Uncoupling proteins: Martin Klingenberg's contributions for 40 years

The uncoupling protein (UCP1) is a proton (H⁺) transporter in the mitochondrial inner membrane. By dissipating the electrochemical H⁺ gradient, UCP1 uncouples respiration from ATP synthesis, which drives an increase in substrate oxidation via the TCA cycle flux that generates more heat. The mitochondrial uncoupling-mediated non-shivering thermogenesis in brown adipose tissue is vital primarily to mammals, such as rodents and new-born humans, but more recently additional functions in adult humans have been described. UCP1 is regulated by β-adrenergic receptors through the sympathetic nervous system and at the molecular activity level by nucleotides and fatty acid to meet thermogenesis needs. The discovery of novel UCP homologs has greatly contributed to the understanding of human diseases, such as obesity and diabetes. In this article, we review the progress made towards the molecular mechanism and function of the UCPs, in particular focusing on the influential contributions from Martin Klingenberg's laboratory. Because all members of the UCP family are potentially promising drug targets, we also present and discuss possible approaches and methods for UCP-related drug discovery.

Early Effects of Prolonged Cardiac Arrest and Ischemic Postconditioning during Cardiopulmonary Resuscitation on Cardiac and Brain Mitochondrial Function in Pigs

BACKGROUND

Out-of-hospital cardiac arrest (CA) is a prevalent medical crisis resulting in severe injury to the heart and brain and an overall survival of less than 10%. Mitochondrial dysfunction is predicted to be a key determinant of poor outcomes following prolonged CA. However, the onset and severity of mitochondrial dysfunction during CA and cardiopulmonary resuscitation (CPR) is not fully understood. Ischemic postconditioning (IPC), controlled pauses during the initiation of CPR, has been shown to improve cardiac function and neurologically favorable outcomes after 15min of CA. We tested the hypothesis that mitochondrial dysfunction develops during prolonged CA and can be rescued with IPC during CPR (IPC-CPR).

METHODS

A total of 63 swine were randomized to no ischemia (Naïve), 19min of ventricular fibrillation (VF) CA without CPR (Untreated VF), or 15min of CA with 4min of reperfusion with either standard CPR (S-CPR) or IPC-CPR. Mitochondria were isolated from the heart and brain to quantify respiration, rate of ATP synthesis, and calcium retention capacity (CRC). Reactive oxygen species (ROS) production was quantified from fresh frozen heart and brain tissue.

RESULTS

Compared to Naïve, Untreated VF induced cardiac and brain ROS overproduction concurrent with decreased mitochondrial respiratory coupling and CRC, as well as decreased cardiac ATP synthesis. Compared to Untreated VF, S-CPR attenuated brain ROS overproduction but had no other effect on mitochondrial function in the heart or brain. Compared to Untreated VF, IPC-CPR improved cardiac mitochondrial respiratory coupling and rate of ATP synthesis, and decreased ROS overproduction in the heart and brain.

CONCLUSIONS

Fifteen minutes of VF CA results in diminished mitochondrial respiration, ATP synthesis, CRC, and increased ROS production in the heart and brain. IPC-CPR attenuates cardiac mitochondrial dysfunction caused by prolonged VF CA after only 4min of reperfusion, suggesting that IPC-CPR is an effective intervention to reduce cardiac injury. However, reperfusion with both CPR methods had limited effect on mitochondrial function in the brain, emphasizing an important physiological divergence in post-arrest recovery between those two vital organs.

Anaesthetics as cardioprotectants: translatability and mechanism

The pharmacological conditioning of the heart with anaesthetics, such as volatile anaesthetics or opioids, is a phenomenon whereby a transient exposure to an anaesthetic agent protects the heart from the harmful consequences of myocardial ischaemia and reperfusion injury. The cellular and molecular mechanisms of anaesthetic conditioning appear largely to mimic those of ischaemic pre- and post-conditioning. Progress has been made on the understanding of the underlying mechanisms although the order of events and the specific targets of anaesthetics that trigger protection are not always clear. In the laboratory, the protection afforded by certain anaesthetics against cardiac ischaemia and reperfusion injury is powerful and reproducible but this has not necessarily translated into similarly robust clinical benefits. Indeed, clinical studies and meta-analyses delivered variable results when comparing in the laboratory setting protective and non-protective anaesthetics. Reasons for this include underlying conditions such as age, obesity and diabetes. Animal models for disease or ageing, human cardiomyocytes derived from stem cells of patients and further clinical studies are employed to better understand the underlying causes that prevent a more robust protection in patients.

Anaesthetics as cardioprotectants: translatability and mechanism

The pharmacological conditioning of the heart with anaesthetics, such as volatile anaesthetics or opioids, is a phenomenon whereby a transient exposure to an anaesthetic agent protects the heart from the harmful consequences of myocardial ischaemia and reperfusion injury. The cellular and molecular mechanisms of anaesthetic conditioning appear largely to mimic those of ischaemic pre- and post-conditioning. Progress has been made on the understanding of the underlying mechanisms although the order of events and the specific targets of anaesthetics that trigger protection are not always clear. In the laboratory, the protection afforded by certain anaesthetics against cardiac ischaemia and reperfusion injury is powerful and reproducible but this has not necessarily translated into similarly robust clinical benefits. Indeed, clinical studies and meta-analyses delivered variable results when comparing in the laboratory setting protective and non-protective anaesthetics. Reasons for this include underlying conditions such as age, obesity and diabetes. Animal models for disease or ageing, human cardiomyocytes derived from stem cells of patients and further clinical studies are employed to better understand the underlying causes that prevent a more robust protection in patients.

Comparison of Cardiomyocyte Differentiation Potential Between Type 1 Diabetic Donor- and Nondiabetic Donor-Derived Induced Pluripotent Stem Cells

Type 1 diabetes mellitus (T1DM) is the most common type of diabetes in children and adolescents. Diabetic subjects are more likely to experience a myocardial infarction compared to nondiabetic subjects. In recent years, induced pluripotent stem cells (iPSCs) have received increasing attention from basic scientists and clinicians and hold promise for myocardial regeneration due to their unlimited proliferation potential and differentiation capacity. However, cardiomyogenesis of type 1 diabetic donor-derived iPSCs (T1DM-iPSCs) has not been investigated yet. The aim of the study was to comparatively analyze cardiomyocyte (CM) differentiation capacity of nondiabetic donor-derived iPSCs (N-iPSCs) and T1DM-iPSCs. The differentiated CMs were confirmed by both expression of cardiac-specific markers and presence of cardiac action potential. Since mitochondrial bioenergetics is vital to every aspect of CM function, extracellular acidification rates and oxygen consumption rates were measured using Seahorse extracellular flux analyzer. The results showed that N-iPSCs and T1DM-iPSCs demonstrated similar capacity of differentiation into spontaneously contracting CMs exhibiting nodal-, atrial-, or ventricular-like action potentials. Differentiation efficiency was up to 90%. In addition, the CMs differentiated from N-iPSCs and T1DM-iPSCs (N-iPSC-CMs and T1DM-iPSC-CMs, respectively) showed 1) well-regulated glucose utilization at the level of glycolysis and mitochondrial oxidative phosphorylation and 2) the ability to switch metabolic pathways independent of extracellular glucose concentration. Collectively, we demonstrate for the first time that T1DM-iPSCs can differentiate into functional CMs with well-regulated glucose utilization as shown in N-iPSCs, suggesting that T1DM-iPSC-CMs might be a promising autologous cell source for myocardial regeneration in type 1 diabetes patients.

Anaesthetic Postconditioning at the Initiation of CPR Improves Myocardial and Mitochondrial Function in a Pig Model of Prolonged Untreated Ventricular Fibrillation

BACKGROUND

Anaesthetic postconditioning (APoC) attenuates myocardial injury following coronary ischaemia/reperfusion. We hypothesised that APoC at the initiation of cardiopulmonary resuscitation (CPR) will improve post resuscitation myocardial function along with improved mitochondrial function in a pig model of prolonged untreated ventricular fibrillation.

METHODS

In 32 pigs isoflurane anaesthesia was discontinued prior to induction of ventricular fibrillation that was left untreated for 15 min. At the initiation of CPR, 15 animals were randomised to controls (CON), and 17 to APoC with 2 vol% sevoflurane during the first 3 min CPR. Pigs were defibrillated after 4 min of CPR. After return of spontaneous circulation (ROSC), isoflurane was restarted at 0.8-1.5 vol% in both groups. Systolic and diastolic blood pressures were measured continuously. Of the animals that achieved ROSC, eight CON and eight APoC animals were randomised to have their left ventricular ejection fraction (LVEF%) assessed by echocardiography at 4h. Seven CON and nine APoC were randomised to euthanasia 15 min after ROSC to isolate mitochondria from the left ventricle for bioenergetic studies.

RESULTS

ROSC was achieved in 10/15 CON and 15/17 APoC animals. APoC improved haemodynamics during CPR and post-CPR LVEF%. Mitochondrial ATP synthesis, coupling of oxidative phosphorylation and calcium retention capacity were improved in cardiac mitochondria isolated after APoC.

CONCLUSIONS

In a porcine model of prolonged untreated cardiac arrest, APoC with inhaled sevoflurane at the initiation of CPR, is associated with preserved mitochondrial function and improved post resuscitation myocardial dysfunction. Approved by the Institutional Animal Care Committee of the Minneapolis Medical Research Foundation of Hennepin County Medical Center (protocol number 11-05).

Divergent effects of painful nerve injury on mitochondrial Ca(2+) buffering in axotomized and adjacent sensory neurons

Mitochondria critically regulate cytoplasmic Ca(2+) concentration ([Ca(2+)]c), but the effects of sensory neuron injury have not been examined. Using FCCP (1µM) to eliminate mitochondrial Ca(2+) uptake combined with oligomycin (10µM) to prevent ATP depletion, we first identified features of depolarization-induced neuronal [Ca(2+)]c transients that are sensitive to blockade of mitochondrial Ca(2+) buffering in order to assess mitochondrial contributions to [Ca(2+)]c regulation. This established the loss of a shoulder during the recovery of the depolarization (K(+))-induced transient, increased transient peak and area, and elevated shoulder level as evidence of diminished mitochondrial Ca(2+) buffering. We then examined transients in Control neurons and neurons from the 4th lumbar (L4) and 5th lumbar (L5) dorsal root ganglia after L5 spinal nerve ligation (SNL). The SNL L4 neurons showed decreased transient peak and area compared to control neurons, while the SNL L5 neurons showed increased shoulder level. Additionally, SNL L4 neurons developed shoulders following transients with lower peaks than Control neurons. Application of FCCP plus oligomycin elevated resting [Ca(2+)]c in SNL L4 neurons more than in Control neurons. Whereas application of FCCP plus oligomycin 2s after neuronal depolarization initiated mitochondrial Ca(2+) release in most Control and SNL L4 neurons, this usually failed to release mitochondrial Ca(2+) from SNL L5 neurons. For comparable cytoplasmic Ca(2+) loads, the releasable mitochondrial Ca(2+) in SNL L5 neurons was less than Control while it was increased in SNL L4 neurons. These findings show diminished mitochondrial Ca(2+) buffering in axotomized SNL L5 neurons but enhanced Ca(2+) buffering by neurons in adjacent SNL L4 neurons.

Far red/near infrared light-induced protection against cardiac ischemia and reperfusion injury remains intact under diabetic conditions and is independent of nitric oxide synthase

Far red/near-infrared light (NIR) promotes a wide range of biological effects including tissue protection but whether and how NIR is capable of acutely protecting myocardium against ischemia and reperfusion injury in vivo is not fully elucidated. Our previous work indicates that NIR exposure immediately before and during early reperfusion protects the myocardium against infarction through mechanisms that are nitric oxide (NO)-dependent. Here we tested the hypothesis that NIR elicits protection in a diabetic mouse model where other cardioprotective interventions such as pre- and postconditioning fail, and that the protection is independent of nitric oxide synthase (NOS). NIR reduced infarct size dose dependently. Importantly, NIR-induced protection was preserved in a diabetic mouse model (db/db) and during acute hyperglycemia, as well as in endothelial NOS(-/-) mice and in wild type mice treated with NOS inhibitor L-NAME. In in vitro experiments NIR light liberates NO from nitrosyl hemoglobin (HbNO) and nitrosyl myoglobin (MbNO) in a wavelength-(660-830 nm) and dose-dependent manner. Irradiation at 660 nm yields the highest release of NO, while at longer wavelengths a dramatic decrease of NO release can be observed. Similar wavelength dependence was observed for the protection of mice against cardiac ischemia and reperfusion injury in vivo. NIR-induced NO release from deoxymyoglobin in the presence of nitrite mildly inhibits respiration of isolated mitochondria after hypoxia. In summary, NIR applied during reperfusion protects the myocardium against infarction in an NO-dependent, but NOS-independent mechanisms, whereby mitochondria may be a target of NO released by NIR, leading to reduced reactive oxygen species generation during reperfusion. This unique mechanism preserves protection even during diabetes where other protective strategies fail.

Epoxyeicosatrienoic acids pretreatment improves amyloid β-induced mitochondrial dysfunction in cultured rat hippocampal astrocytes

Amyloid-β (Aβ) has long been implicated as a causative protein in Alzheimer's disease. Cellular Aβ accumulation is toxic and causes mitochondrial dysfunction, which precedes clinical symptoms of Alzheimer's disease pathology. In the present study, we explored the possible use of epoxyeicosatrienoic acids (EETs), epoxide metabolites of arachidonic acid, as therapeutic target against Aβ-induced mitochondrial impairment using cultured neonatal hippocampal astrocytes. Inhibition of endogenous EET production by a selective epoxygenase inhibitor, MS-PPOH, caused a greater reduction in mitochondrial membrane potential in the presence of Aβ (1, 10 μM) exposure versus absence of Aβ. MS-PPOH preincubation also aggravated Aβ-induced mitochondrial fragmentation. Preincubation of the cells with either 14,15- or 11,12-EET prevented this mitochondrial depolarization and fragmentation. EET pretreatment also further improved the reduction observed in mitochondrial oxygen consumption in the presence of Aβ. Preincubation of the cells with EETs significantly improved cellular respiration under basal condition and in the presence of the protonophore, carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone (FCCP). The uncoupling of ATP synthase from the electron transfer chain that occurred in Aβ-treated cells was also prevented by preincubation with EETs. Lastly, cellular reactive oxygen species production, a hallmark of Aβ toxicity, also showed significant reduction in the presence of EETs. We have previously shown that Aβ reduces EET synthesis in rat brain homogenates and cultured hippocampal astrocytes and neurons (Sarkar P, Narayanan J, Harder DR. Differential effect of amyloid beta on the cytochrome P450 epoxygenase activity in rat brain. Neuroscience 194: 241-249, 2011). We conclude that reduction of endogenous EETs may be one of the mechanisms through which Aβ inflicts toxicity and thus supplementing the cells with exogenous EETs improves mitochondrial dynamics and prevents metabolic impairment.

Impairment of endothelial-myocardial interaction increases the susceptibility of cardiomyocytes to ischemia/reperfusion injury

Endothelial-myocardial interactions may be critically important for ischemia/reperfusion injury. Tetrahydrobiopterin (BH4) is a required cofactor for nitric oxide (NO) production by endothelial NO synthase (eNOS). Hyperglycemia (HG) leads to significant increases in oxidative stress, oxidizing BH4 to enzymatically incompetent dihydrobiopterin. How alterations in endothelial BH4 content impact myocardial ischemia/reperfusion injury remains elusive. The aim of this study was to examine the effect of endothelial-myocardial interaction on ischemia/reperfusion injury, with an emphasis on the role of endothelial BH4 content. Langendorff-perfused mouse hearts were treated by triton X-100 to produce endothelial dysfunction and subsequently subjected to 30 min of ischemia followed by 2 h of reperfusion. The recovery of left ventricular systolic and diastolic function during reperfusion was impaired in triton X-100 treated hearts compared with vehicle-treated hearts. Cardiomyocytes (CMs) were co-cultured with endothelial cells (ECs) and subsequently subjected to 2 h of hypoxia followed by 2 h of reoxygenation. Addition of ECs to CMs at a ratio of 1∶3 significantly increased NO production and decreased lactate dehydrogenase activity compared with CMs alone. This EC-derived protection was abolished by HG. The addition of 100 µM sepiapterin (a BH4 precursor) or overexpression of GTP cyclohydrolase 1 (the rate-limiting enzyme for BH4 biosynthesis) in ECs by gene trasfer enhanced endothelial BH4 levels, the ratio of eNOS dimer/monomer, eNOS phosphorylation, and NO production and decreased lactate dehydrogenase activity in the presence of HG. These results demonstrate that increased BH4 content in ECs by either pharmacological or genetic approaches reduces myocardial damage during hypoxia/reoxygenation in the presence of HG. Maintaining sufficient endothelial BH4 is crucial for cardioprotection against hypoxia/reoxygenation injury.

Quantitative characterization of changes in the cardiac mitochondrial proteome during anesthetic preconditioning and ischemia

Changes in mitochondrial bioenergetics have been proposed to be critical for triggering and effecting anesthetic-induced preconditioning (APC) against cardiac ischemia and reperfusion injury. The objective of this study was to analyze changes in mitochondrial protein levels and link those changes to potential functional changes. A (18)O-labeling method was applied for relative comparison of cardiac mitochondrial samples from control and isoflurane exposed rats before and after ischemia and reperfusion. Wistar rats were exposed to isoflurane for 30 min (APC) or did not receive the anesthetic (control). Rats were subjected to 30 min coronary occlusion and 15 min reperfusion without (ischemia) or after APC (ischemia + APC). The following comparisons were made: control vs. APC, control vs. ischemia, and APC vs. ischemia + APC. Proteins were analyzed by liquid chromatography-mass spectrometry. A total of 98 proteins currently annotated as mitochondrial proteins in the UniProt database were positively identified from three replicate experiments. Most of the changes during APC and ischemia occur in complexes of the electron transport chain. Overall, fewer changes in ETC complexes were found when comparing APC with APC + ischemia than when comparing control and ischemia. This corresponds to the preservation of bioenergetics due to APC after ischemia and reperfusion as indicated by preserved ATP level and generation. APC itself induced changes in complex I, but those changes were not correlated with activity changes in mitochondria after APC. Thus, a proteomic mass spectral approach does not only assess quantitative changes without prior knowledge of proteins, but also allows insight into the mechanisms of ischemia and reperfusion injury and APC.

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.

Endothelial-cardiomyocyte crosstalk enhances pharmacological cardioprotection

Endothelial cells (EC) serve a paracrine function to enhance signaling in cardiomyocytes (CM), and conversely, CM secrete factors that impact EC function. Understanding how EC interact with CM may be critically important in the context of ischemia-reperfusion injury, where EC might promote CM survival. We used isoflurane as a pharmacological stimulus to enhance EC protection of CM against hypoxia and reoxygenation injury. Triggering of intracellular signal transduction pathways culminating in the enhanced production of nitric oxide (NO) appears to be a central component of pharmacologically induced cardioprotection. Although the endothelium is well recognized as a regulator for vascular tone, little attention has been given to its potential importance in mediating cardioprotection. In the current investigation, EC-CM in co-culture were used to test the hypothesis that EC contribute to isoflurane-enhanced protection of CM against hypoxia and reoxygenation injury and that this protection depends on hypoxia-inducible factor (HIF1α) and NO. CM were protected against cell injury [lactate dehydrogenase (LDH) release] to a greater extent in the presence vs. absence of isoflurane-stimulated EC (1.7 ± 0.2 vs. 4.58 ± 0.8 fold change LDH release), and this protection was NO-dependent. Isoflurane enhanced release of NO in EC (1103 ± 58 vs. 702 ± 92 pmol/mg protein) and EC-CM in co-culture sustained NO release during reoxygenation. In contrast, lentiviral mediated HIF1α knockdown in EC decreased basal and isoflurane stimulated NO release in an eNOS dependent manner (517 ± 32 vs. 493 ± 38 pmol/mg protein) and prevented the sustained increase in NO during reoxygenation when co-cultured. Opening of mitochondrial permeability transition pore (mPTP), an index of mitochondrial integrity, was delayed in the presence vs. absence of EC (141 ± 2 vs. 128 ± 2.5 arbitrary mPTP opening time). Isoflurane stimulated an increase in HIF1α in EC but not in CM under normal oxygen tension (3.5 ± 0.1 vs. 0.79 ± 0.15 fold change density) and this action was blocked by pretreatment with the Mitogen-activated Protein/Extracellular Signal-regulated Kinase inhibitor U0126. Expression and nuclear translocation of HIF1α were confirmed by Western blot and immunofluorescence. Taken together, these data support the concept that EC are stimulated by isoflurane to produce important cardioprotective factors that may contribute to protection of myocardium during ischemia and reperfusion injury.

Mitochondrial approaches to protect against cardiac ischemia and reperfusion injury

The mitochondrion is a vital component in cellular energy metabolism and intracellular signaling processes. Mitochondria are involved in a myriad of complex signaling cascades regulating cell death vs. survival. Importantly, mitochondrial dysfunction and the resulting oxidative and nitrosative stress are central in the pathogenesis of numerous human maladies including cardiovascular diseases, neurodegenerative diseases, diabetes, and retinal diseases, many of which are related. This review will examine the emerging understanding of the role of mitochondria in the etiology and progression of cardiovascular diseases and will explore potential therapeutic benefits of targeting the organelle in attenuating the disease process. Indeed, recent advances in mitochondrial biology have led to selective targeting of drugs designed to modulate or manipulate mitochondrial function, to the use of light therapy directed to the mitochondrial function, and to modification of the mitochondrial genome for potential therapeutic benefit. The approach to rationally treat mitochondrial dysfunction could lead to more effective interventions in cardiovascular diseases that to date have remained elusive. The central premise of this review is that if mitochondrial abnormalities contribute to the etiology of cardiovascular diseases (e.g., ischemic heart disease), alleviating the mitochondrial dysfunction will contribute to mitigating the severity or progression of the disease. To this end, this review will provide an overview of our current understanding of mitochondria function in cardiovascular diseases as well as the potential role for targeting mitochondria with potential drugs or other interventions that lead to protection against cell injury.

Monitoring mitochondrial electron fluxes using NAD(P)H-flavoprotein fluorometry reveals complex action of isoflurane on cardiomyocytes

Mitochondrial bioenergetic studies mostly rely on isolated mitochondria thus excluding the regulatory role of other cellular compartments important for the overall mitochondrial function. In intact cardiomyocytes, we followed the dynamics of electron fluxes along specific sites of the electron transport chain (ETC) by simultaneous detection of NAD(P)H and flavoprotein (FP) fluorescence intensities using a laser-scanning confocal microscope. This method was used to delineate the effects of isoflurane, a volatile anesthetic and cardioprotective agent, on the ETC. Comparison to the effects of well-characterized ETC inhibitors and uncoupling agent revealed two distinct effects of isoflurane: uncoupling-induced mitochondrial depolarization and inhibition of ETC at the level of complex I. In correlation, oxygen consumption measurements in cardiomyocytes confirmed a dose-dependent, dual effect of isoflurane, and in isolated mitochondria an obstruction of the ETC primarily at the level of complex I. These effects are likely responsible for the reported mild stimulation of mitochondrial reactive oxygen species (ROS) production required for the cardioprotective effects of isoflurane. In conclusion, isoflurane exhibits complex effects on the ETC in intact cardiomyocytes, altering its electron fluxes, and thereby enhancing ROS production. The NAD(P)H-FP fluorometry is a useful method for exploring the effect of drugs on mitochondria and identifying their specific sites of action within the ETC of intact cardiomyocytes.

Mitochondrial depolarization underlies delay in permeability transition by preconditioning with isoflurane: roles of ROS and Ca2+

During reperfusion, the interplay between excess reactive oxygen species (ROS) production, mitochondrial Ca(2+) overload, and mitochondrial permeability transition pore (mPTP) opening, as the crucial mechanism of cardiomyocyte injury, remains intriguing. Here, we investigated whether an induction of a partial decrease in mitochondrial membrane potential (DeltaPsi(m)) is an underlying mechanism of protection by anesthetic-induced preconditioning (APC) with isoflurane, specifically addressing the interplay between ROS, Ca(2+), and mPTP opening. The magnitude of APC-induced decrease in DeltaPsi(m) was mimicked with the protonophore 2,4-dinitrophenol (DNP), and the addition of pyruvate was used to reverse APC- and DNP-induced decrease in DeltaPsi(m). In cardiomyocytes, DeltaPsi(m), ROS, mPTP opening, and cytosolic and mitochondrial Ca(2+) were measured using confocal microscope, and cardiomyocyte survival was assessed by Trypan blue exclusion. In isolated cardiac mitochondria, antimycin A-induced ROS production and Ca(2+) uptake were determined spectrofluorometrically. In cells exposed to oxidative stress, APC and DNP increased cell survival, delayed mPTP opening, and attenuated ROS production, which was reversed by mitochondrial repolarization with pyruvate. In isolated mitochondria, depolarization by APC and DNP attenuated ROS production, but not Ca(2+) uptake. However, in stressed cardiomyocytes, a similar decrease in DeltaPsi(m) attenuated both cytosolic and mitochondrial Ca(2+) accumulation. In conclusion, a partial decrease in DeltaPsi(m) underlies cardioprotective effects of APC by attenuating excess ROS production, resulting in a delay in mPTP opening and an increase in cell survival. Such decrease in DeltaPsi(m) primarily attenuates mitochondrial ROS production, with consequential decrease in mitochondrial Ca(2+) uptake.

KATP channel subunits in rat dorsal root ganglia: alterations by painful axotomy

Background

ATP-sensitive potassium (K_ATP) channels in neurons mediate neuroprotection, they regulate membrane excitability, and they control neurotransmitter release. Because loss of DRG neuronal K_ATP currents is involved in the pathophysiology of pain after peripheral nerve injury, we characterized the distribution of the K_ATP channel subunits in rat DRG, and determined their alterations by painful axotomy using RT-PCR, immunohistochemistry and electron microscopy.

Results

PCR demonstrated Kir6.1, Kir6.2, SUR1 and SUR2 transcripts in control DRG neurons. Protein expression for all but Kir6.1 was confirmed by Western blots and immunohistochemistry. Immunostaining of these subunits was identified by fluorescent and confocal microscopy in plasmalemmal and nuclear membranes, in the cytosol, along the peripheral fibers, and in satellite glial cells. Kir6.2 co-localized with SUR1 subunits. Kir6.2, SUR1, and SUR2 subunits were identified in neuronal subpopulations, categorized by positive or negative NF200 or CGRP staining. K_ATP current recorded in excised patches was blocked by glybenclamide, but preincubation with antibody against SUR1 abolished this blocking effect of glybenclamide, confirming that the antibody targets the SUR1 protein in the neuronal plasmalemmal membrane.

In the myelinated nerve fibers we observed anti-SUR1 immunostaining in regularly spaced funneled-shaped structures. These structures were identified by electron microscopy as Schmidt-Lanterman incisures (SLI) formed by the Schwann cells. Immunostaining against SUR1 and Kir6.2 colocalized with anti-Caspr at paranodal sites.

DRG excised from rats made hyperalgesic by spinal nerve ligation exhibited similar staining against Kir6.2, SUR1 or SUR2 as DRG from controls, but showed decreased prevalence of SUR1 immunofluorescent NF200 positive neurons. In DRG and dorsal roots proximal to axotomy SLI were smaller and showed decreased SUR1 immunofluorescence.

Conclusions

We identified Kir6.2/SUR1 and Kir6.2/SUR2 K_ATP channels in rat DRG neuronal somata, peripheral nerve fibers, and glial satellite and Schwann cells, in both normal state and after painful nerve injury. This is the first report of K_ATP channels in paranodal sites adjacent to nodes of Ranvier and in the SLI of the Schwann cells. After painful axotomy K_ATP channels are downregulated in large, myelinated somata and also in SLI, which are also of smaller size compared to controls.

Because K_ATP channels may have diverse functional roles in neurons and glia, further studies are needed to explore the potential of K_ATP channels as targets of therapies against neuropathic pain and neurodegeneration.

Xenon preconditioning: the role of prosurvival signaling, mitochondrial permeability transition and bioenergetics in rats

BACKGROUND

Similar to volatile anesthetics, the anesthetic noble gas xenon protects the heart from ischemia/reperfusion injury, but the mechanisms responsible for this phenomenon are not fully understood. We tested the hypothesis that xenon-induced cardioprotection is mediated by prosurvival signaling kinases that target mitochondria.

METHODS

Male Wistar rats instrumented for hemodynamic measurements were subjected to a 30 min left anterior descending coronary artery occlusion and 2 h reperfusion. Rats were randomly assigned to receive 70% nitrogen/30% oxygen (control) or three 5-min cycles of 70% xenon/30% oxygen interspersed with the oxygen/nitrogen mixture administered for 5 min followed by a 15 min memory period. Myocardial infarct size was measured using triphenyltetrazolium staining. Additional hearts from control and xenon-pretreated rats were excised for Western blotting of Akt and glycogen synthase kinase 3 beta (GSK-3beta) phosphorylation and isolation of mitochondria. Mitochondrial oxygen consumption before and after hypoxia/reoxygenation and mitochondrial permeability transition pore opening were determined.

RESULTS

Xenon significantly (P < 0.05) reduced myocardial infarct size compared with control (32 +/- 4 and 59% +/- 4% of the left ventricular area at risk; mean +/- sd) and enhanced phosphorylation of Akt and GSK-3beta. Xenon pretreatment preserved state 3 respiration of isolated mitochondria compared with the results obtained in the absence of the gas. The Ca(2+) concentration required to induce mitochondrial membrane depolarization was larger in the presence compared with the absence of xenon pretreatment (78 +/- 17 and 56 +/- 17 microM, respectively). The phosphoinositol-3-kinase-kinase inhibitor wortmannin blocked the effect of xenon on infarct size and respiration.

CONCLUSIONS

These results indicate that xenon preconditioning reduces myocardial infarct size, phosphorylates Akt, and GSK-3beta, preserves mitochondrial function, and inhibits Ca(2+)-induced mitochondrial permeability transition pore opening. These data suggest that xenon-induced cardioprotection occurs because of activation of prosurvival signaling that targets mitochondria and renders them less vulnerable to ischemia-reperfusion injury.

Near infrared light protects cardiomyocytes from hypoxia and reoxygenation injury by a nitric oxide dependent mechanism

Photobiomodulation with near infrared light (NIR) provides cellular protection in various disease models. Previously, infrared light emitted by a low-energy laser has been shown to significantly improve recovery from ischemic injury of the canine heart. The goal of this investigation was to test the hypothesis that NIR (670 nm) from light emitting diodes produces cellular protection against hypoxia and reoxygenation-induced cardiomyocyte injury. Additionally, nitric oxide (NO) was investigated as a potential cellular mediator of NIR. Our results demonstrate that exposure to NIR at the time of reoxygenation protects neonatal rat cardiomyocytes and HL-1 cells from injury, as assessed by lactate dehydrogenase release and MTT assay. Similarly, indices of apoptosis, including caspase 3 activity, annexin binding and the release of cytochrome c from mitochondria into the cytosol, were decreased after NIR treatment. NIR increased NO in cardiomyocytes, and the protective effect of NIR was completely reversed by the NO scavengers carboxy-PTIO and oxyhemoglobin, but only partially blocked by the NO synthase (NOS) inhibitor L-NMMA. Mitochondrial metabolism, measured by ATP synthase activity, was increased by NIR, and NO-induced inhibition of oxygen consumption with substrates for complex I or complex IV was reversed by exposure to NIR. Taken together these data provide evidence for protection against hypoxia and reoxygenation injury in cardiomyocytes by NIR in a manner that is dependent upon NO derived from NOS and non-NOS sources.

Role for SUR2A ED domain in allosteric coupling within the K(ATP) channel complex

Allosteric regulation of heteromultimeric ATP-sensitive potassium (K_ATP) channels is unique among protein systems as it implies transmission of ligand-induced structural adaptation at the regulatory SUR subunit, a member of ATP-binding cassette ABCC family, to the distinct pore-forming K⁺ (Kir6.x) channel module. Cooperative interaction between nucleotide binding domains (NBDs) of SUR is a prerequisite for K_ATP channel gating, yet pathways of allosteric intersubunit communication remain uncertain. Here, we analyzed the role of the ED domain, a stretch of 15 negatively charged aspartate/glutamate amino acid residues (948–962) of the SUR2A isoform, in the regulation of cardiac K_ATP channels. Disruption of the ED domain impeded cooperative NBDs interaction and interrupted the regulation of K_ATP channel complexes by MgADP, potassium channel openers, and sulfonylurea drugs. Thus, the ED domain is a structural component of the allosteric pathway within the K_ATP channel complex integrating transduction of diverse nucleotide-dependent states in the regulatory SUR subunit to the open/closed states of the K⁺-conducting channel pore.

Role of sarcolemmal ATP-sensitive potassium channel in oxidative stress-induced apoptosis: mitochondrial connection

From time of their discovery, sarcolemmal ATP-sensitive K+ (sarcK ATP) channels were thought to have an important protective role in the heart during stress whereby channel opening protects the heart from stress-induced Ca2+ overload and resulting damage. In contrast, some recent studies indicate that sarcK ATP channel closing can lead to cardiac protection. Also, the role of the sarcK ATP channel in apoptotic cell death is unclear. In the present study, the effects of channel inhibition on apoptosis and the specific interaction between the sarcK ATP channel and mitochondria were investigated. Apoptotic cell death of cultured HL-1 and neonatal cardiomyocytes following exposure to oxidative stress was significantly increased in the presence of sarcK ATP channel inhibitor HMR-1098 as evidenced by terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling and caspase-3,7 assays. This was paralleled by an increased release of cytochrome c from mitochondria to cytosol, suggesting activation of the mitochondrial death pathway. sarcK ATP channel inhibition during stress had no effect on Bcl-2, Bad, and phospho-Bad, indicating that the increase in apoptosis cannot be attributed to these modulators of the apoptotic pathway. However, monitoring of mitochondrial Ca2+ with rhod-2 fluorescent indicator revealed that mitochondrial Ca2+ accumulation during stress is potentiated in the presence of HMR-1098. In conclusion, this study provides novel evidence that opening of sarcK ATP channels, through a specific Ca2+-related interaction with mitochondria, plays an important role in preventing cardiomyocyte apoptosis and mitochondrial damage during stress.

Isoflurane preconditioning uncouples mitochondria and protects against hypoxia-reoxygenation

Ischemic cardiac injury can be substantially alleviated by exposing the heart to pharmacological agents such as volatile anesthetics before occurrence of ischemia-reperfusion. A hallmark of this preconditioning phenomenon is its memory, when cardioprotective effects persist even after removal of preconditioning stimulus. Since numerous studies pinpoint mitochondria as crucial players in protective pathways of preconditioning, the aim of this study was to investigate the effects of preconditioning agent isoflurane on the mitochondrial bioenergetic phenotype. Endogenous flavoprotein fluorescence, an indicator of mitochondrial redox state, was elevated to 195 +/- 16% of baseline upon isoflurane application in intact cardiomyocytes, indicating more oxidized state of mitochondria. Isoflurane treatment also elicited partial dissipation of mitochondrial transmembrane potential, which remained depolarized even after anesthetic withdrawal (tetramethylrhodamine fluorescence intensity declined to 83 +/- 3 and 81 +/- 7% of baseline during isoflurane exposure and washout, respectively). Mild uncoupling, with preserved ATP synthesis, was also detected in mitochondria that were isolated from animals that had been previously preconditioned by isoflurane in vivo, revealing its memory nature. These mitochondria, after exposure to hypoxia and reoxygenation, exhibited better preserved respiration and ATP synthesis compared with mitochondria from nonpreconditioned animals. Partial mitochondrial depolarization was paralleled by a diminished Ca(2+) uptake into isoflurane-treated mitochondria, as indicated by the reduced increment in rhod-2 fluorescence when mitochondria were challenged with increased Ca(2+) (180 +/- 24 vs. 258 +/- 14% for the control). In conclusion, isoflurane preconditioning elicits partial mitochondrial uncoupling and reduces mitochondrial Ca(2+) uptake. These effects are likely to reduce the extent of the mitochondrial damage after the hypoxic stress.

Mechanism of cardiac sarcolemmal adenosine triphosphate-sensitive potassium channel activation by isoflurane in a heterologous expression system

BACKGROUND

Activation of the cardiac sarcolemmal adenosine triphosphate-sensitive potassium (KATP) channel during metabolic stress initiates cellular events that preserve cardiac performance. Previous studies showed that halogenated anesthetics prime KATP channels under whole cell voltage clamp and act in intracellular pH (pHi)-dependent manner on KATP channels in excised membrane patches. However, it is not known how halogenated anesthetics interact with these channels.

METHODS

The authors evaluated the effect of pHi and isoflurane on the KATP channel subunits, the pore-forming inward rectifier Kir6.2, and the regulatory sulfonylurea receptor SUR2A, using HEK293 cells as a heterologous expression system. Single channel activity was recorded in the inside-out patch configuration.

RESULTS

At pHi 7.4, isoflurane had negligible effect on activity of wild-type Kir6.2/SUR2A, but at pHi 6.8, the channel open probability was increased by isoflurane (0.177 +/- 0.077 to 0.364 +/- 0.164). By contrast, the open probability of truncated Kir6.2DeltaC26, which forms a functional channel without SUR2A, was attenuated by isoflurane at both pHi 7.4 and pHi 6.8. Coexpression of Kir6.2DeltaC26 with SUR2A restored pHi sensitivity of channel activation by isoflurane. Site-directed mutagenesis within the Walker motifs of SUR2A abolished isoflurane activation of KATP channel at pHi 6.8. In addition, the pancreatic-type channels expressing sulfonylurea receptor SUR1 could not be activated by isoflurane.

CONCLUSIONS

The nucleotide binding domains of SUR2A play a crucial role in isoflurane facilitation of the KATP channel activity at moderately acidic pHi as would occur during early ischemia. These findings support direct and differential interaction of isoflurane with the subunits of the cardiac sarcolemmal KATP channel.

Targeted expression of Kir6.2 in mitochondria confers protection against hypoxic stress

Selective K(+) transport in the inner mitochondrial membrane has been attributed to at least three different channel types: ATP-sensitive, Ca(2+)-regulated and voltage-dependent K(+) channels. Studies utilizing their selective modulators have suggested that an increased activity of these channels plays an important role in the cellular protection from metabolic stress. However, direct evidence for this effect is largely absent, and recent findings on the lack of specificity for several channel openers and blockers have questioned the actual contribution of the mitochondrial K(+) channels in the preservation of cellular viability. In order to directly investigate the role of enhanced mitochondrial K(+) uptake in cellular protection, we selectively expressed the inward rectifying K(+) channel Kir6.2 in the mitochondria of HEK293 and HL-1 cells. Targeted Kir6.2 expression was achieved by cloning the Kir6.2 gene in pCMV/mito/GFP vector and the proper trafficking to mitochondria was confirmed by colocalization studies and Western blot. An increased K(+) influx to mitochondria overexpressing Kir6.2, as evidenced by using the K(+)-sensitive PBFI AM fluorescent dye, substantially improved the cellular viability after hypoxic stress, which was assessed by lactate dehydrogenase (LDH) release. In parallel, monitoring of mitochondrial Ca(2+) during stress, via the specific indicator rhod-2, revealed a significant attenuation of Ca(2+) accumulation in mitochondria overexpressing K(+) channels. This effect was abolished in mitochondria expressing an inactive mutant of Kir6.2. Mitochondria expressing Kir6.2 K(+) channel also exhibited a significant degree of depolarization that became even more pronounced during the stress. In conclusion, this study provides the first non-pharmacological evidence that an increased K(+) influx to mitochondria protects against hypoxic stress by preventing detrimental effects of Ca(2+) overload.

Extracellular signal-regulated kinases trigger isoflurane preconditioning concomitant with upregulation of hypoxia-inducible factor-1alpha and vascular endothelial growth factor expression in rats

INTRODUCTION

Extracellular signal-related kinases 1 and 2 (Erk1/2) are mitogen-activated protein kinases that have been implicated in anesthetic preconditioning; but whether Erk1/2 triggers or mediates this beneficial effect and the mechanisms by which Erk1/2 produces cardioprotection are unknown. We tested the hypothesis that isoflurane preconditioning is triggered by Erk1/2 concomitant with upregulation of hypoxia-inducible factor 1alpha (HIF-1alpha) and vascular endothelial growth factor (VEGF) expression in rats instrumented for hemodynamic measurement and subjected to a 30-min coronary artery occlusion and 2-h reperfusion.

METHODS

Rats randomly received IV 0.9% saline (control) or isoflurane (1.0 minimum alveolar concentration administered for 30 min and discontinued 15 min [memory period] before coronary occlusion) in the absence or presence of the selective Erk1/2 inhibitor PD 098059 (1 mg/kg in dimethylsulfoxide administered IV either 3 min before exposure to isoflurane [trigger] or 3 min after discontinuation of the drug [mediator]). Additional rabbits were pretreated with dimethylsulfoxide alone. Left ventricular tissue samples were obtained at selected intervals from additional groups of rats for Western immunoblot analysis of phospho-Erk1/2, HIF-1alpha, and VEGF protein expression.

RESULTS

Isoflurane significantly (P < 0.05) reduced infarct size (41% +/- 8% of the left ventricular area at risk; triphenyltetrazolium chloride staining) as compared with control (59% +/- 4%). PD 098059 administered before, but not after, isoflurane abolished this cardioprotection (61% +/- 5% and 42% +/- 9%, respectively). Isoflurane-induced increases in phospho-Erk1/2, HIF-1alpha, and VEGF expression were also inhibited by PD 098059 pretreatment.

CONCLUSIONS

The results indicate that Erk1/2 triggers isoflurane preconditioning concomitant with HIF-1alpha and VEGF upregulation in vivo.

Kv1.5 channelopathy due to KCNA5 loss-of-function mutation causes human atrial fibrillation

Atrial fibrillation is a rhythm disorder characterized by chaotic electrical activity of cardiac atria. Predisposing to stroke and heart failure, this common condition is increasingly recognized as a heritable disorder. To identify genetic defects conferring disease susceptibility, patients with idiopathic atrial fibrillation, lacking traditional risk factors, were evaluated. Genomic DNA scanning revealed a nonsense mutation in KCNA5 that encodes Kv1.5, a voltage-gated potassium channel expressed in human atria. The heterozygous E375X mutation, present in a familial case of atrial fibrillation and absent in 540 unrelated control individuals, introduced a premature stop codon disrupting the Kv1.5 channel protein. The truncation eliminated the S4-S6 voltage sensor, pore region and C-terminus, preserving the N-terminus and S1-S3 transmembrane domains that secure tetrameric subunit assembly. Heterologously expressed recombinant E375X mutant failed to generate the ultrarapid delayed rectifier current I(Kur) vital for atrial repolarization and exerted a dominant-negative effect on wild-type current. Loss of channel function translated into action potential prolongation and early after-depolarization in human atrial myocytes, increasing vulnerability to stress-provoked triggered activity. The pathogenic link between compromised Kv1.5 function and susceptibility to atrial fibrillation was verified, at the organism level, in a murine model. Rescue of the genetic defect was achieved by aminoglycoside-induced translational read-through of the E375X premature stop codon, restoring channel function. This first report of Kv1.5 loss-of-function channelopathy establishes KCNA5 mutation as a novel risk factor for repolarization deficiency and atrial fibrillation.

Inhibition of Glycogen Synthase Kinase Enhances Isoflurane-Induced Protection Against Myocardial Infarction During Early Reperfusion In Vivo

Inhibition of glycogen synthase kinase (GSK)-beta protects against ischemia-reperfusion injury. Brief exposure to isoflurane before and during early reperfusion after coronary artery occlusion also protects against infarction. Whether GSK-beta mediates this action is unknown. We tested the hypothesis that GSK inhibition enhances isoflurane-induced postconditioning. Rabbits (n = 88; 6 to 7 per group) subjected to a 30-min coronary occlusion followed by 3 h reperfusion received saline, isoflurane (0.5 or 1.0 minimum alveolar concentration [MAC]) administered for 3 min before and 2 min after reperfusion, the selective GSK inhibitor SB216763 (SB21; 0.2 or 0.6 mg/kg), or 0.5 MAC isoflurane plus 0.2 mg/kg SB21. Other groups of rabbits pretreated with phosphatidylinositol-3 kinase (PI3K) inhibitor wortmannin (0.6 mg/kg), 70-kDa ribosomal protein s6 kinase (p70s6K) inhibitor rapamycin (0.25 mg/kg), or mitochondrial permeability transition pore (mPTP) opener atractyloside (5 mg/kg) received 0.6 mg/kg SB21 or 0.5 MAC isoflurane plus 0.2 mg/kg SB21. Additional groups received the mPTP inhibitor, cyclosporin A (5 mg/kg), plus 0.2 mg/kg SB21 with or without atractyloside pretreatment. Isoflurane (1.0 but not 0.5 MAC) and SB21 (0.6 but not 0.2 mg/kg) reduced (P < 0.05) infarct size (21% +/- 5%, 44% +/- 7%, 23% +/- 4%, and 46% +/- 2%, respectively, of left ventricular area at risk, mean+/- sd; triphenyltetrazolium staining) as compared with control (42% +/- 6%). Isoflurane (0.5 MAC) plus 0.2 mg/kg SB21 and cyclosporin A plus 0.2 mg/kg SB21 produced similar degrees of protection (24% +/- 4% and 27% +/- 6%, respectively). Atractyloside but not wortmannin or rapamycin abolished protection produced by 0.6 mg/kg SB21 and 0.5 MAC isoflurane plus 0.2 mg/kg SB21. Thus, GSK inhibition enhances isoflurane-induced protection against infarction during early reperfusion via a mPTP-dependent mechanism.

The Influence of B-Cell Lymphoma 2 Protein, an Antiapoptotic Regulator of Mitochondrial Permeability Transition, on Isoflurane-Induced and Ischemic Postconditioning in Rabbits

Brief exposure to isoflurane or repetitive, transient ischemia during early reperfusion after prolonged coronary artery occlusion protects against myocardial infarction by inhibiting the mitochondrial permeability transition pore (mPTP). Inhibition of mPTP during delayed ischemic preconditioning occurred concomitant with enhanced expression of the antiapoptotic protein B cell lymphoma-2 (Bcl-2). We tested the hypothesis that Bcl-2 mediates myocardial protection by isoflurane or brief ischemic episodes during reperfusion in rabbits (n = 91) subjected to a 30-min left anterior descending coronary artery occlusion followed by 3 h reperfusion. Rabbits received 0.9% saline, isoflurane (0.5 or 1.0 minimum alveolar concentration, MAC) administered for 3 min before and 2 min after reperfusion, 3 cycles of postconditioning ischemia (10 or 20 s each) during early reperfusion, 0.5 MAC isoflurane plus 3 cycles of postconditioning ischemia (10 s), or the direct mPTP inhibitor cyclosporin A (CsA, 10 mg/kg) in the presence or absence of the selective Bcl-2 inhibitor HA14-1 (2 mg/kg, i.p.). Isoflurane (1.0, but not 0.5, MAC) and postconditioning ischemia (20 s but not 10 s) significantly (P < 0.05) reduced infarct size (mean +/- sd, 21% +/- 4%, 43% +/- 7%, 19% +/- 7%, and 39% +/- 11%, respectively, of left ventricular area at risk) as compared with control (44% +/- 4%). Isoflurane (0.5 MAC) plus 10 s postconditioning ischemia and CsA alone also exerted protection. HA14-1 alone did not affect infarct size nor block protection produced by CsA but abolished reductions in infarct size caused by 1.0 MAC isoflurane, 20 s postconditioning ischemia, and 0.5 MAC isoflurane plus 10 s postconditioning ischemia. The results suggest that Bcl-2 mediates isoflurane-induced and ischemic postconditioning by indirectly modulating mPTP activity in vivo.

Impact of in vivo preconditioning by isoflurane on adenosine triphosphate-sensitive potassium channels in the rat heart: lasting modulation of nucleotide sensitivity during early memory period

BACKGROUND

The early memory of anesthetic-induced preconditioning (APC) is a period when myocardial protection continues even after removal of the anesthetic. Because adenosine triphosphate-sensitive potassium (KATP) channels are important mediators of APC, the authors investigated the hypothesis that the memory involves channel priming by isoflurane via a long-term modulation of the sensitivity to intracellular adenosine nucleotides.

METHODS

Ventricular cardiomyocytes were obtained from the rat hearts after 30 min in vivo APC with 1.4% isoflurane and from control non-APC rat hearts. Whole cell and excised inside-out patch clamp techniques were used to study the sarcolemmal KATP channel. Membrane expression of KATP channel proteins, the pore-forming inward rectifier Kir6.2, and the regulatory sulfonylurea receptor SUR2A were assessed in APC and non-APC hearts by Western blotting.

RESULTS

Activation of whole cell KATP current by isoflurane was enhanced after in vivo APC. At the single-channel level, this was paralleled by a 12-fold decrease in adenosine 5'-triphosphate sensitivity and a 3-fold decrease in adenosine 5'-diphosphate sensitivity, without changing the probability of channel opening or single-channel conductance. The membrane expression of Kir6.2 and SUR2A subunits was not altered by in vivo APC. A direct in vitro application of isoflurane to excised membrane patches increased the channel open probability and produced a 4-fold decrease in adenosine 5'-triphosphate sensitivity only of channels in non-APC myocytes.

CONCLUSIONS

In vivo APC by isoflurane decreases sensitivity of the sarcolemmal KATP channel to inhibition by adenosine 5'-triphosphate and decreases adenosine 5'-diphosphate sensitivity. These effects persist even after discontinuation of the anesthetic, suggesting a possible novel factor that may contribute to the mechanism of early memory of APC.

Role of Erk1/2, p70s6K, and eNOS in isofluraneinduced cardioprotection during early reperfusionin vivo

PURPOSE

Administration of isoflurane during early reperfusion after prolonged coronary artery occlusion decreases myocardial infarct size by activating phosphatidylinositol-3-kinase (PI3K) signal transduction. The extracellular signal-related kinases (Erk1/2) represent a redundant mechanism by which signaling elements downstream from PI3K, including 70-kDA ribosomal protein s6 kinase (p70s6K) and endothelial nitric oxide synthase (eNOS), may be activated to reduce reperfusion injury. We tested the hypothesis Erk1/2, p70s6K, and eNOS mediate isoflurane-induced postconditioning in rabbit myocardium in vivo.

METHODS

Barbiturate-anesthetized rabbits (n = 78) instrumented for measurement of systemic hemodynamics were subjected to a 30-min coronary occlusion followed by three hours reperfusion. Rabbits were randomly assigned to receive 0.9% saline (control), the Erk1/2 inhibitor PD 098059 (2 mg x kg(-1)), the p70s6K inhibitor rapamycin (0.25 mg x kg(-1)), the nonselective nitric oxide synthase (NOS) inhibitor N-nitro-L-arginine methyl ester (L-NAME; 10 mg x kg(-1)), the selective inducible NOS antagonist aminoguanidine hydrochloride (AG, 300 mg x kg(-1)), or the selective neuronal NOS inhibitor 7-nitroindazole (7-NI, 50 mg x kg(-1)) in the presence or absence of 1.0 minimum alveolar concentration isoflurane administered for three minutes before and two minutes after reperfusion.

RESULTS

Brief exposure to 1.0 minimum alveolar concentration isoflurane reduced (P < 0.05) infarct size (21 +/- 4% [mean +/- SD] of left ventricle area at risk, respectively; triphenyltetrazolium staining) as compared to control (41 +/- 5%). PD 098059, rapamycin, and L-NAME, but not AG nor 7-NI, abolished the protection produced by isoflurane.

CONCLUSION

The results suggest that the protective effects of isoflurane against infarction during early reperfusion are mediated by Erk1/2, p70s6K, and eNOS in vivo.

Inhibition of Mitochondrial Permeability Transition Enhances Isoflurane-Induced Cardioprotection During Early Reperfusion: The Role of Mitochondrial KATP Channels

Inhibition of the mitochondrial permeability transition pore (mPTP) mediates the protective effects of brief, repetitive ischemic episodes during early reperfusion after prolonged coronary artery occlusion. Brief exposure to isoflurane immediately before and during early reperfusion also produces cardioprotection, but whether mPTP is involved in this beneficial effect is unknown. We tested the hypothesis that mPTP mediates isoflurane-induced postconditioning and also examined the role of mitochondrial KATP (mKATP) channels in this process. Rabbits (n = 102) subjected to a 30-min coronary occlusion followed by 3 h reperfusion received 0.9% saline (control), isoflurane (0.5 or 1.0 MAC) administered for 3 min before and 2 min after reperfusion, or the mPTP inhibitor cyclosporin A (CsA, 5 or 10 mg/kg) in the presence or absence of the mPTP opener atractyloside (5 mg/kg) or the selective mK(ATP) channel antagonist 5-hydroxydecanoate (5-HD; 10 mg/kg). Other rabbits received 0.5 MAC isoflurane plus 5 mg/kg CsA in the presence and absence of atractyloside or 5-HD. Isoflurane (1.0 but not 0.5 MAC) and CsA (10 but not 5 mg/kg) reduced (P < 0.05) infarct size (21% +/- 4%, 44% +/- 6%, 24% +/- 3%, and 43% +/- 6%, respectively, mean +/- sd of left ventricular area at risk; triphenyltetrazolium staining) as compared with control (42% +/- 7%). Isoflurane (0.5 MAC) plus CsA (5 mg/kg) was also protective (27% +/- 4%). Neither atractyloside nor 5-HD alone affected infarct size, but these drugs abolished protection by 1.0 MAC isoflurane, 10 mg/kg CsA, and 0.5 MAC isoflurane plus 5 mg/kg CsA. The results indicate that mPTP inhibition enhances, whereas opening abolishes, isoflurane-induced postconditioning. This isoflurane-induced inhibition of mitochondrial permeability transition is dependent on activation of mitochondrial KATP channels in vivo.