expression at both mRNA and protein levels. The degree of downregulation of the BK β-subunit was greater than that of the BK α-subunit, and the downregulation of BK was only associated with DO, not BPH. Finally, the small interference (si) RNA-mediated downregulation of the BK β-subunit was employed to study the effect of BK depletion on MLC20 phosphorylation. siRNA-mediated BK channel reduction was associated with an increased MLC20 phosphorylation level in cultured DSM cells. In summary, PBOO-induced DO is associated with downregulation of BK channel expression in the rabbit model, and this finding can be translated to human BPH patients with DO. Furthermore, downregulation of the BK channel may contribute to DO by increasing the basal level of MLC20 phosphorylation.

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