Age-dependent increase in xanthine oxidoreductase differs in various heart cell types

Pro-Aging Effects of Xanthine Oxidoreductase Products

The senescence process is the result of a series of factors that start from the genetic constitution interacting with epigenetic modifications induced by endogenous and environmental causes and that lead to a progressive deterioration at the cellular and functional levels. One of the main causes of aging is oxidative stress deriving from the imbalance between the production of reactive oxygen (ROS) and nitrogen (RNS) species and their scavenging through antioxidants. Xanthine oxidoreductase (XOR) activities produce uric acid, as well as reactive oxygen and nitrogen species, which all may be relevant to such equilibrium. This review analyzes XOR activity through in vitro experiments, animal studies and clinical reports, which highlight the pro-aging effects of XOR products. However, XOR activity contributes to a regular level of ROS and RNS, which appears essential for the proper functioning of many physiological pathways. This discourages the use of therapies with XOR inhibitors, unless symptomatic hyperuricemia is present.

Inhibition of xanthine oxidase reduces oxidative stress and improves skeletal muscle function in response to electrically stimulated isometric contractions in aged mice

Oxidative stress is a putative factor responsible for reducing function and increasing apoptotic signaling in skeletal muscle with aging. This study examined the contribution and functional significance of the xanthine oxidase enzyme as a potential source of oxidant production in aged skeletal muscle during repetitive in situ electrically stimulated isometric contractions. Xanthine oxidase activity was inhibited in young adult and aged mice via a subcutaneously placed time-release (2.5mg/day) allopurinol pellet, 7 days before the start of in situ electrically stimulated isometric contractions. Gastrocnemius muscles were electrically activated with 20 maximal contractions for 3 consecutive days. Xanthine oxidase activity was 65% greater in the gastrocnemius muscle of aged mice compared to young mice. Xanthine oxidase activity also increased after in situ electrically stimulated isometric contractions in muscles from both young (33%) and aged (28%) mice, relative to contralateral noncontracted muscles. Allopurinol attenuated the exercise-induced increase in oxidative stress, but it did not affect the elevated basal level of oxidative stress that was associated with aging. In addition, inhibition of xanthine oxidase activity decreased caspase-3 activity, but it had no effect on other markers of mitochondrial-associated apoptosis. Our results show that compared to control conditions, suppression of xanthine oxidase activity by allopurinol reduced xanthine oxidase activity, H₂O₂ levels, lipid peroxidation, and caspase-3 activity; prevented the in situ electrically stimulated isometric contraction-induced loss of glutathione; prevented the increase in catalase and copper-zinc superoxide dismutase activities; and increased maximal isometric force in the plantar flexor muscles of aged mice after repetitive electrically evoked contractions.

Aging-related changes in myocardial purine metabolism and ischemic tolerance

Impaired tolerance to ischemia-reperfusion in older hearts may stem in part from alterations in purine catabolism, impacting on maintenance of energy state and protective signaling via extracellular adenosine. We characterized effects of aging on normoxic and post-ischemic purine metabolism in hearts from young (2-4 month), middle-aged (12 month), old (18 month), and senescent (24-28 month) C57/Bl6 mice. Normoxic function was similar in all age groups while normoxic purine efflux increased gradually with age. This was the result of enhanced efflux of hypoxanthine, xanthine and uric acid, with extracellular accumulation of adenosine and inosine remaining unchanged. While total purine washout during 60 min reperfusion following 20 min global ischemia was unaltered by aging (1057+/-109 nmoles/g in young vs. 1221+/-127 nmoles/g in senescent hearts), selective changes in purine catabolism were evident. Accumulation of adenosine and inosine were reduced by 50 and 80%, respectively, matched by 400 and 300% elevations in hypoxanthine and xanthine accumulation, respectively. Uric acid remained unchanged. Thus, while adenosine and inosine represented 15+/-2 and 47+/-3% of total purine efflux in young hearts, these values decreased to only 6+/-1 and 9+/-2% in senescent hearts. Efflux of IMP also increased 500% with aging whereas 5'-AMP was unaltered. These changes were associated with a substantial fall in ischemic tolerance, with left ventricular developed pressure recovering to 46+/-3% in young hearts vs. only 24+/-6, 16+/-4, and 19+/-4% in middle-age, old and senescent hearts, respectively. Our data collectively support a pronounced shift in purine catabolism, with reduced accumulation of salvageable and cardioprotective adenosine, and enhanced accumulation of poorly salvaged (and potentially injurious) hypoxanthine and xanthine. Mechanisms underlying this shift have yet to be determined. However, this may play a role in the marked decline in myocardial tolerance to ischemia with aging and senescence.

Interstitial purine metabolites in hearts with LV remodeling

The myocardial ATP concentration is significantly decreased in failing hearts, which may be related to the progressive loss of the myocardial total adenine nucleotide pool. The total myocardial interstitial purine metabolites (IPM) in the dialysate of interstitial fluid could reflect the tissue ATP depletion. In rats, postmyocardial infarction (MI) left ventricular (LV) remodeling was induced by ligation of the coronary artery. Cardiac microdialysis was employed to assess changes of IPM in response to graded beta-adrenergic stimulation with isoproterenol (Iso) in myocardium of hearts with post-MI LV remodeling (MI group) or hearts with sham operation (sham group). The dialysate samples were analyzed for adenosine, inosine, hypoxanthine, xanthine, and uric acid. LV volume was greater in the MI group (2.2 +/- 0.2 ml/kg) compared with the sham group (1.3 +/- 0.2 ml/kg, P < 0.05). Infarct size was 28 +/- 4%. The baseline dialysate level of uric acid was higher in the MI group (18.9 +/- 3.4 micromol) compared with the sham group (4.6 +/- 0.7 micromol, P < 0.01). During and after Iso infusion, the dialysate levels of adenosine, xanthine, and uric acid were all significantly higher in the MI group. Thus the level of IPM is increased in hearts with postinfarction LV remodeling both at baseline and during Iso infusion. These results suggest that the decreased myocardial ATP level in hearts with post-MI LV remodeling may be caused by the chronic depletion of the total adenine nucleotide pool.

Xanthine oxidase activity in the dexamethasone-induced hypertensive rat

Hypertension may be associated with an increase in oxidative stress as a possible mechanism for the increased vascular tone and organ injury. Previously, we reported an increased production of reactive oxygen species and endothelial cell death in the microcirculation of hypertensive rats. We hypothesize that xanthine oxidase (XO) may be a potential source of oxidants induced by glucocorticoid-induced hypertension. Male Wistar rats were administered dexamethasone (0.5 mg/kg/day) for 5 days to induce hypertension. After general anesthesia, cremaster muscle was collected for analysis of XO and xanthine dehydrogenase (XDH) activities. The mean blood pressure and XO levels in cremaster muscle were significantly increased in the dexamethasone-treated rats compared with controls. There was a strong age-dependent rise in total XO + XDH activity in all groups. To inhibit XO, we administered allopurinol (ALLO, 0.4 mg/mL) in the drinking water to a subset of control and dexamethasone-treated rats during a 5-day treatment. The ALLO significantly reduced the mean arterial blood pressure in the dexamethasone-treated rats. Although in the cremaster muscle the total XO + XDH levels were not completely reduced with ALLO, the XO levels of the dexamethasone-treated + ALLO rats were reduced to levels of the control + ALLO group. These results suggest that dexamethasone induces an elevated level of XO activity in the cremaster muscle. The enhanced XO activity can be attenuated by chronic allopurinol treatment.

Relationship between free radicals and adenosine in the mechanism of preconditioning: are they interrelated or independent triggers?

Both free radicals (FRs) and adenosine receptor activation contribute to triggering a mechanism of preconditioning (PC) against infarction. This study examined the possibility that there is some interaction between FRs and adenosine generation during PC. In the first series of experiments, the effects of an FR scavenger, N-2-mercaptopropionyl glycine (MPG), on the interstitial adenosine level during PC and on the infarct size-limiting effect of PC were assessed in the rabbit heart in situ. PC with 5-min ischemia/5-min reperfusion limited infarct size after 30-min coronary occlusion (expressed as a percentage of area at risk, %IS/AR) from 33.2 +/- 4.7% (S.E.) to 10.8 +/- 1.1% (p < 0.05). This cardioprotection was blocked by MPG (1.5 mg/kg/min i.v.) infused before and during PC (%IS/AR = 27.4 +/- 3.6). However, the same dose of MPG did not suppress elevation of the adenosine and inosine levels in the microdialysate from the myocardium during 5-min ischemia/reperfusion. In the second series of experiments, the effect of an FR-generating system (1 mM hypoxanthine and 20 mU/ml xanthine oxidase) on the purine production was compared to that of PC in isolated rabbit hearts. Whereas PC increased the adenosine level in the coronary effluent from 0.17 +/- 0.16 microM under baseline to 1.68 +/- 0.53 microM, infusion of the FR generators over a period of 5 min did not increase the adenosine release. However, infarct size was similarly reduced by PC and by 5-min transient infusion of FR generators, and the cardioprotection by the FR generators was abolished by 300 microM MPG. These results suggest that there is no interaction between free radicals and adenosine during the trigger phase of PC in the rabbit heart.

Detection of hydroxyl free radicals in the reperfused primate heart

Early reperfusion of an ischemic region can result in significant salvage of the area at risk. We show the presence of hydroxyl free radicals at the time of post ischemia reperfusion using electron paramagnetic resonance (EPR) spectroscopy in a macaque model. These free radicals may be formed as a result of reperfusion or may be an un-involved bystander. It is possible that they may be involved in reperfusion injury.

Age-related changes in hepatic and splenic insulin receptors and serum insulin and glucose levels in inbred mice

Inbred mice of strains A/J, DBA/1J, and SJL/J were housed and aged in our animal colony, and parameters of carbohydrate metabolism were assessed at various ages. The patterns of age-related change were both organ- and strain-specific. Age-related changes in two of the strains were associated with relative carbohydrate intolerance. Common to all three strains was a biphasic pattern of change in hepatic insulin receptor number, with a decrease in early life and a return to earlier levels late in life. In both A/J and DBA/1J mice, there was a sharp increase in serum insulin level (twofold to 9.7-fold) that corresponded to the decrease in hepatic insulin receptors and was associated with hyperglycemia; no significant change in serum insulin or glucose levels was seen in SJL/J mice, despite a similar biphasic pattern in hepatic insulin receptor concentration. Age-related changes in splenic insulin receptors resembled changes in the liver in A/J and SJL/J mice, ie, there were synchronous biphasic age-related patterns. This was not the case in the spleens of DBA/1J mice, in which we did not observe age-related changes. There was no change in insulin receptor affinity with age, nor was there any difference in affinity between tissues or mouse strains. The pattern of change in hepatic insulin receptors and serum insulin levels was more complex than has been previously recognized. We do not know the mechanisms responsible for this complex pattern, but it must involve at least two discrete age-related events.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of supplementing hypothermic crystalloid cardioplegia with catalase plus allopurinol in the isolated rabbit heart

The effect of adding allopurinol and catalase to hypothermic cardioplegia for ischemic-reperfusion injury was investigated in the isolated rabbit heart. Hearts were divided into two groups, namely: Group C (n = 7), which received a hypothermic crystalloid cardioplegic solution alone (4 degrees C), and group T (n = 7), which received the hypothermic cardioplegic solution with allopurinol (148 mumol/L)13 and catalase (37 nmol/L).12 The cardioplegic solution was infused continuously into the isolated hearts, which had been placed in ice-cold saline, during a 12 h preservation. Subsequently, the hearts were mounted on a noncirculating, nonpulsatile perfusion circuit using Krebs-Henseleit buffer solution at 37 degrees C for 1 h at a constant perfusion pressure of 75 mm Hg. The left ventricular developed pressure (LVDP), maximum rate of pressure change (max dp/dt), and percent recovery of coronary flow were higher, while the creatine phosphokinase concentration and left ventricular end diastolic pressure (LVEDP) were lower in group T. The tissue malondialdehyde concentration and water content were similar in both groups. Thus, cardiac function after a 12 h preservation was enhanced by the added combination of allopurinol and catalase to the cardioplegic solution, supporting its role in the prevention of free radical reperfusion injury in cardiac preservation.

Superoxide dismutase attenuated post-ischaemic contractile dysfunction in a myocardial xanthine oxidase deficient species

1. We assessed the effect of polyethylene glycol conjugated superoxide dismutase (PEG-SOD) on myocardial stunning in the rabbit heart in which xanthine oxidase level is extremely low. 2. In open-chest anaesthetized rabbits, the left marginal branch of the coronary artery was occluded for 10 min and then reperfused for 30 min. A group of rabbits (PEG-SOD group) received 1000 units/kg of PED-SOD and another group (control group) was given saline 15 min before the coronary occlusion. 3. Regional systolic thickening fraction (TF) was similarly reduced to approximately -25% of baseline value during ischaemia in both groups. However recovery of TF after reperfusion was significantly better in the PEG-SOD group (n = 9) and TF at 30 min after reperfusion was 70.1 +/- 3.9% of baseline value compared with 44.9 +/- 3.4% in the control group (n = 9; P less than 0.05). Rate-pressure products, left ventricular pressure, and LV dP/dt max were not significantly different between the PEG-SOD treated and untreated control rabbits at any time during the experiment. PEG-SOD did not modify the regional myocardial blood flow (coloured microsphere method) during ischaemia/reperfusion, which was assessed by using separate groups of rabbits. 4. These findings indicate that oxygen free radicals are important in the pathogenesis of myocardial stunning in xanthine oxidase deficient hearts.

Myocardial protection by intravenous diltiazem during angioplasty of single-vessel coronary artery disease

The possible cardioprotective effect of diltiazem during ischemia caused by percutaneous transluminal coronary angioplasty was tested. Electrocardiograms and myocardial lactate, hypoxanthine and urate production were determined in 26 patients with a stenosis in the left anterior descending artery without angiographically demonstrable collaterals. Measurements took place before angioplasty, after each of 4 occlusions and 15 minutes after the last balloon inflation. Patients were randomly given placebo or DL-diltiazem (0.4 mg/kg as a bolus intravenously, followed by an infusion of 15 mg/hr). During angioplasty the ST-segment elevation for the anterior wall leads V2, V4 and V6, and the intracoronary lead was similar for both groups, as was lactate release. Diltiazem significantly reduced cardiac hypoxanthine release immediately after angioplasty from 63 to 88% (p less than 0.05). The drug diminished urate production after the last dilatation by 82% (p less than 0.05). In conclusion, intravenous infusion of diltiazem reduced cardiac adenosine triphosphate breakdown during angioplasty as shown by diminished hypoxanthine and urate production. In contrast, diltiazem was unable to attenuate ST-segment elevation and lactate release.

Age-dependent production of mitochondrial hydrogen peroxide, lipid peroxides and fluorescent pigments in the rat heart

Mitochondria were prepared from hearts of 3-, 14-, 18-, and 24-month-old male Wistar rats. Respiratory control ratio (RCR) values did not change with age in the glutamate or succinate-induced respiration except at 24 months in which RCR values significantly increased with both the substrates. Using still glutamate or succinate as substrates the production of H2O2 was measured in the presence of antimycin. A 70% and 25% increase in H2O2 formation was observed at 14 and 18 months of age, respectively, in comparison to the youngest group. Only in the presence of succinate was a 25% elevation in H2O2 found at 24 months of age. These observations parallel with the decrease of the ratio between tissue levels of reduced and oxidized glutathione that was observed at 14 and 18 months of age. The concentration of myocardial malondialdehyde, a secondary product of lipid peroxidation, remained the same at all ages measured, most probably because it is readily metabolized in vivo. On the contrary the myocardial level of lipofuscin, which is not degraded by the cell, progressively increased beginning from 18 months of age.