Effect of inosine on ventricular regional perfusion and infarct size after coronary occlusion

Protection of mice against X-ray injuries by the post-irradiation administration of guanosine and inosine

PURPOSE

To examine the radioprotective action of guanosine (Guo) and inosine (Ino) administered to mice after irradiation with X-rays.

MATERIALS AND METHODS

Survival of mice exposed to lethal and sublethal doses of X-rays was studied. Peripheral blood cells were counted using a light microscope. The damage to bone marrow cells was assessed by micronucleus (MN) test. Damage and repair of DNA in blood leukocytes were estimated using the comet assay.

RESULTS

Mice injected intraperitoneally (i.p.) with Guo or Ino ( approximately 30 microg g(-1), i.e., approximately 0.6 mg per 20-g mouse) 15 min after acute whole-body irradiation with 7 Gy recovered from X-ray injury. On the 30th day after irradiation, 50 and 40% of mice injected with Guo and Ino, respectively, remained alive. The dose reduction factor (DRF) was 1.23 for Guo and 1.15 for Ino. The protective effect gradually decreased as the time interval between the irradiation and injection was increased to 3, 5, 8 h. Guo and Ino facilitated the restoration of peripheral blood cell counts. These compounds protected bone marrow cells from damage and normalized erythropoiesis. Guo and Ino contributed to a more rapid and complete repair of DNA in mouse leukocytes irradiated both in vitro and in vivo.

CONCLUSION

Guo and Ino introduced shortly after irradiation reduce leukopenia and thrombocytopenia and offer promise as therapeutic agents for treatment of radiation injuries.

Extracellular inosine is modulated by H2O2 and protects sertoli cells against lipoperoxidation and cellular injury

Extracellular purines are involved in the regulation of a wide range of physiological processes, including cytoprotection, ischemic preconditioning, and cell death. These actions are usually mediated via triggering of membrane purinergic receptors, which may activate antioxidant enzymes, conferring cytoprotection. Recently, it was demonstrated that the oxidative stress induced by cisplatin up-regulated A1 receptor expression in rat testes, suggesting an involvement of purinergic signaling in the response of testicular cells to oxidant injury. In this article, we report the effect of hydrogen peroxide on purinergic agonist release by cultured Sertoli cells. Extracellular inosine levels are strongly increased in the presence of H2O2, suggesting an involvement of this nucleoside on Sertoli cells response to oxidant treatment. Inosine was observed to decrease H2O2-induced lipoperoxidaton and cellular injury, and it also preserved cellular ATP content during H2O2 exposure. These effects were abolished in the presence of nucleoside uptake inhibitors, indicating that nucleoside internalisation is essential for its action in preventing cell damage.

Ophidian envenomation strategies and the role of purines

Snake envenomation employs three well integrated strategies: prey immobilization via hypotension, prey immobilization via paralysis, and prey digestion. Purines (adenosine, guanosine and inosine) evidently play a central role in the envenomation strategies of most advanced snakes. Purines constitute the perfect multifunctional toxins, participating simultaneously in all three envenomation strategies. Because they are endogenous regulatory compounds in all vertebrates, it is impossible for any prey organism to develop resistance to them. Purine generation from endogenous precursors in the prey explains the presence of many hitherto unexplained enzyme activities in snake venoms: 5'-nucleotidase, endonucleases (including ribonuclease), phosphodiesterase, ATPase, ADPase, phosphomonoesterase, and NADase. Phospholipases A(2), cytotoxins, myotoxins, and heparinase also participate in purine liberation, in addition to their better known functions. Adenosine contributes to prey immobilization by activation of neuronal adenosine A(1) receptors, suppressing acetylcholine release from motor neurons and excitatory neurotransmitters from central sites. It also exacerbates venom-induced hypotension by activating A(2) receptors in the vasculature. Adenosine and inosine both activate mast cell A(3) receptors, liberating vasoactive substances and increasing vascular permeability. Guanosine probably contributes to hypotension, by augmenting vascular endothelial cGMP levels via an unknown mechanism. Novel functions are suggested for toxins that act upon blood coagulation factors, including nitric oxide production, using the prey's carboxypeptidases. Leucine aminopeptidase may link venom hemorrhagic metalloproteases and endogenous chymotrypsin-like proteases with venom L-amino acid oxidase (LAO), accelerating the latter. The primary function of LAO is probably to promote prey hypotension by activating soluble guanylate cyclase in the presence of superoxide dismutase. LAO's apoptotic activity, too slow to be relevant to prey capture, is undoubtedly secondary and probably serves principally a digestive function. It is concluded that the principal function of L-type Ca(2+) channel antagonists and muscarinic toxins, in Dendroaspis venoms, and acetylcholinesterase in other elapid venoms, is to promote hypotension. Venom dipeptidyl peptidase IV-like enzymes probably also contribute to hypotension by destroying vasoconstrictive peptides such as Peptide YY, neuropeptide Y and substance P. Purines apparently bind to other toxins which then serve as molecular chaperones to deposit the bound purines at specific subsets of purine receptors. The assignment of pharmacological activities such as transient neurotransmitter suppression, histamine release and antinociception, to a variety of proteinaceous toxins, is probably erroneous. Such effects are probably due instead to purines bound to these toxins, and/or to free venom purines.

Inosine improves gut permeability and vascular reactivity in endotoxic shock

OBJECTIVE

To investigate the effects of inosine administration on vascular reactivity, gut permeability, neutrophil accumulation and lipid peroxidation in tissues in murine endotoxin shock.

DESIGN

Randomized, prospective laboratory study.

SETTING

Research laboratory.

SUBJECTS

BALB/c mice 6-8 wks age.

INTERVENTIONS

BALB/c mice were randomly assigned to one of five groups: a) vehicle controls, which received saline intraperitoneally; b) inosine controls, which received inosine alone (100 mg/kg, ip); c) lipopolysaccharide (LPS)-treated animals, which received LPS (40 and 100 mg/kg, ip, depending on the experimental protocol); d) inosine pretreatment group, which received inosine (100 mg/kg, ip) 30 mins before LPS; and finally, e) inosine posttreatment group, which received inosine (100 mg/kg, ip) 60 mins after LPS.

MEASUREMENTS AND MAIN RESULTS

The passage of fluorescein isothiocyanate-conjugated dextran (4 kDa, FD4) was analyzed in everted gut ileal sacs incubated ex vivo as an index of gut permeability. LPS induced a significant intestinal hyperpermeability, and inosine exerted protective effects both in pre- and posttreatment regimens. Myeloperoxidase and malondialdehyde were also measured to study neutrophil accumulation and lipid peroxidation in selected tissues. Inosine, both in pre- and posttreatment regimens ameliorated the increases in myeloperoxidase and malondialdehyde in the lung and gut. LPS-treated animals showed decreased contractile and relaxant responses, and inosine pretreatment (but not posttreatment) partially improved these responses.

CONCLUSIONS

Taken together, inosine has organ protective effects during shock. A significant portion of its protective action is maintained even in the posttreatment scenario.

Purines inhibit poly(ADP-ribose) polymerase activation and modulate oxidant-induced cell death

Purines such as adenosine, inosine, and hypoxanthine are known to have potent antiinflammatory effects. These effects generally are believed to be mediated by cell surface adenosine receptors. Here we provide evidence that purines protect against oxidant-induced cell injury by inhibiting the activation of the nuclear enzyme poly(ADP-ribose) polymerase (PARP). Upon binding to broken DNA, PARP cleaves NAD+ into nicotinamide and ADP-ribose and polymerizes the latter on nuclear acceptor proteins such as histones and PARP itself. Overactivation of PARP depletes cellular NAD+ and ATP stores and causes necrotic cell death. We have identified some purines (hypoxanthine, inosine, and adenosine) as potential endogenous PARP inhibitors. We have found that purines (hypoxanthine > inosine > adenosine) dose-dependently inhibited PARP activation in peroxynitrite-treated macrophages and also inhibited the activity of the purified PARP enzyme. Consistently with their PARP inhibitory effects, the purines also protected interferon gamma + endotoxin (IFN/LPS) -stimulated RAW macrophages from the inhibition of mitochondrial respiration and inhibited nitrite production from IFN/LPS-stimulated macrophages. We have selected hypoxanthine as the most potent cytoprotective agent and PARP inhibitor among the three purine compounds, and investigated the mechanism of its cytoprotective effect. We have found that hypoxanthine protects thymocytes from death induced by the cytotoxic oxidant peroxynitrite. In line with the PARP inhibitory effect of purines, hypoxanthine has prevented necrotic cell death while increasing caspase activity and DNA fragmentation. As previously shown with other PARP inhibitors, hypoxanthine acted proximal to mitochondrial alterations as hypoxanthine inhibited the peroxynitrite-induced mitochondrial depolarization and secondary superoxide production. Our data imply that purines may serve as endogenous PARP inhibitors. We propose that, by affecting PARP activation, purines may modulate the pattern of cell death during shock, inflammation, and reperfusion injury.

Inosine inhibits inflammatory cytokine production by a posttranscriptional mechanism and protects against endotoxin-induced shock

Extracellular purines, including adenosine and ATP, are potent endogenous immunomodulatory molecules. Inosine, a degradation product of these purines, can reach high concentrations in the extracellular space under conditions associated with cellular metabolic stress such as inflammation or ischemia. In the present study, we investigated whether extracellular inosine can affect inflammatory/immune processes. In immunostimulated macrophages and spleen cells, inosine potently inhibited the production of the proinflammatory cytokines TNF-alpha, IL-1, IL-12, macrophage-inflammatory protein-1alpha, and IFN-gamma, but failed to alter the production of the anti-inflammatory cytokine IL-10. The effect of inosine did not require cellular uptake by nucleoside transporters and was partially reversed by blockade of adenosine A1 and A2 receptors. Inosine inhibited cytokine production by a posttranscriptional mechanism. The activity of inosine was independent of activation of the p38 and p42/p44 mitogen-activated protein kinases, the phosphorylation of the c-Jun terminal kinase, the degradation of inhibitory factor kappaB, and elevation of intracellular cAMP. Inosine suppressed proinflammatory cytokine production and mortality in a mouse endotoxemic model. Taken together, inosine has multiple anti-inflammatory effects. These findings, coupled with the fact that inosine has very low toxicity, suggest that this agent may be useful in the treatment of inflammatory/ischemic diseases.

Anoxia inhibits guanosine salvage in cardiac myocytes

The adult heart depends largely on salvage synthesis to supply its 5'-nucleotide needs. Previous work from this laboratory established that guanosine is metabolized into guanine 5'-nucleotides in heart cells, but that salvage rates are very slow as compared to adenosine. The author hypothesized that guanosine salvage is regulated according to the needs of the cell for guanine nucleotides. This hypothesis was tested using cardiac myocytes which were rendered anoxic for 0-60 min. During this anoxic period, guanine nucleotides were depleted about 50%. At 0, 30, and 60 min, aliquots were removed for cell counting and nucleotide analysis; 50 microM 3H-guanosine was then added and the incubation continued for 1 min. The cells were then extracted and assayed for radioactivity in the guanine nucleotide products. Anoxia for 60 min, depressed GTP levels by 89%, total guanine nucleotides by 50%, and short-term guanosine salvage by 48% over aerobic controls. Reoxygenation of the myocytes after 30 min of anoxia returned guanosine salvage rates to nearly normal (87% of control). Preincubation of the myocytes with 5 mM ribose for times up to 1 hour modestly increased salvage rates of guanosine in aerobic cells. These results suggest that guanosine salvage in cardiac myocytes is not regulated by the size of the guanine nucleotide pool (that is, not sensitive to the demand for guanine nucleotides). Instead, salvage of guanosine is probably limited by cytosolic levels of ATP or phosphoribosylpyrophosphate, the production of which are dependent on adequate oxygen supplies.

Inosine--a natural modulator of contractility and myocardial blood flow in the ischemic heart?

The energetic role of inosine (INO) remains controversial. The aim of the present study was first to test whether endogenous INO consumption/production correlates with regional myocardial contractile performance and second to test whether locally increased levels of INO influence contractility and blood flow in severely ischemic myocardium. Fentanyl-anesthetized dogs with implanted sonomicrometry crystals and independently perfused left anterior descending coronary arteries were studied. Two relatively load-independent indexes of regional myocardial contractility derived from left ventricular pressure-segment length loops were used: the regional stroke work-end-diastolic segment length relationship (Wr/L(ed)) and the end-systolic pressure-segment length relationship (Plv/L(es)). Very good correlations between myocardial contractile performance (as measured by the slope of the regional Wr/L(ed) relationship) and endogenous INO consumption/production under both nonischemic and ischemic conditions were found. Ischemia severely depressed contractility, significantly shifting rightward the Wr/L(ed) and Plv/L(es) relationships. INO infused into the left anterior descending bypass, in a concentration of 600 to 800 mumol/L, partially restored contractile performance as evidenced by a significant leftward displacement of both relationships. Wr, measured at a common maximum L(ed), increased significantly by 61 +/- 5%. Border-zone collateral flow (microspheres) increased by 35 +/- 7% within the endocardial segments and by 34 +/- 9% in the epicardial segments, but no increase in flow in the ischemic region was measureable. With the current emphasis on recanalization with thrombolytic therapy and considering the apparent safety of INO, this naturally occurring nucleoside might prove to be a useful adjunctive agent in the treatment of acute myocardial ischemia.

Effects of inosine on glycolysis and contracture during myocardial ischemia

The effects of inosine (INO) on substrate metabolism and rigor formation in ischemic myocardium were examined in isolated rabbit hearts. Metabolite content was assessed in tissue extracts by chemical analysis and in the whole heart by 13C and 31P nuclear magnetic resonance spectroscopy. In ischemic hearts metabolizing either [3-13C]pyruvate or [1-13C]glucose, 1 mM INO increased both total and 13C-labeled alanine content; lactate content was unaffected. At 3 minutes of ischemia, tissue alanine was 1.81 +/- 0.11 microM/g wet wt (mean +/- SEM) in hearts perfused with pyruvate+INO versus 1.23 +/- 0.15 microM/g wet wt in hearts perfused with pyruvate alone (p less than 0.05). INO reduced tissue glycogen during ischemia in pyruvate-perfused hearts. Tissue alanine content in ischemic hearts that were supplied glucose+INO (1.29 +/- 0.13 microM/g wet wt) was greater than in ischemic hearts supplied glucose alone (0.65 +/- 0.14 microM/g wet wt). Alanine was found to originate from pyruvate and was a glycolytic end product in glucose-perfused hearts. INO raised the [3-13C]alanine/[3-13C]lactate ratio in ischemic, intact hearts (glucose = 0.24 +/- 0.07 versus glucose+INO = 0.60 +/- 0.09; pyruvate = 0.49 +/- 0.08 versus pyruvate+INO = 0.89 +/- 0.08). At 7 minutes of ischemia, ATP content fell to 70 +/- 3% with glucose+INO versus 58 +/- 5% with glucose alone. Rigor (stone heart) was delayed from 14.7 +/- 1.3 to 23.2 +/- 1.6 minutes with INO. INO did not change ATP content in ischemic hearts that were supplied pyruvate but delayed rigor (pyruvate = 9.9 +/- 1.2 minutes; pyruvate+INO = 15.6 +/- 1.0 minutes), possibly at the expense of glycogen. Supplemental glucose improved the effectiveness of INO with pyruvate to preserve ATP (pyruvate+glucose = 42 +/- 6%; pyruvate+glucose+INO = 72 +/- 6%) and further delayed rigor (pyruvate+glucose = 13.3 +/- 1.5 minutes; pyruvate+glucose+INO = 20.3 +/- 1.8 minutes). Glucose metabolism supported improved energetic and contractile states in ischemic hearts treated with INO. Thus, cardioprotection of the ischemic heart by INO was associated with preservation of functional integrity and improved energy production due to increased glycolytic activity. Activation of glycolysis in the presence of INO was accommodated by augmented alanine production without the additional accumulation of lactate.

Haemodynamic effects of inosine. A new drug for failing human heart?

Haemodynamic effects of inosine were studied in intact dogs and subsequently in patients suffering from prolonged otherwise intractable cardiogenic shock. The nucleoside significantly improve myocardial performance in patients with an extremely low cardiac index by 63 +/- 29% as well as animals with no signs of haemodynamic deterioration, increasing cardiac output by 12.2 +/- 4.3%. These results suggest that inosine may become a promising inotropic agent. Further studies though are necessary to elucidate the mechanism of inosine action.

Metabolic intervention to affect myocardial recovery following ischemia

Myocardial recovery during reperfusion following ischemia is critical to patient survival in a broad spectrum of clinical settings. Myocardial functional recovery following ischemia correlates well with recovery of myocardial adenosine triphosphate (ATP). Adenosine triphosphate recovery is uniformly incomplete during reperfusion following moderate ischemic injury and is therefore subject to manipulation by metabolic intervention. By definition ATP recovery is limited either by (1) energy availability and application in the phosphorylation of adenosine monophosphate (AMP) to ATP or (2) availability of AMP for this conversion. Experimental data suggest that substrate energy and the mechanisms required for its application in the creation of high energy phosphate bonds (AMP conversion to ATP) are more than adequate during reperfusion following moderate ischemic injury. Adenosine monophosphate availability, however, is inadequate following ischemia due to loss of diffusable adenine nucleotide purine metabolites. These purine precursors are necessary to fuel adenine nucleotide salvage pathways. Metabolic interventions that enhance AMP recovery rather than those that improve substrate energy availability during reperfusion are therefore recommended. The mechanisms of various metabolic interventions are discussed in this framework along with the rationale for or against their clinical application.

Management of acute myocardial infarction with natural physiological agents

A number of natural physiological agents deserve evaluation in the treatment of acute myocardial infarction. Prostacyclin and magnesium dilate large coronary arteries and could promote collateral circulation to ischemic regions, especially if used in conjunction with alpha-agonists to prevent a drop in coronary perfusion pressure. In addition, prostacyclin has anti-aggregatory and de-aggregatory effects on platelets and a stabilizing action on hypoxic tissue, while magnesium has anti-arrhythmic, potassium-retaining, and fibrinolytic effects, all of which could improve the outcome in acute MI. Adenosine or ribose infusion could be used to promote rapid repletion of adenine nucleotides in reperfused tissue, but unfortunately arteriolar vasodilation by adenosine might reduce collateral perfusion by "coronary steal". High-dose insulin has positive-inotropic (at minimal oxygen cost) and potent anti-arrhythmic actions that have not been adequately tested in previous clinical trials of "polarizing solutions". Carnitine infusion could improve the bioenergetics of ischemic myocardium by relieving inhibition of mitochondrial adenine nucleotide translocase.

Inosine: a protective agent in an organ culture model of myocardial ischemia

Fetal mouse hearts in organ culture provide a model of ischemic-like injury in which the myocardial protective effect of pharmacological agents can be studied independent of blood flow. To investigate the potential protective effect of a diffusable purine under ischemic-like conditions, we used 4 mM inosine in fetal mouse heart organ cultures deprived of oxygen and oxidizable substrates for 1-10 hours. We studied hearts (n = 258) immediately after simulated ischemia (early) and after a 20-hour recovery period (late), by utilizing three indices of myocardial viability. Thallium-201 accumulation is an early marker of myocardial viability during injury, whereas the percentage of lactic dehydrogenase release from hearts to culture medium and the percentage of irreversibly injured myocytes assessed by planimetry of midventricular histological sections are late markers, used after recovery from injury. At 10 hours of injury, thallium-201 accumulation was 38% greater in inosine-supplied hearts, 3.50 +/- 0.16 vs. 2.54 +/- 0.08 (counts/min per mg wet weight)/(counts/min per microliter medium) (mean +/- SEM) (P less than 0.001). After recovery from 10 hours of injury, lactic dehydrogenase release was 29% less in inosine-supplied hearts, 35 +/- 3% vs. 49 +/- 3% (P less than 0.001). After recovery from 8 hours of injury, the percentage of histologically irreversibly injured tissue was 23% less in inosine-supplied hearts, 60 +/- 7% vs. 78 +/- 3% (P less than 0.05). These data indicate that inosine has a protective effect on fetal mouse myocardium during simulated ischemia and suggest that inosine deserves further evaluation.

The influence of inosine on the size of myocardial ischaemia and myocardial metabolism in the pig

In acute experiments on open chest pigs 15 min occlusions of the left anterior descending coronary artery were performed, each occlusion followed by 45 min reperfusion. Myocardial ischaemia was defined by epicardial electrogram recorded from the border of the ischaemic area. Myocardial extraction of lactate and glucose as well as the extraction of FFA were measured before and at the 15th min of occlusion. Inosine (5 mg/kg/min) or 0.9% NaCl infusion was administered i.v. throughout the occlusion period. Inosine significantly diminished the number of ischaemic points and reduced an increase in R-wave voltage induced by coronary occlusion. Myocardial extraction of measured substrates was not significantly influenced by inosine administration. In conclusion, inosine decreases the area of ischaemic injury induced by acute coronary occlusion in the pig.

Myocardial infarction without coronary occlusion. A morphologic study in sheep

The macroscopic, histologic and enzyme-histochemical characteristics of the myocardial lesion obtained after heating of a thermoprobe in a branch of the left coronary artery in sheep is reported. In 13 sheep such myocardial lesions were produced distal to the location of the thermoprobe. Alterations were observed in accordance with generally accepted morphologic criteria for myocardial infarction. The coronary artery branch in which the thermoprobe was located showed erythrocyte and platelet aggregates immediately after the heating episode, which disappeared within a few min, as demonstrated by coronary arteriography. Injection of radiolabelled microspheres into the coronary circulation after induction of the myocardial lesion, cryosectioning of the heart and autoradiography revealed a lack of blood flow in the damaged myocardial region. We consider this new method a suitable tool for further studies on the complex pathology involved in the development of myocardial infarction.