Activation of mitochondrial large-conductance calcium-activated K+ channels via protein kinase A mediates desflurane-induced preconditioning

A potent and selective activator of large-conductance Ca2+-activated K+ channels induces preservation of mitochondrial function after hypoxia and reoxygenation by handling of calcium and transmembrane potential

AIMS

Ischaemic heart disease remains a significant cause of mortality globally. A pharmacological agent that protects cardiac mitochondria against oxygen deprivation injuries is welcome in therapy against acute myocardial infarction. Here, we evaluate the effect of large-conductance Ca2+-activated K+ channels (BKCa) activator, Compound Z, in isolated mitochondria under hypoxia and reoxygenation.

METHODS

Mitochondria from mice hearts were obtained by differential centrifugation. The isolated mitochondria were incubated with a BKCa channel activator, Compound Z, and subjected to normoxia or hypoxia/reoxygenation. Mitochondrial function was evaluated by measurement of O2 consumption in the complexes I, II, and IV in the respiratory states 1, 2, 3, and by maximal uncoupled O2 uptake, ATP production, ROS production, transmembrane potential, and calcium retention capacity.

RESULTS

Incubation of isolated mitochondria with Compound Z under normoxia conditions reduced the mitochondrial functions and induced the production of a significant amount of ROS. However, under hypoxia/reoxygenation, the Compound Z prevented a profound reduction in mitochondrial functions, including reducing ROS production over the hypoxia/reoxygenation group. Furthermore, hypoxia/reoxygenation induced a large mitochondria depolarization, which Compound Z incubation prevented, but, even so, Compound Z created a small depolarization. The mitochondrial calcium uptake was prevented by the BKCa activator, extruding the mitochondrial calcium present before Compound Z incubation.

CONCLUSION

The Compound Z acts as a mitochondrial BKCa channel activator and can protect mitochondria function against hypoxia/reoxygenation injury, by handling mitochondrial calcium and transmembrane potential.

Beneficial Effects of Halogenated Anesthetics in Cardiomyocytes: The Role of Mitochondria

In the last few years, the use of anesthetic drugs has been related to effects other than those initially related to their fundamental effect, hypnosis. Halogenated anesthetics, mainly sevoflurane, have been used as a therapeutic tool in patients undergoing cardiac surgery, thanks to the beneficial effect of the cardiac protection they generate. This effect has been described in several research studies. The mechanism by which they produce this effect has been associated with the effects generated by anesthetic preconditioning and postconditioning. The mechanisms by which these effects are induced are directly related to the modulation of oxidative stress and the cellular damage generated by the ischemia/reperfusion procedure through the overexpression of different enzymes, most of them included in the Reperfusion Injury Salvage Kinase (RISK) and the Survivor Activating Factor Enhancement (SAFE) pathways. Mitochondria is the final target of the different routes of pre- and post-anesthetic conditioning, and it is preserved from the damage generated in moments of lack of oxygen and after the recovery of the normal oxygen concentration. The final consequence of this effect has been related to better cardiac function in this type of patient, with less myocardial damage, less need for inotropic drugs to achieve normal myocardial function, and a shorter hospital stay in intensive care units. The mechanisms through which mitochondrial homeostasis is maintained and its relationship with the clinical effect are the basis of our review. From a translational perspective, we provide information regarding mitochondrial physiology and physiopathology in cardiac failure and the role of halogenated anesthetics in modulating oxidative stress and inducing myocardial conditioning.

Myocardial Protection by Desflurane: From Basic Mechanisms to Clinical Applications

ABSTRACT

Coronary heart disease is an affliction that is common and has an adverse effect on patients' quality of life and survival while also raising the risk of intraoperative anesthesia. Mitochondria are the organelles most closely associated with the pathogenesis, development, and prognosis of coronary heart disease. Ion abnormalities, an acidic environment, the production of reactive oxygen species, and other changes during abnormal myocardial metabolism cause the opening of mitochondrial permeability transition pores, which disrupts electron transport, impairs mitochondrial function, and even causes cell death. Differences in reliability and cost-effectiveness between desflurane and other volatile anesthetics are minor, but desflurane has shown better myocardial protective benefits in the surgical management of patients with coronary artery disease. The results of myocardial protection by desflurane are briefly summarized in this review, and biological functions of the mitochondrial permeability transition pore, mitochondrial electron transport chain, reactive oxygen species, adenosine triphosphate-dependent potassium channels, G protein-coupled receptors, and protein kinase C are discussed in relation to the protective mechanism of desflurane. This article also discusses the effects of desflurane on patient hemodynamics, myocardial function, and postoperative parameters during coronary artery bypass grafting. Although there are limited and insufficient clinical investigations, they do highlight the possible advantages of desflurane and offer additional suggestions for patients.

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.

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.

Inhibitory effect of desflurane on degranulation of mast cells induced by lateral ventricular injection of stimulator-C48/80 in C57BL/6 male mice

Inhalation of anesthetic agents have been observed to confer neuroprotection for decades. The present study was intended to determine whether desflurane (DES) prohibits mast cells (MCs) from degranulation induced by lateral ventricular injection (LVC) with Compound 48/80 (C48/80) in C57BL/6. Total 100 mice were recruited to this study, but only 88 male mice (20–24 weeks) were survived from the procedure, and randomized and allocated into four groups: (A) the saline group; (B) the C48/80 group; (C) the sodium cromoglycate (CRO + C48/80) group; (D) 7.5% DES preconditioning for 2 h + C48/80 lateral ventricular injection (DES + C48/80) group. The slices of mice brain thalamus were performed for toluidine blue staining (MCs) and immunochemistry (fluorescence of Iba1 and GFAP, respectively), and brain tissues were extracted to probe IL-6, TNF-α, NF-κB (p65), and TLR4 against GAPDH by western blotting. Our results demonstrated that administration of C48/80 provoked degranulation of mast cells at thalamus, increasing the fluorescence intensities of Iba1 and GFAP, and over-expressing IL-6, TNF-α, NF-κB(p65), and TLR4. However, pre-conditioning inhalation of DES prohibited MCs from degranulation, diminishing the fluorescent intensities of Iba1 and GFAP, decreasing expressed levels of IL-6, TNF-α, NF-κB(p65), as well as TLR4. It suggests inhalation DES could inhibit the neuroinflammation and deactivate glial and astrocytes via direct prohibiting degranulation of MCs at thalamus in the central nervous system (CNS).

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.

Signaling pathways targeting mitochondrial potassium channels

In this review, we describe key signaling pathways regulating potassium channels present in the inner mitochondrial membrane. The signaling cascades covered here include phosphorylation, redox reactions, modulation by calcium ions and nucleotides. The following types of potassium channels have been identified in the inner mitochondrial membrane of various tissues: ATP-sensitive, Ca²⁺-activated, voltage-gated and two-pore domain potassium channels. The direct roles of these channels involve regulation of mitochondrial respiration, membrane potential and synthesis of reactive oxygen species (ROS). Changes in channel activity lead to diverse pro-life and pro-death responses in different cell types. Hence, characterizing the signaling pathways regulating mitochondrial potassium channels will facilitate understanding the physiological role of these proteins. Additionally, we describe in this paper certain regulatory mechanisms, which are unique to mitochondrial potassium channels.

The Slo(w) path to identifying the mitochondrial channels responsible for ischemic protection

Mitochondria play an important role in tissue ischemia and reperfusion (IR) injury, with energetic failure and the opening of the mitochondrial permeability transition pore being the major causes of IR-induced cell death. Thus, mitochondria are an appropriate focus for strategies to protect against IR injury. Two widely studied paradigms of IR protection, particularly in the field of cardiac IR, are ischemic preconditioning (IPC) and volatile anesthetic preconditioning (APC). While the molecular mechanisms recruited by these protective paradigms are not fully elucidated, a commonality is the involvement of mitochondrial K⁺ channel opening. In the case of IPC, research has focused on a mitochondrial ATP-sensitive K⁺ channel (mitoK_ATP), but, despite recent progress, the molecular identity of this channel remains a subject of contention. In the case of APC, early research suggested the existence of a mitochondrial large-conductance K⁺ (BK, big conductance of potassium) channel encoded by the Kcnma1 gene, although more recent work has shown that the channel that underlies APC is in fact encoded by Kcnt2 In this review, we discuss both the pharmacologic and genetic evidence for the existence and identity of mitochondrial K⁺ channels, and the role of these channels both in IR protection and in regulating normal mitochondrial function.

Cardiac Slo2.1 Is Required for Volatile Anesthetic Stimulation of K+ Transport and Anesthetic Preconditioning

BACKGROUND

Anesthetic preconditioning (APC) is a clinically important phenomenon in which volatile anesthetics (VAs) protect tissues such as heart against ischemic injury. The mechanism of APC is thought to involve K channels encoded by the Slo gene family, and the authors showed previously that slo-2 is required for APC in Caenorhabditis elegans. Thus, the authors hypothesized that a slo-2 ortholog may mediate APC-induced cardioprotection in mammals.

METHODS

A perfused heart model of ischemia-reperfusion injury, a fluorescent assay for K flux, and mice lacking Slo2.1 (Slick), Slo2.2 (Slack), or both (double knockouts, Slo2.x dKO) were used to test whether these channels are required for APC-induced cardioprotection and for cardiomyocyte or mitochondrial K transport.

RESULTS

In wild-type (WT) hearts, APC improved post-ischemia-reperfusion functional recovery (APC = 39.5 ± 3.7% of preischemic rate × pressure product vs. 20.3 ± 2.3% in controls, means ± SEM, P = 0.00051, unpaired two-tailed t test, n = 8) and lowered infarct size (APC = 29.0 ± 4.8% of LV area vs. 51.4 ± 4.5% in controls, P = 0.0043, n = 8). Protection by APC was absent in hearts from Slo2.1 mice (% recovery APC = 14.6 ± 2.6% vs. 16.5 ± 2.1% in controls, P = 0.569, n = 8 to 9, infarct APC = 52.2 ± 5.4% vs. 53.5 ± 4.7% in controls, P = 0.865, n = 8 to 9). APC protection was also absent in Slo2.x dKO hearts (% recovery APC = 11.0 ± 1.7% vs. 11.9 ± 2.2% in controls, P = 0.725, n = 8, infarct APC = 51.6 ± 4.4% vs. 50.5 ± 3.9% in controls, P = 0.855, n = 8). Meanwhile, Slo2.2 hearts responded similar to WT (% recovery APC = 41.9 ± 4.0% vs. 18.0 ± 2.5% in controls, P = 0.00016, n = 8, infarct APC = 25.2 ± 1.3% vs. 50.8 ± 3.3% in controls, P < 0.000005, n = 8). Furthermore, VA-stimulated K transport seen in cardiomyocytes or mitochondria from WT or Slo2.2 mice was absent in Slo2.1 or Slo2.x dKO.

CONCLUSION

Slick (Slo2.1) is required for both VA-stimulated K flux and for the APC-induced cardioprotection.

Functional Role of Mitochondrial and Nuclear BK Channels

BK channels are important for the regulation of many cell functions. The significance of plasma membrane BK channels in the control of action potentials, resting membrane potential, and neurotransmitter release is well established; however, the composition and functions of mitochondrial and nuclear BK (nBK) channels are largely unknown. In this chapter, we summarize the recent findings on the subcellular localization, biophysical, and pharmacological properties of mitochondrial and nBK channels and discuss their molecular identity and physiological functions.

Morphine-Induced Preconditioning: Involvement of Protein Kinase A and Mitochondrial Permeability Transition Pore

Background

Morphine induces myocardial preconditioning (M-PC) via activation of mitochondrial large conductance Ca²⁺-sensitive potassium (mK_Ca) channels. An upstream regulator of mK_Ca channels is protein kinase A (PKA). Furthermore, mK_Ca channel activation regulates mitochondrial bioenergetics and thereby prevents opening of the mitochondrial permeability transition pore (mPTP). Here, we investigated in the rat heart in vivo whether 1) M-PC is mediated by activation of PKA, and 2) pharmacological opening of the mPTP abolishes the cardioprotective effect of M-PC and 3) M-PC is critically dependent on STAT3 activation, which is located upstream of mPTP within the signalling pathway.

Methods

Male Wistar rats were randomised to six groups (each n = 6). All animals underwent 25 minutes of regional myocardial ischemia and 120 minutes of reperfusion. Control animals (Con) were not further treated. Morphine preconditioning was initiated by intravenous administration of 0.3 mg/kg morphine (M-PC). The PKA blocker H-89 (10 μg/kg) was investigated with and without morphine (H-89+M-PC, H-89). We determined the effect of mPTP opening with atractyloside (5 mg/kg) with and without morphine (Atr+M-PC, Atr). Furthermore, the effect of morphine on PKA activity was tested in isolated adult rat cardiomyocytes. In further experiments in isolated hearts we tested the protective properties of morphine in the presence of STAT3 inhibition, and whether pharmacological prevention of the mPTP-opening by cyclosporine A (CsA) is cardioprotective in the presence of STAT3 inhibition.

Results

Morphine reduced infarct size from 64±5% to 39±9% (P<0.05 vs. Con). H-89 completely blocked preconditioning by morphine (64±9%; P<0.05 vs. M-PC), but H-89 itself had not effect on infarct size (61±10%; P>0.05 vs. Con). Also, atractyloside abolished infarct size reduction of morphine completely (65±9%; P<0.05 vs. M-PC) but had no influence on infarct size itself (64±5%; P>0.05 vs. Con). In isolated hearts STAT3 inhibitor Stattic completely abolished morphine-induced preconditioning. Administration of Stattic and mPTP inhibitor cyclosporine A reduced infarct size to 31±6% (Stat+CsA, P<0.05 vs. Con). Cyclosporine A alone reduced infarct size to 26±7% (CsA P<0.05 vs. Con). In cardiomyocytes, PKA activity was increased by morphine.

Conclusion

Our data suggest that morphine-induced cardioprotection is mediated by STAT3-activation and inhibition of mPTP, with STA3 located upstream of mPTP. There is some evidence that protein kinase A is involved within the signalling pathway.

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.

Peri-operative anaesthetic myocardial preconditioning and protection - cellular mechanisms and clinical relevance in cardiac anaesthesia

Preconditioning has been shown to reduce myocardial damage caused by ischaemia–reperfusion injury peri-operatively. Volatile anaesthetic agents have the potential to provide myocardial protection by anaesthetic preconditioning and, in addition, they also mediate renal and cerebral protection. A number of proof-of-concept trials have confirmed that the experimental evidence can be translated into clinical practice with regard to postoperative markers of myocardial injury; however, this effect has not been ubiquitous. The clinical trials published to date have also been too small to investigate clinical outcome and mortality. Data from recent meta-analyses in cardiac anaesthesia are also not conclusive regarding intra-operative volatile anaesthesia. These inconclusive clinical results have led to great variability currently in the type of anaesthetic agent used during cardiac surgery. This review summarises experimentally proposed mechanisms of anaesthetic preconditioning, and assesses randomised controlled clinical trials in cardiac anaesthesia that have been aimed at translating experimental results into the clinical setting.

BK channel activators and their therapeutic perspectives

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

Mitochondrial targets for volatile anesthetics against cardiac ischemia-reperfusion injury

Mitochondria are critical modulators of cell function and are increasingly recognized as proximal sensors and effectors that ultimately determine the balance between cell survival and cell death. Volatile anesthetics (VA) are long known for their cardioprotective effects, as demonstrated by improved mitochondrial and cellular functions, and by reduced necrotic and apoptotic cell death during cardiac ischemia and reperfusion (IR) injury. The molecular mechanisms by which VA impart cardioprotection are still poorly understood. Because of the emerging role of mitochondria as therapeutic targets in diseases, including ischemic heart disease, it is important to know if VA-induced cytoprotective mechanisms are mediated at the mitochondrial level. In recent years, considerable evidence points to direct effects of VA on mitochondrial channel/transporter protein functions and electron transport chain (ETC) complexes as potential targets in mediating cardioprotection. This review furnishes an integrated overview of targets that VA impart on mitochondrial channels/transporters and ETC proteins that could provide a basis for cation regulation and homeostasis, mitochondrial bioenergetics, and reactive oxygen species (ROS) emission in redox signaling for cardiac cell protection during IR injury.

KCNMA1 encoded cardiac BK channels afford protection against ischemia-reperfusion injury

Mitochondrial potassium channels have been implicated in myocardial protection mediated through pre-/postconditioning. Compounds that open the Ca2+- and voltage-activated potassium channel of big-conductance (BK) have a pre-conditioning-like effect on survival of cardiomyocytes after ischemia/reperfusion injury. Recently, mitochondrial BK channels (mitoBKs) in cardiomyocytes were implicated as infarct-limiting factors that derive directly from the KCNMA1 gene encoding for canonical BKs usually present at the plasma membrane of cells. However, some studies challenged these cardio-protective roles of mitoBKs. Herein, we present electrophysiological evidence for paxilline- and NS11021-sensitive BK-mediated currents of 190 pS conductance in mitoplasts from wild-type but not BK-/- cardiomyocytes. Transmission electron microscopy of BK-/- ventricular muscles fibres showed normal ultra-structures and matrix dimension, but oxidative phosphorylation capacities at normoxia and upon re-oxygenation after anoxia were significantly attenuated in BK-/- permeabilized cardiomyocytes. In the absence of BK, post-anoxic reactive oxygen species (ROS) production from cardiomyocyte mitochondria was elevated indicating that mitoBK fine-tune the oxidative state at hypoxia and re-oxygenation. Because ROS and the capacity of the myocardium for oxidative metabolism are important determinants of cellular survival, we tested BK-/- hearts for their response in an ex-vivo model of ischemia/reperfusion (I/R) injury. Infarct areas, coronary flow and heart rates were not different between wild-type and BK-/- hearts upon I/R injury in the absence of ischemic pre-conditioning (IP), but differed upon IP. While the area of infarction comprised 28±3% of the area at risk in wild-type, it was increased to 58±5% in BK-/- hearts suggesting that BK mediates the beneficial effects of IP. These findings suggest that cardiac BK channels are important for proper oxidative energy supply of cardiomyocytes at normoxia and upon re-oxygenation after prolonged anoxia and that IP might indeed favor survival of the myocardium upon I/R injury in a BK-dependent mode stemming from both mitochondrial post-anoxic ROS modulation and non-mitochondrial localizations.

Pharmacological options to protect the aged heart from ischemia and reperfusion injury by targeting the PKA-BK(Ca) signaling pathway

The beneficial effects of many cardioprotective strategies including ischemic or pharmacological conditioning are reduced in the aged heart. The underlying reason(s) for the age-dependent loss of cardioprotection is unclear. Recently, we demonstrated that protein kinase A (PKA) dependent cardioprotection is lost in the aged heart. However, activation of large-conductance Ca(2+)-sensitive K(+) (BK(Ca)) channels, a putative PKA downstream target, initiated cardioprotection also in the aged heart. Therefore, we aimed to investigate whether 1) BK(Ca) channels are critically involved in PKA activation induced cardioprotection and 2) the age-dependent loss of cardioprotection is caused by differences in PKA regulation. Using an in vivo rat model with regional myocardial ischemia, we treated young (2-4 months) and aged (22-24 months) Wistar rats with PKA activator forskolin, BK(Ca) channel activator NS1619 and/or BK(Ca) channel blocker iberiotoxin. Forskolin induced infarct size reduction was 1) age-dependent and 2) prevented by iberiotoxin. The effect of forskolin on myocardial PKA activity was comparable in young and aged animals. In addition, NS1619 initiated cardioprotection also in the aged heart both when administered before ischemia and during early reperfusion phase. Activation of BK(Ca) channels is critically involved in forskolin induced cardioprotection. The age-dependency of forskolin induced cardioprotection is not caused by age-dependent differences in PKA activation. Pharmacological targeting of BK(Ca) channels before or after myocardial ischemia is a promising therapeutic strategy to protect the aged heart from ischemia and reperfusion injury.

Bkca opener, NS1619 pretreatment protects against shock-induced vascular hyporeactivity through PDZ-Rho GEF-RhoA-Rho kinase pathway in rats

BACKGROUND

Our previous study showed that the ischemic preconditioning and pretreatment of adenosine triphosphate-sensitive potassium channel (KATP) opener, pinacidil, may induce a good protective effect on shock-induced vascular hyporeactivity. Whether the pretreatment of opener/activator of the large-conductance calcium-activated potassium channel (Bkca), NS1619, can also induce a protective effect on vascular reactivity and play a beneficial effect on subsequent hemorrhagic shock is not clear.

METHODS

With Sprague-Dawley rats subjected to hemorrhagic shock and their isolated superior mesenteric artery, the protective effect of NS1619 (0.5, 1, 2, and 4 mg/kg) pretreatment (30 minutes before hemorrhage shock) on vascular reactivity and the underlying mechanisms were observed.

RESULTS

NS1619 pretreatment significantly improved the 72-hour survival of hemorrhagic shock rats, alleviated shock-induced decrease of vascular reactivity and calcium sensitivity, and increased the cardiac output and oxygen delivery. NS1619 2 mg/kg had the best effect. These protective effects of NS1619 pretreatment on vascular reactivity and calcium sensitivity were antagonized by RhoA inhibitor, C3 transferase, and Rho kinase antagonist, Y-27632. NS1619 pretreatment up-regulated the activities of RhoA, Rho-kinase, and PDZ-Rho GEF (guanine nucleotide exchange factor). These effects of NS1619 pretreatment were eliminated by RhoA inhibitor, C3 transferase.

CONCLUSION

Bkca opener, NS1619 pretreatment has good protective effect on vascular reactivity and calcium sensitivity, which plays a good beneficial effect on hemorrhagic shock. The mechanism may be mainly through PDZ-Rho GEF-RhoA-Rho kinase pathway. Bkca channel may be a potential target for the treatment of shock-induced vascular hyporeactivity.

Intermediate conductance Ca2+-activated K+ channels modulate human placental trophoblast syncytialization

Regulation of human placental syncytiotrophoblast renewal by cytotrophoblast migration, aggregation/fusion and differentiation is essential for successful pregnancy. In several tissues, these events are regulated by intermediate conductance Ca²⁺-activated K⁺ channels (IK_Ca), in part through their ability to regulate cell volume. We used cytotrophoblasts in primary culture to test the hypotheses that IK_Ca participate in the formation of multinucleated syncytiotrophoblast and in syncytiotrophoblast volume homeostasis. Cytotrophoblasts were isolated from normal term placentas and cultured for 66 h. This preparation recreates syncytiotrophoblast formation in vivo, as mononucleate cells (15 h) fuse into multinucleate syncytia (66 h) concomitant with elevated secretion of human chorionic gonadotropin (hCG). Cells were treated with the IK_Ca inhibitor TRAM-34 (10 µM) or activator DCEBIO (100 µM). Culture medium was collected to measure hCG secretion and cells fixed for immunofluorescence with anti-IK_Ca and anti-desmoplakin antibodies to assess IK_Ca expression and multinucleation respectively. K⁺ channel activity was assessed by measuring ⁸⁶Rb efflux at 66 h. IK_Ca immunostaining was evident in nucleus, cytoplasm and surface of mono- and multinucleate cells. DCEBIO increased ⁸⁶Rb efflux 8.3-fold above control and this was inhibited by TRAM-34 (85%; p<0.0001). Cytotrophoblast multinucleation increased 12-fold (p<0.05) and hCG secretion 20-fold (p<0.05), between 15 and 66 h. Compared to controls, DCEBIO reduced multinucleation by 42% (p<0.05) and hCG secretion by 80% (p<0.05). TRAM-34 alone did not affect cytotrophoblast multinucleation or hCG secretion. Hyposmotic solution increased ⁸⁶Rb efflux 3.8-fold (p<0.0001). This effect was dependent on extracellular Ca²⁺, inhibited by TRAM-34 and 100 nM charybdotoxin (85% (p<0.0001) and 43% respectively) but unaffected by 100 nM apamin. In conclusion, IK_Ca are expressed in cytotrophoblasts and their activation inhibits the formation of multinucleated cells in vitro. IK_Ca are stimulated by syncytiotrophoblast swelling implicating a role in syncytiotrophoblast volume homeostasis. Inappropriate activation of IK_Ca in pathophysiological conditions could compromise syncytiotrophoblast turnover and volume homeostasis in pregnancy disease.

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

BACKGROUND

Right heart failure is a fatal consequence of chronic pulmonary hypertension (PH). The development of PH is characterized by increased proliferation of vascular cells, in particular pulmonary artery smooth muscle cells (PASMCs) and pulmonary artery endothelial cells. In the course of PH, an escalated right ventricular (RV) afterload occurs, which leads to increased perioperative morbidity and mortality. BK(Ca) channels are ubiquitously expressed in vascular smooth muscle cells and their opening induces cell membrane hyperpolarization followed by vasodilation. Moreover, BK activation induces anti-proliferative effects in a multitude of cell types. On this basis, we hypothesized that treatment with the nebulized BK channel opener NS1619 might be a therapy option for pulmonary hypertension and tested this in rats.

METHODS

(1) Rats received monocrotaline injection for PH induction. Twenty-four days later, rats were anesthetized and NS1619 or the solvent was administered by inhalation. Systemic hemodynamic parameters, RV hemodynamic parameters, and blood gas analyses were measured before as well as 30 and 120 minutes after inhalation. (2) Rat PASMCs were stimulated with PDGF-BB in the presence and absence of NS1619. AKT, ERK1 and ERK2 activation were investigated by western blot analyses, and relative cell number was determined 48 hours after stimulation.

RESULTS

Inhalation of a 12 µM and 100 µM NS1619 solution significantly reduced RV pressure without affecting systemic arterial pressure. Blood gas analyses demonstrated significantly reduced carbon dioxide and improved oxygenation in NS1619-treated animals pointing towards a considerable pulmonary shunt-reducing effect. In PASMC's, NS1619 (100 µM) significantly attenuated PASMC proliferation by a pathway independent of AKT and ERK1/2 activation.

CONCLUSION

NS1619 inhalation reduces RV pressure and improves oxygen supply and its application inhibits PASMC proliferation in vitro. Hence, BK opening might be a novel option for the treatment of pulmonary hypertension.

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.

Intracellular BK(Ca) (iBK(Ca)) channels

The large conductance calcium- and voltage-activated potassium channel (BK(Ca)) is widely expressed at the plasma membrane. This channel is involved in a variety of fundamental cellular functions including excitability, smooth muscle contractility, and Ca(2+) homeostasis, as well as in pathological situations like proinflammatory responses in rheumatoid arthritis, and cancer cell proliferation. Immunochemical, biochemical and pharmacological studies from over a decade have intermittently shown the presence of BK(Ca) in intracellular organelles. To date, intracellular BK(Ca) (iBK(Ca)) has been localized in the mitochondria, endoplasmic reticulum, nucleus and Golgi apparatus but its functional role remains largely unknown except for the mitochondrial BK(Ca) whose opening is thought to play a role in protecting the heart from ischaemic injury. In the nucleus, pharmacology suggests a role in regulating nuclear Ca(2+), membrane potential and eNOS expression. Establishing the molecular correlates of iBK(Ca), the mechanisms defining iBK(Ca) organelle-specific targeting, and their modulation are challenging questions. This review summarizes iBK(Ca) channels, their possible functions, and efforts to identify their molecular correlates.

Aromatase inhibition attenuates desflurane-induced preconditioning against acute myocardial infarction in male mouse heart in vivo

The volatile anesthetic desflurane (DES) effectively reduces cardiac infarct size following experimental ischemia/reperfusion injury in the mouse heart. We hypothesized that endogenous estrogens play a role as mediators of desflurane-induced preconditioning against myocardial infarction. In this study, we tested the hypothesis that desflurane effects local estrogen synthesis by modulating enzyme aromatase expression and activity in the mouse heart. Aromatase metabolizes testosterone to 17β- estradiol (E2) and thereby significantly contributes to local estrogen synthesis. We tested aromatase effects in acute myocardial infarction model in male mice. The animals were randomized and subjected to four groups which were pre-treated with the selective aromatase inhibitor anastrozole (A group) and DES alone (DES group) or in combination (A+DES group) for 15 minutes prior to surgical intervention whereas the control group received 0.9% NaCl (CON group). All animals were subjected to 45 minutes ischemia following 180 minutes reperfusion. Anastrozole blocked DES induced preconditioning and increased infarct size compared to DES alone (37.94±15.5% vs. 17.1±3.62%) without affecting area at risk and systemic hemodynamic parameters following ischemia/reperfusion. Protein localization studies revealed that aromatase was abundant in the murine cardiovascular system with the highest expression levels in endothelial and smooth muscle cells. Desflurane application at pharmacological concentrations efficiently upregulated aromatase expression in vivo and in vitro. We conclude that desflurane efficiently regulates aromatase expression and activity which might lead to increased local estrogen synthesis and thus preserve cellular integrity and reduce cardiac damage in an acute myocardial infarction model.

Desflurane-induced and ischaemic postconditioning against myocardial infarction are mediated by Pim-1 kinase

BACKGROUND

Anaesthetic-induced (APOST) and ischaemic postconditioning (IPOST) against myocardial infarction are mediated via phosphatidylinositol-3-kinase/Akt. Pim-1 kinase is acting downstream of Akt and has recently been demonstrated to enhance cardiomyocyte survival. We tested the hypothesis that both APOST and IPOST are mediated by Pim-1 kinase.

METHODS

Pentobarbital-anaesthetized male C57BL/6 mice were subjected to 45-min coronary artery occlusion (CAO) and 3-h reperfusion. Animals received either no intervention, the Pim-1 kinase inhibitor II (10 μg/g intraperitoneally) or its vehicle dimethy sulfoxide (10 μl/g intraperitoneally). Three minutes prior to the end of CAO, 1.0 minimum alveolar concentration desflurane was administered for 18 min alone or in combination with Pim-1 kinase inhibitor II. IPOST was induced by three cycles of each 10-s ischaemia/reperfusion, and animals received either IPOST alone or in combination with Pim-1 kinase inhibitor II. Infarct size was determined with triphenyltetrazolium chloride and area at risk with Evans blue. Protein expression of Pim-1 kinase, Bad, phospho-Bad(Ser112) and B-cell lymphoma 2 was determined using Western immunoblotting analysis.

RESULTS

Infarct size in control animals (CON) was 46 ± 3%. Dimethylsulfoxide (47 ± 3%) and Pim-1 kinase inhibitor II (44 ± 5%) did not significantly reduce infarct size. Desflurane (16 ± 2%*; *P < 0.05 vs. CON) and IPOST (21 ± 2%*) significantly reduced infarct size compared with CON. Inhibition of Pim-1 kinase abolished desflurane-induced postconditioning (46 ± 4%) and IPOST (44 ± 5%). Western blot analysis revealed that only desflurane enhances phosphorylation of Bad at serine 112 that was abrogated by Pim-1 kinase inhibitor II.

CONCLUSION

These data suggest that Pim-1 kinase mediates both desflurane-induced postconditioning and IPOST in mice.

Endothelial nitric oxide synthase mediates the first and inducible nitric oxide synthase mediates the second window of desflurane-induced preconditioning

OBJECTIVES

Nitric oxide synthases (NOSs) mediate the first window of anesthetic-induced preconditioning (APC). The authors tested the hypothesis that endothelial NOS (eNOS) mediates the first window and inducible NOS (iNOS) mediates the second window of APC.

DESIGN

Randomized, prospective, blinded laboratory investigation.

SETTING

Experimental laboratory.

PARTICIPANTS

Mice.

INTERVENTIONS

Mice were subjected to a 45-minute coronary artery occlusion (CAO) and a 180-minute reperfusion. C57BL/6 mice received desflurane, 1.0 minimum alveolar concentration, for 30 minutes or 12, 24, 48, or 96 hours before CAO. In eNOS(-/-) and iNOS(-/-) mice, desflurane was given 30 minutes and 48 hours before CAO. In the control groups, no desflurane was administered. Myocardial infarct size (IS) was determined after staining with Evans blue and triphenyltetrazolium chloride.

MEASUREMENTS AND MAIN RESULTS

The second window of APC was detectable at 48 hours but not at 12, 24, and 96 hours after preconditioning. In the control groups, IS was not different among the wild-type (50 ± 10%), eNOS(-/-) (52 ± 14%), and iNOS(-/-) (46 ± 10%) mice. The IS decreased significantly (p < 0.05) when desflurane was administered 30 minutes (10 ± 6%) or 48 hours (16 ± 7%) before CAO in wild-type mice, 48 hours (21 ± 13%) before CAO in eNOS(-/-) mice, and 30 minutes (13 ± 6%) before CAO in iNOS(-/-) mice. Desflurane given 30 minutes before CAO in eNOS(-/-) mice (60 ± 10%) and 48 hours before CAO in iNOS(-/-) mice (48 ± 21%) did not decrease the IS significantly compared with controls.

CONCLUSIONS

Endothelial NOS and iNOS work independently to mediate the first and second windows of APC, respectively. Endothelial NOS is not necessary to trigger the second window of APC.

A cell-based phenotypic assay to identify cardioprotective agents

RATIONALE

Tissue ischemia/reperfusion (IR) injury underlies several leading causes of death such as heart-attack and stroke. The lack of clinical therapies for IR injury may be partly due to the difficulty of adapting IR injury models to high-throughput screening (HTS).

OBJECTIVE

To develop a model of IR injury that is both physiologically relevant and amenable to HTS.

METHODS AND RESULTS

A microplate-based respirometry apparatus was used. Controlling gas flow in the plate head space, coupled with the instrument's mechanical systems, yielded a 24-well model of IR injury in which H9c2 cardiomyocytes were transiently trapped in a small volume, rendering them ischemic. After initial validation with known protective molecules, the model was used to screen a 2000-molecule library, with post-IR cell death as an end point. Po2 and pH monitoring in each well also afforded metabolic data. Ten protective, detrimental, and inert molecules from the screen were subsequently tested in a Langendorff-perfused heart model of IR injury, revealing strong correlations between the screening end point and both recovery of cardiac function (negative, r2=0.66) and infarct size (positive, r2=0.62). Relationships between the effects of added molecules on cellular bioenergetics and protection against IR injury were also studied.

CONCLUSIONS

This novel cell-based assay can predict either protective or detrimental effects on IR injury in the intact heart. Its application may help identify therapeutic or harmful molecules.

Desflurane-induced post-conditioning against myocardial infarction is mediated by calcium-activated potassium channels: role of the mitochondrial permeability transition pore

BACKGROUND

Desflurane (DES)-induced preconditioning is mediated by large-conductance calcium-activated potassium channels (BK(Ca)). Whether BK(Ca) are involved in anaesthetic-induced post-conditioning is unknown. We tested the hypothesis that DES-induced post-conditioning is mediated by BK(Ca) upstream of the mitochondrial permeability transition pore (mPTP).

METHODS

Pentobarbital-anaesthetized male C57Black/6 mice were subjected to 45 min coronary artery occlusion (CAO) and 3 h reperfusion. Animals received either no intervention or dimethylsulphoxide (DMSO, 10 µl g(-1)). DES (1.0 MAC, 7.5 vol%) was administered for 18 min, starting 3 min before the end of CAO. The following agents were given either alone or in combination with DES: the BK(Ca) activator NS1619 (1 µg g(-1)), the BK(Ca) inhibitor iberiotoxin (IbTx, 0.05 µg g(-1)), the mPTP opener atractyloside (ATRA, 25 µg g(-1)), and the mPTP inhibitor cyclosporine A (CYC A, 10 µg g(-1)). Infarct size (IS) was determined with triphenyltetrazolium chloride and the area at risk with Evans Blue, respectively.

RESULTS

IS in control animals was 48(6)%. Neither DMSO, IbTx nor ATRA affected myocardial IS. DES alone or NS1619 alone or the combination reduced IS (P<0.05), CYC A alone or in combination with IbTx or DES also reduced IS (P<0.05). DES-induced reduction of myocardial IS was completely abolished by IbTx and was partially blocked by ATRA and ATRA partially blocked IS reduction by NS1619.

CONCLUSIONS

These data suggest that DES-induced post-conditioning against myocardial infarction is mediated by BK(Ca) and mPTP. Cardioprotection by BK(Ca) activator NS1619 might occur, at least in part, independently of mPTP.

SLO-2 is cytoprotective and contributes to mitochondrial potassium transport

Mitochondrial potassium channels are important mediators of cell protection against stress. The mitochondrial large-conductance "big" K(+) channel (mBK) mediates the evolutionarily-conserved process of anesthetic preconditioning (APC), wherein exposure to volatile anesthetics initiates protection against ischemic injury. Despite the role of the mBK in cardioprotection, the molecular identity of the channel remains unknown. We investigated the attributes of the mBK using C. elegans and mouse genetic models coupled with measurements of mitochondrial K(+) transport and APC. The canonical Ca(2+)-activated BK (or "maxi-K") channel SLO1 was dispensable for both mitochondrial K(+) transport and APC in both organisms. Instead, we found that the related but physiologically-distinct K(+) channel SLO2 was required, and that SLO2-dependent mitochondrial K(+) transport was triggered directly by volatile anesthetics. In addition, a SLO2 channel activator mimicked the protective effects of volatile anesthetics. These findings suggest that SLO2 contributes to protection from hypoxic injury by increasing the permeability of the mitochondrial inner membrane to K(+).

Age-related loss of cardiac preconditioning: impact of protein kinase A

Helium induces preconditioning (He-PC) by mitochondrial calcium-sensitive potassium (mK(Ca)) channel-activation, but this effect is lost in the aged myocardium. Both, the upstream signalling pathway of He-PC and the underlying mechanisms for an age-related loss of preconditioning are unknown. A possible candidate as upstream regulator of mK(Ca) channels is protein kinase A (PKA). We investigated whether 1) regulation of PKA is involved in He-PC and 2) regulation of PKA is age-dependent. Young (2-3 months) and aged (22-24 months) Wistar rats were randomised to eight groups (each n=8). All animals underwent 25 min regional myocardial ischemia and 120 min reperfusion. Control (Con, Age Con) animals were not further treated. Young rats inhaled 70% helium for 3×5 min (He-PC). The PKA-blocker H-89 (10 μg/kg) was administered with and without helium (He-PC+H-89, H-89). Furthermore, we tested the effect of direct activation of mK(Ca) channels with NS1619. The adenylyl cyclase activator forskolin (For) was administered in young (300 μg/kg) and aged animals (300 and 1000 μg/kg). He-PC reduced infarct size from 60±4% (Con) to 37±10% (p<0.05). Infarct size reduction was completely abolished by H-89 (58±5%; p<0.05), but H-89 alone had no effect (57±2%). NS1619 reduced infarct size in the same concentration in both, young and aged rats (35±6%; p<0.05 vs. Con and 34±8%; p<0.05 vs. Age Con). Forskolin in a concentration of 300 μg/kg reduced infarct size in young (37±6%; p<0.05) but not in aged rats (48±13%; n.s.). In contrast, 1000 μg/kg Forskolin reduced infarct size also in aged rats (28±3%; p<0.05). He-PC is mediated by activation of PKA. Alterations in PKA regulation might be an underlying mechanism for the age-dependent loss of preconditioning.

Isoflurane differentially modulates mitochondrial reactive oxygen species production via forward versus reverse electron transport flow: implications for preconditioning

BACKGROUND

Reactive oxygen species (ROS) mediate the effects of anesthetic precondition to protect against ischemia and reperfusion injury, but the mechanisms of ROS generation remain unclear. In this study, the authors investigated if mitochondria-targeted antioxidant (mitotempol) abolishes the cardioprotective effects of anesthetic preconditioning. Further, the authors investigated the mechanism by which isoflurane alters ROS generation in isolated mitochondria and submitochondrial particles.

METHODS

Rats were pretreated with 0.9% saline, 3.0 mg/kg mitotempol in the absence or presence of 30 min exposure to isoflurane. Myocardial infarction was induced by left anterior descending artery occlusion for 30 min followed by reperfusion for 2 h and infarct size measurements. Mitochondrial ROS production was determined spectrofluorometrically. The effect of isoflurane on enzymatic activity of mitochondrial respiratory complexes was also determined.

RESULTS

Isoflurane reduced myocardial infarct size (40 ± 9% = mean ± SD) compared with control experiments (60 ± 4%). Mitotempol abolished the cardioprotective effects of anesthetic preconditioning (60 ± 9%). Isoflurane enhanced ROS generation in submitochondrial particles with nicotinamide adenine dinucleotide (reduced form), but not with succinate, as substrate. In intact mitochondria, isoflurane enhanced ROS production in the presence of rotenone, antimycin A, or ubiquinone when pyruvate and malate were substrates, but isoflurane attenuated ROS production when succinate was substrate. Mitochondrial respiratory experiments and electron transport chain complex assays revealed that isoflurane inhibited only complex I activity.

CONCLUSIONS

The results demonstrated that isoflurane produces ROS at complex I and III of the respiratory chain via the attenuation of complex I activity. The action on complex I decreases unfavorable reverse electron flow and ROS release in myocardium during reperfusion.

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

Chronic hypoxia protects the heart against injury caused by acute oxygen deprivation, but its salutary mechanism is poorly understood. The aim was to find out whether cardiomyocytes isolated from chronically hypoxic hearts retain the improved resistance to injury and whether the mitochondrial large-conductance Ca2+-activated K+ (BKCa) channels contribute to the protective effect. Adult male rats were adapted to continuous normobaric hypoxia (inspired O2 fraction 0.10) for 3 wk or kept at room air (normoxic controls). Myocytes, isolated separately from the left ventricle (LVM), septum (SEPM), and right ventricle, were exposed to 25-min metabolic inhibition with sodium cyanide, followed by 30-min reenergization (MI/R). Some LVM were treated with either 30 μM NS-1619 (BKCa opener), or 2 μM paxilline (BKCa blocker), starting 25 min before metabolic inhibition. Cell injury was detected by Trypan blue exclusion and lactate dehydrogenase (LDH) release. Chronic hypoxia doubled the number of rod-shaped LVM and SEPM surviving the MI/R insult and reduced LDH release. While NS-1619 protected cells from normoxic rats, it had no additive salutary effect in the hypoxic group. Paxilline attenuated the improved resistance of cells from hypoxic animals without affecting normoxic controls; it also abolished the protective effect of NS-1619 on LDH release in the normoxic group. While chronic hypoxia did not affect protein abundance of the BKCa channel regulatory β1-subunit, it markedly decreased its glycosylation level. It is concluded that ventricular myocytes isolated from chronically hypoxic rats retain the improved resistance against injury caused by MI/R. Activation of the mitochondrial BKCa channel likely contributes to this protective effect.

Mitochondria and GSK-3beta in cardioprotection against ischemia/reperfusion injury

The mitochondrion is a powerhouse of the cell, a platform of cell signaling and decision-maker of cell death, including death by ischemia/reperfusion. Ischemia shuts off ATP production by mitochondria, and cell viability is compromised by energy deficiency and build-up of cytotoxic metabolites during ischemia. Furthermore, the mitochondrial permeability transition pore (mPTP) is primed by ischemia to open upon reperfusion, leading to reperfusion-induced cell necrosis. mPTP opening can be suppressed by ischemic preconditioning (IPC) and other interventions that induce phosphorylation of GSK-3beta. Activation of the mitochondrial ATP-sensitive K(+) channel (mK(ATP) channel) is an important signaling step in a trigger phase of IPC, which ultimately enhances GSK-3beta phosphorylation upon reperfusion, and this channel functions as a mediator of cytoprotection as well. The mitochondrial Ca(2+)-activated K(+) channel appears to play roles similar to those of the mK(ATP) channel, though regulatory mechanisms of the channels are different. Phosphorylated GSK-3beta inhibits mPTP opening presumably by multiple mechanisms, including preservation of hexokinase II in mPTP complex, prevention of interaction of cyclophilin-D with adenine nucleotide translocase, inhibition of p53 activation and attenuation of ATP hydrolysis during ischemia. However, cytoprotective signaling pathways to GSK-3beta phosphorylation and other mPTP regulatory factors are modified by co-morbidities, including type 2 diabetes, and such modification makes the myocardium refractory to IPC and other cardioprotective agents. Regulatory mechanisms of mPTP, and their alterations by morbidities frequently associated with ischemic heart disease need to be further characterized for translation of mitochondrial and mPTP biology to the clinical arena.

Ischaemic and morphine-induced post-conditioning: impact of mK(Ca) channels

BACKGROUND

Mitochondrial calcium-sensitive potassium (mK(Ca)) channels are involved in cardiac preconditioning. In the present study, we investigated whether also ischaemic-, morphine-induced post-conditioning, or both is mediated by the activation of mK(Ca) channels in the rat heart in vitro.

METHODS

Animals were treated in compliance with institutional and national guidelines. Male Wistar rats were randomly assigned to one of seven groups (each n = 7). Control animals were not further treated. Post-conditioning was induced either by 3 × 30 s of ischaemia/reperfusion (I-PostC) or by administration of morphine (M-PostC, 1 µM) for 15 min at the onset of reperfusion. The mK(Ca)-channel inhibitor paxilline (1 µM) was given with and without post-conditioning interventions (M-PostC+Pax, I-PostC+Pax, and Pax). As a positive control, we determined whether direct activation of mK(Ca) channels with NS1619 (10 µM) induced cardiac post-conditioning (NS1619). Isolated hearts underwent 35 min ischaemia followed by 120 min reperfusion. At the end of reperfusion, infarct sizes were measured by triphenyltetrazolium chloride staining.

RESULTS

In the control group, infarct size was 53 (5)% of the area at risk. Morphine- and ischaemic post-conditioning reduced infarct size in the same range [M-PostC: 37 (4)%, I-PostC: 35 (5)%; each P<0.05 vs control]. The mK(Ca)-channel inhibitor paxilline completely blocked post-conditioning [M-PostC+Pax: 47 (7)%, I-PostC+Pax: 51 (3)%; each P<0.05 vs M-PostC and I-PostC, respectively]. Paxilline itself had no effect on infarct size (NS vs control). NS1619 reduced infarct size to 33 (4)% (P < 0.05 vs control).

CONCLUSIONS

Ischaemic- and morphine-induced post-conditioning is mediated by the activation of mK(Ca) channels.

Morphine induces preconditioning via activation of mitochondrial K(Ca) channels

Purpose

Mitochondrial calcium sensitive potassium (mK_Ca) channels are involved in cardioprotection induced by ischemic preconditioning. In the present study we investigated whether morphine-induced preconditioning also involves activation of mK_Ca channels.

Methods

Isolated rat hearts (six groups; each n = 8) underwent global ischemia for 30 min followed by a 60-min reperfusion. Control animals were not further treated. Morphine preconditioning (MPC) was initiated by two five-minute cycles of morphine 1 μM infusion with one five-minute washout and one final ten-minute washout period before ischemia. The mK_Ca blocker, paxilline 1 μM, was administered, with and without morphine administration (MPC + Pax and Pax). As a positive control, we added an ischemic preconditioning group (IPC) alone and combined with paxilline (IPC + Pax). At the end of reperfusion, infarct sizes were determined by triphenyltetrazoliumchloride staining.

Results

Infarct size was (mean ± SD) 45 ± 9% of the area at risk in the Control group. The infarct size was less in the morphine or ischemic preconditioning groups (MPC: 23 ± 8%, IPC: 20 ± 5%; each P < 0.05 vs Control). Infarct size reduction was abolished by paxilline (MPC + Pax: 37 ± 7%, P < 0.05 vs MPC and IPC + Pax: 36 ± 6%, P < 0.05 vs IPC), whereas paxilline alone had no effect (Pax: 46 ± 7%, not significantly different from Control).

Conclusion

Cardioprotection by morphine-induced preconditioning is mediated by activation of mK_Ca channels.

Comparison of Isoflurane-, Sevoflurane-, and Desflurane-Induced Pre- and Postconditioning Against Myocardial Infarction in Mice In Vivo

The murine in vivo model of acute myocardial infarction is increasingly used to investigate anesthetic-induced preconditioning (APC) and postconditioning (APOST). However, in mice the potency of different volatile anesthetics to reduce myocardial infarct size (IS) has never been investigated systematically nor in a head to head comparison with regard to ischemic preconditioning (IPC) and postconditioning (IPOST). Male C57BL/6 mice were subjected to 45 min of coronary artery occlusion (CAO) and 180 min of reperfusion. To induce APC, 1.0 MAC isoflurane (ISO), sevoflurane (SEVO) or desflurane (DES) was administered 30 min prior to CAO for 15 min. In an additional group, ISO was administered 45 min prior to CAO for 30 min. To induce APOST, 1.0 MAC ISO, SEVO or DES was administered for 18 min starting 3 min prior to the end of CAO. IPC was induced by 3 or 6 cycles of 5 min ischemia/reperfusion, 40 or 60 min prior to CAO, respectively. IPOST was induced by 3 cycles of 30 sec reperfusion/ischemia at the beginning of reperfusion. Area at risk (AAR) and IS were determined with Evans Blue and TTC staining, respectively. IS (IS/AAR) was 50 ± 4% (mean ± SEM) in the control group and was significantly (* P < 0.05) reduced by 3×5 IPC (26 ± 3%*), 6×5 IPC (26 ± 4%*), IPOST (20 ± 2%*), ISO APOST (19 ± 1%*), SEVO APOST (15 ± 1%*), DES APOST (14 ± 2%*) and SEVO APC (27 ± 6%*). ISO APC significantly reduced IS compared to control when administered 30 min (33 ± 4%*), but not when administered 15 min (48 ± 6%). DES APC significantly reduced IS compared to control and to SEVO APC (7 ± 1%*). Within the paradigm of preconditioning, the potency of volatile anesthetics to reduce myocardial infarct size in mice significantly increases from ISO over SEVO to DES, whereas within the paradigm of postconditioning the potency of these volatile anesthetics to reduce myocardial infarct size in mice is similar.

Differences in production of reactive oxygen species and mitochondrial uncoupling as events in the preconditioning signaling cascade between desflurane and sevoflurane

BACKGROUND

Signal transduction cascade of anesthetic-induced preconditioning has been extensively studied, yet many aspects of it remain unsolved. Here, we investigated the roles of reactive oxygen species (ROS) and mitochondrial uncoupling in cardiomyocyte preconditioning by two modern volatile anesthetics: desflurane and sevoflurane.

METHODS

Adult rat ventricular cardiomyocytes were isolated enzymatically. The preconditioning potency of desflurane and sevoflurane was assessed in cell survival experiments by evaluating myocyte protection from the oxidative stress-induced cell death. ROS production and flavoprotein fluorescence, an indicator of flavoprotein oxidation and mitochondrial uncoupling, were monitored in real time by confocal microscopy. The functional aspect of enhanced ROS generation by the anesthetics was assessed in cell survival and confocal experiments using the ROS scavenger Trolox.

RESULTS

Preconditioning of cardiomyocytes with desflurane or sevoflurane significantly decreased oxidative stress-induced cell death. That effect coincided with increased ROS production and increased flavoprotein oxidation detected during acute myocyte exposure to the anesthetics. Desflurane induced significantly greater ROS production and flavoprotein oxidation than sevoflurane. ROS scavenging with Trolox abrogated preconditioning potency of anesthetics and attenuated flavoprotein oxidation.

CONCLUSION

Preconditioning with desflurane or sevoflurane protects isolated rat cardiomyocytes from oxidative stress-induced cell death. Scavenging of ROS abolishes the preconditioning effect of both anesthetics and attenuates anesthetic-induced mitochondrial uncoupling, suggesting a crucial role for ROS in anesthetic-induced preconditioning and implying that ROS act upstream of mitochondrial uncoupling. Desflurane exhibits greater effect on stimulation of ROS production and mitochondrial uncoupling than sevoflurane.

Role of large-conductance Ca(2+) -activated potassium channels in adenosine A(1) receptor-mediated pharmacological preconditioning in H9c2 cells

Large-conductance Ca(2+)-activated potassium channels, located on the inner mitochondrial membrane, have recently been implicated in cytoprotection. Therefore, the primary aim of this study was to determine the role of large-conductance Ca(2+)-activated potassium channels in adenosine A(1) receptor-induced pharmacological preconditioning in the rat embryonic cardiomyoblast-derived cell line H9c2. For pharmacological preconditioning, H9c2 cells were exposed to the adenosine A(1) receptor agonist N(6)-cyclopentyladenosine (100 nM) or the Ca(2+)-activated potassium channel opener NS1619 (10 microM) for 30 min prior to 6 h hypoxia (0.5% O(2)) in glucose-free and serum-free media. Where appropriate cells were treated (15 min) before pharmacological preconditioning with the Ca(2+)-activated potassium channels blockers paxilline (1 microM) or iberiotoxin (100 nM). Cell viability following 6 h hypoxia was assessed by monitoring lactate dehydrogenase (LDH) release and caspase-3 activation. Ca(2+)-activated potassium channel subunit protein expression and cell survival protein kinase (ERK1/2 and PKB/Akt) activation were assessed by Western blotting. The results demonstrate that the adenosine A(1) receptor is functionally expressed in H9c2 cells and when activated protects against hypoxia-induced LDH release and caspase-3 activation. Treatment with paxilline or iberiotoxin attenuated adenosine A(1) receptor and NS1619-induced pharmacological preconditioning. Large-conductance Ca(2+)-activated potassium channel alpha and beta4 protein subunits were detected in mitochondrial fractions isolated from H9c2 cells. NS1619 (10 microM) induced no significant changes in ERK1/2 or PKB phosphorylation. These results have shown for the first time that large-conductance Ca(2+)-activated potassium channels are involved in adenosine A(1) receptor-induced pharmacological preconditioning in a cell model system.

Novel channels of the inner mitochondrial membrane

Along with a large number of carriers, exchangers and "pumps", the inner mitochondrial membrane contains ion-conducting channels which endow it with controlled permeability to small ions. Some have been shown to be the mitochondrial counterpart of channels present also in other cellular membranes. The manuscript summarizes the current state of knowledge on the major inner mitochondrial membrane channels, properties, identity and proposed functions. Considerable attention is currently being devoted to two K(+)-selective channels, mtK(ATP) and mtBK(Ca). Their activation in "preconditioning" is considered by many to underlie the protection of myocytes and other cells against subsequent ischemic damage. We have recently shown that in apoptotic lymphocytes inner membrane mtK(V)1.3 interacts with the pro-apoptotic protein Bax after the latter has inserted into the outer mitochondrial membrane. Whether the just-discovered mtIK(Ca) has similar cellular role(s) remains to be seen. The Ca(2+) "uniporter" has been characterized electrophysiologically, but still awaits a molecular identity. Chloride-selective channels are represented by the 107 pS channel, the first mitochondrial channel to be observed by patch-clamp, and by a approximately 400 pS pore we have recently been able to fully characterize in the inner membrane of mitochondria isolated from a colon tumour cell line. This we propose to represent a component of the Permeability Transition Pore. The available data exclude the previous tentative identification with porin, and indicate that it coincides instead with the still molecularly unidentified "maxi" chloride channel.