S-nitrosoglutathione induces formation of nitrosylmyoglobin in isolated hearts during cardioplegic ischemia--an electron spin resonance study

Keeping the heart in balance: the functional interactions of myoglobin with nitrogen oxides

Myoglobin (Mb) is an important intracellular oxygen-binding hemoprotein found in the cytoplasm of skeletal and cardiac muscle tissue playing a well-known role in O(2) storage and delivery. Within the last decade the knowledge about Mb's function has been considerably extended by the generation of myoglobin-deficient (myo(-/-)) mice, which for the first time enabled the analysis of Mb's role in physiology without pharmacological intervention. Utilizing the myo(-/-) mice, it has been demonstrated that beyond its function in O(2) supply Mb substantially contributes to nitric oxide (NO) homeostasis in the heart. By a dynamic cycle, in which a decrease in tissue O(2) tension drives the conversion of Mb from being a NO scavenger under normoxia to a NO producer during hypoxia, mitochondrial respiration is reversibly adapted to the intracellular O(2) tension. Therefore, Mb may act as an important O(2) sensor through which NO can regulate muscle energetics and function. As Mb is widespread throughout the fauna, the diverse oxygen-dependent interactions between Mb and nitrogen oxides may not only be of relevance for mammals but also for other vertebrates as evidenced by comparable phenotypes of 'artificial' (myo(-/-) mice) and 'natural' Mb knockouts (icefish and amphibians). In conclusion, it seems likely that Mb's multifunctional properties create an environment characterized by a tightly adapted aerobic mitochondrial respiration and low levels of free radicals, and thus serve an essential and beneficial role within the myocardium, which appears to be functionally important over a wide range of species.

Multimodality imaging of nitric oxide and nitric oxide synthases

Nitric oxide (NO) and NO synthases (NOSs) are crucial factors in many pathophysiological processes such as inflammation, vascular/neurological function, and many types of cancer. Noninvasive imaging of NO or NOS can provide new insights in understanding these diseases and facilitate the development of novel therapeutic strategies. In this review, we will summarize the current state-of-the-art multimodality imaging in detecting NO and NOSs, including optical (fluorescence, chemiluminescence, and bioluminescence), electron paramagnetic resonance (EPR), magnetic resonance (MR), and positron emission tomography (PET). With continued effort over the last several years, these noninvasive imaging techniques can now reveal the biodistribution of NO or NOS in living subjects with high fidelity which will greatly facilitate scientists/clinicians in the development of new drugs and/or patient management. Lastly, we will also discuss future directions/applications of NO/NOS imaging. Successful development of novel NO/NOS imaging agents with optimal in vivo stability and desirable pharmacokinetics for clinical translation will enable the maximum benefit in patient management.

Electron paramagnetic resonance (EPR) spin trapping of biological nitric oxide

Nitric oxide (NO) is a free radical species with multiple physiological functions. Because of low concentrations and short half-life of NO, its direct measurement in living tissues remains a difficult task. Electron paramagnetic resonance (EPR) spin trapping is probably one of the best suitable platforms for development of new methods for quantification of biological NO. The most reliable EPR-based approaches developed so far are based on the reaction of NO with various iron complexes, both intrinsic and exogenously applied. This review is focused on the current state and perspectives of EPR spin trapping for experimental and clinical NO biology.

Spin trapping of nitric oxide by aci anions of nitroalkanes

In alkaline solutions, nitroalkanes (RCH2NO2) undergo deprotonation and rearrange to an aci anion (RHC=NO2-), which may function as a spin trap. Using electron paramagnetic resonance (EPR) spectroscopy, we have investigated suitability of aci anions of a series of nitroalkanes (CH3NO2, CH3CH2NO2, CH3(CH2)2NO2, and CH3(CH2)3NO2) to spin trap nitric oxide (*NO). Based on the observed EPR spectra, the general structure of the adducts, formed by addition of *NO to RHC=NO2-, was identified as nitronitroso dianion radicals of general formula [RC(NO)NO2]*2- in strong base (0.5 M NaOH), and as a mono-anion radical [RCH(NO)NO2]*- in alkaline buffers, pH 10-13. The hyperfine splitting on 14N in the -NO2 moiety (11.2-12.48 G) is distinctly different from the splitting on 14N in the -NO moiety of the adducts (5.23-6.5 G). The structure of the adducts was verified using 15N-labeled *NO, which produced radicals, in which triplet due to splitting on 14N (I = 1) in 14NO/aci nitro adducts was replaced by a doublet due to 15N (I = 1/2) in 15NO/aci nitro adducts. EPR spectra of aci nitromethane/NO adduct recorded in NaOH and NaOD (0.5 M) showed that the hydrogen at alpha-carbon can be exchanged for deuterium, consistent with structures of the adducts being [CH(NO)NO2]*2- and [CD(NO)NO2]*2-, respectively. These results indicate that nitroalkanes could potentially be used as prototypes for development of *NO-specific spin traps suitable for EPR analysis.

Evaluation of nitric oxide release in the coronary effluent by a novel EPR technique: A study on isolated rat hearts subjected to cold cardioplegia and reperfusion

Aim of this study was to investigate the cardiac release of nitric oxide (NO) before and after cold cardioplegia by a novel electron paramagnetic resonance (EPR) technique. Isolated rat hearts were perfused for 20 min in a Langendorff apparatus and then subjected to 3 hours potassium-hypotermic cardioplegia, followed by 20 min reperfusion. The coronary effluent was collected in a flask containing ferrous-bis-diethyldithiocarbamate as a spin trap of NO. Since the trapping agent was not delivered to the heart with the perfusion medium, we avoided that an abnormal extraction of NO from the tissue could inhibit its biological activity. The EPR signal was well detectable after equilibration (25.6 +/- 3.0 nmol/L +/- S.E.M.) and significantly increased following perfusion with 10 micromol/L serotonin (41.1 +/- 3.2 nmol/L) or 10 micromol/L nitroprusside (43.5 +/- 2.9 nmol/L). The basal level of NO did not change after reperfusion, but serotonin administration was not able to stimulate its release. Serotonin failure to stimulate NO production was not due to a loss of endothelial NO synthase, since its protein expression was not modified after reperfusion. The perfusion pressure increased by 51% after reperfusion and was quite completely restored following serotonin or nitroprusside treatment, with respect to the non-stimulated equilibration condition. Therefore, we suggest that the coronary spasm following a cold cardioplegic arrest is not due to an impaired production of basal NO and that NO-donors can be effective in relaxing vascular smooth muscle cells.

The catabolic fate of nitric oxide: the nitric oxide oxidase and peroxynitrite reductase activities of cytochrome oxidase

Stimulation of cardiomyocytes to endogenously evolve nitric oxide is shown by microsensor measurements on single cells to lead to transient nitric oxide concentrations of a few hundred nanomolar. At these submicromolar concentrations, no evidence could be found for the expected reaction between nitric oxide generated and the oxymyoglobin present in the cells: nitric oxide + oxymyoglobin --> nitrate + metmyoglobin. No metmyoglobin formation was detected by electron paramagnetic resonance spectroscopy, and microsensor measurements revealed near quantitative conversion of the nitric oxide to nitrite rather than nitrate ion. Moreover, the rate of nitrite formation is shown to be too rapid to be accounted for by non-enzymatic means. The essentially quantitative and rapid catabolism of nitric oxide to nitrite ion can plausibly be explained on the basis of a cycle of reactions catalyzed by cytochrome c oxidase. It is demonstrated with the purified hemoproteins in vitro that the terminal oxidase can outcompete oxymyoglobin for available nitric oxide. It is proposed that under normal physiological and most pathological (non-inflammatory) conditions, reaction with cytochrome c oxidase is the major route by which NO is removed from mitochondria-rich cells.

Effect of nitrosylmyoglobin and saturated fatty acid anions on metmyoglobin-catalyzed oxidation of aqueous methyl linoleate emulsions

In aqueous methyl linoleate emulsions (pH 7.4, 25 degrees C, air-saturated), nitrosylmyoglobin and saturated fatty acid anions (palmitate and stearate investigated) each showed antioxidant effect on metmyoglobin-induced peroxidation as measured by oxygen depletion rate. For equimolar concentration of nitrosylmyoglobin and metmyoglobin and for metmyoglobin in moderate excess, a reduction in oxygen consumption rate of approximately 70% was observed. Fatty acid anions reduced oxygen consumption rate most significantly for palmitate (up to 60% for a fatty acid:heme protein ratio of 90:1). No further antioxidative effect was seen for fatty acid anions in the presence of nitrosylmyoglobin, whereas nitrosylmyoglobin showed a further antioxidant effect in presence of fatty acid anions in the metmyoglobin-catalyzed process. The antioxidative mechanism of nitrosylmyoglobin and fatty acid anions is different, and while the fatty acid anions seem active in inhibiting initiation of oxidation through protection against metmyoglobin activation into perferrylmyoglobin, as shown by freeze-quench Electron Spin Resonance (ESR) spectroscopy, nitrosylmyoglobin is rather active in the oxygen consuming (propagation) phase.

Nitrosylhemoglobin, an unequivocal index of nitric oxide release from nitroaspirin: in vitro and in vivo studies in the rat by ESR spectroscopy

Electron spin resonance (ESR) spectroscopy was applied for the unequivocal detection/quantitation of nitric oxide (NO) as nitrosylhemoglobin (HbFe(II)NO) released from nitroaspirin, benzoic acid,2-(acetyloxy)-3-[(nitrooxy)methyl]phenyl ester (NCX-4016; NO-ASA), the lead of a new class of nonsteroidal anti-inflammatory drugs. In both in vitro and in vivo experiments, the paramagnetic complex was detected at 100 K in the venous blood of the rat (microwave power, 20 mW) and characterized by a three-line hyperfine structure with coupling constants (A(x) and A(z)) of 17 G at g(x)=2.066 and g(z)=2.009. The kinetics of NO release from the drug were first determined in vitro by incubating rat blood with 1 mM NO-ASA and confirmed by the two-line hyperfine structure obtained with the labeled compound ((15)N-NO-ASA). In in vivo studies, the hematic levels of HbFe(II)NO were determined after oral (p.o.) and intraperitoneal (i.p.) administration of the drug (100 and 200 mg kg(-1)). In p.o. treated animals, the complex was detectable at 1 h post-dosing and its formation was maximal at 4-6 h, where the antithrombotic activity peaks. In i.p. treated animals, HbFe(II)NO complex peaks at the second hour to decline thereafter: in these animals, the ESR technique was applied to also detect nitrosylmyoglobin as an index of NO diffusion/compartmentalization in myocardial tissue. The results of this study emphasize the great potentiality of ESR spectroscopy for the study of the release, the metabolic fate and distribution of NO from nitrovasodilators.

Nitric oxide activates the sarcolemmal K(ATP) channel in normoxic and chronically hypoxic hearts by a cyclic GMP-dependent mechanism

Chronic myocardial hypoxia results in elevated nitric oxide (NO) production and increased current through the sarcolemmal K(ATP) channel. We hypothesized these two processes are related and determined whether NO alters the electrophysiology of Purkinje fibers obtained from rabbits (n=12/group) raised in a normoxic (F(I)O2=0.21) or hypoxic (F(I)O2=0.12) environment from birth to 9 days of age. Action potential duration (APD)(90) was shorter (112+/-3 ms v 126+/-3 ms) and maximum diastolic potential (MDP) was more negative (-84+/-2 mV v-80+/-1 mV) in hypoxic hearts compared with normoxic controls. In normoxic hearts the NO donors, S-nitrosoglutathione (GSNO) 50 microM and spermine NONOate (50 microM) shortened APD(90) and increased MDP to levels present in chronically hypoxic hearts. This effect was completely abolished by the K(ATP) channel blocker glibenclamide (3 microM) and by a nitric oxide trap, Carboxy-PTIO (100 microM). The NO carrier glutathione (50 microM) and decomposed spermine NONOate had no effect on APD(90) or MDP. GSNO had no effect in hypoxic hearts; however, when GSNO was combined with glibenclamide APD(90) increased, and MDP decreased to normoxic values. 8-Bromo cGMP (100 microM) shortened APD(90) and increased MDP to levels present in chronically hypoxic hearts. This effect was abolished by glibenclamide. A soluble guanylyl cyclase inhibitor, ODQ (10 microM), had no effect on action potentials in normoxic hearts but in hypoxic hearts resulted in an increase in APD(90) to levels present in normoxic hearts and a decrease in MDP. The effect of ODQ could not be reversed by GSNO. We conclude that NO activates the sarcolemmal K(ATP) channel in normoxic and chronically hypoxic hearts by a cyclic GMP-dependent mechanism.

Reaction of S-nitrosoglutathione with the heme group of deoxyhemoglobin

The mechanism of interaction between S-nitrosoglutathione (GSNO) and hemoglobin is a crucial component of hypotheses concerning the role played by S-nitrosohemoglobin in vivo. We previously demonstrated (Patel, R. P., Hogg, N., Spencer, N. Y., Kalyanaraman, B., Matalon, S., and Darley-Usmar, V. M. (1999) J. Biol. Chem. 274, 15487-15492) that transnitrosation between oxygenated hemoglobin and GSNO is a slow, reversible process, and that the reaction between GSNO and deoxygenated hemoglobin (deoxyHb) did not conform to second order reversible kinetics. In this study we have reinvestigated this reaction and show that GSNO reacts with deoxyHb to form glutathione, nitric oxide, and ferric hemoglobin. Nitric oxide formed from this reaction is immediately autocaptured to form nitrosylated hemoglobin. GSNO reduction by deoxyHb is essentially irreversible. The kinetics of this reaction depended upon the conformation of the protein, with more rapid kinetics occurring in the high oxygen affinity state (i.e. modification of the Cysbeta-93) than in the low oxygen affinity state (i.e. treatment with inositol hexaphosphate). A more rapid reaction occurred when deoxymyoglobin was used, further supporting the observation that the kinetics of reduction are directly proportional to oxygen affinity. This observation provides a mechanism for how deoxygenation of hemoglobin/myoglobin could facilitate nitric oxide release from S-nitrosothiols and represents a potential physiological mechanism of S-nitrosothiol metabolism.

Reduction of myocardial nitrosyl complex formation by a nitric oxide scavenger prolongs cardiac allograft survival

Nitric oxide synthase (NOS) inhibitors have been shown to reduce NO but yield conflicting results on cardiac allograft survival. In this study, we provide an alternative approach specifically to examine the efficacy of a NO scavenger on nitrosyl complex formation and graft survival in a model of heterotopic cardiac transplantation. Efficacy was examined under both acute and chronic conditions (i.e., without or with immunosuppression, respectively). Electron paramagnetic resonance (EPR) spectroscopy of frozen myocardial tissue from untreated allografts showed progressive increases in nitrosylheme and nitrosomyoglobin before graft failure. These signals were not seen in either isografts or native hearts of allograft recipients. Both plasma nitrate plus nitrite and myocardial nitrosyl complex formation in cardiac allografts were significantly decreased in recipient animals treated with the NO scavenger, NOX-100, or by low-dose cyclosporine (CsA). Both interventions were nearly equivalent in significantly prolonging graft survival. The short-term combination treatment of both NOX-100 plus CsA completely eliminated myocardial nitrosyl complex formation and synergistically prolonged graft survival. Long-term combination drug treatment (days 0-100) followed by cessation of therapy resulted in permanent graft acceptance with no evidence for nitrosyl complex formation. These studies support a role of NO in cardiac allograft rejection. Furthermore, these studies indicate a potential therapeutic value of NO scavengers in preventing organ rejection.

Reversible and irreversible hemichrome generation by the oxygenation of nitrosylmyoglobin

The repeated oxygenation/reduction/nitrosylation of nitrosylmyoglobin produces low-spin ferric heme hemichromes which have been characterized by electron spin resonance spectroscopy. The predominant myoglobin hemichrome is a chemically reversible dihistidyl complex identified by the g values 1.53, 2.21, and 2.97. Also present is a low-spin ferric hydroxide derivative which is represented by the g values 1.83, 2.18, and 2.59. The formation of these species goes undetected by UV-vis spectroscopy, but the oxygenation of myoglobin to metmyoglobin is correlated with complete conversion of nitric oxide to nitrate which is released following a clear induction period. These results are interpreted in terms of the intermediates generated during the MbNO oxygenation reaction.

The peroxynitrite generator, SIN-1, becomes a nitric oxide donor in the presence of electron acceptors

SIN-1 has been used, in vitro, to simultaneously generate nitric oxide (*NO) and superoxide (O*-2). However, the pharmacological activity of SIN-1 resembles that of a *NO donor. SIN-1 decays by a three-step mechanism. After initial isomerization to an open ring form, SIN-1A reduces oxygen by a one-electron transfer reaction to give O*-2 and the SIN-1 cation radical, which decomposes to form SIN-1C and *NO. Here we report that one-electron oxidizing agents, in addition to oxygen, can oxidize SIN-1A, resulting in the release of *NO without the concomitant formation of O*-2. We demonstrate that easily reducible nitroxides, such as the nitronyl and imino nitroxides, are able to oxidize SIN-1. Biological oxidizing agents such as ferricytochrome c also stimulate *NO production from SIN-1. In addition, decomposition of SIN-1 by human plasma or by the homogenate of rat liver, kidney, and heart tissues results in the formation of *NO. Our findings suggest that SIN-1 may react with heme proteins and other electron acceptors in biological systems to produce *NO. Thus, at the relatively low in vivo oxygen concentrations, SIN-1 is likely to behave more like an *NO donor than a peroxynitrite donor. The relevance of this reaction to myocardial protection afforded by SIN-1 in ischemia/reperfusion-induced injury is discussed.

Myocardial nuclear factor-kappaB activity and nitric oxide production in rejecting cardiac allografts

BACKGROUND

Nuclear factor-kappaB (NF-kappaB) is a rapid response transcription factor for genes whose products are critical for inflammation and immunity. In a rat model of heterotopic cardiac transplantation, we studied NF-kappaB DNA binding activity and nitric oxide (.NO) production in untreated allografts and whether inhibition of NF-kappaB suppresses .NO production and prolongs graft survival.

METHODS

In allograft recipients and isograft controls, NF-kappaB was assayed by electrophoretic mobility shift assay, daily from transplant until rejection. Myocardial .NO was directly detected in explanted allografts by electron spin resonance spectroscopy on day 6 after transplant. The potent inhibitor of NF-kappaB, pyrrolidine dithiocarbamate (PDTC; 250 mg/kg s.c.) was administered daily from transplant until day of rejection. The extent of graft lymphocytic infiltrate was assessed by routine hematoxylin and eosin staining. Immunohistochemical staining of NF-kappaB was per formed to identify the cell type responsible for NF-kappaB activity.

RESULTS

A time-dependent increase in myocardial NF-kappaB activity was seen in untreated allografts as compared with isografts as determined by PhosphorImage analysis. Peak NF-kappaB activity occurred in allografts on day 4 with a ninefold increase as compared with isografts (24.0+/-3.7% vs. 2.7+/-0.5; P<0.05). On posttransplant day 6, electron spin resonance spectroscopy analysis of allografts demonstrated .NO identified by a triplet nitrogen signal centered at g=2.012 with hyperfine splitting of 17.5 Gauss, which is consistent with nitrosoheme formation and low-field signals at g=2.08 and g=2.03 consistent with nitrosomyoglobin. These signals were not seen in native hearts of allograft recipients. With PDTC administration, a threefold decrease in NF-kappaB activity within the transplanted heart was observed on posttransplant day 5 as compared with untreated allografts (9.7+/-1.6% vs. 23.5+/-2.5%; P<0.01). PDTC prolonged graft survival as compared with untreated allografts (11.7+/-0.3 vs. 6.6+/-0.2 days; P<0.05) and reduced the intensity of the nitrosoheme and nitrosomyoglobin signals. Allograft mononuclear cell infiltrate correlated with peak NF-kappaB activity with peak infiltrate on posttransplant day 4. PDTC treatment had no effect on the extent of infiltrate. Immunohistochemical staining localized NF-kappaB to the infiltrating mononuclear cells on posttransplant day 5.

CONCLUSION

These data support a role for NF-kappaB in allograft rejection.

Electron spin resonance analysis of heme-nitrosyl and reduced iron-sulfur centered complexes in allogeneic, heterotopic cardiac transplants: effects of treatment with pyrrolidine dithiocarbamate

Inhibition of inducible nitric oxide synthase (iNOS) prolongs allograft survival suggesting a role for nitric oxide (.NO) in allograft rejection. Induction of iNOS is regulated by the oxidant-sensitive, nuclear factor kappa B (NF-kappaB) in many cell types. In the present study using electron spin resonance (ESR) spectroscopy, we evaluated whether pyrrolidine dithiocarbamate (PDTC), a metal chelator and antioxidant, might limit .NO production during the development of rejection in cardiac allografts. We performed either isogeneic (Lewis to Lewis) or allogeneic (Wistar-Furth to Lewis) heterotopic abdominal cardiac transplantation. Allograft recipients received daily injections of PDTC or aminoguanidine (a known inhibitor of iNOS). At postoperative days 4 or 6, grafted and native hearts of transplant recipients were flushed with cardioplegic solution to remove blood contamination. ESR data of allografts revealed a triplet nitrogen signal (aN=17.5 G) and centered at g=2.012 and an additional broad signal at g=2.08. This signal was not seen in either isografts or native hearts of either isograft or allograft recipients. Based upon these parameters, these signals are attributed to nitrosomyoglobin. This signal was inhibited by treatment with aminoguanidine or PDTC. Under these conditions, PDTC also prolonged graft survival from 6.6+/-0.2 to 11.7+/-0.3 days. Thus, it is conceivable that nitrosylmyoglobin formation precedes rejection in cardiac allografts and inhibition of nitrosomyoglobin with agents such as PDTC contribute to improved graft survival.

Nitric oxide regulates oxygen uptake and hydrogen peroxide release by the isolated beating rat heart

Isolated rat heart perfused with 1.5-7.5 microM NO solutions or bradykinin, which activates endothelial NO synthase, showed a dose-dependent decrease in myocardial O2 uptake from 3.2 +/- 0.3 to 1.6 +/- 0.1 (7.5 microM NO, n = 18, P < 0.05) and to 1.2 +/- 0.1 microM O2.min-1.g tissue-1 (10 microM bradykinin, n = 10, P < 0.05). Perfused NO concentrations correlated with an induced release of hydrogen peroxide (H2O2) in the effluent (r = 0.99, P < 0.01). NO markedly decreased the O2 uptake of isolated rat heart mitochondria (50% inhibition at 0.4 microM NO, r = 0.99, P < 0.001). Cytochrome spectra in NO-treated submitochondrial particles showed a double inhibition of electron transfer at cytochrome oxidase and between cytochrome b and cytochrome c, which accounts for the effects in O2 uptake and H2O2 release. Most NO was bound to myoglobin; this fact is consistent with NO steady-state concentrations of 0.1-0.3 microM, which affect mitochondria. In the intact heart, finely adjusted NO concentrations regulate mitochondrial O2 uptake and superoxide anion production (reflected by H2O2), which in turn contributes to the physiological clearance of NO through peroxynitrite formation.

In vivo production of nitric oxide in rats after administration of hydroxyurea

The metabolism of nitrovasodilators such as glyceryl trinitrate and nitroprusside provides the active moiety of these drugs (that is, nitric oxide). This process is not limited to the known nitrovasodilators, but also occurs with nitroaromatic antimicrobials. Here we report that the administration of hydroxyurea, an antitumor drug, to rats at pharmacological doses formed detectable nitrosyl hemoglobin, which increased with dose. At higher doses, nitrosyl hemoprotein complexes could also be detected in liver tissue. [15N]hydroxyurea was synthesized and compared with [14N]hydroxyurea. These observations verified that nitric oxide detected as nitrosyl hemoglobin or nitrosyl hemoprotein complexes in rats was the result of the metabolism of hydroxyurea. The time course and dose-dependence of nitric oxide generation were also investigated. Hydroxyurea's antineoplastic activity is caused by its direct action on ribonucleotide reductase, the rate-limiting enzyme in DNA synthesis. Because nitric oxide also inhibits ribonucleotide reductase, this metabolite may supplement this action of hydroxyurea. In addition, the known ability of hydroxyurea to ease the pain of sickle cell anemia patients may be the result of vasodilation by the drug-derived nitric oxide.

The mechanism of cardioprotection by S-nitrosoglutathione monoethyl ester in rat isolated heart during cardioplegic ischaemic arrest

1. This study was designed (i) to assess the effect of S-nitrosoglutathione monoethyl ester (GSNO-MEE), a membrane-permeable analogue of S-nitrosoglutathione (GSNO), on rat isolated heart during cardioplegic ischaemia, and (ii) to monitor the release of nitric oxide (.NO) from GSNO-MEE in intact hearts using endogenous myoglobin as an intracellular .NO trap and the hydrophilic N-methyl glucamine dithiocarbamate-iron (MGD-Fe2+) complex as an extracellular .NO trap. 2. During aerobic perfusion of rat isolated heart with GSNO-MEE (20 mumol 1(-1), there was an increase in cyclic GMP from 105 +/- 11 to 955 +/- 193 pmol g-1 dry wt. (P < 0.05), and a decrease in glycogen content from 119 +/- 3 to 96 +/- 2 mumol g-1 dry wt. (P < 0.05), and glucose-6-phosphate concentration from 258 +/- 22 in control to 185 +/- 17 nmol g-1 dry wt. (P < 0.05). During induction of cardioplegia, GSNO-MEE caused the accumulation of cyclic GMP (100 +/- 6 in control vs. 929 +/- 168 pmol g-1 dry wt. in GSNO-MEE-treated group, P < 0.05), and depletion of glycogen from 117 +/- 3 to 103 +/- 2 mumol g-1 dry wt. (P < 0.05) in myocardial tissue. 3. Inclusion of GSNO-MEE (20 mumol l-1) in the cardioplegic solution improved the recovery of developed pressure (46 +/- 8 vs. 71 +/- 3% of baseline, P < 0.05), and rate-pressure product from 34 +/- 6 to 63 +/- 5% of baseline (P < 0.05), and reduced the diastolic pressure during reperfusion from 61 +/- 7 in control to 35 +/- 5 mmHg (P < 0.05) after 35 min ischaemic arrest. GSH-MEE (20 mumol l-1) in the cardioplegic solution did not elicit the protective effect. 4. During cardioplegic ischaemia, GSNO-MEE (20-200 mumol l-1) induced the formation of nitrosylmyoglobin (MbNO), which was detected by electron spin resonance (ESR) spectroscopy. Inclusion of MGD-Fe2+ (50 mumol l-1 Fe2+ and 500 mumol l-1 MGD) in the cardioplegic solution along with GSNO-MEE yielded an ESR signal characteristic of the MGD-Fe2+ -NO adduct. However, the MGD-Fe2+ trap did not prevent the formation of the intracellular MbNO complex in myocardial tissue. During aerobic reperfusion, denitrosylation of the MbNO complex slowly occurred as shown by the decrease in ESR spectral intensity. GSNO-MEE treatment did not affect ubisemiquinone radical formation during reperfusion. 5. GSNO-MEE (20 microliters l-1) treatment elevated the myocardial cyclic GMP during ischaemia (47 +/- 3 in control vs. 153 +/- 34 pmol g-1 dry wt. after 35 min ischaemia, P < 0.05). The cyclic GMP levels decreased in the control group during ischaemia from 100 +/- 6 after induction of cardioplegia to 47 +/- 3 pmol g-1 dry wt. at the end of ischaemic duration. 6. Glycogen levels were lower in GSNO-MEE (20 mumol l-1)-treated hearts throughout the ischaemic duration (26.7 +/- 3.1 in control vs. 19.7 +/- 2.4 mumol g dry-t wt. in GSNO-MEE-treated group at the end of ischaemic duration), because of rapid depletion of glycogen during induction of cardioplegia. During ischaemia, the amounts of glycogen consumed in both groups were similar. Equivalent amounts of lactate were produced in both groups (148 +/- 4 in control vs. 141 +/- 4 mumol g-1 dry wt. in GSNO-MEE-treated group after 35 min in ischaemia). 7. The mechanism(s) of myocardial protection by GSNO-MEE against ischaemic injury may involve preischaemic glycogen reduction and/or elevated cyclic GMP levels in myocardial tissue during ischaemia.