Maintenance of the myocardial thiol pool by N-acetylcysteine. An effective means of improving cardioplegic protection.

The reduced thiol pool of myocardial tissue represents an important defense mechanism against oxygen toxicity. Since the ischemia-induced depletion of this pool might favor the cytotoxicity of oxygen-derived free radicals produced during reperfusion, we assessed the effects of the thiol group donor N-acetylcysteine in an isolated buffer-perfused rat heart model of ischemia/reperfusion. Fifty hearts were studied. A first series of experiments that consisted of two groups (n = 10) was designed to simulate the conditions of standard cardioplegic arrest. Hearts were subjected to 180 minutes of cold (15 degrees to 18 degrees C) global ischemia and 1 hour of reperfusion. The control group received crystalloid hyperkalemic cardioplegic solution given every 30 minutes during arrest, and the treated group received the same solution supplemented with N-acetylcysteine (0.04 mol/L). On the basis of comparisons of postreperfusion left ventricular developed pressure, maximal dP/dt, and diastolic pressure, N-acetylcysteine-containing cardioplegic solution afforded significantly better protection. A second series of experiments was then undertaken to assess the effects of N-acetylcysteine in hearts subjected to the sequence of ischemic events that is inherent in transplantation procedures. Hearts were cardioplegically arrested, stored for 5 hours at 2 degrees C, subjected to 1 additional hour of ischemic arrest at 15 degrees to 18 degrees C, and reperfused for 60 minutes. Three groups (n = 10) were studied that differed by the modalities of cardioplegic preservation used during the poststorage ischemic interval. One group received multidose unmodified cardioplegic solution. A second group received multidose cardioplegic solution supplemented with N-acetylcysteine (0.04 mol/L), and the third group was given only a single dose of N-acetylcysteine-enriched (0.07 mol/L) cardioplegic reperfusate at the end of arrest. Multidose N-acetylcysteine-containing cardioplegic solution resulted in a significantly better hemodynamic recovery than unmodified cardioplegic solution. The protection afforded by N-acetylcysteine was lost when the drug was given only at the time of reperfusion. We conclude that supplementation of cardioplegic solution with N-acetylcysteine markedly improves postarrest recovery of function, presumably through an enhancement of the reduced thiol pool, which increases the capacity of reperfused myocardium to handle the postischemic burst of free radical production. The clinical relevance of these findings stems from the fact that thiol-containing drugs are available for human use.

Enzyme release and mitochondrial activity in reoxygenated cardiac muscle: relationship with oxygen-induced lipid peroxidation.

The aim of this work was to precisely determine the sites of the peroxidative action on unsatured lipids by oxygen-derived free radicals and the lytic cell damage on reoxygenated perfused hearts. The cellular load of lipid peroxidation products (malondialdehyde) during the reoxygenation was dependent on PO2. This unfavorable biochemical response was linked to creatine kinase leakage, alteration of coronary flow and mitochondrial injury. When an enzymatic (superoxide dismutase, 290 IU/minute) or tripeptide scavenger of oxygen radicals (reduced glutathione, 0.5 mmol/l) was administered at the end of hypoxia and during reoxygenation, the abnormal intolerance of hypoxic heart to molecular oxygen was significantly weakened; the load of lipid peroxides load, enzyme release, and vascular alteration were all reduced. Moreover, mitochondrial activity was enhanced and the oxygen-induced uncoupling of mitochondrial remained limited: both the respiratory control ratio (RCR) and the ADP/O ratio were higher than in control reoxygenated hearts. The inhibition by rotenone (100 mumol/l) of reoxidation of electron chain transfer during oxygen readmission also reduced the unfavorable cardiac accumulation of lipid peroxidation products and the release of creatine kinase. These data demonstrate that in the oxygen paradox, the peroxidative attack on lipids plays an important role in inducing alterations of sarcolemmal permeability and mitochondrial activity. An uncontrolled reactivation of oxidative function of mitochondria during reoxygenation enhances the synthesis of oxygen-derived free radicals and triggers the peroxidation of cardiac lipids resulting in irreversible injury to cellular and intracellular membranes.

[A new concept of cardioplegic protection in cardiac surgery: iron chelation]. / Un nouveau concept de la protection cardioplégique en chirurgie cardiaque: la chélation du fer.

Hydroxyl is one of the most cytotoxic of all oxygen-derived free radicals produced during the myocardial ischaemia-reperfusion sequence. The purpose of the present study was to determine the effects of various interventions aimed at diminishing the production of hydroxyl radicals by reducing either one of their precursors (hydrogen peroxide) or the metal (ferric iron) which catalyzes the reaction generating these radicals. Sixty isolated and perfused rat hearts with isovolaemic contraction were studied. Except for non-ischaemic controls, these hearts were subjected to a 3-hour cardioplegic arrest in hypothermia (15-18 degrees C) followed by a 45-min reperfusion. The following interventions were performed: pretreatment with peroxidase, a hydrogen peroxide scavenger; pretreatment with peroxidase combined with deferoxamine, an ironchelating agent; pretreatment with peroxidase followed by addition of deferoxamine to the cardioplegic solution; addition of deferoxamine to the cardioplegic solution without pretreatment with the enzyme. Judging from the post-ischaemic values of developed pressure (maximum systolic pressure--diastolic pressure), left ventricular dP/dt and diastolic pressure and coronary flow rate, it appeared that the best myocardial protection was provided by deferoxamine-enriched cardioplegia. This study confirms that hydroxyl radicals most probably play a role in the genesis of the myocardial lesions associated with global ischaemia followed by reperfusion. Moreover, our results highlight the potential value of deferoxamine added to cardioplegic protection in heart surgery performed under extracorporeal circulation.

Femtosecond charge separation in organized assemblies: free-radical reactions with pyridine nucleotides in micelles.

Femtosecond laser UV pulse-induced charge separation and electron transfer across a polar interface have been investigated in anionic aqueous micells (sodium lauryl sulfate) containing an aromatic hydrocarbon (phenothiazine). The early events of the photoejection of the electron from the micellized chromophore and subsequent reaction of electron with the aqueous perimicellar phase have been studied by ultrafast infrared and visible absorption spectroscopy. The charge separation (chromophore +...e-) inside the micelle occurs in less than 10(-13) s (100 fs). The subsequent thermalization and localization of the photoelectron in the aqueous phase are reached in 250 fs. This results in the appearance of an infrared band assigned to a nonrelaxed solvated electron (presolvated state). This transient species relaxes toward the fully solvated state of the electron in 270 fs. In anionic aqueous micelles containing pyridine dinucleotides at high concentration (0.025-0.103 M), a single electron transfer can be initiated by femtosecond photoionization of phenothiazine. The one-electron reduction of the oxidized pyridine dinucleotide leads to the formation of a free pyridinyl radical. The bimolecular rate constant of this electron transfer depends on both the pH of the micellar system and the concentration of oxidized acceptor. The free-radical reaction is analyzed in terms of the time dependence of a diffusion-controlled process. In the first 2 ps following the femtosecond photoionization of PTH inside the micelle, an early formation of a free pyridinyl radical is observed. This suggests that an ultrafast free-radical reaction with an oxidized form of pyridine nucleotide can be triggered by a single electron transfer in less than 5 X 10(11) s-1.

Prevention of hydroxyl radical formation: a critical concept for improving cardioplegia. Protective effects of deferoxamine.

The hydroxyl radical is one of the most damaging oxygen metabolites that are thought to be produced during ischemia and reperfusion of cardiac tissue. Therefore, we used the isolated, isovolumetric, buffer-perfused rat heart preparation of cardioplegic arrest to assess the effects of interventions targeted at inhibiting production of the hydroxyl radical by decreasing either the availability of one of its precursors (hydrogen peroxide) or that of the metal catalyst (ferric iron) involved in the radical formation. Sixty hearts were studied and, except for nonischemic controls, were subjected to 3 hr of hypothermic (15 degrees to 18 degrees C) cardioplegic arrest, followed by 45 min of reperfusion. The following interventions were tested: pretreatment with peroxidase, a scavenger of hydrogen peroxide, pretreatment with a combination of peroxidase and the iron chelator deferoxamine, pretreatment with peroxidase followed by supplementation of the cardioplegic solution with deferoxamine, and supplementation of the cardioplegic solution with deferoxamine without preischemic enzymatic treatment. Based on comparisons of postreperfusion pressure development, maximal ventricular dP/dt, left ventricular compliance, and coronary flow, deferoxamine-containing cardioplegic solution alone afforded the best myocardial protection. This may be due to the ability of deferoxamine to act both as an iron chelator and as a direct scavenger of superoxide anion, an activated oxygen species that participates in hydroxyl radical formation. This study confirms that an important component of the cardiac damage sustained during global ischemia and reperfusion may involve injury caused by the hydroxyl radical. Furthermore, our results point out the potential therapeutic usefulness of deferoxamine in the context of cardioplegic protection during open-heart procedures.

[Protective effects of inhibitors of oxygen free radicals on the ischemic and reperfused heart. Applications to cardioplegia]. / Effets protecteurs des piégeurs des radicaux libres de l'oxygène sur le coeur ischémique et reperfusé. Applications à la cardioplégie.

Oxygen free radicals play an important role in the induction of myocardial lesions by the sequence ischaemic-reperfusion. The aim of this study was to determine whether the protection afforded by a cardioplegic solution could be improved by the addition of different anti-oxygen free radical agents. Forty isolated, perfused, rat hearts' isovolumic contraction systems were divided into 5 groups of 8. In 4 groups, cardioplegia was stopped for 90 minutes in normothermia and then reperfused for 45 minutes. These hearts received a single initial injection of either standard cardioplegic solution or a solution enriched with dismutase peroxide (200,000 U/l), reduced glutathione (0.1 mM) or peroxidase (6,000 U/l). The fifth group of hearts was continually aerobically reperfused and served as a non-ischaemic control group. Based on post-ischaemic values of the pressure developed (maximal systolic-diastolic pressure), LVdP/dt, diastolic pressure and coronary flow, the best myocardial protection was observed in those hearts given cardioplegic solution enriched with peroxidase, the haemodynamic indices being comparable to those of the non-ischaemic controls. These results confirm that myocardial protection with cardioplegic solutions can be improved by the addition of anti-oxygen free radical agents, especially peroxidase which inactivates both hydrogen peroxide (precursor of the very cytotoxic hydroxyl radical) and some hydroperoxides, so interrupting the self-sustaining chain of lipidoperoxidation and limiting the damaging effects of this reaction on the cardiac cell membranes.

Enhancement of cardioplegic protection with the free-radical scavenger peroxidase.

This study assesses the ability of the free-radical scavenger peroxidase to enhance cardioplegic protection when given during or before myocardial ischemia. Forty-four isolated isovolumetric buffer-perfused rat hearts were studied. In a first series of experiments that consisted of three groups, hearts were subjected to 90 min of normothermic global ischemia followed by 45 min of reperfusion. One group received a crystalloid cardioplegic solution given as a single dose at the onset of arrest. A second group received cardioplegic solution supplemented with superoxide dismutase (200,000 U/liter), and a third group received cardioplegic solution supplemented with peroxidase (6000 U/liter). Based on comparisons of postreperfusion coronary flow, left ventricular developed pressure, maximum dP/dt, and diastolic pressure, we found that the best protection was provided by peroxidase-enriched cardioplegia. A second series of experiments was then undertaken to assess the effects of the latter enzyme given as a pretreatment. Hearts were subjected to 3 hr of global ischemia, during which myocardial protection was provided by hypothermia (15 degrees C) along with multidose cardioplegia. The treatment group was given peroxidase (10,000 U/liter) added to the perfusate fluid for 15 min before the onset of cardioplegic arrest without further enzyme supplementation during ischemia or reperfusion. Hearts perfused with standard buffer for an equal period of time served as controls. While the two groups demonstrated the same degree of postischemic increase in myocardial stiffness, peroxidase-pretreated hearts had a significantly better recovery of contractile indexes at 30 and 45 min of reflow.(ABSTRACT TRUNCATED AT 250 WORDS)

[Free radical scavenging agents in myocardial protection during cardiac surgery]. / Les piégeurs de radicaux libres dans la protection myocardique en chirurgie cardiaque.

Oxygen free radicals are very unstable metabolites which are produced in abundant quantity during the reoxygenation of an ischemic organ. Oxidation, by these radicals, of the structural lipids of the membranes, is at the origin of cellular lesions all the more extensive as the ischemia, by itself, decreases the ischemic tissue content in "trapping" molecules which usually inactivate those free radicals. Thus, was introduced the concept of an exogenous supply of trappers intended to bring under control the production of radicals and consequently preserve the membrane integrity in the revascularized tissue. This review summarizes, in light of our experience, the results obtained with free radicals trappers in the scope of myocardial preservation, especially in cardiac surgery, and analyzes some of the problems that remain to be resolved before considering the clinical use of these trappers.

A comparative study of free radical scavengers in cardioplegic solutions. Improved protection with peroxidase.

Oxygen-derived free radicals play an important role in the myocardial injury associated with ischemia and reperfusion. This study was designed to assess whether the protection afforded by a K+ rich, Mg2+ rich cardioplegic solution could be enhanced by the addition of free radical scavengers acting at different levels of the radical generating pathway. Forty isolated isovolumic rat hearts were divided into five groups (n = 8). Four groups of hearts were subjected to 90 minutes of normothermic cardioplegic arrest followed by 45 minutes of reperfusion. Hearts were given an initial bolus of either unmodified cardioplegic solution or cardioplegic solution enriched with superoxide dismutase (200,000 U/L) reduced glutathione (0.1 mmol/L), or peroxidase (6,000 U/L). One group of hearts was aerobically perfused throughout the experimental protocol and served as nonischemic controls. Based on comparisons of postreperfusion ventricular pressure development, maximal ventricular dP/dt, left ventricular compliance and coronary flow, peroxidase-containing cardioplegic solution afforded the best myocardial protection, with values of these indicators not significantly different from those of nonischemic perfused control heart. Glutathione afforded protection slightly inferior to that of peroxidase but still markedly better than in groups receiving superoxide dismutase or unmodified cardioplegic solution. This study confirms that cardioplegic protection can be enhanced by the addition of free radical scavengers, in particular peroxidase.

Hemodynamic and metabolic responses of the working heart in relation to the oxygen carrying capacity of the perfusion medium.

Hemodynamic and metabolic adaptations of isolated working heart perfused alternatively with normal or low oxygen carrying capacity medium were studied in an experimental model. A step change in arterial oxygen content (1.75 to 15.3 ml O2/100 ml) was followed by a decrease in coronary flow, an increase in aortic flow, external work, myocardial oxygen consumption and efficiency, respectively. Metabolic investigations (steady state values) showed the activities of both glycolysis and the Krebs cycle to increase with the oxygen carrying capacity of the perfusion medium. Within the limits of these aerobic conditions, most of the cardiac changes were reversible. The use of reconstituted blood provides physiological conditions of oxygenation, allows a dynamic equilibrium between oxygen supply and oxygen requirements and maintains a near physiological regulation between cardiac dynamic and metabolic functions. These conclusions stress the importance of optimal O2 carrying capacity of perfusion medium in metabolic studies on isolated working heart.

Transient responses of coronary flow in the blood-perfused isolated rat heart submitted to changes in oxygen content.

This study examines the transient response of coronary blood flow to acute changes in O2 content at normal and high arterial PO2 (Pa, O2) in the blood-perfused, working isolated rat heart. The perfusion system used in this study presents the following advantages: it eliminates the gas/blood interface, includes a peripheral circulation for control of pre-load and after-load, and allows for rapid change of perfusates and continuous recording of aortic and coronary blood flow. With this system the isolated rat heart is capable of stable haemodynamic performance for periods in excess of 4 h. A sudden decrease in O2 content from 0.147 to 0.067 11(-11) at constant Pa,O2 (133 mmHg; n = 15) was associated with a marked increase in coronary blood flow (QCOR). This increase showed two phases: a rapid phase which reached 200% of the control value in 20 s, followed by a slow phase (235% in 90 s). When the same decrease in O2 content (0.135 to 0.057 11(-1] was associated with an increase in Pa, O2 (n = 22; 143 to 412 mmHg), the response of QCOR was limited both in amplitude (175% rather than 235%) and in rate of onset (response time of 15.6 instead of 9.2 s). These results are consistent with the majority of currently popular hypotheses regarding control of QCOR including the adenosine hypothesis and that of vessel wall PO2 being a direct mediator. The time course of changes in coronary vascular resistance, with a Pa, O2-dependent rapid phase, suggests the simultaneous function of the two mechanisms.

Role of oxygen radicals in cardiac injury due to reoxygenation.

The ability of oxygen derived free radicals to induce irreversible cellular injuries during reoxygenation was studied on isolated potassium-arrested heart preparation. Enzymatic scavengers of hydrogen peroxide (H2O2) and superoxide anion (O-2), catalase and superoxide dismutase, were not effective in reversing the cardiac alterations induced by hypoxia. Cellular injuries induced by reoxygenation, 'Oxygen paradox', were partially prevented by scavengers of H2O2 (glutathione reduced form, catalase) and O-2 (superoxide dismutase). The 'oxygen paradox' was associated with a release of malonaldehyde. The inhibition of lipid peroxidation by alpha-tocopherol prevented the toxic effect of molecular oxygen on hypoxic hearts. The specific quenchers of singlet oxygen (histidine) and hydroxyl radical (mannitol) reduced the peroxidation of unsaturated lipids and the intensity of the 'oxygen paradox' phenomenon. The results indicate that in cardiac muscle (i) oxygen derived free radicals are important byproducts of abnormal oxidative metabolism present during the post hypoxic period; (ii) the 'oxygen paradox' phenomenon is related to the formation of lipid hydroperoxides leading to the cellular membrane disruption and to the irreversible alteration of cardiac integrity.

[Early cellular alterations induced by myocardial hypoxia: possible role of cyclic AMP (author's transl)]. / Lésions cellulaires précoces induites par l'hypoxie myocardique: rôle éventuel de l'AMP cyclique.

The ability of endogenous myocardial catecholamines to participate in the development of myocardial cellular alterations during a short period of severe hypoxia (30 min) was studied in isolated, Langendorff-perfused rat heart preparation, arrested by a high potassium concentration (16 mM) and perfused in the absence of exogenous substrate (Table I). Tyramine, which accelerated catecholamine depletion, also increased myocardial cell damage as assessed by a higher lactate dehydrogenase (LDH) release and a more marked reduction in cellular levels of high energy phosphates and glycogen (Table II). On the other hand, under conditions of beta-blockade (atenolol), hypoxia-induced tissular damage was reduced (Table II). These changes could be related to modifications in the cellular content of cyclic AMP (cAMP) since cAMP was consistently higher during the first 30 min of hypoxic perfusion than in control normoxic hearts (Table III) whereas cyclic GMP content remained unchanged. Moreover, interventions increasing cellular content of cAMP (theophylline, dibutyryl-cAMP) also increased hypoxic damage (Table IV), whereas N-methyl imidazole which reduced cellular content of cAMP lessened hypoxia-induced cellular alterations (Table IV). It is concluded that cellular lesions developing during the first 30 min of hypoxia in isolated arrested rat heart preparation perfused without exogenous substrate could be related to intracellular accumulation of cAMP occurring under the effect of endogenous catecholamine release.