Mitochondrial BKCa channels contribute to protection of cardiomyocytes isolated from chronically hypoxic rats

HIF-1α limits myocardial infarction by promoting mitophagy in mouse hearts adapted to chronic hypoxia

AIM

The transcriptional factor HIF-1α is recognized for its contribution to cardioprotection against acute ischemia/reperfusion injury. Adaptation to chronic hypoxia (CH) is known to stabilize HIF-1α and increase myocardial ischemic tolerance. However, the precise role of HIF-1α in mediating the protective effect remains incompletely understood.

METHODS

Male wild-type (WT) mice and mice with partial Hif1a deficiency (hif1a+/-) were exposed to CH for 4 weeks, while their respective controls were kept under normoxic conditions. Subsequently, their isolated perfused hearts were subjected to ischemia/reperfusion to determine infarct size, while RNA-sequencing of isolated cardiomyocytes was performed. Mitochondrial respiration was measured to evaluate mitochondrial function, and western blots were performed to assess mitophagy.

RESULTS

We demonstrated enhanced ischemic tolerance in WT mice induced by adaptation to CH compared with their normoxic controls and chronically hypoxic hif1a+/- mice. Through cardiomyocyte bulk mRNA sequencing analysis, we unveiled significant reprogramming of cardiomyocytes induced by CH emphasizing mitochondrial processes. CH reduced mitochondrial content and respiration and altered mitochondrial ultrastructure. Notably, the reduced mitochondrial content correlated with enhanced autophagosome formation exclusively in chronically hypoxic WT mice, supported by an increase in the LC3-II/LC3-I ratio, expression of PINK1, and degradation of SQSTM1/p62. Furthermore, pretreatment with the mitochondrial division inhibitor (mdivi-1) abolished the infarct size-limiting effect of CH in WT mice, highlighting the key role of mitophagy in CH-induced cardioprotection.

CONCLUSION

These findings provide new insights into the contribution of HIF-1α to cardiomyocyte survival during acute ischemia/reperfusion injury by activating the selective autophagy pathway.

Lack of activity of the mitochondrial large-conductance calcium-regulated potassium channels in senescent vascular smooth muscle cells

A limited number of studies have shown functional changes in mitochondrial ion channels in aging and senescent cells. We have identified, for the first time, mitochondrial large-conductance calcium-regulated potassium channels in human smooth muscle mitochondria. This channel, with a conductance of 273 pS, was regulated by calcium ions and membrane potential. Additionally, it was activated by the potassium channel opener NS11021 and blocked by paxilline. Importantly, we have shown that senescence of these cells induced by hydrogen peroxide treatment leads to the disappearance of potassium channel protein levels and channel activity measured by the single channel patch-clamp technique. Our data suggest that disturbances in the expression of mitochondrial large conductance calcium-regulated potassium channels may be hallmarks of cellular senescence and contribute to the misregulation of mitochondrial function in senescent cells.

External Hemin as an Inhibitor of Mitochondrial Large-Conductance Calcium-Activated Potassium Channel Activity

The mitochondrial large-conductance calcium-activated potassium channel (mitoBKCa) is located in the inner mitochondrial membrane and seems to play a crucial role in cytoprotection. The mitoBKCa channel is regulated by many modulators, including activators, such as calcium ions and inhibitors, such as heme and its oxidized form hemin. Heme/hemin binds to the heme-binding motif (CXXCH) located between two RCK domains present in the mitochondrial matrix. In the present study, we used the patch-clamp technique in the outside-out configuration to record the activity of mitoBKCa channels. This allowed for the application of channel modulators to the intermembrane-space side of the mitoBKCa. We found that hemin applied in this configuration inhibits the activity of mitoBKCa. In addition, we proved that the observed hemin effect is specific and it is not due to its interaction with the inner mitochondrial membrane. Our data suggest the existence of a new potential heme/hemin binding site in the structure of the mitoBKCa channel located on the mitochondrial intermembrane space side, which could constitute a new way for the regulation of mitoBKCa channel activity.

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

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.

The effect of adaptation to hypoxia on cardiac tolerance to ischemia/reperfusion

The acute myocardial infarction (AMI) and sudden cardiac death (SCD), both associated with acute cardiac ischemia, are one of the leading causes of adult death in economically developed countries. The development of new approaches for the treatment and prevention of AMI and SCD remains the highest priority for medicine. A study on the cardiovascular effects of chronic hypoxia (CH) may contribute to the development of these methods. Chronic hypoxia exerts both positive and adverse effects. The positive effects are the infarct-reducing, vasoprotective, and antiarrhythmic effects, which can lead to the improvement of cardiac contractility in reperfusion. The adverse effects are pulmonary hypertension and right ventricular hypertrophy. This review presents a comprehensive overview of how CH enhances cardiac tolerance to ischemia/reperfusion. It is an in-depth analysis of the published data on the underlying mechanisms, which can lead to future development of the cardioprotective effect of CH. A better understanding of the CH-activated protective signaling pathways may contribute to new therapeutic approaches in an increase of cardiac tolerance to ischemia/reperfusion.

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

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.

Effects of Shenfu injection on myocardial adenosine receptors in rats with myocardial ischemia-reperfusion postconditioning

OBJECTIVE

Shenfu injection (SFI) has been reported to have a protection against myocardial ischemia-reperfusion (MI/R) injury. However, the changes of adenosine receptors in MI/R postconditioning when pretreated with SFI are unclear.

METHODS

Forty-five rats were randomly divided into sham group (sham), MI/R postconditioning group (MI/R-post), low-dose SFI group (1 mL/kg), middle-dose SFI group (2.5 mL/kg), and high-dose SFI group (5 mL/kg). In SFI groups, SFI was intravenously injected before reperfusion, and rats were treated with ischemic postconditioning after ischemia for 30 min. After 24 h of reperfusion, the levels of Ca²⁺ and cAMP in blood platelets were analyzed. Myocardial infarct volume and myocardial pathology were observed. The levels of adenosine receptor subtypes A₁, A_2b, and A₃ in myocardium were analyzed using immunohistochemistry and Western blot. The oxidative stress-related indicators were also observed.

RESULTS

Compared with the MI/R-post group, SFI ameliorated the MI/R injury by decreasing the myocardial infarct area, oxidative stress, and concentration of Ca²⁺ and cAMP (p < 0.01). Pretreatment with SFI enhanced the expression of adenosine receptors A₁ and A_2b in a dose manner compared with the MI/R-post group. In contrast, the levels of adenosine receptor A₃ were increased after MI/R postconditioning compared with the sham group, and its expression continued to increase with the increase of SFI. Furthermore, the oxidative stress reduced with the concentrations of SFI.

CONCLUSION

These results demonstrated that pretreatment with SFI might regulate the expression of adenosine receptors to improve the MI/R postconditioning.

Multidimensional Regulation of Cardiac Mitochondrial Potassium Channels

Mitochondria play a fundamental role in the energetics of cardiac cells. Moreover, mitochondria are involved in cardiac ischemia/reperfusion injury by opening the mitochondrial permeability transition pore which is the major cause of cell death. The preservation of mitochondrial function is an essential component of the cardioprotective mechanism. The involvement of mitochondrial K+ transport in this complex phenomenon seems to be well established. Several mitochondrial K+ channels in the inner mitochondrial membrane, such as ATP-sensitive, voltage-regulated, calcium-activated and Na+-activated channels, have been discovered. This obliges us to ask the following question: why is the simple potassium ion influx process carried out by several different mitochondrial potassium channels? In this review, we summarize the current knowledge of both the properties of mitochondrial potassium channels in cardiac mitochondria and the current understanding of their multidimensional functional role. We also critically summarize the pharmacological modulation of these proteins within the context of cardiac ischemia/reperfusion injury and cardioprotection.

Single channel properties of mitochondrial large conductance potassium channel formed by BK-VEDEC splice variant

The activation of mitochondrial large conductance calcium-activated potassium (mitoBKCa) channels increases cell survival during ischemia/reperfusion injury of cardiac cells. The basic biophysical and pharmacological properties of mitoBKCa correspond to the properties of the BKCa channels from the plasma membrane. It has been suggested that the VEDEC splice variant of the KCNMA1 gene product encoding plasma membrane BKCa is targeted toward mitochondria. However there has been no direct evidence that this protein forms a functional channel in mitochondria. In our study, we used HEK293T cells to express the VEDEC splice variant and observed channel activity in mitochondria using the mitoplast patch-clamp technique. For the first time, we found that transient expression with the VEDEC isoform resulted in channel activity with the conductance of 290 ± 3 pS. The channel was voltage-dependent and activated by calcium ions. Moreover, the activity of the channel was stimulated by the potassium channel opener NS11021 and inhibited by hemin and paxilline, which are known BKCa channel blockers. Immunofluorescence experiments confirmed the partial colocalization of the channel within the mitochondria. From these results, we conclude that the VEDEC isoform of the BKCa channel forms a functional channel in the inner mitochondrial membrane. Additionally, our data show that HEK293T cells are a promising experimental model for expression and electrophysiological studies of mitochondrial potassium channels.

Flavonoids and Mitochondria: Activation of Cytoprotective Pathways?

A large number of diverse mechanisms that lead to cytoprotection have been described to date. Perhaps, not surprisingly, the role of mitochondria in these phenomena is notable. In addition to being metabolic centers, due to their role in cell catabolism, ATP synthesis, and biosynthesis these organelles are triggers and/or end-effectors of a large number of signaling pathways. Their role in the regulation of the intrinsic apoptotic pathway, calcium homeostasis, and reactive oxygen species signaling is well documented. In this review, we aim to characterize the prospects of influencing cytoprotective mitochondrial signaling routes by natural substances of plant origin, namely, flavonoids (e.g., flavanones, flavones, flavonols, flavan-3-ols, anthocyanidins, and isoflavones). Flavonoids are a family of widely distributed plant secondary metabolites known for their beneficial effects on human health and are widely applied in traditional medicine. Their pharmacological characteristics include antioxidative, anticarcinogenic, anti-inflammatory, antibacterial, and antidiabetic properties. Here, we focus on presenting mitochondria-mediated cytoprotection against various insults. Thus, the role of flavonoids as antioxidants and modulators of antioxidant cellular response, apoptosis, mitochondrial biogenesis, autophagy, and fission and fusion is reported. Finally, an emerging field of flavonoid-mediated changes in the activity of mitochondrial ion channels and their role in cytoprotection is outlined.

Participation of opioid receptors in the cytoprotective effect of chronic normobaric hypoxia

We studied the role of the delta, micro, and kappa opioid receptor (OR) subtypes in the cardioprotective effect of chronic continuous normobaric hypoxia (CNH) in the model of acuteanoxia-reoxygenation of isolated cardiomyocytes. Adaptation of rats to CNH was performed by their exposure to atmosphere containing 12% of O(2) for 21 days. Anoxia-reoxygenation of cardiomyocytes isolated from normoxiccontrol rats caused the death of 51 % of cells and lactate dehydrogenase (LDH) release. Adaptation of rats to CNH resulted in the anoxia/reoxygenation-induced cardiomyocyte death of only 38 %, and reduced the LDH release by 25 %. Pre-incubation of the cells with either the non-selective OR (opioid receptor) blocker naloxone (300 nM/l), the delta OR antagonist TIPP(psi) (30 nM/l), the selective delta(2) OR antagonist naltriben (1 nM/l) or the micro OR antagonist CTAP (100 nM/l) for 25 minutes before anoxia abolished the reduction of cell death and LDH release afforded by CNH. The antagonist of delta(1) OR BNTX (1 nM/l) or the kappa OR antagonist nor-binaltorphimine (3 nM/l) did not influence the cytoprotective effects of CNH. Taken together, the cytoprotective effect of CNH is associated with the activation of the delta(2) and micro OR localized on cardiomyocytes.

Endogenous and Agonist-induced Opening of Mitochondrial Big Versus Small Ca2+-sensitive K+ Channels on Cardiac Cell and Mitochondrial Protection

Both big (BKCa) and small (SKCa) conductance Ca-sensitive K channels are present in mammalian cardiac cell mitochondria (m). We used pharmacological agonists and antagonists of BKCa and SKCa channels to examine the importance of endogenous opening of these channels and the relative contribution of either or both of these channels to protect against contractile dysfunction and reduce infarct size after ischemia reperfusion (IR) injury through a mitochondrial protective mechanism. After global cardiac IR injury of ex vivo perfused Guinea pig hearts, we found the following: both agonists NS1619 (for BKCa) and DCEB (for SKCa) improved contractility; BKCa antagonist paxilline (PAX) alone or with SKCa antagonist NS8593 worsened contractility and enhanced infarct size; both antagonists PAX and NS8593 obliterated protection by their respective agonists; BKCa and SKCa antagonists did not block protection afforded by SKCa and BKCa agonists, respectively; and all protective effects by the agonists were blocked by scavenging superoxide anions (O2) with Mn(III) tetrakis (4-benzoic acid) porphyrin (TBAP). Contractile function was inversely associated with global infarct size. In in vivo rats, infusion of NS8593, PAX, or both antagonists enhanced regional infarct size while infusion of either NS1619 or DCEB reduced infarct size. In cardiac mitochondria isolated from ex vivo hearts after IR, combined SKCa and BKCa agonists improved respiratory control index and Ca retention capacity compared with IR alone, whereas the combined antagonists did not alter respiratory control index but worsened Ca retention capacity. Although the differential protective bioenergetics effects of endogenous or exogenous BKCa and SKCa channel opening remain unclear, each channel likely responds to different sensing Ca concentrations and voltage gradients over time during oxidative stress-induced injury to individually or together protect cardiac mitochondria and myocytes.

Mitochondrial Ca2+-activated K+ channels and their role in cell life and death pathways

Ca²⁺-activated K⁺ channels (K_Ca) are expressed at the plasma membrane and in cellular organelles. Expression of all K_Ca channel subtypes (BK, IK and SK) has been detected at the inner mitochondrial membrane of several cell types. Primary functions of these mitochondrial K_Ca channels include the regulation of mitochondrial ROS production, maintenance of the mitochondrial membrane potential and preservation of mitochondrial calcium homeostasis. These channels are therefore thought to contribute to cellular protection against oxidative stress through mitochondrial mechanisms of preconditioning. In this review, we summarize the current knowledge on mitochondrial KCa channels, and their role in mitochondrial function in relation to cell death and survival pathways. More specifically, we systematically discuss studies on the role of these mitochondrial K_Ca channels in pharmacological preconditioning, and according protective effects on ischemic insults to the brain and the heart.

Mitochondrial nucleoid clusters protect newly synthesized mtDNA during Doxorubicin- and Ethidium Bromide-induced mitochondrial stress

Mitochondrial DNA (mtDNA) is compacted in ribonucleoprotein complexes called nucleoids, which can divide or move within the mitochondrial network. Mitochondrial nucleoids are able to aggregate into clusters upon reaction with intercalators such as the mtDNA depletion agent Ethidium Bromide (EB) or anticancer drug Doxorobicin (DXR). However, the exact mechanism of nucleoid clusters formation remains unknown. Resolving these processes may help to elucidate the mechanisms of DXR-induced cardiotoxicity. Therefore, we addressed the role of two key nucleoid proteins; mitochondrial transcription factor A (TFAM) and mitochondrial single-stranded binding protein (mtSSB); in the formation of mitochondrial nucleoid clusters during the action of intercalators. We found that both intercalators cause numerous aberrations due to perturbing their native status. By blocking mtDNA replication, both agents also prevented mtDNA association with TFAM, consequently causing nucleoid aggregation into large nucleoid clusters enriched with TFAM, co-existing with the normal nucleoid population. In the later stages of intercalation (>48h), TFAM levels were reduced to 25%. In contrast, mtSSB was released from mtDNA and freely distributed within the mitochondrial network. Nucleoid clusters mostly contained nucleoids with newly replicated mtDNA, however the nucleoid population which was not in replication mode remained outside the clusters. Moreover, the nucleoid clusters were enriched with p53, an anti-oncogenic gatekeeper. We suggest that mitochondrial nucleoid clustering is a mechanism for protecting nucleoids with newly replicated DNA against intercalators mediating genotoxic stress. These results provide new insight into the common mitochondrial response to mtDNA stress and can be implied also on DXR-induced mitochondrial cytotoxicity.

Upregulated ATF6 contributes to chronic intermittent hypoxia-afforded protection against myocardial ischemia/reperfusion injury

In the present study, we investigated the role of activating transcription factor 6 (ATF6) in the mechanism by which chronic intermittent hypoxia (CIH) increases tolerance to myocardial ischemia/reperfusion (I/R). Experiments were conducted using a rat model of I/R injury in vivo and isolated Langendorff-perfused rat hearts ex vivo. The role of Akt in this process was also investigated in vitro using rat myoblast H9c2 cells. Cell viability was measured using a cell counting kit-8 assay. Lactate dehydrogenase (LDH) and creatine kinase cardiac isoenzyme activity were also measured as markers of cellular damage. ATF6, Akt and phosphorylated (p)-Akt expression was analyzed by western blot analysis. RNA interference (RNAi) was used to suppress ATF6 expression. We noted that ATF6 expression in the ventricular myocardium was significantly increased in rats exposed to CIH. Furthermore, we noted that CIH preserved cardiac function after I/R in vivo and improved post-ischemic recovery of myocardial performance in isolated rat hearts. ATF6 and p-Akt expression was upregulated in cultured H9c2 cells exposed to chronic mild hypoxia compared with those cultured under normoxic conditions. Chronic mild hypoxia attenuated subsequent simulated I/R injury in H9c2 cells (48 h), as evidenced by increased cell viability and decreased LDH activity. By contrast, decreased cell viability and increased LDH activity were observed in siRNA-ATF6-transfected H9c2 cells, with a concomitant reduction in p-Akt levels. These results indicated that ATF6 upregulation is involved in the mechanism by which CIH attenuates myocardial I/R injury, possibly through upregulation of p-Akt, which is a key regulator of cardiomyocyte survival.

Calcium-Activated Potassium Channels: Potential Target for Cardiovascular Diseases

Ca(2+)-activated K(+) channels (KCa) are classified into three subtypes: big conductance (BKCa), intermediate conductance (IKCa), and small conductance (SKCa) KCa channels. The three types of KCa channels have distinct physiological or pathological functions in cardiovascular system. BKCa channels are mainly expressed in vascular smooth muscle cells (VSMCs) and inner mitochondrial membrane of cardiomyocytes, activation of BKCa channels in these locations results in vasodilation and cardioprotection against cardiac ischemia. IKCa channels are expressed in VSMCs, endothelial cells, and cardiac fibroblasts and involved in vascular smooth muscle proliferation, migration, vessel dilation, and cardiac fibrosis. SKCa channels are widely expressed in nervous and cardiovascular system, and activation of SKCa channels mainly contributes membrane hyperpolarization. In this chapter, we summarize the physiological and pathological roles of the three types of KCa channels in cardiovascular system and put forward the possibility of KCa channels as potential target for cardiovascular diseases.

Oxidative Stress and Maxi Calcium-Activated Potassium (BK) Channels

All cells contain ion channels in their outer (plasma) and inner (organelle) membranes. Ion channels, similar to other proteins, are targets of oxidative impact, which modulates ion fluxes across membranes. Subsequently, these ion currents affect electrical excitability, such as action potential discharge (in neurons, muscle, and receptor cells), alteration of the membrane resting potential, synaptic transmission, hormone secretion, muscle contraction or coordination of the cell cycle. In this chapter we summarize effects of oxidative stress and redox mechanisms on some ion channels, in particular on maxi calcium-activated potassium (BK) channels which play an outstanding role in a plethora of physiological and pathophysiological functions in almost all cells and tissues. We first elaborate on some general features of ion channel structure and function and then summarize effects of oxidative alterations of ion channels and their functional consequences.

Tumour necrosis factor-α contributes to improved cardiac ischaemic tolerance in rats adapted to chronic continuous hypoxia

AIM

It has been demonstrated that tumour necrosis factor-alpha (TNF-α) via its receptor 2 (TNFR2) plays a role in the cardioprotective effects of preconditioning. It is also well known that chronic hypoxia is associated with activation of inflammatory response. With this background, we hypothesized that TNF-α signalling may contribute to the improved ischaemic tolerance of chronically hypoxic hearts.

METHODS

Adult male Wistar rats were kept either at room air (normoxic controls) or at continuous normobaric hypoxia (CNH; inspired O2 fraction 0.1) for 3 weeks; subgroups of animals were treated with infliximab (monoclonal antibody against TNF-α; 5 mg kg(-1), i.p., once a week). Myocardial levels of oxidative stress markers and the expression of selected signalling molecules were analysed. Infarct size (tetrazolium staining) was assessed in open-chest rats subjected to acute coronary artery occlusion/reperfusion.

RESULTS

CNH increased myocardial TNF-α level and expression of TNFR2; this response was abolished by infliximab treatment. CNH reduced myocardial infarct size from 50.8 ± 4.3% of the area at risk in normoxic animals to 35.5 ± 2.4%. Infliximab abolished the protective effect of CNH (44.9 ± 2.0%). CNH increased the levels of oxidative stress markers (3-nitrotyrosine and malondialdehyde), the expression of nuclear factor κB and manganese superoxide dismutase, while these effects were absent in infliximab-treated animals. CNH-elevated levels of inducible nitric oxide synthase and cyclooxygenase 2 were not affected by infliximab.

CONCLUSION

TNF-α plays a role in the induction of ischaemia-resistant cardiac phenotype of CNH rats, possibly via the activation of protective redox signalling.

Involvement of PKCε in Cardioprotection Induced by Adaptation to Chronic Continuous Hypoxia

Continuous normobaric hypoxia (CNH) renders the heart more tolerant to acute ischemia/reperfusion injury. Protein kinase C (PKC) is an important component of the protective signaling pathway, but the contribution of individual PKC isoforms under different hypoxic conditions is poorly understood. The aim of this study was to analyze the expression of PKCε after the adaptation to CNH and to clarify its role in increased cardiac ischemic tolerance with the use of PKCε inhibitory peptide KP-1633. Adult male Wistar rats were exposed to CNH (10 % O2, 3 weeks) or kept under normoxic conditions. The protein level of PKCε and its phosphorylated form was analyzed by Western blot in homogenate, cytosolic and particulate fractions; the expression of PKCε mRNA was measured by RT-PCR. The effect of KP-1633 on cell viability and lactate dehydrogenase (LDH) release was analyzed after 25-min metabolic inhibition followed by 30-min re-energization in freshly isolated left ventricular myocytes. Adaptation to CNH increased myocardial PKCε at protein and mRNA levels. The application of KP-1633 blunted the hypoxia-induced salutary effects on cell viability and LDH release, while control peptide KP-1723 had no effect. This study indicates that PKCε is involved in the cardioprotective mechanism induced by CNH.

A hypoxic episode during cardiogenesis downregulates the adenosinergic system and alters the myocardial anoxic tolerance

To what extent hypoxia alters the adenosine (ADO) system and impacts on cardiac function during embryogenesis is not known. Ectonucleoside triphosphate diphosphohydrolase (CD39), ecto-5'-nucleotidase (CD73), adenosine kinase (AdK), adenosine deaminase (ADA), equilibrative (ENT1,3,4), and concentrative (CNT3) transporters and ADO receptors A1, A2A, A2B, and A3 constitute the adenosinergic system. During the first 4 days of development chick embryos were exposed in ovo to normoxia followed or not followed by 6 h hypoxia. ADO and glycogen content and mRNA expression of the genes were determined in the atria, ventricle, and outflow tract of the normoxic (N) and hypoxic (H) hearts. Electrocardiogram and ventricular shortening of the N and H hearts were recorded ex vivo throughout anoxia/reoxygenation ± ADO. Under basal conditions, CD39, CD73, ADK, ADA, ENT1,3,4, CNT3, and ADO receptors were differentially expressed in the atria, ventricle, and outflow tract. In H hearts ADO level doubled, glycogen decreased, and mRNA expression of all the investigated genes was downregulated by hypoxia, except for A2A and A3 receptors. The most rapid and marked downregulation was found for ADA in atria. H hearts were arrhythmic and more vulnerable to anoxia-reoxygenation than N hearts. Despite downregulation of the genes, exposure of isolated hearts to ADO 1) preserved glycogen through activation of A1 receptor and Akt-GSK3β-GS pathway, 2) prolonged activity and improved conduction under anoxia, and 3) restored QT interval in H hearts. Thus hypoxia-induced downregulation of the adenosinergic system can be regarded as a coping response, limiting the detrimental accumulation of ADO without interfering with ADO signaling.

Bax inhibitor-1-mediated inhibition of mitochondrial Ca2+ intake regulates mitochondrial permeability transition pore opening and cell death

A recently studied endoplasmic reticulum (ER) stress regulator, Bax inhibitor-1 (BI-1) plays a regulatory role in mitochondrial Ca²⁺ levels. In this study, we identified ER-resident and mitochondria-associated ER membrane (MAM)-resident populations of BI-1. ER stress increased mitochondrial Ca²⁺ to a lesser extent in BI-1–overexpressing cells (HT1080/BI-1) than in control cells, most likely as a result of impaired mitochondrial Ca²⁺ intake ability and lower basal levels of intra-ER Ca²⁺. Moreover, opening of the Ca²⁺-induced mitochondrial permeability transition pore (PTP) and cytochrome c release were regulated by BI-1. In HT1080/BI-1, the basal mitochondrial membrane potential was low and also resistant to Ca²⁺ compared with control cells. The activity of the mitochondrial membrane potential-dependent mitochondrial Ca²⁺ intake pore, the Ca²⁺ uniporter, was reduced in the presence of BI-1. This study also showed that instead of Ca²⁺, other cations including K⁺ enter the mitochondria of HT1080/BI-1 through mitochondrial Ca²⁺-dependent ion channels, providing a possible mechanism by which mitochondrial Ca²⁺ intake is reduced, leading to cell protection. We propose a model in which BI-1–mediated sequential regulation of the mitochondrial Ca²⁺ uniporter and Ca²⁺-dependent K⁺ channel opening inhibits mitochondrial Ca²⁺ intake, thereby inhibiting PTP function and leading to cell protection.

Mitochondrial channels: ion fluxes and more

The field of mitochondrial ion channels has recently seen substantial progress, including the molecular identification of some of the channels. An integrative approach using genetics, electrophysiology, pharmacology, and cell biology to clarify the roles of these channels has thus become possible. It is by now clear that many of these channels are important for energy supply by the mitochondria and have a major impact on the fate of the entire cell as well. The purpose of this review is to provide an up-to-date overview of the electrophysiological properties, molecular identity, and pathophysiological functions of the mitochondrial ion channels studied so far and to highlight possible therapeutic perspectives based on current information.

K(+) channels: function-structural overview

Potassium channels are particularly important in determining the shape and duration of the action potential, controlling the membrane potential, modulating hormone secretion, epithelial function and, in the case of those K(+) channels activated by Ca(2+), damping excitatory signals. The multiplicity of roles played by K(+) channels is only possible to their mammoth diversity that includes at present 70 K(+) channels encoding genes in mammals. Today, thanks to the use of cloning, mutagenesis, and the more recent structural studies using x-ray crystallography, we are in a unique position to understand the origins of the enormous diversity of this superfamily of ion channels, the roles they play in different cell types, and the relations that exist between structure and function. With the exception of two-pore K(+) channels that are dimers, voltage-dependent K(+) channels are tetrameric assemblies and share an extremely well conserved pore region, in which the ion-selectivity filter resides. In the present overview, we discuss in the function, localization, and the relations between function and structure of the five different subfamilies of K(+) channels: (a) inward rectifiers, Kir; (b) four transmembrane segments-2 pores, K2P; (c) voltage-gated, Kv; (d) the Slo family; and (e) Ca(2+)-activated SK family, SKCa.

Pharmacological activation of mitochondrial BK(Ca) channels protects isolated cardiomyocytes against simulated reperfusion-induced injury

The aim of this study was to find out whether opening of mitochondrial large-conductance Ca(2+)-activated potassium channels (BK(Ca)) protects cardiomyocytes against injury caused by simulated ischemia and reperfusion. This study also aimed to determine whether the protective mechanism involves signaling by reactive oxygen species (ROS) and phosphatidylinositol-3-kinase (PI3K). We used isolated ventricular myocytes, which are believed to contain no functional BK(Ca) channels in the sarcolemma. Cells were isolated from the left ventricles of adult male Wistar rats and subjected to 25-min metabolic inhibition with NaCN and 2-deoxyglucose followed by 30-min re-energization. NS11021 (0.1 μmol/L), a novel BK(Ca) channel opener, or hydrogen peroxide (2 μmol/L) added at re-energization, increased cell survival (the number of rod-shaped cells) and markedly reduced the release of lactate dehydrogenase (LDH). These cytoprotective effects of NS11021 were completely abolished by paxilline, a BK(Ca) inhibitor, or tempol, an antioxidant, but not by wortmannin, an inhibitor of PI3K. NS11021 slightly but significantly increased the fluorescence signal in 2'7'-dichlorodihydrofluorescein diacetate (DCF-DA)-loaded myocytes, indicating an increased ROS formation. The NS11021-induced ROS formation was abolished by paxilline or tempol. NS13558 (0.1 μmol/L), an inactive structural analogue of NS11021, affected neither cell survival/LDH release nor DCF-DA fluorescence. These results suggest that pharmacological activation of mitochondrial BK(Ca) channels effectively protects isolated cardiomyocytes against injury associated with simulated reperfusion. The mechanism for this form of protection requires ROS signaling, but not the activation of the PI3K pathway.

Large-conductance Ca²⁺-activated potassium channel in mitochondria of endothelial EA.hy926 cells

In the present study, we describe the existence of a large-conductance Ca²⁺-activated potassium (BKCa) channel in the mitochondria of the human endothelial cell line EA.hy926. A single-channel current was recorded from endothelial mitoplasts (i.e., inner mitochondrial membrane) using the patch-clamp technique in the mitoplast-attached mode. A potassium-selective current was recorded with a mean conductance equal to 270 ± 10 pS in a symmetrical 150/150 mM KCl isotonic solution. The channel activity, which was determined as the open probability, increased with the addition of calcium ions and the potassium channel opener NS1619. Conversely, the activity of the channel was irreversibly blocked by paxilline and iberiotoxin, BKCa channel inhibitors. The open-state probability was found to be voltage dependent. The substances known to modulate BKCa channel activity influenced the bioenergetics of mitochondria isolated from human endothelial EA.hy926 cells. In isolated mitochondria, 100 μM Ca²⁺, 10 μM NS1619, and 0.5 μM NS11021 depolarized the mitochondrial membrane potential and stimulated nonphosphorylating respiration. These effects were blocked by iberiotoxin and paxilline in a potassium-dependent manner. Under phosphorylating conditions, NS1619-induced, iberiotoxin-sensitive uncoupling diverted energy from ATP synthesis during the phosphorylating respiration of the endothelial mitochondria. Immunological analysis with antibodies raised against proteins of the plasma membrane BKCa channel identified a pore-forming α-subunit and an auxiliary β₂-subunit of the channel in the endothelial mitochondrial inner membrane. In conclusion, we show for the first time that the inner mitochondrial membrane in human endothelial EA.hy926 cells contains a large-conductance calcium-dependent potassium channel with properties similar to those of the surface membrane BKCa channel.

BK Channels in Cardiovascular Diseases and Aging

Aging is a major risk factor for cardiovascular diseases, one of the main world-wide causes of death. Several structural and functional changes occur in the cardiovascular system during the aging process and the mechanisms involved in such alterations are yet to be completely described. BK channels are transmembrane proteins that play a key role in many physiological processes, including regulation of vascular tone. In vascular smooth muscle cells, BK opening and the consequent efflux of potassium (K(+)) leads to membrane hyperpolarization, which is followed by the closure of voltage-dependent Ca(2+) channels, reduction of Ca(2+) entry and vasodilatation. BK regulates nitric oxide-mediated vasodilatation and thus is crucial for normal endothelial function. Herein we will briefly review general structural properties of BK and focus on their function in the cardiovascular system emphasizing their role in cardiovascular aging and diseases.

AMP-activated protein kinase in BK-channel regulation and protection against hearing loss following acoustic overstimulation

The energy-sensing AMP-activated serine/threonine protein kinase (AMPK) confers cell survival in part by stimulation of cellular energy production and limitation of cellular energy utilization. AMPK-sensitive functions further include activities of epithelial Na+ channel ENaC and voltage-gated K+ channel KCNE1/KCNQ1. AMPK is activated by an increased cytosolic Ca2+ concentration. The present study explored whether AMPK regulates the Ca2+-sensitive large conductance and voltage-gated potassium (BK) channel. cRNA encoding BK channel was injected into Xenopus oocytes with and without additional injection of wild-type AMPK (AMPKα1+AMPKβ1+AMPKγ1), constitutively active AMPKγR70Q, or inactive AMPKαK45R. BK-channel activity was determined utilizing the 2-electrode voltage-clamp. Moreover, BK-channel protein abundance in the cell membrane was determined by confocal immunomicroscopy. As BK channels are expressed in outer hair cells (OHC) of the inner ear and lack of BK channels increases noise vulnerability, OHC BK-channel expression was examined by immunohistochemistry and hearing function analyzed by auditory brain stem response measurements in AMPKα1-deficient mice (ampk-/-) and in wild-type mice (ampk+/+). As a result, coexpression of AMPK or AMPKγR70Q but not of AMPKαK45R significantly enhanced BK-channel-mediated currents and BK-channel protein abundance in the oocyte cell membrane. BK-channel expression in the inner ear was lower in ampk-/- mice than in ampk+/+ mice. The hearing thresholds prior to and immediately after an acoustic overexposure were similar in ampk-/- and ampk+/+ mice. However, the recovery from the acoustic trauma was significantly impaired in ampk-/- mice compared to ampk+/+ mice. In summary, AMPK is a potent regulator of BK channels. It may thus participate in the signaling cascades that protect the inner ear from damage following acoustic overstimulation.

Physiology of potassium channels in the inner membrane of mitochondria

The inner membrane of the ATP-producing organelles of endosymbiotic origin, mitochondria, has long been considered to be poorly permeable to cations and anions, since the strict control of inner mitochondrial membrane permeability is crucial for efficient ATP synthesis. Over the past 30 years, however, it has become clear that various ion channels--along with antiporters and uniporters--are present in the mitochondrial inner membrane, although at rather low abundance. These channels are important for energy supply, and some are a decisive factor in determining whether a cell lives or dies. Their electrophysiological and pharmacological characterisations have contributed importantly to the ongoing elucidation of their pathophysiological roles. This review gives an overview of recent advances in our understanding of the functions of the mitochondrial potassium channels identified so far. Open issues concerning the possible molecular entities giving rise to the observed activities and channel protein targeting to mitochondria are also discussed.

Rottlerin increases cardiac contractile performance and coronary perfusion through BKCa++ channel activation after cold cardioplegic arrest in isolated hearts

BACKGROUND

Cardioplegia and cardiopulmonary bypass (CP/CPB) subjects myocardium to complex injurious stimuli that can result in cardiomyocyte and vascular contractile abnormalities. Rottlerin, originally identified as a delta-protein kinase C inhibitor, has a number of known additional effects that may be beneficial in the setting of CP/CPB. We tested the hypothesis that rottlerin mitigates deleterious effects associated with CP/CPB.

METHODS AND RESULTS

Langendorff-perfused isolated rat hearts were subjected to 2 hours intermittent cold (10°C) CP (St Thomas II) followed by 30 minutes normothermic reperfusion. CP was delivered every 30 minutes for 1 minute. Hearts were treated with rottlerin 1 μmol/L (CP+R) (n=7) or without rottlerin (CP) (n=9), and the BK(Ca++) channel inhibitor paxilline 100 nmol/L was supplied in the CP. Hearts constantly perfused with KHB served as controls (n=6). Baseline parameters of cardiac function were similar between groups. CP resulted in reduced cardiac function (left ventricular diastolic pressure, 39 ± 3.8%; ± dP/dt, 32 ± 4.4%, -41 ± 5.1% decrease compared to baseline). Treatment with rottlerin 1 μmol/L significantly improved CP-induced cardiac function (left ventricular diastolic pressure, 20 ± 5.9%; ± dP/dt, 5.2 ± 4.5%, -11.6 ± 4.7% decrease versus baseline; P<0.05 CP+R versus CP). Rottlerin also caused a significant increase in coronary flow postreperfusion (CP, 34 ± 4.2% decrease from baseline; CP+R, 26 ± 9.6% increase over baseline; P=0.01). Independent of vascular effects, CP significantly decreased isolated myocyte contraction, which was restored by rottlerin treatment. The BK(Ca++) channel inhibitor greatly reduced the majority of beneficial effects associated with rottlerin.

CONCLUSIONS

Rottlerin significantly improves cardiac performance after CP arrest through improved cardiomyocyte contraction and coronary perfusion.