Search

The primary steps of a one-electron reduction of a disulfide molecule (cystamine) by the infrared p-like state of an excited hydrated electron have been investigated by femtosecond laser spectroscopy. The initial electron photodetachment from chloride ion or phenothiazine is triggered by a two-photon ultraviolet excitation. In homogeneous aqueous cystamine solution, a subpicosecond univalent reduction of the disulfide molecule by an infrared prehydrated electron competes with the nonadiabatic relaxation of trapped electrons (electron solvation process). This presolvation one-electron reduction occurs with a characteristic time of 160 ± 20 fs at 294 K. Within the electron solvation regime, this elementary redox process is totally achieved in less than 1 × 10-12 s and exhibits a probability 9 times higher than the radiationless relaxation of an infrared excited electron. In aqueous organized assemblies (cationic micelles) the partitioning of reactants does not influence the frequency rate of a prehydration ...

With the intensive development of ultrafast spectroscopic techniques, reaction dynamics and transition states can be investigated at the subpicosecond time scale (0.05–3 ps). Aqueous ionic solutions represent a paradigm for the characterization of elementary chemical steps at the microscopic level. The discrimination of very short-lived electronic states during electron transfers in molecular liquids provide guidance for further theoretical developments on non-reactive and reactive dynamics in complicated many body systems.

Dans le cadre d'une etude ab initio, menee au niveau MCSCF, le radical hydronium H 3 O. est hydrate au milieu d'une cavite dielectrique spherique de differents rayons, en utilisant les constantes dielectriques statique ou optique de l'eau liquide. Le spectre d'excitation electronique a ete calcule avec l'aide d'une methode utilisant les propagateurs a 2 particules. La solvatation non specifique du radical hydronium se traduit par un decalage general du spectre vers le rouge et des transitions absorbant a 1.8 eV et 2.4 eV sont predites. Le spectre d'absorption apparent de (H 3 O.) aq a l'etat fondamental est comparable a celui de l'electron hydrate (e - ) aq . En revanche, la bande d'absorption du premier etat excite (H 3 O.) * aq apparait centree autour de 1.50 eV. L'analyse comparative de la position de cette bande avec celle observee experimentalement a 1.45 eV par spectroscopie femtoseconde est menee. Un modele de l'electron hydrate impliquant un radical itinerant est egalement brievement discute.

The elucidation of detailed mechanisms of ultrafast events that occur inreactions has been made possible by recent advances in spectroscopy techniquesthat use ultrashort laser pulse generation. Ultrashort laser pulses (100femtoseconds duration, 1 fs = 10−15 s) allow to initiate selectivephotochemical processes (single charge transfer for instance) and to obtainunique informations on the dynamics of primary steps of radical reactionsinvolving ultrafast electron or proton transfer: transient electronicrelaxation, formation of solvation cage, encounter pair formation, ion-moleculereaction. Recent investigations on the femtochemistry of neat liquid water andionic aqueous solutions are reviewed and discussed.

The elucidation of detailed mechanisms of ultrafast events that occur in molecular charge transfer or reaction dynamics has been made possible by recent advances in spectroscopy techniques that use ultrashort laser pulse generation. Ultrashort laser pulses (100 femtoseconds duration, 1 fs=10-15 s) allow initiation of selective photochemical processes (photoejection of epithermal electrons), and obtaining of unique information on the dynamics of primary steps of radical reactions involving ultrafast electron or proton-transfer: formation of the hydration cage around an electron, encounter pair formation, ion-molecule reactions. Recent investigations on the non-equilibrium reaction processes in the universal protic solvent (water) are presented.

International audience; In liquids and electrolyte solutions, elementary radical reactions may be investigated on the time scale of molecular motions by femtosecond visible-IR absorption spectroscopy. Some studies devoted to the microscopic understanding of sub-picosecond prehydration electron transfer between an aqueous halide ion (Cl-) and ionic acceptors (Cd++, H+) are presented. Within an electron hydration regime, the one-electron reduction of a metallic ion or a hydrated proton (hydronium or dihydronium ion) is totally achieved in less than 2 × 10^-12 s at 294 K. The consequences of an early partition between reactive and nonreactive IR electronic pathways on the electron hydration process are discussed.

The aim of this experimental work is to discriminate, in real time, the first steps of a S-S bond formation in solution (figure 1). One electron reduction or oxidation of a simple thioether, the dimethylsulfide (CH3SCH3), has been investigated by femtosecond UV-IR spectroscopy. Indeed, when an electron of excess energy is injected in a liquid organic sulfide characterized by a high electron affinity, an irreversible electron attachment competes with the solvation process. This electron transfer yields a primary anion (RSR-, R = CH3) which exhibits a high reactivity through an ultrafast ion-molecule reaction. This contribution will focus mainly on S-S bridge formation triggered by a primary anion/cation-molecule reaction.

The understanding of chemical reactions at the microscopic level correspond to fundamental challenges for the physical-chemistry and the biophysics. The elementary steps of a reaction often correspond to very short lived transitional states between reactants and products [1, 2, 3, 4, 5, 6, 7, 8]. At the molecular level, the lifetime of transient steps (t ≤ 10-12 s) correspond to angstrom or subangstrom displacements.

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.

In this paper we develop the contribution of femtosecond spectroscopy to our understanding of the primary events involved in single-electron reactions in homogeneous aqueous solutions. Electron localization and hydration, electron—cation recombination and radical pair formation have been investigated at this time scale.

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.

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.

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 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.

Oxygen deprivation induces severe damage in cardiac muscle. The ultrastructural leions, which include mitochondrial and cellular swellings and structural defects in cell plasma membranes, are accentuated by the duration of the hypoxic period (Hearse et al., 1973).

The mechanisms governing the localization and solvation of excess electrons in liquids and glasses are one of the fundamental and still unsolved problems in the physics and chemistry of condensed phase. Laser photolysis of solute molecules and pulse radiolysis of polar fluids have been extensively used to generate excess electron and to investigate the initial steps leading to the long lifetime state of solvated electron. However, very little is known about the dynamics of electron solvation in liquid water. The best available measurement gives an upper limite of 0.3 ps for the appearance of solvated electron after a UV photolysis of an aqueous solution of ferrocyanure[1]. It has not been possible up to now to clearly demonstrate whether the infrared absorbing state, identified in alcools as a localized and a precursor of the fully relaxed solvated state[2], has its analog in pure liquid water.

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.

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.

Publisher Summary This chapter presents a study on the oxygen carrying capacity dependence of cardiac energy metabolism. The influence of oxygen carrying capacity on cardiac energy metabolism was evaluated on isolated working hearts. Three groups of six hearts were studied: (1) Group A (control) was perfused with reconstituted blood, hematocrit 35%, and arterial oxygen content: 13.5 ml O 2 /100 ml, (2) Group B includes heart first perfused with blood and red cell free electrolyte solution, hematocrit 0%, and arterial O 2 content: 1.5 ml O 2 /100 ml, and (3) Groups C includes hearts that were alternatively perfused with blood, electrolyte solution and, then, blood again. When hematocrit changed from 35% to 0%, myocardial oxygen consumption decreased from 7.64 to 3.4 μg Atom 0 min −1 mg −1 mitochondrial protein, external work decreased from 0.82 to 0.28 Joule −1 min −1 g −1 wet weight, and cellular adenosine triphosphate concentration varied from 5.1 to 2.9 μmol g −1 wet weight. These modifications were associated with a significant reduction in the calculated free forms of cytosolic adenine nucleotides concentration. From the study, it was concluded that creatine phosphate concentration does not necessarily reflect the quality of myocardial oxygenation. The oxygen supply becomes a limiting factor of mitochondrial respiratory rate in absence of hemoglobin in the perfusate of isolated rat hearts. The reconstituted blood permits to use normal arterial oxygen pressure and is able to maintain physiological capacity of cardiac energy metabolism.

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.

Some aspects of oxygen supply to tissues are studied, using an original method to perfuse with blood a working isolated heart of small animals, and a mathematical model to analyse experimental results. The role of hemorheological factors and heterogeneity of capillary diameters upon oxygen distribution could explain the effects of hemodilution on the microcirculation.

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.

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)

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.