Flumazenil preconditions cardiomyocytes via oxygen radicals and K(ATP) channels

The Roles of GABA in Ischemia-Reperfusion Injury in the Central Nervous System and Peripheral Organs

Ischemia-reperfusion (I/R) injury is a common pathological process, which may lead to dysfunctions and failures of multiple organs. A flawless medical way of endogenous therapeutic target can illuminate accurate clinical applications. γ-Aminobutyric acid (GABA) has been known as a marker in I/R injury of the central nervous system (mainly in the brain) for a long time, and it may play a vital role in the occurrence of I/R injury. It has been observed that throughout cerebral I/R, levels, syntheses, releases, metabolisms, receptors, and transmissions of GABA undergo complex pathological variations. Scientists have investigated the GABAergic enhancers for attenuating cerebral I/R injury; however, discussions on existing problems and mechanisms of available drugs were seldom carried out so far. Therefore, this review would summarize the process of pathological variations in the GABA system under cerebral I/R injury and will cover corresponding probable issues and mechanisms in using GABA-related drugs to illuminate the concern about clinical illness for accurately preventing cerebral I/R injury. In addition, the study will summarize the increasing GABA signals that can prevent I/R injuries occurring in peripheral organs, and the roles of GABA were also discussed correspondingly.

High-throughput screening of FDA-approved drugs using oxygen biosensor plates reveals secondary mitofunctional effects

Repurposing of FDA-approved drugs with effects on mitochondrial function might shorten the critical path to mitochondrial disease drug development. We improved a biosensor-based assay of mitochondrial O2 consumption, and identified mitofunctional defects in cell models of LHON and FXTAS. Using this platform, we screened a 1600-compound library of clinically used drugs. The assay identified drugs known to affect mitochondrial function, such as metformin and decoquinate. We also identified several drugs not previously known to affect mitochondrial respiration including acarbose, metaraminol, gallamine triethiodide, and acamprosate. These previously unknown 'mitoactives' represent novel links to targets for mitochondrial regulation and potentially therapy, for mitochondrial disease.

Physiopathology of the embryonic heart (with special emphasis on hypoxia and reoxygenation)

The adaptative response of the developing heart to adverse intrauterine environment such as reduced O2 delivery can result in alteration of gene expression with short- and long-term consequences including adult cardiovascular diseases. The tolerance of the developing heart of acute or chronic oxygen deprivation, its capacity to recover during reperfusion and the mechanisms involved in reoxygenation injury are still under debate. Indeed, the pattern of response of the immature myocardium to hypoxia-reoxygenation differs from that of the adult. This review deals with the structural and metabolic characteristics of the embryonic heart and the functional consequences of hypoxia and reoxygenation. The relative contribution of calcium and sodium overload, pH disturbances and oxidant stress to the hypoxia-induced cardiac dysfunction is examined, as well as various cellular signaling pathways (e.g. MAP kinases) involved in cell survival or death. In the context of the recent advances in developmental cardiology and fetal cardiac surgery, a better understanding of the physiopathology of the stressed developing heart is required.

Brief hypoxia conditions monocytes to protect reperfused cardiocytes against cell death via the CD11b receptor

BACKGROUND

Traditionally leukocytes have been viewed as the mediators and effectors of cell injury after tissue ischemia and reperfusion through the indiscriminate release of toxic cytokines and oxygen free radicals. This can be detrimental to functioning of the transplanted heart when reperfused after implantation. Paradoxically, evidence now suggests that certain cytotoxic cytokines and even oxygen free radicals can be cytoprotective in smaller concentrations. This study sought to determine whether cultured human monocytes can be pre-conditioned by brief hypoxia to protect cardiomyocytes from cell death after hypoxia and re-oxygenation.

METHODS

Cultured human monocytes were exposed to transient hypoxia (10 minutes), after which we determined CD11b expression using flow cytometry. The 3 control groups comprised immunoglobulin G-negative controls, fMLP-positive controls, and virgin monocyte (VM) controls. We studied chick embryonic ventricular myocytes in a flow-through chamber while controlling flow rate, pH, O(2), and CO2 tension. We quantified cardiomyocyte viability using propidium iodide (5 micromol/liter). Cell systems of cardiomyocytes alone, cardiomyocytes and VM, human monocytes exposed to transient hypoxia before coculture with cardiomyocytes (PCHM-cardiomyocyte), and PCHM cocultured with anti-CD11b antibodies for 30 minutes before coculture with cardiomyocytes (CDHM-cardiomyocyte) were subjected to 1 hour of hypoxia and 3 hours of re-oxygenation, and cell death was determined. The experiment was repeated and the cell systems fixed, stained, and examined for monocytes adhering to cardiomyocytes.

RESULTS

CD11b expression increased significantly with both transient hypoxia and fMLP (18.39% +/- 4.116%, n = 5, and 37.04% +/- 7.783%, n = 5, respectively, p < 0.05 vs VM 100%, n = 5). Coculture of cardiomyocytes with VM had no effects on cardiocyte death (40.6% +/- 6.1%, n = 6) compared with controls (46.5% +/- 4.0%, n = 10). The PCHM cocultured with cardiomyocytes significantly decreased cardiomyocyte death (25.2% +/- 4.7%, n = 6, p < 0.05). This protection was abrogated by the addition of CD11b-blocking antibodies to PCHM before coculture with cardiomyocytes (51.0% +/- 6.1%, n = 6, p < 0.05). The PCHM showed increased adhesion to cardiomyocytes (5.4 +/- 0.38/high-power [HP] field vs 0.67 +/- 0.24/HP field in VM, p < 0.05). The increased adhesion was abolished by CD11b-blocking antibody (0.78 +/- 0.28 vs 0.67 +/- 0.24 cells/HP field, p < 0.05).

CONCLUSIONS

These results suggest that monocytes activated by transient hypoxia protect cardiomyocytes during hypoxia and re-oxygenation through expression of CD11b receptors. These cells seem to adhere to myocytes through this receptor to achieve this effect. The exact mechanism is unclear and requires further study. Autologous recipient monocytes may be pre-conditioned to protect the donor heart during reperfusion.

Ectopic pacing at physiological rate improves postanoxic recovery of the developing heart

Recently, rapid and transient cardiac pacing was shown to induce preconditioning in animal models. Whether the electrical stimulation per se or the concomitant myocardial ischemia affords such a protection remains unknown. We tested the hypothesis that chronic pacing of a cardiac preparation maintained in a normoxic condition can induce protection. Hearts of 4-day-old chick embryos were electrically paced in ovo over a 12-h period using asynchronous and intermittent ventricular stimulation (5 min on-10 min off) at 110% of the intrinsic rate. Sham (n = 6) and paced hearts (n = 6) were then excised, mounted in vitro, and subjected successively to 30 min of normoxia (20% O(2)), 30 min of anoxia (0% O(2)), and 60 min of reoxygenation (20% O(2)). Electrocardiogram and atrial and ventricular contractions were simultaneously recorded throughout the experiment. Reoxygenation-induced chrono-, dromo-, and inotropic disturbances, incidence of arrhythmias, and changes in electromechanical delay (EMD) in atria and ventricle were systematically investigated in sham and paced hearts. Under normoxia, the isolated heart beat spontaneously and regularly, and all baseline functional parameters were similar in sham and paced groups (means +/- SD): heart rate (190 +/- 36 beats/min), P-R interval (104 +/- 25 ms), mechanical atrioventricular propagation (20 +/- 4 mm/s), ventricular shortening velocity (1.7 +/- 1 mm/s), atrial EMD (17 +/- 4 ms), and ventricular EMD (16 +/- 2 ms). Under anoxia, cardiac function progressively collapsed, and sinoatrial activity finally stopped after approximately 9 min in both groups. During reoxygenation, paced hearts showed 1) a lower incidence of arrhythmias than sham hearts, 2) an increased rate of recovery of ventricular contractility compared with sham hearts, and 3) a faster return of ventricular EMD to basal value than sham hearts. However, recovery of heart rate, atrioventricular conduction, and atrial EMD was not improved by pacing. Activity of all hearts was fully restored at the end of reoxygenation. These findings suggest that chronic electrical stimulation of the ventricle at a near-physiological rate selectively alters some cellular functions within the heart and constitutes a nonischemic means to increase myocardial tolerance to a subsequent hypoxia-reoxygenation.

Ischemic preconditioning alters real-time measure of O2 radicals in intact hearts with ischemia and reperfusion

Reactive oxygen species (ROS) are believed to be involved in triggering cardiac ischemic preconditioning (IPC). Decreased formation of ROS on reperfusion after prolonged ischemia may in part underlie protection by IPC. In heart models, these contentions have been based either on the effect of ROS scavengers to abrogate IPC-induced preservation or on a measurement of oxidation products on reperfusion. Using spectrophotofluorometry at the left ventricular wall and the fluorescent probe dihydroethidium (DHE), we measured intracellular ROS superoxide (O(2)(-).) continuously in isolated guinea pig heart and tested the effect of IPC and the O(2)(-). scavenger manganese(III) tetrakis (4-benzoic acid) porphyrin chloride (MnTBAP) on O(2)(-). formation throughout the phases of preconditioning (PC), 30-min ischemia and 60-min reperfusion (I/R). IPC was evidenced by improved contractile function and reduced infarction; MnTBAP abrogated these effects. Brief PC pulses increased O(2)(-). during the ischemic but not the reperfusion phase. O(2)(-). increased by 35% within 1 min of ischemia, increased further to 95% after 20 min of ischemia, and decreased slowly on reperfusion. In the IPC group, O(2)(-). was not elevated over 35% during index ischemia and was not increased at all on reperfusion; these effects were abrogated by MnTBAP. Our results directly demonstrate how intracellular ROS increase in intact hearts during IPC and I/R and clarify the role of ROS in triggering and mediating IPC.

H(2)O(2) opens mitochondrial K(ATP) channels and inhibits GABA receptors via protein kinase C-epsilon in cardiomyocytes

Oxygen radicals and protein kinase C (PKC) mediate ischemic preconditioning. Using a cultured chick embryonic cardiomyocyte model of hypoxia and reoxygenation, we found that the oxygen radicals generated by ischemic preconditioning were H(2)O(2). Like preconditioning, H(2)O(2) selectively activated the epsilon-isoform of PKC in the particulate compartment and increased cell viability after 1 h of hypoxia and 3 h of reoxygenation. The glutathione peroxidase ebselen (converting H(2)O(2) to H(2)O) and the superoxide dismutase inhibitor diethyldithiocarbamic acid abolished the increased H(2)O(2) and the protection of preconditioning. PKC activation with phorbol 12-myristate 13-acetate increased cell survival; the protection of preconditioning was blocked by epsilonV(1-2), a selective PKC-epsilon antagonist. Similar to preconditioning, the protection of PKC activation was abolished by mitochondrial K(ATP) channel blockade with 5-hydroxydecanoate or by GABA receptor stimulation with midazolam or diazepam. In addition, PKC, mitochondrial ATP-sensitive K(+) (K(ATP)) channels, and GABA receptors had no effects on H(2)O(2) generated by ischemic preconditioning before prolonged hypoxia and reoxygenation. We conclude that H(2)O(2) opens mitochondrial K(ATP) channels and inhibits GABA receptors via activating PKC-epsilon. Through this signal transduction, preconditioning protects ischemic cardiomyocytes.

Preconditioning attenuates apoptosis and necrosis: role of protein kinase C epsilon and -delta isoforms

Preconditioning reduces cardiomyocyte necrosis in vivo and in vitro, but it is unknown whether preconditioning blocks apoptosis. We wanted to compare the effects of preconditioning on necrosis and apoptosis in cardiomyocytes. Necrosis was detected with propidium iodide, and apoptosis was quantified by three complementary techniques: flow cytometry, TdT-mediated dUTP nick-end labeling assay, and DNA-laddering electrophoresis. Apoptosis increased with simulated ischemia time (6 h, 19 +/- 1%; 12 h, 27 +/- 2%; 18 h, 40 +/- 4%; 24 h, 54 +/- 4%; and 36 h, 83 +/- 4%; n = 6 for each group). Simulated ischemia and reoxygenation contributed equally to apoptosis (12-h ischemia, 27 +/- 2%, n = 6; 12-h ischemia and 12-h reoxygenation, 51 +/- 4%, n = 6; and 24-h ischemia, 54 +/- 5%, n = 8). Necrosis occurred primarily during reoxygenation; none was detected during simulated ischemia. Preconditioning with 10 min of simulated ischemia reduced necrosis (18 +/- 6%, n = 8) but had no effect on apoptosis. However, three 1-min cycles of simulated ischemia separated by 5 min of reoxygenation reduced necrosis and apoptosis similarly. The protein kinase C (PKC) inhibitors Go6976 (0.1 microM) or chelerythrene (4 microM) abolished the effect of preconditioning. Preconditioning selectively activated PKC epsilon but had no effect on PKC delta and on total PKC enzyme activity. Preconditioning protected against necrosis and apoptosis, but the preconditioning ischemia required for blocking apoptosis was less than that for reducing necrosis. Activation of PKC epsilon isoform is important in mediating the protection.

Signal transduction of flumazenil-induced preconditioning in myocytes

The objective of this study was to examine the role of oxygen radicals, protein kinase C (PKC), and ATP-sensitive K(+) (K(ATP)) channels in mediating flumazenil-produced preconditioning. Chick cardiomyocyte death was quantified using propidium iodide, and oxygen radical generation was assessed using 2',7'-dichlorofluorescin oxidation. Preconditioning was initiated with 10 min of ischemia followed by 10 min of reoxygenation. Alternatively, flumazenil was infused for 10 min and removed 10 min before ischemia. Flumazenil (10 microM) and preconditioning increased oxygen radicals [1,693 +/- 101 (n = 3) and 1,567 +/- 98 (n = 3), respectively, vs. 345 +/- 53 (n = 3) in control] and reduced cell death similarly [22 +/- 3% (n = 5) and 18 +/- 2% (n = 6), respectively, vs. controls 49 +/- 5% (n = 8)]. Protection and increased oxygen radicals by flumazenil were abolished by pretreatment with the antioxidant thiol reductant 2-mercaptopropionyl glycine (800 microM; 52 +/- 10%, n = 6). Specific PKC inhibitors Go-6976 (0.1 microM) and chelerythrine (2 microM), given during ischemia and reoxygenation, blocked flumazenil-produced protection (47 +/- 5%, n = 6). The PKC activator phorbol 12-myristate 13-acetate (0.2 microM), given during ischemia and reoxygenation, reduced cell death similarly to that with flumazenil [17 +/- 4% (n = 6) and 22 +/- 3% (n = 5)]. Finally, 5-hydroxydecanoate (1 mM), a selective mitochondrial K(ATP) channel antagonist given during ischemia and reoxygenation, abolished the protection of flumazenil and phorbol 12-myristate 13-acetate. Thus flumazenil mimics preconditioning to reduce cell death in cardiomyocytes. Oxygen radicals activate mitochondrial K(ATP) channels via PKC during the process.