Adenosine: a sensitive marker of myocardial ischaemia in man

The clinical application of purine nucleosides as biomarkers of tissue Ischemia and hypoxia in humans in vivo

During periods of ischemia and hypoxia, intracellular adenosine triphosphate stores are rapidly depleted. Its metabolism results in release of purine nucleosides into the systemic circulation. While the potential of purine nucleosides as a biomarker of ischemia has long been recognized, this has been limited by their complex physiological role and inherent instability leading to problematic sampling and prolonged, complex analysis procedures. Purine release has been demonstrated from cerebral tissue in patients undergoing carotid endarterectomy and patients presenting to hospital with stroke and transient ischemic attack. Rises in purine nucleosides have also been demonstrated in patients with angina and myocardial infarction, during systemic hypoxia, exercise, in patients with peripheral arterial disease and during surgery. This article reviews purine nucleoside production in ischemia, the development of purine analysis technology and details results of the studies investigating purine nucleosides as a biomarker of ischemia with suggestions for areas of future research.

Blockade of the Adenosine 2A Receptor Mitigates the Cardiomyopathy Induced by Loss of Plakophilin-2 Expression

Background: Mutations in plakophilin-2 (PKP2) are the most common cause of familial Arrhythmogenic Right Ventricular Cardiomyopathy, a disease characterized by ventricular arrhythmias, sudden death, and progressive fibrofatty cardiomyopathy. The relation between loss of PKP2 expression and structural cardiomyopathy remains under study, though paracrine activation of pro-fibrotic intracellular signaling cascades is a likely event. Previous studies have indicated that ATP release into the intracellular space, and activation of adenosine receptors, can regulate fibrosis in various tissues. However, the role of this mechanism in the heart, and in the specific case of a PKP2-initiated cardiomyopathy, remains unexplored.

Objectives: To investigate the role of ATP/adenosine in the progression of a PKP2-associated cardiomyopathy.

Methods: HL1 cells were used to study PKP2- and Connexin43 (Cx43)-dependent ATP release. A cardiac-specific, tamoxifen-activated PKP2 knock-out murine model (PKP2cKO) was used to define the effect of adenosine receptor blockade on the progression of a PKP2-dependent cardiomyopathy.

Results: HL1 cells silenced for PKP2 showed increased ATP release compared to control. Knockout of Cx43 in the same cells blunted the effect. PKP2cKO transcriptomic data revealed overexpression of genes involved in adenosine-receptor cascades. Istradefylline (an adenosine 2A receptor blocker) tempered the progression of fibrosis and mechanical failure observed in PKP2cKO mice. In contrast, PSB115, a blocker of the 2B adenosine receptor, showed opposite effects.

Conclusion: Paracrine adenosine 2A receptor activation contributes to the progression of fibrosis and impaired cardiac function in animals deficient in PKP2. Given the limitations of the animal model, translation to the case of patients with PKP2 deficiency needs to be done with caution.

Development and application of a LC-MS/MS method to quantify basal adenosine concentration in human plasma from patients undergoing on-pump CABG surgery

A sensitive and robust LC-MS/MS method was developed to quantify basal adenosine concentrations in human plasma of patients undergoing on-pump coronary artery bypass grafting (CABG) surgery. A strong cation exchange (SCX) monolithic cartridge was used to enrich analyte, improve robustness, and reduce biological complexity. A simple modifier-free mobile phase was employed to improve sensitivity and reproducibility. This method exhibits consistent precision and accuracy, and the RSDs or REs of all the intraday and interday determinations were within 10%. The calibration curve was linear across the examined dynamic range from 1nM to 500nM (r(2)=0.996). LOD and LOQ were determined to be 0.257nM and 0.857nM respectively, while LLOQ was below 10nM. This method was used to monitor changes of adenosine levels in patient plasma drawn intraoperatively during on-pump CABG surgery. The analysis of 84 patients revealed that the mean concentration of adenosine in coronary sinus plasma after cardiopulmonary bypass (CPB) is higher than that in coronary sinus before CPB (p=0.0024; two-tailed t-test) and that in radial artery plasma after CPB (p=0.0409; two-tailed t-test). These findings suggest that the equilibrium between adenosine production and elimination has favored the elevation of adenosine basal level during on-pump CABG surgery and the change is specific to heart tissues. Evaluation of adenosine with a sensitive and robust analytical method has important implications on providing consistent results and meaningful insights into adenosine regulation, as well as its steady state and sustained action on the heart. Relating patient characteristics or clinical outcomes with basal adenosine concentration can be used to optimize the CABG-CPB maneuver by regulating adenosine level via pharmacological intervention, and differentiating adenosine's contribution to cardioprotection from other modulatory factors.

Endothelial dysfunction during acute alcohol withdrawal syndrome

BACKGROUND

Endothelial dysfunction (EF) is a central phenomenon in a variety of conditions associated with increased cardiovascular morbidity. Here, we investigated EF during acute alcohol withdrawal syndrome before and 24h after medication. We aimed to analyze microcirculation, applying the post-occlusive reactive hyperemia (PORH) test and spectral analysis of skin vasomotion as markers of EF. Additionally, we explored whether segmentation of spectral analysis data may disclose more detailed information on dynamic blood flow behavior.

METHODS

We investigated 30 unmedicated patients during acute alcohol withdrawal syndrome and matched controls. Patients were reinvestigated after 24h when half of them had been treated with clomethiazole. Capillary blood flow was assessed on the right forearm after compression of the brachial artery. Parameters of PORH such as time to peak (TP), slope and PORH indices were calculated. Spectral analysis was performed in order to study five different frequency bands. Withdrawal symptoms were quantified by means of the alcohol withdrawal scale (AW scale).

RESULTS

We observed a blunted hyperemic response in patients after occlusion of the brachial artery indicated by significantly increased TP and decreased PORH indices. In contrast, vasomotion as investigated by spectral analysis was not altered. Segmentation analysis revealed some alterations in the cardiac band at rest, and indicated differences between treated and untreated patients after 24h.

CONCLUSION

Our results suggest peripheral endothelial dysfunction in patients during acute alcohol withdrawal. No major influence of treatment was observed. Future studies need to address the relation of EF to cardiac morbidity during alcohol withdrawal.

Peripheral endothelial dysfunction in patients suffering from acute schizophrenia: a potential marker for cardiovascular morbidity?

Patients suffering from schizophrenia have an increased standardized ratio for cardiovascular mortality compared to the general population. Endothelial function was identified as a prominent parameter for cardiac risk stratification in patients with heart disease. Here, we aimed to analyze the reactivity of the microcirculation applying the post-occlusive reactive hyperemia (PORH) test and spectral analysis of skin vasomotion as markers of endothelial function. We investigated 21 unmedicated patients suffering from paranoid schizophrenia as well as 21 matched controls. The capillary blood flow was assessed on the right forearm after compression of the brachial artery. Parameters of PORH such as time to peak (TP) or PORH index were calculated. In addition, spectral analysis of skin vasomotion was performed and five frequency bands (endothelial, sympathetic, vascular myogenic, respiratory and heart beat activity) were studied. Psychotic symptoms were quantified using the Positive and Negative Syndrome Scale (PANSS) and correlated to the parameters obtained. We report a blunted hyperemic response in patients after occlusion of the brachial artery indicated by significantly increased TP and decreased PORH indices. In contrast, vasomotion as investigated by spectral analysis of skin flow was rather sparsely altered showing differences at rest for the sympathetic and cardiac components only. Our results are suggestive of peripheral endothelial dysfunction in unmedicated patients suffering from schizophrenia. Future, prospective studies should address the relation of endothelial dysfunction to cardiac morbidity in patients with schizophrenia.

Adenosine A1 and A2 receptor agonists reduce endotoxin-induced cellular energy depletion and oedema formation in the lung

BACKGROUND AND OBJECTIVE

Tissue depletion of adenosine during endotoxaemia has previously been described in the lung. Therapeutic approaches to prevent adenosine depletion and the role of A1 and A2 receptor agonists, however, have not been investigated until now.

METHODS

In isolated and ventilated rabbit lungs, it was tested whether pretreatment with adenosine A1 agonist 2-chloro-N6-cyclopentyladenosine (CCPA; 10(-7) mol, n = 6) or A2 receptor agonist 5'-(N-cyclopropyl)-carboxyamido adenosine (CPCA; 10(-7) mol, n = 6) prior to injection of lipopolysaccharide (LPS) (500 pg mL-1) influenced pulmonary artery pressure (PAP), pulmonary energy content and oedema formation as compared with controls, solely infused with LPS (n = 6). Release rates of adenosine and uric acid were determined by high-performance liquid chromatography. Pulmonary tissue concentrations of high-energy phosphates were measured and the adenine nucleotide pool, adenosine 5'-triphosphate (ATP)/adenosine 5'-diphosphate (ADP) ratio and adenylate energy charge of the pulmonary tissue were calculated.

RESULTS

Administration of LPS induced increases in PAP within 2 h up to 20.8 +/- 2.9 mmHg (P < 0.01). While pretreatment with the A1 agonist merely decelerated pressure increase (13.8 +/- 1.1 mmHg, P < 0.05), the A2 agonist completely suppressed the pulmonary pressure reaction (9.6 +/- 1.0 mmHg, P < 0.01). Emergence of lung oedema after exclusive injection of LPS up to 12.0 +/- 2.9 g was absent after A1 (0.6 +/- 0.5 g) and A2 (-0.3 +/- 0.2 g) agonists. These observations were paralleled by increased adenosine release rates compared with LPS controls (P < 0.05). Moreover, tissue concentrations of ADP, ATP, guanosine 5'-diphosphate, guanosine 5'-triphosphate, nicotinamide-adenine-dinucleotide and creatine phosphate were significantly reduced after LPS. Consequently, the calculated tissue adenine nucleotide pool and the adenylate energy charge increased after adenosine receptor stimulation (P = 0.001).

CONCLUSIONS

Adenosine A1- and A2-receptor agonists reduced LPS-induced vasoconstriction and oedema formation by maintenance of tissue energy content. Thus, adenosine receptor stimulation, in particular of the A2 receptor, might be beneficial during acute lung injury.

Clinical methods for the evaluation of endothelial function - a focus on resistance arteries

Endothelial dysfunction is a key event in the pathophysiology of cardiovascular diseases and appears as a strong independent predictor of cardiovascular events. In this context, biological evaluation of endothelial circulating markers can be helpful. However, functional tests using pharmacological stimuli appear more specific for the study of resistance arteries. These methods consist in the evaluation of the endothelium-dependent changes in regional vascular flow in response to local infusion of substances that act through endothelial receptors without modification of systemic arterial pressure and in comparison with a non endothelium-dependent relaxation. Flow is measured by Doppler and intravascular ultrasound in coronary circulation, laser Doppler in skin and by venous occlusion plethysmography in peripheral muscular arteries. Similar studies can be performed ex vivo using isolated resistance arteries obtained from fat subcutaneous biopsies. In addition, other information can be obtained from reactive hyperemia and the study of the flow-mediated dilatation of conduit arteries to enable a selective and comprehensive approach of the heterogeneity of endothelial function in pathophysiology.

Effect of dopexamine on intestinal tissue concentrations of high-energy phosphates and intestinal release of purine compounds in endotoxemic rats

OBJECTIVES

To determine the effect of dopexamine, a synthetic catecholamine ligand for dopaminergic and beta2-adrenergic receptors, on intestinal release of adenosine 5'-triphosphate (ATP) degradation products and on intestinal tissue concentrations of high-energy phosphates during endotoxemia.

DESIGN

Randomized, controlled trial.

SETTING

Experimental laboratory.

SUBJECTS

Twenty-one male Wistar rats.

INTERVENTIONS

Rats given endotoxin (Escherichia coli lipopolysaccharide [LPS]; 1.5 mg/kg i.v. over 60 mins) were treated with a continuous infusion of dopexamine (DPX; 2.5 microg/kg/min, n = 7, group LPS + DPX) or 0.9% saline (n = 7, group LPS) during a study period of 120 mins. Animals in the control group (n = 7) received a volume-equivalent infusion of 0.9% saline without endotoxin.

MEASUREMENTS AND MAIN RESULTS

In all groups, arterial and portal venous concentrations of adenosine, hypoxanthine, and uric acid were measured at baseline and at 60 and 120 mins after the endotoxin challenge, and we calculated the portal venous/arterial concentration differences as an indicator of the intestinal release of the purine compounds. Furthermore, at the end of the study, the intestinal tissue concentrations of the high-energy phosphates ATP, adenosine 5'-diphosphate (ADP), adenosine 5'-monophosphate (AMP), creatine phosphate, and adenosine were determined, and we calculated the adenine nucleotide pool, the ATP/ADP and AMP/adenosine ratios, and the adenylate energy charge of the intestinal tissue. Endotoxemia decreases intestinal tissue ATP, ADP, AMP, and creatine phosphate concentrations, increases tissue adenosine content, and increases the release of hypoxanthine and uric acid from the intestinal tract. Dopexamine attenuates the endotoxin-induced decrease of the intestinal tissue adenine nucleotide pool, the AMP/adenosine ratio, and the release of the ATP-degradation products hypoxanthine and uric acid from the intestinal tract.

CONCLUSIONS

Normotensive endotoxemia is associated with a deterioration of the intestinal energy balance and an increased release of ATP degradation products, indicating intestinal tissue ischemia. Furthermore, these results suggest the beneficial effects of dopexamine on pathophysiologic alterations of the intestinal energy metabolism during endotoxemia.

Enhanced accumulation of pericardial fluid adenosine and inosine in patients with coronary artery disease

Adenosine and inosine are believed to have cardioprotective effects. However, little is known about their possible role in the metabolic autoregulation of human coronaries and in pathologic conditions with supply/demand imbalance of the heart such as coronary artery disease. Since these low molecular weight nucleosides freely diffuse through the monolayer of the visceral pericardium, adenosine and inosine concentrations in pericardial fluid may well reflect the conditions in cardiac interstitium. The pericardial fluid and systemic venous blood adenosine and inosine concentrations were measured in 98 human subjects undergoing heart surgery for coronary artery disease or valvular heart disease. Adenosine and inosine concentrations were measured by HPLC with UV detection. In subjects with coronary artery disease pericardial fluid nucleoside concentrations were significantly higher than in patients with valvular heart disease (adenosine: 1545 (996-3146) nmol/L [median (25th-75th quartiles)] vs. 738 (390-2527) nmol/L, P<0.01; inosine: 658 (321-1331) nmol/L vs. 347 (159-1037) nmol/L, P<0.05), while in both patient groups pericardial fluid nucleoside concentrations were higher by an order of magnitude than in venous plasma. Our results show the enhanced release of adenosine and inosine by the ischemic myocardium as a marker of supply/demand imbalance and support the hypothesis that these cardiac nucleosides may have an important role in the adaptation of coronary blood flow in human coronary artery disease.

Concentration changes of malondialdehyde across the cerebral vascular bed and shedding of L-selectin during carotid endarterectomy

BACKGROUND AND PURPOSE

Oxidative stress has been postulated to account for delayed neuronal death due to ischemia/reperfusion. We investigated cerebral formation of malondialdehyde as an index of lipid peroxidation in relation to different sources of reactive oxygen species in patients undergoing carotid endarterectomy.

METHODS

In 25 patients undergoing carotid endarterectomy, jugular venous-arterial concentration differences of brain metabolites, malondialdehyde, plasma total antioxidant status, and soluble P-selectin and L-selectin were measured. A carotid artery shunt (n=5) was placed only after complete loss of somatosensory evoked potentials, indicating a focal cerebral blood flow <15 mL/min per 100 g.

RESULTS

As an indication of cerebral lipid peroxidation, jugular venous-arterial malondialdehyde concentration differences were significantly enhanced before reperfusion, and an additional rise was observed 15 minutes after reperfusion. Plasma total antioxidant status significantly decreased during carotid artery occlusion only in patients with carotid artery shunt. This decrease was matched by cerebral formation of adenosine, hypoxanthine, and nitrite/nitrate. While jugular venous-arterial concentration differences of soluble P-selectin showed changes similar to those of malondialdehyde, the concentration difference for soluble L-selectin was enhanced exclusively at 15 minutes after reperfusion.

CONCLUSIONS

Short-term incomplete cerebral ischemia/reperfusion significantly enhanced cerebral lipid peroxidation, as indicated by malondialdehyde formation. The generation of reactive oxygen species by xanthine oxidase or nitric oxide metabolism might be involved in the induction of lipid peroxidation. The additional rise in cerebral release of malondialdehyde was found to coincide with a significant activation of polymorphonuclear leukocytes across the cerebral circulation.

Intestinal formation of hypoxanthine and uric acid during endotoxemia

The objective of this study was to examine the intestinal metabolism of high-energy purine compounds as sensitive indicators of tissue ischemia during endotoxemia. Arterial (art) and portal venous (PV) concentrations as well as the intestinal net concentration changes of adenosine (ADO), hypoxanthine (Hypo), and uric acid (UA) were measured at baseline and after 60 and 120 min in rats that were subjected to a 1-hr continuous infusion of endotoxin (1.5 mg/kg; group E), and in control animals (group C). Furthermore, the arterial (SaO2) and portal venous oxygen saturation (S(PV)O2) was determined at the same time points. Animals in both groups remained normotensive throughout the study period and no differences in mean arterial blood pressure were observed. In both groups, adenosine concentrations remained constant throughout the study and no changes in the net concentration difference (NCD) of adenosine between arterial and portal venous blood were observed [ADO(NCD); baseline: group E, -23 +/- 46 nmole/L; group C, 17 +/- 84 nmole/L; 120 min: group E, 14 +/- 38 nmole/L; group C, 5 +/- 40 nmole/L]. In contrast to control animals, hypoxanthine and uric acid concentrations increased in arterial and portal venous blood in endotoxemic rats after 120 min. This was accompanied with an increase in the intestinal net concentration differences of both hypoxanthine and uric acid, indicating the gut as the predominant source of these two compounds during endotoxemia [Hypo(NCD); baseline: group E, -36 +/- 53 nmole/L; group C, -53 +/- 185 nmole/L; 120 min: group E, 538 +/- 211 nmole/L; group C, 99 +/- 100 nmole/L] [UA(NCD); baseline: group E, 2.04 +/- 1.62 micromole/L; group C, -0.04 +/- 1.11 micromole/L; 120 min: group E, 9.58 +/- 3.04 micromole/L; group C, 0.35 +/- 1.34 micromole/L]. Furthermore, in endotoxemic rats the portal venous oxygen saturation decreased despite unaltered arterial oxygen saturation [SaO2; baseline: group E, 95.2 +/- 0.9%; group C, 94.2 +/- 0.9%; 120 min: group E, 95.4 +/- 0.7%; group C, 96.4 +/- 0.9%] [S(PV)O2; baseline: group E, 86.2 +/- 3.1%; group C, 85.7 +/- 1.4%; 120 min: group E, 69.1 +/- 4.5%; group C, 82.3 +/- 1.9%]. These results indicate the presence of tissue ischemia in the intestinal tract during early, normotensive endotoxemia. Furthermore, because of the direct toxic damage mediated by oxygen radicals that are generated during the production of uric acid, intestinal mucosal injury observed during endotoxemia may be related to an enhancement of the ATP-degradation pathway.

Evaluation of the determinants of flow-mediated radial artery vasodilatation in humans

The relative importance of the early peak response during hyperaemia and of the duration of the hyperaemic phase (t1/2: blood flow velocity half time and AUCt1/2: area under the curve of flow velocity at t1/2) in the magnitude of the flow-dependent vasodilatation of the radial artery was determined in humans. Radial artery diameter was measured continuously in 18 healthy volunteers using an echo-tracking system coupled to a Doppler device for the measurement of the radial blood flow. In 9 subjects, arterial parameters were measured at baseline and during 3 hyperaemic tests performed after 2, 5 or 10 minutes of ischaemia. Reproducibility of the measured parameters was studied in 9 other subjects. Radial artery diameter, AUCt1/2 and t1/2 increased proportionally with the duration of ischaemia. In contrast, the peak flow response was already maximal after 5 minutes of ischaemia. The regression analysis showed that the best fit model after stepwise analysis only included t1/2 (r = 0.85, p < 0.001). There was no correlation between the peak flow values and the duration of hyperaemia (r = 0.29, p = 0.14). These results demonstrate that conduit arteries postischaemic flow-dependent vasodilatation in humans is both determined by the peak value and by the duration of the hyperaemic phase and suggest that these two components must be considered when comparing this index of NO release between different groups of subjects.

Aminophylline reduces cardiac ischemic pain during percutaneous transluminal coronary angioplasty

OBJECTIVES

We investigated the effect of aminophylline, an antagonist of the adenosine P1 receptor, on cardiac pain experienced during percutaneous transluminal coronary angioplasty (PTCA).

BACKGROUND

Adenosine may mediate cardiac pain because the administration of adenosine provokes cardiac pain like angina. However, it is not known whether endogenous adenosine released during myocardial ischemia is responsible for cardiac pain.

METHODS

This was a single-blind, placebo-controlled randomized study. Of 21 men with stable effort angina with one-vessel coronary artery disease who underwent balloon inflation four times during PTCA, 11 received intravenously administered aminophylline before the fourth balloon inflation and the other 10 were given saline solution. The severity of cardiac pain based on the pain score and ST segment elevation on standard surface and intracoronary electrocardiograms were assessed.

RESULTS

All patients experienced cardiac pain during balloon inflation. Aminophylline significantly prolonged the duration of both the symptom-free interval (from 42 +/- 13 to 64 +/- 27 s, mean +/- SD, p < 0.05) and inflation time (from 79 +/- 23 to 103 +/- 20 s, p < 0.05), and it significantly reduced the pain score from 7.6 +/- 1.4 to 4.6 +/- 2.3 (p < 0.01). However, aminophylline did not affect ST segment elevation. Saline solution did not affect any of these variables. Balloon diameter and pressure were not different between the third and the fourth inflation in either group.

CONCLUSIONS

Aminophylline significantly reduced the severity of cardiac pain during PTCA without affecting ST segment elevation. These findings suggest that the activation of P1 receptors by endogenous adenosine may be partially responsible for cardiac pain during ischemia.

Bimakalim, an ATP-sensitive potassium channel opener, mimics the effects of ischemic preconditioning to reduce infarct size, adenosine release, and neutrophil function in dogs

BACKGROUND

The primary goal of the present study was to determine whether the infarct size-reducing effect of preconditioning is associated with an increase in adenosine release from the ischemic myocardium during a prolonged occlusion period or the subsequent reperfusion period and by a decrease in neutrophil infiltration. A second objective was to determine whether bimakalim, a KATP channel opener, mimics the effects of ischemic preconditioning.

METHODS AND RESULTS

Barbital-anesthetized open-chest dogs were subjected to 60 minutes of left anterior descending coronary artery (LAD) occlusion followed by 3 hours of reperfusion. In the preconditioning group, 5 minutes of LAD occlusion followed by 10 minutes of reperfusion was elicited before the 60-minute occlusion period. In two other groups, bimakalim 1 microgram/kg bolus followed by a 0.05 micrograms.kg-1.min-1 infusion or an equivalent volume of saline was administered intravenously 15 minutes before occlusion and continued until the time of reperfusion. In a final group, bimakalim was administered 10 minutes before reperfusion and continued until the end of the experiment. To measure the release of adenosine from the ischemic region, coronary venous blood samples were collected at various times during ischemia and after reperfusion, and the concentration of adenosine was measured. Myocardial infarct size was determined by triphenyl tetrazolium chloride; transmural myocardial blood flow, by radioactive microspheres. Transmural myeloperoxidase (MPO) activity, an index of neutrophil infiltration in the area at risk, was also measured. Preconditioning produced a marked reduction in infarct size (9.8 +/- 3.0% versus 28.6 +/- 5.2% in the control group, mean +/- SEM); adenosine release at 5, 10, 15, and 30 minutes of the 3-hour reperfusion period; and transmural MPO activity in the risk area. Similarly, pretreatment with bimakalim resulted in reductions in infarct size, adenosine release, and transmural MPO activity to an extent almost identical to that of preconditioning. When bimakalim was administered 10 minutes before reperfusion, the drug also produced a significant reduction in infarct size and transmural MPO activity; however, no significant reduction in coronary venous adenosine concentrations was observed. There were no significant differences in collateral blood flow between groups.

CONCLUSIONS

These results indicate that myocardial preconditioning in the canine heart produced by a short period of ischemia or a KATP channel opener is not mediated by an increase in adenosine release, as measured by coronary venous adenosine concentrations, during 60 minutes of occlusion or the initial 30 minutes of reperfusion. A significant reduction in transmural MPO activity in the ischemic area also appears to result from KATP channel activation and may play a role, at least in part, in the reduction in infarct size observed, particularly when a KATP channel opener is administered just before reperfusion.