Adenosine: a sensitive indicator of cerebral ischemia during carotid endarterectomy

More than a drug target: Purinergic signalling as a source for diagnostic tools in epilepsy

Epilepsy is one of the most common and disabling chronic neurological diseases affecting people of all ages. Major challenges of epilepsy management include the persistently high percentage of drug-refractoriness among patients, the absence of disease-modifying treatments, and its diagnosis and prognosis. To date, long-term video-electroencephalogram (EEG) recordings remain the gold standard for an epilepsy diagnosis. However, this is very costly, has low throughput, and in some instances has very limited availability. Therefore, much effort is put into the search for non-invasive diagnostic tests. Purinergic signalling, via extracellularly released adenosine triphosphate (ATP), is gaining increasing traction as a therapeutic strategy for epilepsy treatment which is supported by evidence from both experimental models and patients. This includes in particular the ionotropic P2X7 receptor. Besides that, other components from the ATPergic signalling cascade such as the metabotropic P2Y receptors (e.g., P2Y₁ receptor) and ATP-release channels (e.g., pannexin-1), have also been shown to contribute to seizures and epilepsy. In addition to the therapeutic potential of purinergic signalling, emerging evidence has also shown its potential as a diagnostic tool. Following seizures and epilepsy, the concentration of purines in the blood and the expression of different compounds of the purinergic signalling cascade are significantly altered. Herein, this review will provide a detailed discussion of recent findings on the diagnostic potential of purinergic signalling for epilepsy management and the prospect of translating it for clinical application. This article is part of the Special Issue on 'Purinergic Signaling: 50 years'.

Novel Point-of-Care Diagnostic Method for Neonatal Encephalopathy Using Purine Nucleosides

Background: Evidence suggests that earlier diagnosis and initiation of treatment immediately after birth is critical for improved neurodevelopmental outcomes following neonatal encephalopathy (NE). Current diagnostic tests are, however, mainly restricted to clinical diagnosis with no molecular tests available. Purines including adenosine are released during brain injury such as hypoxia and are also present in biofluids. Whether blood purine changes can be used to diagnose NE has not been investigated to date.

Methods: Blood purines were measured in a mouse model of neonatal hypoxia and infants with NE using a novel point-of-care diagnostic technology (SMARTChip) based on the summated electrochemical detection of adenosine and adenosine metabolites in the blood.

Results: Blood purine concentrations were ∼2–3-fold elevated following hypoxia in mice [2.77 ± 0.48 μM (Control) vs. 7.57 ± 1.41 μM (post-hypoxia), p = 0.029]. Data in infants with NE had a 2–3-fold elevation when compared to healthy controls [1.63 ± 0.47 μM (Control, N = 5) vs. 4.87 ± 0.92 μM (NE, N = 21), p = 0.0155]. ROC curve analysis demonstrates a high sensitivity (81%) and specificity (80%) for our approach to identify infants with NE. Moreover, blood purine concentrations were higher in infants with NE and seizures [8.13 ± 3.23 μM (with seizures, N = 5) vs. 3.86 ± 0.56 μM (without seizures, N = 16), p = 0.044].

Conclusion: Our data provides the proof-of-concept that measurement of blood purine concentrations via SMARTChip technology may offer a low-volume bedside test to support a rapid diagnosis of NE.

Biological insights from the direct measurement of purine release

Although purinergic signalling has been a well-established and accepted mechanism of chemical communication for many years, it remains important to measure the extracellular concentration of ATP and adenosine in real time. In this review I summarize the reasons why such measurements are still needed, how they provide additional mechanistic insight and give an overview of the techniques currently available to make spatially localised measurements of ATP and adenosine in real time. To illustrate the impact of direct real-time measurements, I explore CO2 and nutrient sensing in the medulla oblongata and hypothalamus. In both of these examples, the sensing involves hemichannel mediated ATP release from glial cells. For CO2 the hemichannels involved, connexin26, are directly CO2-sensitive. This mechanism contributes to the chemosensory control of breathing. In the hypothamalus, specialised glial cells, tanycytes, directly contact the cerebrospinal fluid in the 3rd ventricle and sense nutrients via sweet and umami taste receptors. Nutrient sensing by tanycytes is likely to contribute to the control of body weight as their selective stimulation alters food intake. To illustrate the importance of direct adenosine measurements, I consider the complex and multiple mechanisms of activity-dependent adenosine release in different brain regions. This activity dependent release of adenosine is likely to mediate important feedback regulation and may also be involved in controlling the sleep-wake state. I finish by briefly considering the potential of whole blood purine measurements in clinical practice.

Elevated blood purine levels as a biomarker of seizures and epilepsy

OBJECTIVE

There is a major unmet need for a molecular biomarker of seizures or epilepsy that lends itself to fast, affordable detection in an easy-to-use point-of-care device. Purines such as adenosine triphosphate and adenosine are potent neuromodulators released during excessive neuronal activity that are also present in biofluids. Their biomarker potential for seizures and epilepsy in peripheral blood has, however, not yet been investigated. The aim of the present study was to determine whether blood purine nucleoside measurements can serve as a biomarker for the recent occurrence of seizures and to support the diagnosis of epilepsy.

METHODS

Blood purine concentrations were measured via a point-of-care diagnostic technology based on the summated electrochemical detection of adenosine and adenosine breakdown products (inosine, hypoxanthine, and xanthine; SMARTChip). Measurements of blood purine concentrations were carried out using samples from mice subjected to intra-amygdala kainic acid-induced status epilepticus and in video-electroencephalogram (EEG)-monitored adult patients with epilepsy.

RESULTS

In mice, blood purine concentrations were rapidly increased approximately two- to threefold after status epilepticus (2.32 ± .40 µmol·L^-1 [control] vs. 8.93 ± 1.03 µmol·L^-1 [after status epilepticus]), and levels correlated with seizure burden and postseizure neurodegeneration in the hippocampus. Blood purine concentrations were also elevated in patients with video-EEG-diagnosed epilepsy (2.39 ± .34 µmol·L^-1 [control, n = 13] vs. 4.35 ± .38 µmol·L^-1 [epilepsy, n = 26]).

SIGNIFICANCE

Our data provide proof of concept that the measurement of blood purine concentrations may offer a rapid, low-volume bedside test to support the diagnosis of seizures and epilepsy.

Phosphorus spectroscopy in acute TBI demonstrates metabolic changes that relate to outcome in the presence of normal structural MRI

Metabolic dysfunction is a key pathophysiological process in the acute phase of traumatic brain injury (TBI). Although changes in brain glucose metabolism and extracellular lactate/pyruvate ratio are well known, it was hitherto unknown whether these translate to downstream changes in ATP metabolism and intracellular pH. We have performed the first clinical voxel-based in vivo phosphorus magnetic resonance spectroscopy (³¹P MRS) in 13 acute-phase major TBI patients versus 10 healthy controls (HCs), at 3T, focusing on eight central 2.5 × 2.5 × 2.5 cm³ voxels per subject. PCr/γATP ratio (a measure of energy status) in TBI patients was significantly higher (median = 1.09) than that of HCs (median = 0.93) (p < 0.0001), due to changes in both PCr and ATP. There was no significant difference in PCr/γATP between TBI patients with favourable and unfavourable outcome. Cerebral intracellular pH of TBI patients was significantly higher (median = 7.04) than that of HCs (median = 7.00) (p = 0.04). Alkalosis was limited to patients with unfavourable outcome (median = 7.07) (p < 0.0001). These changes persisted after excluding voxels with > 5% radiologically visible injury. This is the first clinical demonstration of brain alkalosis and elevated PCr/γATP ratio acutely after major TBI. ³¹P MRS has potential for non-invasively assessing brain injury in the absence of structural injury, predicting outcome and monitoring therapy response.

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.

Purine nucleoside use as surrogate markers of cerebral ischaemia during local and general anaesthetic carotid endarterectomy

Objectives:

In periods of cerebral ischaemia, adenosine triphosphate is metabolised, leading to accumulation of adenosine inosine and hypoxanthine. These can be measured in real time using peripheral blood samples intraoperatively. The primary aim of this study was to describe changes in purine concentrations in a cohort of patients undergoing carotid endarterectomy under general anaesthetic, and to evaluate correlation between changes in values with major perioperative steps. The secondary aim was to compare changes in concentrations with a previous cohort of patients who had undergone carotid endarterectomy under local anaesthetic.

Methods:

This was a prospective observational study. Purine concentrations were determined from arterial line samples and measured via an amperometric biosensor at specific time points during carotid endarterectomy. Mean arterial pressure was manipulated to maintain steady cerebral perfusion pressure throughout the procedure. These results were analysed against data from a cohort of patients who underwent carotid endarterectomy under local anaesthetic in previously published work.

Results:

Valid results were obtained for 37 patients. Purine concentrations at baseline were 3.02 ± 1.11 µM and 3.16 ± 1.85 µM for the unshunted and shunted cohorts, respectively. There was no significant change after 30 min of carotid clamping at 2.07 ± 0.89 and 2.4 ± 3.09 µM, respectively (both p > 0.05). Peak purine during the clamp phase in the loco-regional anaesthetic cohort was 6.70 ± 3.4 µM, which was significantly raised compared to both general anaesthetic cohorts (p = 0.004). There were no perioperative neurological events in either cohort.

Conclusion:

This small study does not demonstrate conclusive evidence that purine nucleosides can be used as a marker of cerebral ischaemia; the comparisons to the loco-regional anaesthetic data offer information about differences in the cerebral adenosine triphosphate metabolism between general anaesthetic and loco-regional anaesthetic. We hypothesise that the lack of a rise in purine nucleosides under general anaesthetic may be caused by a decrease in the cerebral metabolic rate and loss of metabolic rate-blood flow coupling caused by general anaesthetic agents.

Point-of-care measurements reveal release of purines into venous blood of stroke patients

Stroke is a leading cause of death and disability. Here, we examine whether point-of-care measurement of the purines, adenosine, inosine and hypoxanthine, which are downstream metabolites of ATP, has potential to assist the diagnosis of stroke. In a prospective observational study, patients who were suspected of having had a stroke, within 4.5 h of symptom onset and still displaying focal neurological symptoms at admission, were recruited. Clinical research staff in the Emergency Departments of two hospitals used a prototype biosensor array, SMARTCap, to measure the purines in the venous blood of stroke patients and healthy controls. In controls, the baseline purines were 7.1 ± (SD) 4.2 μM (n = 52), while in stroke patients, they were 11.6 ± 8.9 μM (n = 76). Using the National Institutes for Stoke Scale (NIHSS) to band the severity of stroke, we found that minor, moderate and severe strokes all gave significant elevation of blood purines above the controls. The purine levels fall over 24 h. This was most marked for patients with haemorrhagic strokes (5.1 ± 3.6 μM, n = 9 after 24 h). The purine levels measured on admission show a significant correlation with the volume of affected brain tissue determined by medical imaging in patients who had not received thrombolysis or mechanical thrombectomy.

ClinicalTrials.gov Identifier: NCT02308605

The Purine Salvage Pathway and the Restoration of Cerebral ATP: Implications for Brain Slice Physiology and Brain Injury

Brain slices have been the workhorse for many neuroscience labs since the pioneering work of Henry McIlwain in the 1950s. Their utility is undisputed and their acceptance as appropriate models for the central nervous system is widespread, if not universal. However, the skeleton in the closet is that ATP levels in brain slices are lower than those found in vivo, which may have important implications for cellular physiology and plasticity. Far from this being a disadvantage, the ATP-impoverished slice can serve as a useful and experimentally-tractable surrogate for the injured brain, which experiences similar depletion of cellular ATP. We have shown that the restoration of cellular ATP in brain slices to in vivo values is possible with a simple combination of D-ribose and adenine (RibAde), two substrates for ATP synthesis. Restoration of ATP in slices to physiological levels has implications for synaptic transmission and plasticity, whilst in the injured brain in vivo RibAde shows encouraging positive results. Given that ribose, adenine, and a third compound, allopurinol, are all separately in use in man, their combined application after acute brain injury, in accelerating ATP synthesis and increasing the reservoir of the neuroprotective metabolite, adenosine, may help reduce the morbidity associated with stroke and traumatic brain injury.

The combination of ribose and adenine promotes adenosine release and attenuates the intensity and frequency of epileptiform activity in hippocampal slices: Evidence for the rapid depletion of cellular ATP during electrographic seizures

In addition to being the universal cellular energy source, ATP is the primary reservoir for the neuromodulator adenosine. Consequently, adenosine is produced during ATP-depleting conditions, such as epileptic seizures, during which adenosine acts as an anticonvulsant to terminate seizure activity and raise the threshold for subsequent seizures. These actions protect neurones from excessive ionic fluxes and hence preserve the remaining cellular content of ATP. We have investigated the consequences of manipulation of intracellular ATP levels on adenosine release and epileptiform activity in hippocampal slices by pre-incubating slices (3 h) with creatine (1 mM) and the combination of ribose (1 mM) and adenine (50 μM; RibAde). Creatine buffers and protects the concentration of cellular ATP, whereas RibAde restores the reduced cellular ATP in brain slices to near physiological levels. Using electrophysiological recordings and microelectrode biosensors for adenosine, we find that, while having no effect on basal synaptic transmission or paired-pulse facilitation, pre-incubation with creatine reduced adenosine release during Mg2+- free/4-aminopyridine-induced electrographic seizure activity, whereas RibAde increased adenosine release. This increased release of adenosine was associated with an attenuation of both the intensity and frequency of seizure activity. Given the depletion of ATP after injury to the brain, the propensity for seizures after trauma and the risk of epileptogenesis, therapeutic strategies elevating the cellular reservoir of adenosine may have value in the traumatized brain. Ribose and adenine are both in use in man and thus their combination merits consideration as a potential therapeutic for the acutely injured central nervous system.

Blood purine measurements as a rapid real-time indicator of reversible brain ischaemia

To preserve the disequilibrium between ATP and ADP necessary to drive cellular metabolism, enzymatic pathways rapidly convert ADP to adenosine and the downstream purines inosine and hypoxanthine. During ischaemia, these same pathways result in the production of purines. We performed a prospective observational study to test whether purine levels in arterial blood might correlate with brain ischaemia. We made real-time perioperative measurements, via microelectrode biosensors, of the purine levels in untreated arterial blood from 18 patients undergoing regional anaesthetic carotid endarterectomy. Pre-operatively, the median purine level was 2.4 μM (95% CI 1.3-4.0 μM); during the cross-clamp phase, the purines rose to 6.7 μM (95% CI 4.7-11.5 μM) and fell back to 1.9 μM (95% CI 1.4-2.7 μM) in recovery. Three patients became unconscious during carotid clamping, necessitating insertion of a temporary carotid shunt to restore cerebral blood flow. In these, the pre-operative median purine level was 5.4 μM (range 4.7-6.1 μM), on clamping, 9.6 μM (range 9.4-16.1 μM); during shunting, purines fell to below the pre-operative level (1.4 μM, range 0.4-2.9 μM) and in recovery 1.8 μM (range 1.8-2.6 μM). Our results suggest that blood purines may be a sensitive real-time and rapidly produced indicator of brain ischaemia, even when there is no accompanying neurological obtundation.

Proof of concept and feasibility studies examining the influence of combination ribose, adenine and allopurinol treatment on stroke outcome in the rat

Background

Cerebral ischaemia results in a rapid and profound depletion of adenosine triphosphate (ATP), the energy currency of the cell. This depletion leads to disruption of cellular homeostasis and cell death. Early replenishment of ATP levels might therefore have a neuroprotective effect in the injured brain. We have previously shown that the ATP precursors, D-ribose and adenine (RibAde), restored the reduced ATP levels in rat brain slices to values similar to those measured in the intact rodent brain. The aim of this study was to assess whether RibAde, either alone or in combination with the xanthine oxidase inhibitor allopurinol (RibAdeAll; to further increase the availability of ATP precursors), could improve outcome in an in vivo rodent model of transient cerebral ischaemia.

Methods

After 60 min occlusion of the middle cerebral artery, and upon reperfusion, rats were administered saline, RibAde, or RibAdeAll for 6 h. Baseline lesion volume was determined by diffusion-weighted MRI prior to reperfusion and final infarct volume determined by T2-weighted MRI at Day 7. Neurological function was assessed at Days 1, 3 and 7.

Results

Ischaemic lesion volume decreased between Days 1 and 7: a 50% reduction was observed for the RibAdeAll group, 38% for the RibAde group and 18% in the animals that received saline. Reductions in lesion size in treatment groups were accompanied by a trend for faster functional recovery.

Conclusion

These data support the potential use of ribose, adenine and allopurinol in the treatment of cerebral ischaemic injury, especially since all compounds have been used in man.

Thoracotomy in the emergency department for resuscitation of the mortally injured

PURPOSE

Emergency department resuscitative thoracotomy is an intervention of last resort for the acutely dying victim of trauma. In light of improvements in pre-hospital emergency systems, improved operative strategies for survival such as damage control and improvements in critical care medicine, the most extreme of resuscitation efforts should be re-evaluated for the potential survivor, with success properly defined as the return of vital signs which allow transport of the patient to the operating room.

METHODS

A retrospective review of all patients at a suburban level I trauma center who underwent emergency department resuscitative thoracotomy as an adjunct to the resuscitation efforts normally delivered in the trauma receiving area over a 22 year period was performed. Survival of emergency department resuscitative thoracotomy was defined as restoration of vital signs and transport out of the trauma resuscitation area to the operating room.

RESULTS

Sixty-eight patients were identified, of whom 27 survived the emergency department resuscitative thoracotomy and were transported to the operating room. Review of pre-hospital and initial hospital data between these potential long term survivors and those who died in the emergency department failed to demonstrate trends which were predictive of survival of emergency department resuscitative thoracotomy. The only subgroup which failed to respond to emergency department resuscitative thoracotomy was patients without signs of life at the scene who arrived to the treatment facility without signs of life.

CONCLUSION

The patient population of the "potential survivor" has been expanded due to advances in critical care practices, technology, and surgical technique and every opportunity for survival should be provided at the outset. Emergency department resuscitative thoracotomy is warranted for any patient with thoracic or subdiaphragmatic trauma who presents in extremis with a history of signs of life at the scene or organized cardiac activity upon arrival. Patients who have no evidence of signs of life at the scene and have no organized cardiac activity upon arrival should be pronounced.

Real-time monitoring of extracellular adenosine using enzyme-linked microelectrode arrays

Throughout the central nervous system extracellular adenosine serves important neuroprotective and neuromodulatory functions. However, current understanding of the in vivo regulation and effects of adenosine is limited by the spatial and temporal resolution of available measurement techniques. Here, we describe an enzyme-linked microelectrode array (MEA) with high spatial (7500 µm(2)) and temporal (4 Hz) resolution that can selectively measure extracellular adenosine through the use of self-referenced coating scheme that accounts for interfering substances and the enzymatic breakdown products of adenosine. In vitro, the MEAs selectively measured adenosine in a linear fashion (r(2)=0.98±0.01, concentration range=0-15 µM, limit of detection =0.96±0.5 µM). In vivo the limit of detection was 0.04±0.02 µM, which permitted real-time monitoring of the basal extracellular concentration in rat cerebral cortex (4.3±1.5 µM). Local cortical injection of adenosine through a micropipette produced dose-dependent transient increases in the measured extracellular concentration (200 nL: 6.8±1.8 µM; 400 nL: 19.4±5.3 µM) [P<0.001]. Lastly, local injection of dipyridamole, which inhibits transport of adenosine through equilibrative nucleoside transporter, raised the measured extracellular concentration of adenosine by 120% (5.6→12.3 µM) [P<0.001]. These studies demonstrate that MEAs can selectively measure adenosine on temporal and spatial scales relevant to adenosine signaling and regulation in normal and pathologic states.

Modulation of intracellular ATP determines adenosine release and functional outcome in response to metabolic stress in rat hippocampal slices and cerebellar granule cells

Cerebral ischaemia rapidly depletes cellular ATP. Whilst this deprives brain tissue of a valuable energy source, the concomitant production of adenosine mitigates the damaging effects of energy failure by suppressing neuronal activity. However, the production of adenosine and other metabolites, and their loss across the blood-brain barrier, deprives the brain of substrates for the purine salvage pathway, the primary means by which the brain makes ATP. Because of this, cerebral ATP levels remain depressed after brain injury. To test whether manipulating cellular ATP levels in brain tissue could affect functional neuronal outcomes in response to oxygen/glucose deprivation (OGD), we examined the effects of creatine and d-ribose and adenine (RibAde). In hippocampal slices creatine delayed ATP breakdown, reduced adenosine release, retarded both the depression of synaptic transmission and the anoxic depolarization caused by OGD, and improved the recovery of transmission. In contrast, RibAde increased cellular ATP, caused increased OGD-induced adenosine release and accelerated the depression of synaptic transmission, but did not improve functional recovery. However, RibAde improved the viability of cerebellar granule cells when administered after OGD. Our data indicate that RibAde may be effective in promoting recovery of brain tissue after injury, potentially via enhancement of salvage-mediated ATP production.

Measurement of purine release with microelectrode biosensors

Purinergic signalling departs from traditional paradigms of neurotransmission in the variety of release mechanisms and routes of production of extracellular ATP and adenosine. Direct real-time measurements of these purinergic agents have been of great value in understanding the functional roles of this signalling system in a number of diverse contexts. Here, we review the methods for measuring purine release, introduce the concept of microelectrode biosensors for ATP and adenosine and explain how these have been used to provide new mechanistic insight in respiratory chemoreception, synaptic physiology, eye development and purine salvage. We finish by considering the association of purine release with pathological conditions and examine the possibilities that biosensors for purines may one day be a standard part of the clinical diagnostic tool chest.

Endogenous adenosine A1 receptor activation underlies the transient post-ischemic rhythmic delta EEG activity

OBJECTIVE

Emergence of slow EEG rhythms within the delta frequency band following an ischemic insult of the brain has long been considered a marker of irreversible anatomical damage. Here we investigated whether ischemic adenosine release and subsequent functional inhibition via the adenosine A(1) receptor (A(1)R) contributes to post-ischemic delta activity.

METHODS

Rats were subjected to episodes of non-injuring transient global cerebral ischemia (GCI) under chloral hydrate anesthesia.

RESULTS

We found that a GCI lasting only 10s was enough to induce a brief discharge of rhythmic delta activity (RDA) with a peak frequency just below 1 Hz quantified as an increase by twofold of the 0.5-1.5 Hz spectral power. This post-ischemic RDA did not occur following administration of the A(1)R antagonist 8-cyclopentyl-1,3-dipropylxanthine. Nevertheless, a similar RDA could be induced in rats not subjected to GCI, by systemic administration of the A(1)R agonist N(6)-cyclopentyladenosine.

CONCLUSIONS

Our data suggest that A(1)R activation at levels that occur following cerebral ischemia underlies the transient post-ischemic RDA.

SIGNIFICANCE

It is likely that the functional, thus potentially reversible, synaptic disconnection by A(1)R activation promotes slow oscillations in the cortical networks. This should be accounted for in the interpretation of early post-ischemic EEG delta activity.

Release of adenosine and ATP during ischemia and epilepsy

Eighty years ago Drury & Szent-Györgyi described the actions of adenosine, AMP (adenylic acid) and ATP (pyrophosphoric or diphosphoric ester of adenylic acid) on the mammalian cardiovascular system, skeletal muscle, intestinal and urinary systems. Since then considerable insight has been gleaned on the means by which these compounds act, not least of which in the distinction between the two broad classes of their respective receptors, with their many subtypes, and the ensuing diversity in cellular consequences their activation invokes. These myriad actions are of course predicated on the release of the purines into the extracellular milieu, but, surprisingly, there is still considerable ambiguity as to how this occurs in various physiological and pathophysiological conditions. In this review we summarise the release of ATP and adenosine during seizures and cerebral ischemia and discuss mechanisms by which the purines adenosine and ATP may be released from cells in the CNS under these conditions.

The accuracy of jugular bulb venous monitoring in detecting cerebral ischemia in awake patients undergoing carotid endarterectomy

To investigate the accuracy of jugular bulb venous monitoring in detecting cerebral ischemia, we performed ipsilateral jugular bulb venous monitoring in 48 patients undergoing carotid surgery under regional anesthesia. Cerebral ischemia was assumed when neurologic deterioration occurred. During carotid clamping, the maximal arterial-jugular venous oxygen content difference [AJDO2 (max)], the minimal jugular venous oxygen saturation [SjO2 (min)], the maximal arterial-jugular venous lactate content difference [AJDL (max)], the maximal lactate oxygen index [LOI (max)], and the maximal modified LOI [mLOI (max)] were determined. To quantify the selectivity of each parameter, we performed receiver operating characteristic analysis and determined the area under the curve. The cutoff points providing the highest accuracy and the corresponding sensitivity (Se) and specificity (Spec) were determined. Neurologic deterioration occurred in 12 patients. All parameters, except AJDO2 (max), showed significant ability to distinguish between ischemic and nonischemic patients. The area under the curve for AJDL (max) was 0.840, for SjO2 (min) 0.766, for LOI 0.745, for mLOI 0.748, and for AJDO2 (max) 0.672. We found cutoff points of > or =0.16 mmol/L for AJDL (max) (Se=67%; Spec=86%) and < or =55% for SjO2 (Se=75%; Spec=83%). In conclusion, the present investigation shows that AJDL, SjO2, LOI, and mLOI provide the ability to detect cerebral hypoperfusion. The highest accuracy was found for AJDL. Neither the calculation of LOI nor of mLOI showed improved results.

Early temporal changes in ecto-nucleotidase activity after cortical stab injury in rat

During a variety of insults to the brain adenine nucleotides are released in large quantities from damaged cells, triggering multiple cellular responses to injury. Here, we evaluated changes in extracellular ATP, ADP and AMP hydrolysis at different times (0-24 hours) after unilateral cortical stab injury (CSI) in adult rats. Results demonstrated that 24 hours following CSI, ATP and ADP hydrolyzing activities were not significantly altered in injured cortex. Based on calculated V (ATP)/V (ADP) ratio it was concluded that ATP/ADP hydrolysis was primarily catalyzed by NTPDase1 enzyme form. In contrast, AMP hydrolysis, catalyzed by 5'-nucleotidase, was significantly reduced at least 4 hours following CSI. Kinetic analysis and Lineweaver-Burk transformation of the enzyme velocities obtained over the range of AMP concentrations (0.05-1.50 mM) revealed that inhibition of 5'-nucleotidase activity after CSI was of the uncompetitive type. Taken together our data suggest that injured tissue has reduced potential for extracellular metabolism of adenine nucleotides in early stages after CSI.

EFFECTS OF ADENOSINE ON FUNCTIONS OF POLYMORPHONUCLEAR LEUKOCYTES FROM PATIENTS WITH SEPTIC SHOCK

Inasmuch as polymorphonuclear leukocytes (PMNs) play a major role in antibacterial defense but can also cause substantial tissue injury, drugs are needed which are able to attenuate tissue-toxic PMN reactions without inhibiting bactericidal mechanisms. Adenosine as a retaliatory metabolite is produced in response to metabolically unfavorable conditions like inflammation. However, it is not known whether adenosine can selectively downregulate adverse PMN reactions in sepsis. In this prospective clinical study, we characterized the effects of adenosine ex vivo on PMN functions in patients with septic shock ([SS] n = 33) and healthy volunteers ([HV] n = 33). The PMNs were primed by tumor necrosis factor-alpha (TNF-alpha) and subsequently stimulated with N-formyl methionyl-leucyl-phenylalanine (fMLP) to test for the formation of hydrogen peroxide (H2O2) in response to soluble inflammatory stimuli. The PMNs were also challenged by opsonized zymosan particles to assess adhesion, phagocytosis, and the associated H2O2 production. As compared with HV, PMNs from SS patients showed strongly enhanced tissue-toxic H2O2 production elicited by TNF-alpha/fMLP. Increasing concentrations of adenosine dose-dependently reduced this tissue-toxic H2O2 production in both groups with a half-maximal inhibitory concentration of 25 nmol/L and 114 nmol/L in HV and SS patients, respectively. This 4.6-fold decrease in the adenosine-mediated inhibition of PMNs from patients with septic shock was compensated by a 3-fold increase in the plasma concentrations of the nucleoside (HV, 42.5 +/- 2.9 nmol/L vs. SS, 125.6 +/- 18.2 nmol/L; mean +/- SEM). When the effects of adenosine were tested at a very high A2A receptor saturating concentration of 10 mol/L, neither adhesion, phagocytosis, nor the associated H2O2 production induced by opsonized zymosan was affected in both groups. These results were confirmed by the highly selective A2A agonist, CGS21680.Thus, adenosine or A2A agonists may be useful to selectively inhibit the potentially tissue-toxic H2O2 production elicited by soluble inflammatory mediators in patients with septic shock.

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.

Delayed ischemic electrocortical suppression during rapid repeated cerebral ischemia and kainate-induced seizures in rat

Global cerebral ischemia induces, within seconds, suppression of spontaneous electrocortical activity, partly due to alterations in synaptic transmission. In vitro studies have found that repeated brief hypoxic episodes prolong the persistence of synaptic transmission due to weakened adenosine release. The aim of this study was to investigate in vivo whether the time to ischemic electrocortical suppression (T(ES)) could be altered during energy stress conditions such as rapid repeated global cerebral ischemia and kainate-induced seizures. Experiments were carried out in adult rats under chloral hydrate anaesthesia. Repeated episodes of 1 min of ischemia were induced by transiently clamping the carotid arteries in a 'four-vessel occlusion' model. We devised an automatic method of T(ES) estimation based on the decay of the root mean square of two-channel electrocorticographic recordings. To distinguish the alterations in spontaneous electrocortical activity we compared T(ES) with the ischemic suppression of visual evoked potentials (VEP). During the first ischemic episode, T(ES) was approximately 15 s and remained unchanged when five ischemic episodes were separated by 10-min reperfusion intervals. When ischemia was repeated after 2 min of reperfusion T(ES) progressively increased, reaching a plateau value of approximately 24 s. A similar plateau was reached during kainate-induced seizures. The T(ES) plateau occurred prior to ischemic suppression of VEP. Our data suggest that, under conditions of acute metabolic stress in vivo, the ischemic suppression of spontaneous electrocortical activity may be delayed up to a plateau value. These findings are consistent with the hypothesis of a depletable adenosine pool; however, the restoration of synaptic transmission may be faster in vivo than in vitro.

Rapid measurement of S-100B serum protein levels by Elecsys S100 immunoassay in patients undergoing carotid artery stenting or endarterectomy

OBJECTIVES

This study was designed to apply the rapid Elecsys S100 immunoassay for real-time measurement of S100 protein serum levels indicating acute brain damage in patients undergoing carotid artery stenting (CAS) or endarterectomy (CEA).

DESIGN AND METHODS

Data of 14 CAS patients were compared to those of 43 CEA and 14 control patients undergoing coronary angiography (CA). S100 serum levels were measured by the full-automatic Elecsys S100 immunoassay and compared to those obtained by the well-established LIA-mat S100 system.

RESULTS

In contrast to CAS and CA patients, median S100 serum levels of CEA patients significantly increased to 0.24 ng/mL before declamping, but subsequently returned to baseline. Three CEA patients with neurological deficits showed sustained elevated S100 levels 6 h after extubation. Absolute S100 values were not significantly different between the two methods. Bland-Altman plot analyses displayed a good agreement, mostly indicating slightly smaller values applying the Elecsys S100 system.

CONCLUSIONS

The Elecsys S100 system appears to be suitable for rapid real-time detection of neurological deficits in patients undergoing CAS and CEA. Persistent elevations of Elecsys S100 levels during CEA were associated with prolonged neurological disorders, whereas transient increases seem to represent impaired blood-brain barrier integrity without neurological deficits.

Plasticity of purine release during cerebral ischemia: clinical implications?

Adenosine is a powerful modulator of neuronal function in the mammalian central nervous system. During a variety of insults to the brain, adenosine is released in large quantities and exerts a neuroprotective influence largely via the A(1) receptor, which inhibits glutamate release and neuronal activity. Using novel enzyme-based adenosine sensors, which allow high spatial and temporal resolution recordings of adenosine release in real time, we have investigated the release of adenosine during hypoxia/ischemia in the in vitro hippocampus. Our data reveal that during the early stages of hypoxia adenosine is likely released per se and not as a precursor such as cAMP or an adenine nucleotide. In addition, repeated hypoxia results in reduced production of extracellular adenosine and this may underlie the increased vulnerability of the mammalian brain to repetitive or secondary hypoxia/ischemia.

Systemic hypoxia and the depression of synaptic transmission in rat hippocampus after carotid artery occlusion

The relationship between step reductions in inspired oxygen and the amplitude of evoked field excitatory postsynaptic potentials (fEPSPs) recorded from hippocampal CA1 neurons was examined in anaesthetized rats with a unilateral common carotid artery occlusion. The amplitudes of fEPSPs recorded from the hippocampus ipsilateral to the occlusion were significantly more depressed with hypoxia than were the fEPSPs recorded from the contralateral hippocampus. The adenosine A1-selective antagonist, 8-cyclopentyl-1,3-dimethylxanthine (8-CPT), blunted the hypoxic depression of the fEPSP. Tissue partial pressure of oxygen (Ptiss,O2) was measured in the ipsilateral and contralateral hippocampus using glass Clark-style microelectrodes. Ptiss,O2 fell to similar levels as a function of inspired oxygen in the ipsilateral and contralateral hippocampus, and in the ipsilateral hippocampus after administration of 8-CPT. Hippocampal blood flow (HBF) was measured using laser Doppler flowmetry. A decline in HBF was associated with systemic hypoxia in both hippocampi. HBF, as a function of inspired oxygen, fell significantly more in the ipsilateral than in the contralateral hippocampus. We conclude that endogenous adenosine acting at the neuronal A1 receptor plays a major role in the depression of synaptic transmission during hypoxic ischaemia. The greater susceptibility of the fEPSP in the ipsilateral hippocampus to systemic hypoxia cannot be explained entirely by differences in Ptiss,O2 or HBF between the two hemispheres.

The effect of acute hypoxemia and hypotension on adenosine-mediated depression of evoked hippocampal synaptic transmission

The present study was designed to investigate the relative contributions of arterial P(O(2)), local cerebral blood flow, and oxygen delivery to the adenosine A(1) receptor-mediated depression of evoked synaptic transmission recorded in the rat hippocampus. Urethane-anesthetized rats were given a unilateral common carotid artery occlusion and then placed in a stereotaxic apparatus for stimulation and recording of bilateral hippocampal field excitatory postsynaptic potentials (fEPSPs). Arterial blood gases, mean arterial blood pressure (MAP), and bilateral hippocampal blood flow (HBF) were also measured. Arterial P(O(2)), HBF, and oxygen delivery were manipulated using normoxic hypotension, hypoxic hypotension, and hypoxic normotension. Both hypoxic hypotension and normoxic hypotension resulted in decreased HBF, decreased oxygen delivery, and a depression of the evoked fEPSP limited to the hippocampus ipsilateral to the occlusion. The enhanced HBF and oxygen delivery associated with increased MAP resulted in a restoration and maintenance of hippocampal fEPSPs despite sustained hypoxemia. The adenosine A(1) receptor-mediated depression of the fEPSP was more strongly correlated with changes in HBF and oxygen delivery than with arterial P(O(2)). We propose that adenosine plays an important role mediating the depression of neuronal activity associated with reduced oxygen delivery characteristically observed in ischemic brain tissue.

A depletable pool of adenosine in area CA1 of the rat hippocampus

Adenosine plays a major modulatory and neuroprotective role in the mammalian CNS. During cerebral metabolic stress, such as hypoxia or ischemia, the increase in extracellular adenosine inhibits excitatory synaptic transmission onto vulnerable neurons via presynaptic adenosine A(1) receptors, thereby reducing the activation of postsynaptic glutamate receptors. Using a combination of extracellular and whole-cell recordings in the CA1 region of hippocampal slices from 12- to 24-d-old rats, we have found that this protective depression of synaptic transmission weakens with repeated exposure to hypoxia, thereby allowing potentially damaging excitation to both persist for longer during oxygen deprivation and recover more rapidly on reoxygenation. This phenomenon is unlikely to involve A(1) receptor desensitization or impaired nucleoside transport. Instead, by using the selective A(1) antagonist 8-cyclopentyl-1,3-dipropylxanthine and a novel adenosine sensor, we demonstrate that adenosine production is reduced with repeated episodes of hypoxia. Furthermore, this adenosine depletion can be reversed at least partially either by the application of exogenous adenosine, but not by a stable A(1) agonist, N(6)-cyclopentyladenosine, or by endogenous means by prolonged (2 hr) recovery between hypoxic episodes. Given the vital neuroprotective role of adenosine, these findings suggest that depletion of adenosine may underlie the increased neuronal vulnerability to repetitive or secondary hypoxia/ischemia in cerebrovascular disease and head injury.

Adenosine in the treatment of stroke: yes, maybe, or absolutely not?

Agonist stimulation of adenosine A(1) receptors has been consistently shown to result in reduction of brain damage following experimentally induced global and focal brain ischaemia in animals. Unsurprisingly, the use of adenosine A(1) receptors as targets for the development of clinical therapeutics suitable for treatment of ischaemic brain disorders has been suggested by many authors. The latest studies of adenosine and its receptors indicate that adenosine-mediated actions might be far more complex than originally anticipated, casting some doubt about the rapid development of stroke treatment based on adenosine. This review discusses the possible role of adenosine receptor subtypes (A(1), A(2) and A(3)) in the context of their potential as therapeutics in stroke.