Mitochondria-derived reactive oxygen species dilate cerebral arteries by activating Ca2+ sparks

Activation of mitoKATP channels induces penile vasodilation and inhibits mitochondrial respiration and ROS production: Role of NO

OBJECTIVE

Mitochondrial ATP-sensitive K+ (mitoKATP) channels are involved in neuronal and cardiac protection from ischemia and oxidative stress. Penile erection is a neurovascular event mediated by relaxation of the erectile tissue via nitric oxide (NO) released from nerves and endothelium. In the present study, we investigated whether mitoKATP channels play a role in the control of penile vascular tone and mitochondrial dynamics, and the involvement of NO.

METHODS

The effect of the selective mitoKATP activator BMS191095 was examined on vascular tone, on mitochondrial bioenergetics by real-time measurements with Agilent Seahorse and on ROS production by MitoSOX fluorescence in freshly isolated microarteries.

RESULTS

BMS191095 and diazoxide relaxed penile arteries, BMS191095 being one order of magnitude more potent. BMS191095-induced relaxations were reduced by mechanical endothelium removal and by inhibitors of the nitric oxide synthase (NOS) and PI3K enzymes. The NO-dependent component of the relaxation to BMS191095 was impaired in penile arteries from insulin resistant obese rats. The blockers of mitoKATP channel 5-HD, sarcolemma KATP (sarcKATP) channel glibenclamide, and large conductance Ca2+-activated K+ (BKCa) channel iberiotoxin, inhibited relaxations to BMS191095 and to the NO donor SNAP. BMS191095 reduced the mitochondrial bioenergetic profile of penile arteries and attenuated mitochondrial ROS production. Blockade of endogenous NO impaired and exogenous NO mimicked, respectively, the inhibitory effects of BMS191095 on basal respiration and oxygen consumed for ATP synthesis. Exogenous NO exhibited dual inhibitory/stimulatory effects on mitochondrial respiration.

CONCLUSIONS

These results demonstrate that selective activation of mitoKATP channels causes penile vasodilation, attenuates ROS production and inhibits mitochondrial respiration in part by releasing endothelial NO. These mechanisms couple blood flow and metabolism in penile arterial wall and suggest that activation of vascular mitoKATP channels may protect erectile tissue against ischemic injury.

Pulmonary hypertension associated with diazoxide: the SUR1 paradox

The ATP-sensitive potassium channels and their regulatory subunits, sulfonylurea receptor 1 (SUR1/Kir6.2) and SUR2/Kir6.1, contribute to the pathophysiology of pulmonary hypertension (PH). Loss-of-function pathogenic variants in the ABCC8 gene, which encodes for SUR1, have been associated with heritable pulmonary arterial hypertension. Conversely, activation of SUR1 and SUR2 leads to the relaxation of pulmonary arteries and reduces cell proliferation and migration. Diazoxide, a SUR1 activator, has been shown to alleviate experimental PH, suggesting its potential as a therapeutic option. However, there are paradoxical reports of diazoxide-induced PH in infants. This review explores the role of SUR1/2 in the pathophysiology of PH and the contradictory effects of diazoxide on the pulmonary vascular bed. Additionally, we conducted a comprehensive literature review of cases of diazoxide-associated PH and analysed data from the World Health Organization pharmacovigilance database (VigiBase). Significant disproportionality signals link diazoxide to PH, while no other SUR activators have been connected with pulmonary vascular disease. Diazoxide-associated PH seems to be dose-dependent and potentially related to acute effects on the pulmonary vascular bed. Further research is required to decipher the differing pulmonary vascular consequences of diazoxide in different age populations and experimental models.

Oxidative Stress in a Mother Consuming Alcohol during Pregnancy and in Her Newborn: A Case Report

Fetal alcohol spectrum disorder (FASD) is a set of conditions resulting from prenatal alcohol exposure (PAE). FASD is estimated to affect between 2% and 5% of people in the United States and Western Europe. The exact teratogenic mechanism of alcohol on fetal development is still unclear. Ethanol (EtOH) contributes to the malfunctioning of the neurological system in children exposed in utero by decreasing glutathione peroxidase action, with an increase in the production of reactive oxygen species (ROS), which causes oxidative stress. We report a case of a mother with declared alcohol abuse and cigarette smoking during pregnancy. By analyzing the ethyl glucuronide (EtG, a metabolite of alcohol) and the nicotine/cotinine in the mother's hair and meconium, we confirmed the alcohol and smoking abuse magnitude. We also found that the mother during pregnancy was a cocaine abuser. As a result, her newborn was diagnosed with fetal alcohol syndrome (FAS). At the time of the delivery, the mother, but not the newborn, had an elevation in oxidative stress. However, the infant, a few days later, displayed marked potentiation in oxidative stress. The clinical complexity of the events involving the infant was presented and discussed, underlining also the importance that for cases of FASD, it is crucial to have more intensive hospital monitoring and controls during the initial days.

Calcium and Reactive Oxygen Species Signaling Interplays in Cardiac Physiology and Pathologies

Mitochondria are key players in energy production, critical activity for the smooth functioning of energy-demanding organs such as the muscles, brain, and heart. Therefore, dysregulation or alterations in mitochondrial bioenergetics primarily perturb these organs. Within the cell, mitochondria are the major site of reactive oxygen species (ROS) production through the activity of different enzymes since it is one of the organelles with the major availability of oxygen. ROS can act as signaling molecules in a number of different pathways by modulating calcium (Ca²⁺) signaling. Interactions among ROS and calcium signaling can be considered bidirectional, with ROS regulating cellular Ca²⁺ signaling, whereas Ca²⁺ signaling is essential for ROS production. In particular, we will discuss how alterations in the crosstalk between ROS and Ca²⁺ can lead to mitochondrial bioenergetics dysfunctions and the consequent damage to tissues at high energy demand, such as the heart. Changes in Ca²⁺ can induce mitochondrial alterations associated with reduced ATP production and increased production of ROS. These changes in Ca²⁺ levels and ROS generation completely paralyze cardiac contractility. Thus, ROS can hinder the excitation-contraction coupling, inducing arrhythmias, hypertrophy, apoptosis, or necrosis of cardiac cells. These interplays in the cardiovascular system are the focus of this review.

Naringenin ameliorates homocysteine induced endothelial damage via the AMPKα/Sirt1 pathway

Introduction

Endothelial damage (ED) has been implicated in accelerating the development of atherosclerosis. The latter condition is a risk factor for developing several cardiovascular diseases (CVDs) associated with high morbidity and mortality rates worldwide.

Objectives

In our previous studies, we found naringenin (Nar), a bioactive flavanone compound, to protect against mitochondrial damage and oxidative stress. Though the pleiotropic effects of Nar have been well described, precise cytoprotective mechanisms of Nar against homocysteine (Hcy) induced ED remains elusive. Understanding these events may give an insight in to prevention and treatment of CVDs.

Methods

After ruling out the NMDA-R1 mediated pathway, RNA-Seq, a novel transcriptomic technique uncovered AMPK signaling pathway was identified as the mechanism with which Nar corrects ED. Further in vivo and in vitro tests validated the role of Nar against ED.

Results

In particular, Nar activates AMPKα/Sirt1 signaling pathway, which restores mitochondrial Ca2+ balance and ultimately lowered production of reactive oxygen species (ROS). Activated AMPKα/Sirt1 signaling pathway also up-regulates endothelial nitric oxide synthase (eNOS) activity, and then increasing the production of nitric oxide (NO), ultimately ameliorating ED.

Conclusion

Nar could increase the ROS elimination and decrease eNOS uncoupling, subsequently upregulate the NO bioavailability and endothelial function by activating AMPKα/Sirt1 signaling pathway.

Lactate is an energy substrate for rodent cortical neurons and enhances their firing activity

Glucose is the mandatory fuel for the brain, yet the relative contribution of glucose and lactate for neuronal energy metabolism is unclear. We found that increased lactate, but not glucose concentration, enhances the spiking activity of neurons of the cerebral cortex. Enhanced spiking was dependent on ATP-sensitive potassium (KATP) channels formed with KCNJ11 and ABCC8 subunits, which we show are functionally expressed in most neocortical neuronal types. We also demonstrate the ability of cortical neurons to take-up and metabolize lactate. We further reveal that ATP is produced by cortical neurons largely via oxidative phosphorylation and only modestly by glycolysis. Our data demonstrate that in active neurons, lactate is preferred to glucose as an energy substrate, and that lactate metabolism shapes neuronal activity in the neocortex through KATP channels. Our results highlight the importance of metabolic crosstalk between neurons and astrocytes for brain function.

The Reactive Species Interactome in the Brain

Significance: Redox pioneer Helmut Sies attempted to explain reactive species' challenges faced by organelles, cells, tissues, and organs via three complementary definitions: (i) oxidative stress, that is, the disturbance in the prooxidant-antioxidant defense balance in favor of the prooxidants; (ii) oxidative eustress, the low physiological exposure to prooxidants; and (iii) oxidative distress, the supraphysiological exposure to prooxidants. Recent Advances: Identification, concentration, and interactions are the most important elements to improve our understanding of reactive species in physiology and pathology. In this context, the reactive species interactome (RSI) is a new multilevel redox regulatory system that identifies reactive species families, reactive oxygen species (ROS), reactive nitrogen species (RNS), and reactive sulfur species, and it integrates their interactions with their downstream biological targets. Critical Issues: We propose a united view to fully combine reactive species identification, oxidative eustress and distress, and the RSI system. In this view, we also propose including the forgotten reactive carbonyl species, an increasingly rediscovered reactive species family related to the other reactive families, and key enzymes within the RSI. We focus on brain physiology and pathology to demonstrate why this united view should be considered. Future Directions: More studies are needed for an improved understanding of the contributions of reactive species through their identification, concentration, and interactions, including in the brain. Appreciating the RSI in its entirety should unveil new molecular players and mechanisms in physiology and pathology in the brain and elsewhere.

Aging-Induced Impairment of Vascular Function: Mitochondrial Redox Contributions and Physiological/Clinical Implications

Significance: The vasculature responds to the respiratory needs of tissue by modulating luminal diameter through smooth muscle constriction or relaxation. Coronary perfusion, diastolic function, and coronary flow reserve are drastically reduced with aging. This loss of blood flow contributes to and exacerbates pathological processes such as angina pectoris, atherosclerosis, and coronary artery and microvascular disease. Recent Advances: Increased attention has recently been given to defining mechanisms behind aging-mediated loss of vascular function and development of therapeutic strategies to restore youthful vascular responsiveness. The ultimate goal aims at providing new avenues for symptom management, reversal of tissue damage, and preventing or delaying of aging-induced vascular damage and dysfunction in the first place. Critical Issues: Our major objective is to describe how aging-associated mitochondrial dysfunction contributes to endothelial and smooth muscle dysfunction via dysregulated reactive oxygen species production, the clinical impact of this phenomenon, and to discuss emerging therapeutic strategies. Pathological changes in regulation of mitochondrial oxidative and nitrosative balance (Section 1) and mitochondrial dynamics of fission/fusion (Section 2) have widespread effects on the mechanisms underlying the ability of the vasculature to relax, leading to hyperconstriction with aging. We will focus on flow-mediated dilation, endothelial hyperpolarizing factors (Sections 3 and 4), and adrenergic receptors (Section 5), as outlined in Figure 1. The clinical implications of these changes on major adverse cardiac events and mortality are described (Section 6). Future Directions: We discuss antioxidative therapeutic strategies currently in development to restore mitochondrial redox homeostasis and subsequently vascular function and evaluate their potential clinical impact (Section 7). Antioxid. Redox Signal. 35, 974-1015.

Metabolic Reprogramming: Strategy for Ischemic Stroke Treatment by Ischemic Preconditioning

Stroke is one of the leading causes of death and permanent disability worldwide. Ischemic preconditioning (IPC) is an endogenous protective strategy, which has been reported to exhibit a significant neuroprotective effect in reducing the incidence of ischemic stroke. However, the underlying neuroprotective mechanisms of IPC remain elusive. An increased understanding of the pathogenic mechanisms of stroke and IPC serves to highlight the importance of metabolic reprogramming. In this review, we summarize the metabolic disorder and metabolic plasticity in the incidence and progression of ischemic stroke. We also elaborate how IPC fully mobilizes the metabolic reprogramming to maintain brain metabolic homeostasis, especially for energy and redox homeostasis, and finally protects brain function in the event of an ischemic stroke.

Intermittent Hypoxia Augments Pulmonary Vasoconstrictor Reactivity through PKCβ/Mitochondrial Oxidant Signaling

Pulmonary vasoconstriction resulting from intermittent hypoxia (IH) contributes to pulmonary hypertension (pHTN) in patients with sleep apnea (SA), although the mechanisms involved remain poorly understood. Based on prior studies in patients with SA and animal models of SA, the objective of this study was to evaluate the role of PKCβ and mitochondrial reactive oxygen species (mitoROS) in mediating enhanced pulmonary vasoconstrictor reactivity after IH. We hypothesized that PKCβ mediates vasoconstriction through interaction with the scaffolding protein PICK1 (protein interacting with C kinase 1), activation of mitochondrial ATP-sensitive potassium channels (mitoK_ATP), and stimulated production of mitoROS. We further hypothesized that this signaling axis mediates enhanced vasoconstriction and pHTN after IH. Rats were exposed to IH or sham conditions (7 h/d, 4 wk). Chronic oral administration of the antioxidant Tempol or the PKCβ inhibitor LY-333531 abolished IH-induced increases in right ventricular systolic pressure and right ventricular hypertrophy. Furthermore, scavengers of O₂^- or mitoROS prevented enhanced PKCβ-dependent vasoconstrictor reactivity to endothelin-1 in pulmonary arteries from IH rats. In addition, this PKCβ/mitoROS signaling pathway could be stimulated by the PKC activator PMA in pulmonary arteries from control rats, and in both rat and human pulmonary arterial smooth muscle cells. These responses to PMA were attenuated by inhibition of mitoK_ATP or PICK1. Subcellular fractionation and proximity ligation assays further demonstrated that PKCβ acutely translocates to mitochondria upon stimulation and associates with PICK1. We conclude that a PKCβ/mitoROS signaling axis contributes to enhanced vasoconstriction and pHTN after IH. Furthermore, PKCβ mediates pulmonary vasoconstriction through interaction with PICK1, activation of mitoK_ATP, and subsequent mitoROS generation.

Cerebrovascular function and mitochondrial bioenergetics after ischemia-reperfusion in male rats

The underlying factors promoting increased mitochondrial proteins, mtDNA, and dilation to mitochondrial-specific agents in male rats following tMCAO are not fully elucidated. Our goal was to determine the morphological and functional effects of ischemia/reperfusion (I/R) on mitochondria using electron microscopy, Western blot, mitochondrial oxygen consumption rate (OCR), and Ca2+ sparks activity measurements in middle cerebral arteries (MCAs) from male Sprague Dawley rats (Naïve, tMCAO, Sham). We found a greatly increased OCR in ipsilateral MCAs (IPSI) compared with contralateral (CONTRA), Sham, and Naïve MCAs. Consistent with our earlier findings, the expression of Mitofusin-2 and OPA-1 was significantly decreased in IPSI arteries compared with Sham and Naïve. Mitochondrial morphology was disrupted in vascular smooth muscle, but morphology with normal and perhaps greater numbers of mitochondria were observed in IPSI compared with CONTRA MCAs. Consistently, there were significantly fewer baseline Ca2+ events in IPSI MCAs compared with CONTRA, Sham, and Naïve. Mitochondrial depolarization significantly increased Ca2+ sparks activity in the IPSI, Sham, Naïve, but not in the CONTRA group. Our data indicate that altered mitochondrial structure and function occur in MCAs exposed to I/R and that these changes impact not only OCR but Ca2+ sparks activity in both IPSI and CONTRA MCAs.

Fetal Cerebral Artery Mitochondrion as Target of Prenatal Alcohol Exposure

Prenatal alcohol exposure results in an array of developmental abnormalities known as fetal alcohol spectrum disorders (FASDs). Despite the high prevalence of FASDs, therapeutic interventions against accidental or intended exposure of developing fetuses to alcohol are limited. This review outlines current knowledge about mitochondria in cerebral blood vessels as a potential target for anti-FASDs intervention. First, it describes the multifaceted role of mitochondria in maintaining the cerebral artery diameter as shown in adult tissue. Second, current literature on alcohol-driven damage of mitochondrial morphology and function in several fetal tissues, including liver, heart, and brain is summarized. The functional consequences of alcohol exposure in these organs include morphological enlargement of mitochondria, increased oxidative stress, and alteration of cellular respiration. These studies point to a tissue-specific effect of alcohol on mitochondrial function and a particular vulnerability of fetal mitochondria to alcohol exposure when compared to adult counterparts. Third, recent work from our group describing persistent changes in fetal baboon cerebral artery proteome following three episodes of prenatal alcohol exposure is reviewed. In conclusion, the consequences of prenatal alcohol exposure on cerebral artery mitochondria constitute an open field of investigation and, eventually, a point of therapeutic intervention against FASDs.

SR-mitochondria communication in adult cardiomyocytes: A close relationship where the Ca2+ has a lot to say

In adult cardiomyocytes, T-tubules, junctional sarcoplasmic reticulum (jSR), and mitochondria juxtapose each other and form a unique and highly repetitive functional structure along the cell. The close apposition between jSR and mitochondria creates high Ca²⁺ microdomains at the contact sites, increasing the efficiency of the excitation-contraction-bioenergetics coupling, where the Ca²⁺ transfer from SR to mitochondria plays a critical role. The SR-mitochondria contacts are established through protein tethers, with mitofusin 2 the most studied SR-mitochondrial "bridge", albeit controversial. Mitochondrial Ca²⁺ uptake is further optimized with the mitochondrial Ca²⁺ uniporter preferentially localized in the jSR-mitochondria contact sites and the mitochondrial Na⁺/Ca²⁺ exchanger localized away from these sites. Despite all these unique features facilitating the privileged transport of Ca²⁺ from SR to mitochondria in adult cardiomyocytes, the question remains whether mitochondrial Ca²⁺ concentrations oscillate in synchronicity with cytosolic Ca²⁺ transients during heartbeats. Proper Ca²⁺ transfer controls not only the process of mitochondrial bioenergetics, but also of mitochondria-mediated cell death, autophagy/mitophagy, mitochondrial fusion/fission dynamics, reactive oxygen species generation, and redox signaling, among others. Our review focuses specifically on Ca²⁺ signaling between SR and mitochondria in adult cardiomyocytes. We discuss the physiological and pathological implications of this SR-mitochondrial Ca²⁺ signaling, research gaps, and future trends.

Mitochondrial ATP is required for the maintenance of membrane integrity in stallion spermatozoa, whereas motility requires both glycolysis and oxidative phosphorylation

To investigate the hypothesis that oxidative phosphorylation is a major source of ATP to fuel stallion sperm motility, oxidative phosphorylation was suppressed using the mitochondrial uncouplers CCCP and 2,4,-dinitrophenol (DNP) and by inhibiting mitochondrial respiration at complex IV using sodium cyanide or at the level of ATP synthase using oligomycin-A. As mitochondrial dysfunction may also lead to oxidative stress, production of reactive oxygen species was monitored simultaneously. All inhibitors reduced ATP content, but oligomycin-A did so most profoundly. Oligomycin-A and CCCP also significantly reduced mitochondrial membrane potential. Sperm motility almost completely ceased after the inhibition of mitochondrial respiration and both percentage of motile sperm and sperm velocity were reduced in the presence of mitochondrial uncouplers. Inhibition of ATP synthesis resulted in the loss of sperm membrane integrity and increased the production of reactive oxygen species by degenerating sperm. Inhibition of glycolysis by deoxyglucose led to reduced sperm velocities and reduced ATP content, but not to loss of membrane integrity. These results suggest that, in contrast to many other mammalian species, stallion spermatozoa rely primarily on oxidative phosphorylation to generate the energy required for instance to maintain a functional Na⁺/K⁺ gradient, which is dependent on an Na⁺-K⁺ antiporter ATPase, which relates directly to the noted membrane integrity loss. Under aerobic conditions, however, glycolysis also provides the energy required for sperm motility.

Local Ca2+ coupling between mitochondria and sarcoplasmic reticulum following depolarization in guinea pig urinary bladder smooth muscle cells

Spatiotemporal changes in cytosolic Ca²⁺ concentration ([Ca²⁺]_c) trigger a number of physiological functions in smooth muscle cells (SMCs). We previously imaged Ca²⁺-induced Ca²⁺ release following membrane depolarization as local Ca²⁺ transients, Ca²⁺ hotspots, in subplasmalemmal regions. In this study, the physiological significance of mitochondria on local Ca²⁺ signaling was examined. Cytosolic and mitochondrial Ca²⁺ images following depolarization or action potentials were recorded in single SMCs from the guinea pig urinary bladder using a fast-scanning confocal fluorescent microscope. Depolarization- and action potential-induced [Ca²⁺]_c transients occurred at several discrete sites in subplasmalemmal regions, peaked within 30 ms, and then spread throughout the whole-cell. In contrast, Ca²⁺ concentration in the mitochondria matrix ([Ca²⁺]_m) increased after a delay of ~50 ms from the start of depolarization, and then peaked within 500 ms. Following repolarization, [Ca²⁺]_c returned to the resting level with a half-decay time of ~500 ms, while [Ca²⁺]_m recovered more slowly (∼1.5 s). Carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone, a mitochondrial uncoupler, abolished depolarization-induced [Ca²⁺]_m elevations and slowed [Ca²⁺]_c changes. Importantly, short depolarization-induced changes in [Ca²⁺]_m and transmembrane potential in mitochondria coupled to Ca²⁺ hotspots were significantly larger than those in other mitochondria. Total internal reflection fluorescence imaging revealed that a subset of mitochondria closely localized with ryanodine receptors and voltage-dependent Ca²⁺ channels. These results indicate that particular mitochondria are functionally coupled to ion channels and sarcoplasmic reticulum fragments within the local Ca²⁺ microdomain, and thus, strongly contribute to [Ca²⁺]_c regulation in SMCs.

Molecular Bases of Brain Preconditioning

Preconditioning of the brain induces tolerance to the damaging effects of ischemia and prevents cell death in ischemic penumbra. The development of this phenomenon is mediated by mitochondrial adenosine triphosphate-sensitive potassium (KATP+) channels and nitric oxide signaling (NO). The aim of this study was to investigate the dynamics of molecular changes in mitochondria after ischemic preconditioning (IP) and the effect of pharmacological preconditioning (PhP) with the KATP+-channels opener diazoxide on NO levels after ischemic stroke in rats. Immunofluorescence-histochemistry and laser-confocal microscopy were applied to evaluate the cortical expression of electron transport chain enzymes, mitochondrial KATP+-channels, neuronal and inducible NO-synthases, as well as the dynamics of nitrosylation and nitration of proteins in rats during the early and delayed phases of IP. NO cerebral content was studied with electron paramagnetic resonance (EPR) spectroscopy using spin trapping. We found that 24 h after IP in rats, there is a two-fold decrease in expression of mitochondrial KATP+-channels (p = 0.012) in nervous tissue, a comparable increase in expression of cytochrome c oxidase (p = 0.008), and a decrease in intensity of protein S-nitrosylation and nitration (p = 0.0004 and p = 0.001, respectively). PhP led to a 56% reduction of free NO concentration 72 h after ischemic stroke simulation (p = 0.002). We attribute this result to the restructuring of tissue energy metabolism, namely the provision of increased catalytic sites to mitochondria and the increased elimination of NO, which prevents a decrease in cell sensitivity to oxygen during subsequent periods of severe ischemia.

Calcium Channels in Vascular Smooth Muscle

Calcium (Ca²⁺) plays a central role in excitation, contraction, transcription, and proliferation of vascular smooth muscle cells (VSMs). Precise regulation of intracellular Ca²⁺ concentration ([Ca²⁺]_i) is crucial for proper physiological VSM function. Studies over the last several decades have revealed that VSMs express a variety of Ca²⁺-permeable channels that orchestrate a dynamic, yet finely tuned regulation of [Ca²⁺]_i. In this review, we discuss the major Ca²⁺-permeable channels expressed in VSM and their contribution to vascular physiology and pathology.

Mitochondrial uncoupler carbonyl cyanide m-chlorophenylhydrazone induces vasorelaxation without involving KATP channel activation in smooth muscle cells of arteries

BACKGROUND AND PURPOSE

The effects and mechanisms of chemical mitochondrial uncouplers on vascular function have never been identified. Here, we characterized the effects of the typical mitochondrial uncoupler carbonyl cyanide m-chlorophenylhydrazone (CCCP) on vascular function in rat mesenteric arteries and aorta and elucidated the potential mechanisms.

EXPERIMENTAL APPROACH

Isometric tension of mesenteric artery and thoracic aorta was recorded by using a multiwire myograph system. Protein levels were measured by western blot analyses. Cytosolic [Ca²⁺ ]_i , mitochondrial ROS (mitoROS) and mitochondrial membrane potential of smooth muscle cells (A10) were measured by laser scanning confocal microscopy.

KEY RESULTS

Acute treatment with CCCP relaxed phenylephrine (PE)- and high K⁺ (KPSS)-induced constriction of rat mesenteric arteries with intact and denuded endothelium. Pretreatment with CCCP prevented PE- and KPSS-induced constriction of rat mesenteric arteries with intact and denuded endothelium. Similarly, CCCP prevented PE- and KPSS-induced constriction of rat thoracic aorta. CCCP increased the cellular ADP/ATP ratio in vascular smooth muscle cells (A10) and activated AMPK in A10 cells and rat thoracic aorta tissues. CCCP-induced aorta relaxation was attenuated in AMPK α1 knockout (-/-) mice. SERCA inhibitors thapsigargin and cyclopiazonic acid (CPA) but not the K_ATP channel blocker glibenclamide partially inhibited CCCP-induced vasorelaxation in endothelium-denuded rat mesenteric arteries. CCCP increased cytosolic [Ca²⁺ ]_i , mitoROS production and depolarized mitochondrial membrane potential in A10 cells. FCCP, the analogue of CCCP, had similar vasoactivity as CCCP in rat mesenteric arteries.

CONCLUSIONS AND IMPLICATIONS

CCCP induces vasorelaxation by a mechanism that does not involve K_ATP channel activation in smooth muscle cells of arteries.

Mitochondrial Fission of Smooth Muscle Cells Is Involved in Artery Constriction

Mitochondria are dynamic organelles and continuously undergo fission and fusion processes. Mitochondrial fission is involved in multiple physiological or pathological processes, but the role of mitochondrial fission of smooth muscle cells in artery constriction is unknown. The role of mitochondrial fission of smooth muscle cells in arterial function was investigated by measuring the tension of rat mesenteric arteries and thoracic aorta and by evaluating mitochondrial fission, mitochondrial reactive oxygen species, and cytosolic [Ca²⁺]_i in rat vascular smooth muscle cells. Mitochondrial fission inhibitors mdivi-1 and dynasore antagonized phenylephrine- and high K⁺-induced constriction of rat mesenteric arteries. Mdivi-1 relaxed phenylephrine-induced constriction, and mdivi-1 pretreatment prevented phenylephrine-induced constriction in mice, rat aorta, and human mesenteric arteries. Phenylephrine- and high K⁺-induced increase of mitochondrial fission in smooth muscle cells of rat aorta and the increase was inhibited by mdivi-1. Mdivi-1 inhibited high K⁺-induced increases of mitochondrial fission, mitochondrial reactive oxygen species, and cytosolic [Ca²⁺]_i in rat vascular smooth muscle cells. Prechelation of cytosolic Ca²⁺ prevented high K⁺-induced cytosolic [Ca²⁺]_i increase, mitochondrial fission, and mitochondrial reactive oxygen species overproduction. Mitochondria-targeted antioxidant mito-TEMPO antagonized phenylephrine- and high K⁺-induced constriction of rat mesenteric arteries. Nitroglycerin and ROCK (Rho-associated protein kinase) inhibitor Y27632, the 2 vasodilators with different vasorelaxant mechanisms, relaxed high K⁺-induced vasoconstriction and inhibited high K⁺-induced mitochondrial fission. In conclusion, the mitochondrial fission of smooth muscle cells is involved in artery constriction.

Altered KATP Channel Subunits Expression and Vascular Reactivity in Spontaneously Hypertensive Rats With Age

ATP-sensitive potassium (KATP) channels link membrane excitability to metabolic state to regulate a series of biological activities including the vascular tone. However, their ability to influence hypertension is controversial. Here we aim to investigate possible alteration of KATP channel in vascular smooth muscles (VSMs) during hypertension development process. In this study, we used 16-week-old spontaneously hypertensive rats (SHRs), 49-week-old SHRs, and their age-matched Wistar-Kyoto rats to study the expression of VSM KATP subunits at the mRNA and protein level and the function of VSM KATP by observing the relaxation reactivity of isolated aorta rings to KATP modulators. We found that the expression of VSM KATP subunits Kir6.1 and sulfonylurea receptor (SUR2B) decreased during hypertension. Moreover, the expression of SUR2B and Kir6.1 in 49-week-old SHRs decreased much more than that in 16-week-old SHRs. Furthermore, the aorta rings of 49-week-old SHRs showed lower reactivity to diazoxide than 16-week-old SHRs. This study suggests that KATP channels in VSM subunits Kir6.1 and SUR2B contribute to modify the functionality of this channel in hypertension with age.

Iowa Mutant Apolipoprotein A-I (ApoA-IIowa) Fibrils Target Lysosomes

The single amino acid mutation G26R in human apolipoprotein A-I (apoA-I_Iowa) is the first mutation that was associated with familial AApoA1 amyloidosis. The N-terminal fragments (amino acid residues 1–83) of apoA-I containing this mutation deposit as amyloid fibrils in patients’ tissues and organs, but the mechanisms of cellular degradation and cytotoxicity have not yet been clarified. In this study, we demonstrated degradation of apoA-I_Iowa fibrils via the autophagy-lysosomal pathway in human embryonic kidney 293 cells. ApoA-I_Iowa fibrils induced an increase in lysosomal pH and the cytosolic release of the toxic lysosomal protease cathepsin B. The mitochondrial dysfunction caused by apoA-I_Iowa fibrils depended on cathepsin B and was ameliorated by increasing the degradation of apoA-I_Iowa fibrils. Thus, although apoA-I_Iowa fibril transport to lysosomes and fibril degradation in lysosomes may have occurred, the presence of an excess number of apoA-I_Iowa fibrils, more than the lysosomes could degrade, may be detrimental to cells. Our results thus provide evidence that the target of apoA-I_Iowa fibrils is lysosomes, and we thereby gained a novel insight into the mechanism of AApoA1 amyloidosis.

The Human Microcirculation: Regulation of Flow and Beyond

The microcirculation is responsible for orchestrating adjustments in vascular tone to match local tissue perfusion with oxygen demand. Beyond this metabolic dilation, the microvasculature plays a critical role in modulating vascular tone by endothelial release of an unusually diverse family of compounds including nitric oxide, other reactive oxygen species, and arachidonic acid metabolites. Animal models have provided excellent insight into mechanisms of vasoregulation in health and disease. However, there are unique aspects of the human microcirculation that serve as the focus of this review. The concept is put forth that vasculoparenchymal communication is multimodal, with vascular release of nitric oxide eliciting dilation and preserving normal parenchymal function by inhibiting inflammation and proliferation. Likewise, in disease or stress, endothelial release of reactive oxygen species mediates both dilation and parenchymal inflammation leading to cellular dysfunction, thrombosis, and fibrosis. Some pathways responsible for this stress-induced shift in mediator of vasodilation are proposed. This paradigm may help explain why microvascular dysfunction is such a powerful predictor of cardiovascular events and help identify new approaches to treatment and prevention.

Role of Mitochondria in Cerebral Vascular Function: Energy Production, Cellular Protection, and Regulation of Vascular Tone

Mitochondria not only produce energy in the form of ATP to support the activities of cells comprising the neurovascular unit, but mitochondrial events, such as depolarization and/or ROS release, also initiate signaling events which protect the endothelium and neurons against lethal stresses via pre-/postconditioning as well as promote changes in cerebral vascular tone. Mitochondrial depolarization in vascular smooth muscle (VSM), via pharmacological activation of the ATP-dependent potassium channels on the inner mitochondrial membrane (mitoKATP channels), leads to vasorelaxation through generation of calcium sparks by the sarcoplasmic reticulum and subsequent downstream signaling mechanisms. Increased release of ROS by mitochondria has similar effects. Relaxation of VSM can also be indirectly achieved via actions of nitric oxide (NO) and other vasoactive agents produced by endothelium, perivascular and parenchymal nerves, and astroglia following mitochondrial activation. Additionally, NO production following mitochondrial activation is involved in neuronal preconditioning. Cerebral arteries from female rats have greater mitochondrial mass and respiration and enhanced cerebral arterial dilation to mitochondrial activators. Preexisting chronic conditions such as insulin resistance and/or diabetes impair mitoKATP channel relaxation of cerebral arteries and preconditioning. Surprisingly, mitoKATP channel function after transient ischemia appears to be retained in the endothelium of large cerebral arteries despite generalized cerebral vascular dysfunction. Thus, mitochondrial mechanisms may represent the elusive signaling link between metabolic rate and blood flow as well as mediators of vascular change according to physiological status. Mitochondrial mechanisms are an important, but underutilized target for improving vascular function and decreasing brain injury in stroke patients. © 2016 American Physiological Society. Compr Physiol 6:1529-1548, 2016.

Phosphine toxicity: a story of disrupted mitochondrial metabolism

Rodenticides and pesticides pose a significant threat not only to the environment but also directly to humans by way of accidental and/or intentional exposure. Metal phosphides, such as aluminum, magnesium, and zinc phosphides, have gained popularity owing to ease of manufacture and application. These agents and their hydrolysis by-product phosphine gas (PH3 ) are more than adequate for eliminating pests, primarily in the grain storage industry. In addition to the potential for accidental exposures in the manufacture and use of these agents, intentional exposures must also be considered. As examples, ingestion of metal phosphides is a well-known suicide route, especially in Asia; and intentional release of PH3 in a populated area cannot be discounted. Metal phosphides cause a wide array of effects that include cellular poisoning, oxidative stress, cholinesterase inhibition, circulatory failure, cardiotoxicity, gastrointestinal and pulmonary toxicity, hepatic damage, neurological toxicity, electrolyte imbalance, and overall metabolic disturbances. Mortality rates often exceed 70%. There are no specific antidotes against metal phosphide poisoning. Current therapeutic intervention is limited to supportive care. The development of beneficial medical countermeasures will rely on investigative mechanistic toxicology; the ultimate goal will be to identify specific treatments and therapeutic windows for intervention.

Depolarization of mitochondria in neurons promotes activation of nitric oxide synthase and generation of nitric oxide

The diverse signaling events following mitochondrial depolarization in neurons are not clear. We examined for the first time the effects of mitochondrial depolarization on mitochondrial function, intracellular calcium, neuronal nitric oxide synthase (nNOS) activation, and nitric oxide (NO) production in cultured neurons and perivascular nerves. Cultured rat primary cortical neurons were studied on 7-10 days in vitro, and endothelium-denuded cerebral arteries of adult Sprague-Dawley rats were studied ex vivo. Diazoxide and BMS-191095 (BMS), activators of mitochondrial KATP channels, depolarized mitochondria in cultured neurons and increased cytosolic calcium levels. However, the mitochondrial oxygen consumption rate was unaffected by mitochondrial depolarization. In addition, diazoxide and BMS not only increased the nNOS phosphorylation at positive regulatory serine 1417 but also decreased nNOS phosphorylation at negative regulatory serine 847. Furthermore, diazoxide and BMS increased NO production in cultured neurons measured with both fluorescence microscopy and electron spin resonance spectroscopy, which was sensitive to inhibition by the selective nNOS inhibitor 7-nitroindazole (7-NI). Diazoxide also protected cultured neurons against oxygen-glucose deprivation, which was blocked by NOS inhibition and rescued by NO donors. Finally, BMS induced vasodilation of endothelium denuded, freshly isolated cerebral arteries that was diminished by 7-NI and tetrodotoxin. Thus pharmacological depolarization of mitochondria promotes activation of nNOS leading to generation of NO in cultured neurons and endothelium-denuded arteries. Mitochondrial-induced NO production leads to increased cellular resistance to lethal stress by cultured neurons and to vasodilation of denuded cerebral arteries.

The mitochondrial function of the cerebral vasculature in insulin-resistant Zucker obese rats

Little is known about mitochondrial functioning in the cerebral vasculature during insulin resistance (IR). We examined mitochondrial respiration in isolated cerebral arteries of male Zucker obese (ZO) rats and phenotypically normal Zucker lean (ZL) rats using the Seahorse XFe24 analyzer. We investigated mitochondrial morphology in cerebral blood vessels as well as mitochondrial and nonmitochondrial protein expression levels in cerebral arteries and microvessels. We also measured reactive oxygen species (ROS) levels in cerebral microvessels. Under basal conditions, the mitochondrial respiration components (nonmitochondrial respiration, basal respiration, ATP production, proton leak, and spare respiratory capacity) showed similar levels among the ZL and ZO groups with the exception of maximal respiration, which was higher in the ZO group. We examined the role of nitric oxide by measuring mitochondrial respiration following inhibition of nitric oxide synthase with N(ω)-nitro-l-arginine methyl ester (l-NAME) and mitochondrial activation after administration of diazoxide (DZ). Both ZL and ZO groups showed similar responses to these stimuli with minor variations.l-NAME significantly increased the proton leak, and DZ decreased nonmitochondrial respiration in the ZL group. Other components were not affected. Mitochondrial morphology and distribution within vascular smooth muscle and endothelium as well as mitochondrial protein levels were similar in the arteries and microvessels of both groups. Endothelial nitric oxide synthase (eNOS) and ROS levels were increased in cerebral microvessels of the ZO. Our study suggests that mitochondrial function is not significantly altered in the cerebral vasculature of young ZO rats, but increased ROS production might be due to increased eNOS in the cerebral microcirculation during IR.

Inhibition of Mitochondrial Complex I Leads to Decreased Motility and Membrane Integrity Related to Increased Hydrogen Peroxide and Reduced ATP Production, while the Inhibition of Glycolysis Has Less Impact on Sperm Motility

Mitochondria have been proposed as the major source of reactive oxygen species in somatic cells and human spermatozoa. However, no data regarding the role of mitochondrial ROS production in stallion spermatozoa are available. To shed light on the role of the mitochondrial electron transport chain in the origin of oxidative stress in stallion spermatozoa, specific inhibitors of complex I (rotenone) and III (antimycin-A) were used. Ejaculates from seven Andalusian stallions were collected and incubated in BWW media at 37 °C in the presence of rotenone, antimycin-A or control vehicle. Incubation in the presence of these inhibitors reduced sperm motility and velocity (CASA analysis) (p<0.01), but the effect was more evident in the presence of rotenone (a complex I inhibitor). These inhibitors also decreased ATP content. The inhibition of complexes I and III decreased the production of reactive oxygen species (p<0.01) as assessed by flow cytometry after staining with CellRox deep red. This observation suggests that the CellRox probe mainly identifies superoxide and that superoxide production may reflect intense mitochondrial activity rather than oxidative stress. The inhibition of complex I resulted in increased hydrogen peroxide production (p<0.01). The inhibition of glycolysis resulted in reduced sperm velocities (p<0.01) without an effect on the percentage of total motile sperm. Weak and moderate (but statistically significant) positive correlations were observed between sperm motility, velocity and membrane integrity and the production of reactive oxygen species. These results indicate that stallion sperm rely heavily on oxidative phosphorylation (OXPHOS) for the production of ATP for motility but also require glycolysis to maintain high velocities. These data also indicate that increased hydrogen peroxide originating in the mitochondria is a mechanism involved in stallion sperm senescence.

Arterial Smooth Muscle Mitochondria Amplify Hydrogen Peroxide Microdomains Functionally Coupled to L-Type Calcium Channels

RATIONALE

Mitochondria are key integrators of convergent intracellular signaling pathways. Two important second messengers modulated by mitochondria are calcium and reactive oxygen species. To date, coherent mechanisms describing mitochondrial integration of calcium and oxidative signaling in arterial smooth muscle are incomplete.

OBJECTIVE

To address and add clarity to this issue, we tested the hypothesis that mitochondria regulate subplasmalemmal calcium and hydrogen peroxide microdomain signaling in cerebral arterial smooth muscle.

METHODS AND RESULTS

Using an image-based approach, we investigated the impact of mitochondrial regulation of L-type calcium channels on subcellular calcium and reactive oxygen species signaling microdomains in isolated arterial smooth muscle cells. Our single-cell observations were then related experimentally to intact arterial segments and to living animals. We found that subplasmalemmal mitochondrial amplification of hydrogen peroxide microdomain signaling stimulates L-type calcium channels, and that this mechanism strongly impacts the functional capacity of the vasoconstrictor angiotensin II. Importantly, we also found that disrupting this mitochondrial amplification mechanism in vivo normalized arterial function and attenuated the hypertensive response to systemic endothelial dysfunction.

CONCLUSIONS

From these observations, we conclude that mitochondrial amplification of subplasmalemmal calcium and hydrogen peroxide microdomain signaling is a fundamental mechanism regulating arterial smooth muscle function. As the principle components involved are fairly ubiquitous and positioning of mitochondria near the plasma membrane is not restricted to arterial smooth muscle, this mechanism could occur in many cell types and contribute to pathological elevations of intracellular calcium and increased oxidative stress associated with many diseases.

Dynamics of enhanced mitochondrial respiration in female compared with male rat cerebral arteries

Mitochondrial respiration has never been directly examined in intact cerebral arteries. We tested the hypothesis that mitochondrial energetics of large cerebral arteries ex vivo are sex dependent. The Seahorse XFe24 analyzer was used to examine mitochondrial respiration in isolated cerebral arteries from adult male and female Sprague-Dawley rats. We examined the role of nitric oxide (NO) on mitochondrial respiration under basal conditions, using N(ω)-nitro-l-arginine methyl ester, and following pharmacological challenge using diazoxide (DZ), and also determined levels of mitochondrial and nonmitochondrial proteins using Western blot, and vascular diameter responses to DZ. The components of mitochondrial respiration including basal respiration, ATP production, proton leak, maximal respiration, and spare respiratory capacity were elevated in females compared with males, but increased in both male and female arteries in the presence of the NOS inhibitor. Although acute DZ treatment had little effect on mitochondrial respiration of male arteries, it decreased the respiration in female arteries. Levels of mitochondrial proteins in Complexes I-V and the voltage-dependent anion channel protein were elevated in female compared with male cerebral arteries. The DZ-induced vasodilation was greater in females than in males. Our findings show that substantial sex differences in mitochondrial respiratory dynamics exist in large cerebral arteries and may provide the mechanistic basis for observations that the female cerebral vasculature is more adaptable after injury.

Diversity of mitochondria-dependent dilator mechanisms in vascular smooth muscle of cerebral arteries from normal and insulin-resistant rats

Mitochondrial depolarization following ATP-sensitive potassium (mitoKATP) channel activation has been shown to induce cerebral vasodilation by generation of mitochondrial reactive oxygen species (ROS), which sequentially promotes frequency of calcium sparks and activation of large conductance calcium-activated potassium channels (BKCa) in vascular smooth muscle (VSM). We previously demonstrated that cerebrovascular insulin resistance accompanies aging and obesity. It is unclear whether mitochondrial depolarization without the ROS generation enhances calcium sparks and vasodilation in phenotypically normal [Sprague Dawley (SD); Zucker lean (ZL)] and insulin-resistant [Zucker obese (ZO)] rats. We compared the mechanisms underlying the vasodilation to ROS-dependent (diazoxide) and ROS-independent [BMS-191095 (BMS)] mitoKATP channel activators in normal and ZO rats. Arterial diameter studies from SD, ZL, and ZO rats showed that BMS as well as diazoxide induced vasodilation in endothelium-denuded cerebral arteries. In normal rats, BMS-induced vasodilation was mediated by mitochondrial depolarization and calcium sparks generation in VSM and was reduced by inhibition of BKCa channels. However, unlike diazoxide-induced vasodilation, scavenging of ROS had no effect on BMS-induced vasodilation. Electron spin resonance spectroscopy confirmed that diazoxide but not BMS promoted vascular ROS generation. BMS- as well as diazoxide-induced vasodilation, mitochondrial depolarization, and calcium spark generation were diminished in cerebral arteries from ZO rats. Thus pharmacological depolarization of VSM mitochondria by BMS promotes ROS-independent vasodilation via generation of calcium sparks and activation of BKCa channels. Diminished generation of calcium sparks and reduced vasodilation in ZO arteries in response to BMS and diazoxide provide new insights into mechanisms of cerebrovascular dysfunction in insulin resistance.

Mitochondria-derived ROS bursts disturb Ca²⁺ cycling and induce abnormal automaticity in guinea pig cardiomyocytes: a theoretical study

Mitochondria are in close proximity to the redox-sensitive sarcoplasmic reticulum (SR) Ca(2+) release [ryanodine receptors (RyRs)] and uptake [Ca(2+)-ATPase (SERCA)] channels. Thus mitochondria-derived reactive oxygen species (mdROS) could play a crucial role in modulating Ca(2+) cycling in the cardiomyocytes. However, whether mdROS-mediated Ca(2+) dysregulation translates to abnormal electrical activities under pathological conditions, and if yes what are the underlying ionic mechanisms, have not been fully elucidated. We hypothesize that pathological mdROS induce Ca(2+) elevation by modulating SR Ca(2+) handling, which activates other Ca(2+) channels and further exacerbates Ca(2+) dysregulation, leading to abnormal action potential (AP). We also propose that the morphologies of elicited AP abnormality rely on the time of mdROS induction, interaction between mitochondria and SR, and intensity of mitochondrial oxidative stress. To test the hypotheses, we developed a multiscale guinea pig cardiomyocyte model that incorporates excitation-contraction coupling, local Ca(2+) control, mitochondrial energetics, and ROS-induced ROS release. This model, for the first time, includes mitochondria-SR microdomain and modulations of mdROS on RyR and SERCA activities. Simulations show that mdROS bursts increase cytosolic Ca(2+) by stimulating RyRs and inhibiting SERCA, which activates the Na(+)/Ca(2+) exchanger, Ca(2+)-sensitive nonspecific cationic channels, and Ca(2+)-induced Ca(2+) release, eliciting abnormal AP. The morphologies of AP abnormality are largely influenced by the time interval among mdROS burst induction and AP firing, dosage and diffusion of mdROS, and SR-mitochondria distance. This study defines the role of mdROS in Ca(2+) overload-mediated cardiac arrhythmogenesis and underscores the importance of considering mitochondrial targets in designing new antiarrhythmic therapies.

Sustained mitochondrial functioning in cerebral arteries after transient ischemic stress in the rat: a potential target for therapies

The objective of the present study was to determine whether mitochondrial function in the cerebral vasculature is maintained after transient middle cerebral artery (MCA) occlusion (tMCAO) in rats. Sprague-Dawley rats were exposed to 90 min of tMCAO followed by 4 or 48 h of reperfusion. MCAs from ischemic (ipsilateral) and nonischemic (contralateral) sides were compared with control MCAs from sham-operated rats. We determined 1) vasoreactivity to diazoxide (DZ; a mitochondrial ATP-activated K(+) channel opener), ACh, bradykinin (BK), serotonin, and sodium nitroprusside; 2) levels of mitochondrial and nonmitochondrial proteins and mitochondrial DNA; and 3) vascular levels of tetramethylrhodamine ethyl ester (an indicator of mitochondrial membrane potential). All dilator responses, including those with DZ, were intact 4 h post-tMCAO. Dilator responses to ACh, BK, and sodium nitroprusside were reduced in ipsilateral MCAs at 48 h compared with contralateral MCAs, but DZ responses were comparable with control MCAs. Surprisingly, contralateral responses to ACh, BK, and serotonin were reduced compared with control MCAs at 48 h. Ipsilateral vasodilation to DZ at 48 h was eliminated by endothelial denudation and endothelial nitric oxide synthase (eNOS) inhibition but was only reduced in control MCAs. Mitochondrial proteins, phosphorylated eNOS, mitochondrial DNA, and mitochondrial membrane potential were higher in ipsilateral compared with contralateral MCAs. In conclusion, contrary to conventional wisdom, mitochondria remain functional for at least 48 h after severe ischemic stress in MCAs, and DZ-induced dilation is preserved due to maintained mitochondrial mass, probably in the endothelium, and eNOS signaling. Our findings support the concept that functioning vascular mitochondria are an unexpected target for novel stroke therapies.

Mitochondrial reactive oxygen species: a double edged sword in ischemia/reperfusion vs preconditioning

Reductions in the blood supply produce considerable injury if the duration of ischemia is prolonged. Paradoxically, restoration of perfusion to ischemic organs can exacerbate tissue damage and extend the size of an evolving infarct. Being highly metabolic organs, the heart and brain are particularly vulnerable to the deleterious effects of ischemia/reperfusion (I/R). While the pathogenetic mechanisms contributing to I/R-induced tissue injury and infarction are multifactorial, the relative importance of each contributing factor remains unclear. However, an emerging body of evidence indicates that the generation of reactive oxygen species (ROS) by mitochondria plays a critical role in damaging cellular components and initiating cell death. In this review, we summarize our current understanding of the mechanisms whereby mitochondrial ROS generation occurs in I/R and contributes to myocardial infarction and stroke. In addition, mitochondrial ROS have been shown to participate in preconditioning by several pharmacologic agents that target potassium channels (e.g., ATP-sensitive potassium (mKATP) channels or large conductance, calcium-activated potassium (mBKCa) channels) to activate cell survival programs that render tissues and organs more resistant to the deleterious effects of I/R. Finally, we review novel therapeutic approaches that selectively target mROS production to reduce postischemic tissue injury, which may prove efficacious in limiting myocardial dysfunction and infarction and abrogating neurocognitive deficits and neuronal cell death in stroke.

Mitochondrial mechanisms in cerebral vascular control: shared signaling pathways with preconditioning

Mitochondrial-initiated events protect the neurovascular unit against lethal stress via a process called preconditioning, which independently promotes changes in cerebrovascular tone through shared signaling pathways. Activation of adenosine triphosphate (ATP)-dependent potassium channels on the inner mitochondrial membrane (mitoKATP channels) is a specific and dependable way to induce protection of neurons, astroglia, and cerebral vascular endothelium. Through the opening of mitoKATP channels, mitochondrial depolarization leads to activation of protein kinases and transient increases in cytosolic calcium (Ca(2+)) levels that activate terminal mechanisms that protect the neurovascular unit against lethal stress. The release of reactive oxygen species from mitochondria has similar protective effects. Signaling elements of the preconditioning pathways also are involved in the regulation of vascular tone. Activation of mitoKATP channels in cerebral arteries causes vasodilation, with cell-specific contributions from the endothelium, vascular smooth muscles, and nerves. Preexisting chronic conditions, such as insulin resistance and/or diabetes, prevent preconditioning and impair relaxation to mitochondrial-centered responses in cerebral arteries. Surprisingly, mitochondrial activation after anoxic or ischemic stress appears to protect cerebral vascular endothelium and promotes the restoration of blood flow; therefore, mitochondria may represent an important, but underutilized target in attenuating vascular dysfunction and brain injury in stroke patients.

Glibenclamide decreases ATP-induced intracellular calcium transient elevation via inhibiting reactive oxygen species and mitochondrial activity in macrophages

Increasing evidence has revealed that glibenclamide has a wide range of anti-inflammatory effects. However, it is unclear whether glibenclamide can affect the resting and adenosine triphosphate (ATP)-induced intracellular calcium ([Ca²⁺]_i) handling in Raw 264.7 macrophages. In the present study, [Ca²⁺]_i transient, reactive oxygen species (ROS) and mitochondrial activity were measured by the high-speed TILLvisION digital imaging system using the indicators of Fura 2-am, DCFDA and rhodamine-123, respectively. We found that glibenclamide, pinacidil and other unselective K⁺ channel blockers had no effect on the resting [Ca²⁺]_i of Raw 264.7 cells. Extracellular ATP (100 µM) induced [Ca²⁺]_i transient elevation independent of extracellular Ca²⁺. The transient elevation was inhibited by an ROS scavenger (tiron) and mitochondria inhibitor (rotenone). Glibenclamide and 5-hydroxydecanoate (5-HD) also decreased ATP-induced [Ca²⁺]_i transient elevation, but pinacidil and other unselective K⁺ channel blockers had no effect. Glibenclamide also decreased the peak of [Ca²⁺]_i transient induced by extracellular thapsigargin (Tg, 1 µM). Furthermore, glibenclamide decreased intracellular ROS and mitochondrial activity. When pretreated with tiron and rotenone, glibenclamide could not decrease ATP, and Tg induced maximal [Ca²⁺]_i transient further. We conclude that glibenclamide may inhibit ATP-induced [Ca²⁺]_i transient elevation by blocking mitochondria K_ATP channels, resulting in decreased ROS generation and mitochondrial activity in Raw 264.7 macrophages.

Nox2-induced production of mitochondrial superoxide in angiotensin II-mediated endothelial oxidative stress and hypertension

AIMS

Angiotensin II (AngII)-induced superoxide (O2(•-)) production by the NADPH oxidases and mitochondria has been implicated in the pathogenesis of endothelial dysfunction and hypertension. In this work, we investigated the specific molecular mechanisms responsible for the stimulation of mitochondrial O2(•-) and its downstream targets using cultured human aortic endothelial cells and a mouse model of AngII-induced hypertension.

RESULTS

Western blot analysis showed that Nox2 and Nox4 were present in the cytoplasm but not in the mitochondria. Depletion of Nox2, but not Nox1, Nox4, or Nox5, using siRNA inhibits AngII-induced O2(•-) production in both mitochondria and cytoplasm. Nox2 depletion in gp91phox knockout mice inhibited AngII-induced cellular and mitochondrial O2(•-) and attenuated hypertension. Inhibition of mitochondrial reverse electron transfer with malonate, malate, or rotenone attenuated AngII-induced cytoplasmic and mitochondrial O2(•-) production. Inhibition of the mitochondrial ATP-sensitive potassium channel (mitoK(+)ATP) with 5-hydroxydecanoic acid or specific PKCɛ peptide antagonist (EAVSLKPT) reduced AngII-induced H2O2 in isolated mitochondria and diminished cytoplasmic O2(•-). The mitoK(+)ATP agonist diazoxide increased mitochondrial O2(•-), cytoplasmic c-Src phosphorylation and cytoplasmic O2(•-) suggesting feed-forward regulation of cellular O2(•-) by mitochondrial reactive oxygen species (ROS). Treatment of AngII-infused mice with malate reduced blood pressure and enhanced the antihypertensive effect of mitoTEMPO. Mitochondria-targeted H2O2 scavenger mitoEbselen attenuated redox-dependent c-Src and inhibited AngII-induced cellular O2(•-), diminished aortic H2O2, and reduced blood pressure in hypertensive mice.

INNOVATION AND CONCLUSIONS

These studies show that Nox2 stimulates mitochondrial ROS by activating reverse electron transfer and both mitochondrial O2(•-) and reverse electron transfer may represent new pharmacological targets for the treatment of hypertension.

Inducible endothelium-derived hyperpolarizing factor: role of the 15-lipoxygenase-EDHF pathway

Endothelium-derived hyperpolarizing factors (EDHFs) regulate vascular tone by contributing to the vasorelaxations to shear stress and endothelial agonists such as bradykinin and acetylcholine. 15(S)-Hydroxy-11,12-epoxyeicosatrienoic acid (15-H-11,12-EETA) and 11(R),12(S),15(S)-trihydroxyeicosatrienoic acid (11,12,15-THETA) are endothelial metabolites of the 15-lipoxygenase (15-LO) pathway of arachidonic acid metabolism and are EDHFs. 11,12,15-THETA activates small conductance, calcium-activated potassium channels on smooth muscle cells causing membrane hyperpolarization, and relaxation. Expression levels of 15-LO in the endothelium regulate the activity of the 15-LO/15-H-11,12-EETA/11,12,15-THETA pathway and its contribution to vascular tone. Regulation of its expression is by transcriptional, translational, and epigenetic mechanisms. Hypoxia, hypercholesterolemia, atherosclerosis, anemia, estrogen, interleukins, and possibly other hormones increase 15-LO expression. An increase in 15-LO results in increased synthesis of 15-H-11,12-EETA and 11,12,15-THETA, increased membrane hyperpolarization, and enhanced contribution to relaxation by endothelial agonists. Thus, the 15-LO pathway represents the first example of an inducible EDHF. In addition to 15-LO metabolites, a number of chemicals have been identified as EDHFs and their contributions to vascular tone vary with species and vascular bed. The reason for multiple EDHFs has evaded explanation. However, EDHF functioning as constitutive EDHFs or inducible EDHFs may explain the need for chemically and biochemically distinct pathways for EDHF activity and the variation in EDHFs between species and vascular beds. This new EDHF classification provides a framework for understanding EDHF activity in physiological and pathological conditions.

The effects of hypertension on the cerebral circulation

Maintenance of brain function depends on a constant blood supply. Deficits in cerebral blood flow are linked to cognitive decline, and they have detrimental effects on the outcome of ischemia. Hypertension causes alterations in cerebral artery structure and function that can impair blood flow, particularly during an ischemic insult or during periods of low arterial pressure. This review will focus on the historical discoveries, novel developments, and knowledge gaps in 1) hypertensive cerebral artery remodeling, 2) vascular function with emphasis on myogenic reactivity and endothelium-dependent dilation, and 3) blood-brain barrier function. Hypertensive artery remodeling results in reduction in the lumen diameter and an increase in the wall-to-lumen ratio in most cerebral arteries; this is linked to reduced blood flow postischemia and increased ischemic damage. Many factors that are increased in hypertension stimulate remodeling; these include the renin-angiotensin-aldosterone system and reactive oxygen species levels. Endothelial function, vital for endothelium-mediated dilation and regulation of myogenic reactivity, is impaired in hypertension. This is a consequence of alterations in vasodilator mechanisms involving nitric oxide, epoxyeicosatrienoic acids, and ion channels, including calcium-activated potassium channels and transient receptor potential vanilloid channel 4. Hypertension causes blood-brain barrier breakdown by mechanisms involving inflammation, oxidative stress, and vasoactive circulating molecules. This exposes neurons to cytotoxic molecules, leading to neuronal loss, cognitive decline, and impaired recovery from ischemia. As the population ages and the incidence of hypertension, stroke, and dementia increases, it is imperative that we gain a better understanding of the control of cerebral artery function in health and disease.

Depolarization of mitochondria in endothelial cells promotes cerebral artery vasodilation by activation of nitric oxide synthase

OBJECTIVE

Mitochondrial depolarization after ATP-sensitive potassium channel activation has been shown to induce cerebral vasodilation by the generation of calcium sparks in smooth muscle. It is unclear, however, whether mitochondrial depolarization in endothelial cells is capable of promoting vasodilation by releasing vasoactive factors. Therefore, we studied the effect of endothelial mitochondrial depolarization by mitochondrial ATP-sensitive potassium channel activators, BMS-191095 (BMS) and diazoxide, on endothelium-dependent vasodilation.

APPROACH AND RESULTS

Diameter studies in isolated rat cerebral arteries showed BMS- and diazoxide-induced vasodilations that were diminished by endothelial denudation. Mitochondrial depolarization-induced vasodilation was reduced by inhibition of mitochondrial ATP-sensitive potassium channels, phosphoinositide-3 kinase, or nitric oxide synthase. Scavenging of reactive oxygen species, however, diminished vasodilation induced by diazoxide, but not by BMS. Fluorescence studies in cultured rat brain microvascular endothelial cells showed that BMS elicited mitochondrial depolarization and enhanced nitric oxide production; diazoxide exhibited largely similar effects, but unlike BMS, increased mitochondrial reactive oxygen species production. Measurements of intracellular calcium ([Ca(2+)]i) in cultured rat brain microvascular endothelial cells and arteries showed that both diazoxide and BMS increased endothelial [Ca(2+)]i. Western blot analyses revealed increased phosphorylation of protein kinase B and endothelial nitric oxide synthase (eNOS) by BMS and diazoxide. Increased phosphorylation of eNOS by diazoxide was abolished by phosphoinositide-3 kinase inhibition. Electron spin resonance spectroscopy confirmed vascular nitric oxide generation in response to diazoxide and BMS.

CONCLUSIONS

Pharmacological depolarization of endothelial mitochondria promotes activation of eNOS by dual pathways involving increased [Ca(2+)]i as well as by phosphoinositide-3 kinase-protein kinase B-induced eNOS phosphorylation. Both mitochondrial reactive oxygen species-dependent and -independent mechanisms mediate activation of eNOS by endothelial mitochondrial depolarization.

Mitochondrial regulation of cytosolic Ca²⁺ signals in smooth muscle

The cytosolic Ca²⁺ concentration ([Ca²⁺]c) controls virtually every activity of smooth muscle, including contraction, migration, transcription, division and apoptosis. These processes may be activated by large (>10 μM) amplitude [Ca²⁺]c increases, which occur in small restricted regions of the cell or by smaller (<1 μM) amplitude changes throughout the bulk cytoplasm. Mitochondria contribute to the regulation of these signals by taking up Ca²⁺. However, mitochondria's reported low affinity for Ca²⁺ is thought to require the organelle to be positioned close to ion channels and within a microdomain of high [Ca²⁺]. In cultured smooth muscle, mitochondria are highly dynamic structures but in native smooth muscle mitochondria are immobile, apparently strategically positioned organelles that regulate the upstroke and amplitude of IP₃-evoked Ca²⁺ signals and IP₃ receptor (IP₃R) cluster activity. These observations suggest mitochondria are positioned within the high [Ca²⁺] microdomain arising from an IP₃R cluster to exert significant local control of channel activity. On the other hand, neither the upstroke nor amplitude of voltage-dependent Ca²⁺ entry is modulated by mitochondria; rather, it is the declining phase of the transient that is regulated by the organelle. Control of the declining phase of the transient requires a high mitochondrial affinity for Ca²⁺ to enable uptake to occur over the normal physiological Ca²⁺ range (<1 μM). Thus, in smooth muscle, mitochondria regulate Ca²⁺ signals exerting effects over a large range of [Ca²⁺] (∼200 nM to at least tens of micromolar) to provide a wide dynamic range in the control of Ca²⁺ signals.

Hydrogen sulfide activates Ca²⁺ sparks to induce cerebral arteriole dilatation

Hydrogen sulfide (H₂S) is a gaseous vasodilator produced by endothelial cells. Mechanisms by which H₂S induces vasodilatation are unclear. We tested the hypothesis that H₂S dilates cerebral arterioles by modulating local and global intracellular Ca²⁺ signals in smooth muscle cells. High-speed confocal imaging revealed that Na₂S, an H₂S donor, increased Ca²⁺ spark frequency ∼1.43-fold and decreased global intracellular Ca²⁺ concentration ([Ca²⁺]i) by ∼37 nM in smooth muscle cells of intact piglet cerebral arterioles. In contrast, H₂S did not alter Ca²⁺ wave frequency. In voltage-clamped (-40 mV) cells, H₂S increased the frequency of iberiotoxin-sensitive, Ca²⁺ spark-induced transient Ca²⁺-activated K⁺ (KCa) currents ∼1.83-fold, but did not alter the amplitude of these events. H₂S did not alter the activity of single KCa channels recorded in the absence of Ca²⁺ sparks in arteriole smooth muscle cells. H₂S increased SR Ca²⁺ load ([Ca²⁺]SR), measured as caffeine (10 and 20mM)-induced [Ca²⁺]i transients, ∼1.5-fold. H₂S hyperpolarized (by ∼18 mV) and dilated pressurized (40 mmHg) cerebral arterioles. Iberiotoxin, a KCa channel blocker, reduced H₂S-induced hyperpolarization by ∼51%. Iberiotoxin and ryanodine, a ryanodine receptor channel inhibitor, reduced H₂S-induced vasodilatation by ∼38 and ∼37%, respectively. In summary, our data indicate that H₂S elevates [Ca²⁺]SR, leading to Ca²⁺ spark activation in cerebral arteriole smooth muscle cells. The subsequent elevation in transient KCa current frequency leads to membrane hyperpolarization, a reduction in global [Ca²⁺]i and vasodilatation.

Mitochondrial mitophagic mechanisms of myocardial matrix metabolism and remodelling

High levels of homocysteine (Hcy), known as hyperhomocysteinmia (HHcy), are correlated with an increase in extracellular matrix remodelling (ECM) via the matrix metalloproteinases (MMPs) and plasminogen/plasmin system. This results in an increase deposition of collagen that leads to endothelial-myocyte (EM) and myocyte-myocyte (MM) uncoupling; the physiological consequences are a plethora of cardiovascular pathologies. Homocysteine-induced increase in intracellular and mitochondrial Ca(2+) plays an important role in increasing reactive oxygen species (ROS) within mitochondria and instigating mitophagy within the cell. This occurs via several Hcy-mitigated processes: agonizing N-methyl-d-aspartate receptor-1 (NMDA-R1), decreasing expression of peroxisome proliferator activator receptor (PPAR) [thereby increasing oxidation], impairing Ca(2+) handling via Na(+)/Ca(2+) exchanger (NCX1) and Sarco endoplasmic reticulum Ca(2+) ATPase (SERCA-2a). The end result is an increase in ROS that directly or indirectly lead to MMP activation within mitochondria or the cytoplasm. Hcy induces a mitochondrial permeability transition that allows MMPs to be released from mitochondria thereby metabolizing matrix and impairing cardiac function. Further work remains to be elucidated concerning the specific mitochondrial mitophagic mechanisms under which matrix metabolism and remodelling occurs. Moreover, the therapeutic implications of NMDA and PPAR ligands are some promise to patient.

Different potassium channels are involved in relaxation of rat renal artery induced by P1075

The ATP-sensitive K(+) channels opener (K(ATP)CO), P1075 [N-cyano-N'-(1,1-dimethylpropyl)-N″-3-pyridylguanidine], has been shown to cause relaxation of various isolated animal and human blood vessels by opening of vascular smooth muscle ATP-sensitive K(+) (K(ATP)) channels. In addition to the well-known effect on the opening of K(ATP) channels, it has been reported that vasorelaxation induced by some of the K(ATP)COs includes some other K(+) channel subtypes. Given that there is still no information on other types of K(+) channels possibly involved in the mechanism of relaxation induced by P1075, this study was designed to examine the effects of P1075 on the rat renal artery with endothelium and with denuded endothelium and to define the contribution of different K(+) channel subtypes in the P1075 action on this blood vessel. Our results show that P1075 induced a concentration-dependent relaxation of rat renal artery rings pre-contracted by phenylephrine. Glibenclamide, a selective K(ATP) channels inhibitor, partly antagonized the relaxation of rat renal artery induced by P1075. Tetraethylammonium (TEA), a non-selective inhibitor of Ca(2+)-activated K(+) channels, as well as iberiotoxin, a most selective blocker of large-conductance Ca(2+) -activated K(+) (BK(Ca)) channels, did not abolish the effect of P1075 on rat renal artery. In contrast, a non-selective blocker of voltage-gated K(+) (K(V)) channels, 4-aminopyridine (4-AP), as well as margatoxin, a potent inhibitor of K(V)1.3 channels, caused partial inhibition of the P1075-induced relaxation of rat renal artery. In addition, in this study, P1075 relaxed contractions induced by 20 mM K(+) , but had no effect on contractions induced by 80 mM K(+). Our results showed that P1075 induced strong endothelium-independent relaxation of rat renal artery. It seems that K(ATP), 4-AP- and margatoxin-sensitive K(+) channels located in vascular smooth muscle mediated the relaxation of rat renal artery induced by P1075.