Interstitial purine metabolites and lactate during regional myocardial hypoxia.

OBJECTIVE: Adenosine is a well known vasodilator believed to contribute to metabolic adjustments of the coronary circulation. The purpose of this study was to assess changes in interstitial fluid adenosine, adenosine metabolites, and lactate during prolonged regional, non-ischaemic myocardial hypoxia. METHODS: To induce regional hypoxia, the left anterior descending coronary artery of anaesthetised dogs (n = 9) was perfused at constant pressure (100 mm Hg) with deoxygenated blood (PO2 approximately 2.6 kPa) for 60 min via an extracorporeal shunt. Cardiac interstitial fluid was sampled by cardiac microdialysis, using dialysate metabolite levels as indices of interstitial fluid concentrations. RESULTS: During hypoxia, coronary blood flow increased 3.9-fold, while myocardial oxygen consumption was maintained relatively constant. There were no changes in global cardiac function, systemic arterial pressure, or heart rate during regional hypoxia, indicating that the hypoxic stimulus did not augment sympathetic nervous system activity. Dialysate adenosine was not increased at any point of the hypoxic period, but was decreased by 25 min hypoxia. Dialysate levels of inosine, hypoxanthine, and xanthine were increased transiently during the first 10 min of hypoxia while there was a sustained increase in dialysate lactate. In the presence of erythro-9-(2-hydroxy-3-nonyl) adenine, an adenosine deaminase inhibitor, adenosine was the predominant purine metabolite and increased transiently during hypoxia. CONCLUSIONS: Flux through the adenosine production and degradation pathways is transiently increased during hypoxia. However, the lack of an increase in interstitial fluid adenosine does not support a role for adenosine in the sustained hyperaemic response to regional myocardial hypoxia.

Adenosine deaminase inhibition augments interstitial adenosine but does not attenuate myocardial infarction.

OBJECTIVE: The objectives were to determine the effects of the adenosine deaminase inhibitor pentostatin (deoxycoformycin) on interstitial fluid (ISF) adenosine before, during, and after myocardial ischaemia and to ascertain whether augmented endogenous ISF adenosine reduces myocardial infarction. METHODS: Untreated anaesthetised dogs (n = 11) were compared to dogs treated with intravenous pentostatin 30 min before ischaemia (0.2 mg.kg-1; n = 11). The changes in ISF adenosine, adenosine metabolites, and lactate were assessed by cardiac microdialysis, using dialysate concentrations as indices of ISF levels. Both groups were exposed to 60 min of regional myocardial ischaemia followed by 3 h of reperfusion. RESULTS: Although ISF adenosine increased during ischaemia in untreated animals, inosine and hypoxanthine were the predominant purine metabolites which accumulated in the ISF. Pentostatin increased dialysate adenosine 3.5-fold before ischaemia, resulted in a sustained and pronounced augmentation of adenosine during ischaemia, and maintained the raised ISF adenosine during early reperfusion. However, the augmentation of ISF adenosine was not associated with a reduction in infarct size [untreated = 33.3(SEM 4.8)% of the area at risk; pentostatin treated = 35.6(4.6)% of the area at risk], nor did pentostatin alter the ischaemia induced increase in ISF lactate. Plasma adenosine, as measured by a microdialysis probe in the femoral artery, increased in pentostatin treated animals upon reperfusion, leading to systemic hypotension, increased blood flow in the non-ischaemic region, and an attenuated reactive hyperaemia in the ischaemic region. CONCLUSIONS: Although inhibition of adenosine deaminase effectively enhances ISF adenosine before and during ischaemia, the increase before ischaemia does not "precondition" the myocardium, nor does the augmentation of adenosine during and after ischaemia attenuate necrosis in this model of ischaemia. Therefore, the enhancement of ISF adenosine to the extent provided by adenosine deaminase inhibition alone is not sufficient to protect the heart in the way seen with ischaemic preconditioning.

Changes in interstitial adenosine during hypoxia: relationship to oxygen supply:demand imbalance, and effects of adenosine deaminase.

OBJECTIVE: The aim was to determine the changes in coronary blood flow and intramyocardial interstitial fluid (ISF) adenosine and adenosine metabolites during systemic hypoxia, and to evaluate (1) whether the increase in ISF adenosine during hypoxia is augmented if the hypoxic hyperaemia is prevented, and (2) the effects of adenosine deaminase on ISF adenosine and coronary blood flow during sustained hypoxia. METHODS: Anaesthetised dogs were instrumented with a flow probe around the left anterior descending coronary artery to measure coronary blood flow and with a microdialysis probe in the myocardium perfused by this artery to sample intramyocardial ISF. Dialysate purine metabolite levels were used as indices of ISF levels. Systemic hypoxia was induced by a reduction in the FIO2. RESULTS: Acute systemic hypoxia (PaO2 approximately 3.9 kPa) resulted in a 60% increase in dialysate adenosine, along with increases in dialysate levels of the adenosine metabolites inosine (74%), hypoxanthine (33%), and xanthine (32%). If the hypoxic hyperaemia was prevented, dialysate adenosine increased by 180% during hypoxia, and the increases in adenosine metabolites were augmented as well. During sustained hypoxia, intracoronary administration of adenosine deaminase decreased dialysate adenosine below prehypoxia levels, but did not alter the hypoxic hyperaemia. CONCLUSIONS: While ISF adenosine is increased during acute systemic hypoxia and is increased in relation to the oxygen supply:demand imbalance, consistent with a role of adenosine in hypoxic hyperaemia in the heart, adenosine is not necessary for the maintenance of a sustained increase in coronary blood flow during hypoxia.

Dual cardiac microdialysis to assess drug-induced changes in interstitial purine metabolites: adenosine deaminase inhibition versus adenosine kinase inhibition.

OBJECTIVE: The purpose of this study was: 1) to evaluate a dual microdialysis technique coupled with local drug administration in the regionally ischemic rabbit heart, and; 2) to assess the ischemia-induced changes in interstitial fluid (ISF) adenosine during inhibition of adenosine deaminase or adenosine kinase. METHODS: Two microdialysis probes were implanted parallel to each other and separated by 5 mm in myocardium perfused by a branch of the left coronary artery. Probes were used to sample myocardial ISF and to deliver drugs locally to the myocardium; purine metabolite concentrations in the collected dialysate were used as indices of ISF levels. Three groups of pentobarbital-anesthetized rabbits were studied. In a control group (n = 6), both probes were perfused with Krebs-Henseleit buffer. In the second and third groups, one probe was perfused with buffer, whereas the other probe was perfused with buffer containing 1 mM erythro-2-(2-hydroxy-3-nonyl)adenine (EHNA) (n = 5), an adenosine deaminase inhibitor, or 10 microM iodotubercidin (n = 9), an adenosine kinase inhibitor. All animals were exposed to 30 min of regional myocardial ischemia followed by 60 min of reperfusion. RESULTS: In the control group, similar increases in dialysate purine metabolites during ischemia were observed in both probes. Locally administered EHNA increased dialysate adenosine prior to ischemia and decreased dialysate inosine and hypoxanthine. During ischemia, the increase in dialysate adenosine in the EHNA-perfused probe was markedly augmented, while the increases in inosine and hypoxanthine were attenuated. In contrast, local infusion of iodotubercidin did not alter dialysate purine metabolites before ischemia, but there was a modest augmentation of adenosine during ischemia. These data illustrate the feasibility of dual microdialysis for assessing the effect of locally administered compounds on interstitial metabolite concentration in the regionally ischemic rabbit heart. Furthermore, adenosine deaminase inhibition has a more profound adenosine augmenting effect than adenosine kinase inhibition in the rabbit heart.

Acadesine reduces myocardial infarct size by an adenosine mediated mechanism.

OBJECTIVE: The aim was to test the hypotheses that acadesine (1) augments endogenous interstitial fluid (ISF) adenosine during ischaemia, and (2) reduces infarct size by adenosine receptor mediated mechanisms. METHODS: To test these hypotheses, the left coronary artery of anaesthetised rabbits (n = 33) was occluded for 30 min and reperfused for 120 min. Acadesine (1 mg.kg-1.min-1 for 5 min, then 0.2 mg.kg-1.min-1) was infused intravenously beginning 30 min before coronary occlusion and ending 30 min after reperfusion. The area at risk was comparable in all groups, averaging 34.7 (SEM 2.2%) of the left ventricle. In separate studies (n = 22), estimates of ISF adenosine and adenosine metabolites were obtained by cardiac microdialysis. Although dialysate adenosine levels increased significantly in the area at risk during ischaemia in the untreated group [from 0.044(0.008) to 0.339(0.146) microM], acadesine did not significantly augment dialysate adenosine levels before or during ischaemia [preischaemia = 0.094(0.032) microM; ischaemia = 0.542(0.262) microM]. In addition, there was no significant difference in dialysate adenosine concentrations during the first 10 min of reperfusion, after which adenosine levels returned to baseline levels. A 2.5-fold large dose failed to increase interstitial fluid adenosine. However, the adenosine receptor blocker 8-p-sulphophenyltheophylline (SPT) in the presence of acadesine increased ISF adenosine fourfold. Acadesine significantly (P < 0.05) reduced infarct size [n = 8, 19.7(2.9)% of risk area] compared with the untreated group [n = 8, 29.4(1.3)%]. This infarct size reduction with acadesine was antagonised by SPT given during ischaemia-reperfusion [n = 8, 46.2(3.0)%] or only during reperfusion [n = 9, 42.7(2.6)%. CONCLUSIONS: Acadesine reduces infarct size by an adenosine mediated mechanism, but this cardioprotective action is not associated with significantly augmented interstitial fluid adenosine levels.

Myocardial interstitial purine metabolites and lactate with increased work in swine.

OBJECTIVE: Dobutamine stimulates the beta-receptors in the heart and increases myocardial blood flow and oxygen consumption 2-3-fold, similar to effects seen with exercise. The purpose of this study was to assess temporal changes in myocardial interstitial purine metabolites, adenosine monophosphate (AMP) and lactate during and following 30 min of dobutamine infusion. METHODS: Dobutamine (15 micrograms/kg/min) was infused via the jugular vein into 9 anesthetized, open-chest, domestic swine. Interstitial fluid was sampled with microdialysis probes placed in the midmyocardium. The effluent from the probes, referred to as the dialysate, was used to estimate myocardial interstitial purine metabolites, AMP, and lactate levels before, during, and following a dobutamine-induced increased work state. RESULTS: Dobutamine infusion resulted in a 77% increase in heart rate, a 258% increase in left ventricular dP/dt, a 208% increase in myocardial oxygen consumption, and a 155% increase in rate x pressure product. Myocardial blood flow was increased in the subepicardium, midmyocardium, and subendocardium by 207, 268, and 268%, respectively, compared to the control period. Neither coronary venous nor dialysate lactate concentrations changed throughout the protocol. Dialysate adenosine and AMP levels were both significantly elevated (P < 0.05) during the dobutamine period and fell back to control values during the recovery period. CONCLUSIONS: The dobutamine-induced increases in myocardial oxygen consumption, rate x pressure product, and blood flow, without an increase in coronary venous or interstitial lactate suggest that energy balance is maintained during dobutamine infusion. Thus an increase in myocardial work, in the absence of demand-induced ischemia, resulted in accumulation of adenosine and AMP in the interstitium.

Increases in interstitial adenosine and cerebral blood flow with inhibition of adenosine kinase and adenosine deaminase.

The purpose of this study was to determine the changes in interstitial fluid (ISF) adenosine and cerebral blood flow (CBF) during inhibition of adenosine kinase or adenosine deaminase. Brain microdialysis was used to (a) measure CBF (H2 clearance), (b) sample cerebral ISF, and (c) deliver drugs locally to the brain. Microdialysis probes were implanted bilaterally in the caudate nucleus of halothane-anesthetized rats (n = 11). One probe was perfused with artificial cerebrospinal fluid (CSF) containing iodotubercidin (IODO), an adenosine kinase inhibitor, while the other probe was perfused with erythro-2-(2-hydroxy-3-nonyl)adenine (EHNA), an adenosine deaminase inhibitor. Both probes were subsequently perfused with EHNA+IODO. Finally, 8-(p-sulfophenyl)theophylline (SPT), an adenosine receptor antagonist, was added to EHNA + IODO in one probe, while the other probe continued to receive only EHNA + IODO. CBF and dialysate adenosine levels increased with either EHNA or IODO; however, the increases were greater with IODO. EHNA + IODO further increased CBF and dialysate adenosine. The hyperemia observed with EHNA + IODO was abolished by adenosine receptor blockade. These data suggest that basal adenosine levels are influenced to a greater extent by adenosine kinase than by adenosine deaminase. In addition, the increased CBF observed with inhibition of adenosine metabolism and the attenuation of this vasodilatory response with adenosine receptor blockade support a role for adenosine in CBF regulation.

Attenuation of ischemia-induced extracellular adenosine accumulation by homocysteine.

The purpose of this study was to determine the effects of homocysteine, which consumes intracellular adenosine via formation of S-adenosylhomocysteine, on interstitial fluid (ISF) adenosine and cerebral blood flow (CBF) before, during, and after cerebral ischemia. Microdialysis probes, used to measure local CBF (H2 clearance) and to sample ISF, were implanted bilaterally into the caudate nucleus of halothane-anesthetized rats (n = 8). L-Homocysteine thiolactone was administered locally via one of the probes. Animals were exposed to 20 min of ischemia, induced by bilateral carotid occlusion plus hemorrhage to an arterial blood pressure of 50 mm Hg, followed by 60 min of reperfusion. Before ischemia, CBF and dialysate adenosine were decreased with homocysteine. During ischemia and early reperfusion, dialysate purine metabolites increased on both sides of the brain; however, the ischemia-induced increase in adenosine was attenuated on the side of local homocysteine. CBF was lower on the side of homocysteine throughout reperfusion. These data demonstrate that homocysteine (a) decreases basal ISF adenosine and CBF, (b) attenuates the increase in dialysate adenosine during ischemia, and (c) reduces hyperemia during early reperfusion.

Increased brain interstitial fluid adenosine concentration during hypoxia in newborn piglet.

The effects of arterial hypoxia on interstitial fluid adenosine concentrations were studied in the frontal cortex and thalamus by the brain dialysis technique and in CSF from the cisterna magna of the newborn piglet. Acute hypoxia (PaO2 = 20 +/- 1 mm Hg) increased the interstitial fluid adenosine concentrations significantly from 0.68 +/- 0.29 (SEM) to 1.60 +/- 0.35 microM in the frontal cortex and from 1.03 +/- 0.32 to 2.60 +/- 0.86 microM in the thalamus (n = 8). Interstitial fluid inosine and hypoxanthine also increased significantly during hypoxia. In separate groups of piglets, the adenosine concentration in the cisterna magna CSF under normoxic conditions was 0.04 +/- 0.01 microM (n = 5), which increased significantly to 0.17 +/- 0.04 microM (n = 6) with hypoxia (PaO2 = 4.7 +/- 1.2 mm Hg). Cisterna magna CSF inosine levels did not change significantly during the severe hypoxia. Adenosine concentrations found in the interstitial space and CSF of newborn piglets under normoxic and hypoxic conditions are within the vasodilator range. These results thus suggest that in the neonatal brain adenosine may play a role in regulating blood flow during hypoxia.

Increases in cerebral interstitial fluid adenosine concentration during hypoxia, local potassium infusion, and ischemia.

This study used the brain dialysis technique to test the hypothesis that the adenosine concentration of cerebral interstitial fluid increases during situations in which cerebral oxygen supply is inadequate for oxygen demand. Sealed 300-micron hollow dialysis fibers were implanted in the caudate nucleus of pentobarbital-anesthetized rats and perfused at 2 microliter/min with artificial cerebrospinal fluid. In vitro tests indicated the recovery of adenosine, inosine, and hypoxanthine from the external medium to be approximately 20% at 2 microliter/min and close to 100% at 0.1 microliter/min. Three in vivo interventions were tested: hypoxia/hypotension (PaO2 = 41.9 mm Hg; MABP = 42.8 mm Hg; n = 9), local potassium infusion (n = 4), and cerebral anoxia/ischemia (n = 10). These interventions produced 10-, 4-, and 30-fold increases in perfusate adenosine concentration, respectively, as well as increases in perfusate concentrations of inosine and hypoxanthine. A separate group of rats (n = 9) perfused at 0.1 microliter/min yielded estimates of cerebral interstitial fluid adenosine, inosine, and hypoxanthine concentrations of 1.26, 3.30, and 7.19 microM, respectively. These results are consistent with the adenosine hypothesis for the regulation of CBF.

Interstitial fluid adenosine and sagittal sinus blood flow during bicuculline-seizures in newborn piglets.

Changes of interstitial fluid adenosine concentrations and effects of O2 supply on interstitial fluid adenosine were studied by the brain dialysis technique in the frontal cortex of newborn piglets subjected to bicuculline-induced seizures. The O2 supply was changed globally by changing MABP and locally by varying PO2 in the artificial CSF perfusing the dialysis cannula. Sagittal sinus blood flow (SSBF), cerebrovascular resistance (CVR), and CMRO2 were also examined in the same animals. Seizures increased interstitial fluid adenosine 7.9-fold (p less than 0.05) when ictal MABP was maintained at preictal level and perfusate PO2 was 24 mm Hg (group 1, n = 6). Interstitial fluid adenosine increased 11.8-fold (p less than 0.05) during seizures associated with moderate systemic hypotension and the low perfusate PO2 (group 2, n = 6). By contrast, seizures increased interstitial fluid adenosine three-fold (p less than 0.05) when perfusate PO2 was increased to 182 mm Hg and ictal MABP was maintained at preictal level (group 3, n = 8). When ictal MABP was elevated from the preictal level and the perfusate was rich in oxygen, seizures failed to increase interstitial fluid adenosine (group 4, n = 7). In groups 1 and 3, the increase in interstitial fluid adenosine during seizures was associated with significant increases in SSBF and CMRO2, as well as significant decreases in CVR. These data suggest that the increase in O2 supply during seizures in piglets did not match completely the increase in O2 demand and resulted in enhanced release of adenosine into the interstitial space.

Adenosine receptor blockade augments interstitial fluid levels of excitatory amino acids during cerebral ischemia.

The excitotoxic hypothesis suggests that cerebral ischemic damage results in part from the accumulation of the excitatory and potentially toxic neurotransmitters glutamate and aspartate. Adenosine, which also increases during cerebral ischemia, is proposed to inhibit neurotransmitter release. The purpose of this study was to determine if adenosine receptor blockade exacerbates the accumulation of glutamate and aspartate during cerebral ischemia. Microdialysis probes, implanted bilaterally in the caudate nucleus of halothane-anesthetized rats, were used to (1) assess changes in interstitial fluid (ISF) glutamate, aspartate, adenosine, and adenosine metabolites; (2) measure local cerebral blood flow (H2 clearance); and (3) deliver 8-(p-sulfophenyl)theophylline (SPT), an adenosine receptor antagonist, locally to the brain. The probe on one side of the brain was perfused with artificial cerebrospinal fluid (CSF) containing 10(-3) M SPT, while the probe on the opposite side received only artificial CSF. Animals were exposed to 20 min of ischemia (carotid occlusion+arterial blood pressure = 50 mm Hg) followed by 60 min of reperfusion. Dialysate glutamate and aspartate increased during and after cerebral ischemia, but were increased to a greater extent in the presence of adenosine receptor blockade. Likewise, the increase in dialysate adenosine and adenosine metabolites was enhanced on the side of locally administered SPT. These data suggest that endogenous adenosine attenuates the accumulation of glutamate and aspartate during cerebral ischemia.

Brain interstitial adenosine and sagittal sinus blood flow during systemic hypotension in piglet.

We sampled, using the brain dialysis technique, interstitial fluid adenosine from the frontal cortex of newborn piglets subjected to hemorrhagic hypotension while measuring sagittal sinus blood flow, cerebrovascular resistance (CVR), and cerebral O2 delivery. In group 1 (n = 8), MABP was reduced in successive steps from 76 to 30 mm Hg with decrements of approximately 10 mm Hg. At 60 mm Hg, CVR decreased by 19% (p less than 0.001), but sagittal sinus blood flow and interstitial fluid adenosine remained unchanged. At 50 mm Hg, both sagittal sinus blood flow and CVR decreased by 19% (p less than 0.001) and interstitial fluid adenosine rose 4.7-fold (p less than 0.05). At 40 and 30 mm Hg, sagittal sinus blood flow decreased further but CVR remained steady, whereas interstitial fluid adenosine rose 10- and 16-fold, respectively. In group 2 (n = 7), an abrupt reduction of MABP from 80 to 47 mm Hg produced no change in sagittal sinus blood flow and a 29% decrease in CVR (p less than 0.01). Interstitial fluid adenosine increased twofold (p less than 0.01). In group 3 (n = 7), an abrupt reduction of MABP from 79 to 40 mm Hg decreased sagittal sinus blood flow and CVR by 24 and 30%, respectively (p less than 0.01). Interstitial fluid adenosine rose threefold (p less than 0.01). In groups 1, 2, and 3, the increases in interstitial fluid adenosine accompanied decreases in cerebral O2 delivery. In group 4 (n = 7), artificial CSF with a PO2 of 152 mm Hg was perfused through the brain dialysis cannula during graded hypotension.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of local infusion of adenosine and adenosine analogues on local cerebral blood flow.

The purpose of this study was to determine the effects of local infusion of adenosine (ADO) and non-metabolized ADO analogues on local cerebral blood flow (CBF) and interstitial fluid (ISF) ADO levels. The brain dialysis technique was used to (a) deliver drugs locally to brain tissue, (b) estimate cerebral ISF ADO levels, and (c) measure local CBF (hydrogen clearance). Dialysis probes were implanted bilaterally in the caudate nuclei of ketamine-anesthetized rats. The probe on one side was perfused with artificial CSF while the contralateral probe was perfused with artificial CSF containing ADO (n = 5), or the ADO agonists 2-chloroadenosine (2-CADO; n = 4) or 5'-N-ethylcarboxamide adenosine (NECA; n = 4). When ADO was included in the artificial CSF at 10(-5), 10(-4), or 10(-3) M, a 30% increase in local CBF was detected only with 10(-3) M ADO. During perfusion with ADO, dialysate inosine and hypoxanthine levels increased, indicating that the cells adjacent to the probe metabolized the exogenous ADO. With 2-CADO included in the artificial CSF at 10(-6), 10(-5), or 10(-4) M, local CBF increased 18, 131, and 201%, respectively. Perfusion with artificial CSF containing 10(-7), 10(-6), or 10(-5) M NECA resulted in a 35, 112, and 187% increase in local CBF, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhanced interstitial fluid adenosine attenuates myocardial stunning.

Reversible myocardial dysfunction associated with transient ischemia has been termed the stunned myocardium. Because exogenous adenosine has been shown to protect the ischemic myocardium, we hypothesized that augmentation of endogenous adenosine levels would attenuate myocardial stunning. To induce stunning, anesthetized dogs were subjected to 15 minutes of ischemia (left anterior descending artery occlusion) followed by 60 minutes of reperfusion. Erythro-9-(2-hydroxy-3-nonyl) adenine (EHNA; 5 mg/kg/hr), an adenosine deaminase inhibitor, was used to augment adenosine levels. The effect of EHNA on interstitial fluid (ISF) adenosine levels, coronary blood flow, and regional systolic wall thickening was compared with that of an untreated group (n = 8). EHNA increased preischemia ISF adenosine levels threefold and was associated with a corresponding increase in coronary blood flow. EHNA administration did not alter preischemia systolic wall thickening. Although ISF adenosine increased fourfold during ischemia in the untreated group, ISF adenosine increased nearly sixtyfold above preischemia values in the EHNA-treated group and remained elevated throughout reperfusion. Postischemic regional function was enhanced significantly in the group treated with EHNA. These data show that adenosine deaminase inhibition increased ISF adenosine levels and attenuated myocardial stunning. Metabolic manipulation of myocardial ISF nucleoside levels may be beneficial in limiting postischemic myocardial dysfunction.

Neurochemical and morphological responses to acutely and chronically implanted brain microdialysis probes.

The purpose of this study was to compare, in rats, brain microdialysis results obtained using microdialysis probes implanted acutely for 2 h versus probes implanted chronically for 24 h in the caudate. Specific comparisons included: (1) dialysate purine and amino acid profiles during cerebral ischemia; (2) diffusional characteristics of the microdialysis probe; and (3) tissue morphology surrounding the probe. During ischemia, the increase in dialysate levels of adenosine, inosine, and hypoxanthine was less pronounced from probes implanted chronically, while dialysate xanthine levels increased to a greater extent. An increase in dialysate amino acid neurotransmitters during cerebral ischemia was observed in the acutely implanted probes within 10 min of the onset of cerebral ischemia; in the chronically implanted probes this increase did not occur until after 50 min of severe ischemia. Both in vitro and in vivo tests revealed a diffusional barrier in chronically implanted probes. Moreover, the tissue surrounding chronically implanted probes exhibited a high degree of inflammation, and fibrin deposits were substantial. In addition, uric acid levels (an indicator of tissue injury) sampled from chronically implanted probes were 7-fold greater than levels sampled from acutely implanted probes. These data raise concerns about the use of chronically implanted microdialysis probes for the measurement of purine and amino acid profiles during cerebral ischemia.

Intravascular adenosine at reperfusion reduces infarct size and neutrophil adherence.

BACKGROUND: Adenosine has been shown to reduce infarct size predominantly during reperfusion by adenosine A2-receptor-mediated processes. This cardioprotection may involve inhibition of events in the vascular compartment, such as adherence-independent and adherence-dependent actions of neutrophils. This study tested the hypothesis that adenosine exerts its cardioprotection during reperfusion by targeting effectors in the vascular compartment. METHODS: Polyadenylic acid (molecular weight, 230,000 daltons) was used as an intravascularly confined adenosine mimetic. In anesthetized New Zealand white rabbits, the left coronary artery was occluded for 30 minutes and reperfused for 120 minutes. RESULTS: Polyadenylic acid (1 mg/kg bolus, 0.5 mg kg-1 h-1) given 5 minutes before reperfusion significantly (p < 0.05) reduced infarct size compared with vehicle (23% +/- 2% versus 37% +/- 2% area at risk). The A1-antagonist KW-3902 had no effect on this polyadenylic acid-induced protection (17% +/- 3%), whereas the A1-A2 antagonist sulfophenytheophylline blocked this infarct size reduction (41% +/- 2%). In vitro adherence of platelet-activating factor-activated neutrophils to thoracic aortic endothelium was significantly diminished by polyadenylic acid (185 +/- 12 neutrophils/mm2 versus 36 +/- 4 neutrophils/mm2 endothelial surface). Sulfophenytheophylline inhibited this effect (280 +/- 6 neutrophils/mm2), whereas KW-3902 did not (31 +/- 7 neutrophils/mm2). CONCLUSIONS: An intravascular adenosine mimetic agent exerts cardioprotection during reperfusion by targeting receptor-mediated mechanisms in the intravascular compartment, possibly involving inhibition of neutrophil-related processes.

Pentostatin-augmented interstitial adenosine prevents postcardioplegia injury in damaged hearts.

This study tests the hypothesis that the adenosine deaminase inhibitor pentostatin (2-deoxycoformycin), when given before ischemia or during infusions of blood cardioplegia, augments interstitial adenosine levels and prevents postcardioplegia dysfunction in hearts with antecedent ischemia. Twenty-one anesthetized dogs were placed on cardiopulmonary bypass, and the hearts were made globally ischemic for 30 minutes. Dogs received blood cardioplegia with no pentostatin (BCP group, n = 6), pretreatment pentostatin (0.2 mg/kg) infused 5 minutes before global ischemia (PS-PTx group, n = 7), or pentostatin included only in the blood cardioplegia without pretreatment (PS-BCP group, n = 8). Microdialysate myocardial adenosine levels (an index of interstitial fluid levels) increased only modestly in the BCP group (from 0.55 +/- 0.13 microM to 2.64 +/- 0.50 microM) and the PS-BCP group (from 0.55 +/- 0.18 microM to 1.08 +/- 0.48 microM) during normothermic ischemia, but interstitial adenosine levels were not augmented further during cardioplegic arrest in either group. In contrast, the adenosine level in the PS-PTx group was significantly (p < 0.05) augmented during global ischemia (from 0.50 +/- 0.13 microM to 63.16 +/- 28.08 microM) and cardioplegia infusion (to 15.26 microM +/- 5.61 microM). Relative to baseline, postischemic left ventricular performance (end-systolic pressure-volume relation) was depressed in both the BCP (from 5.5 +/- 1.2 mm Hg/mL to 3.8 +/- 0.4 mm Hg/mL) and PS-BCP groups (from 7.1 +/- 0.9 mm Hg/mL to 3.8 +/- 0.7 mm Hg/mL). In contrast, PS-PTx restored postischemic performance (from 6.2 +/- 0.5 mm Hg/mL to 7.5 +/- 0.9 mm Hg/mL).(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in extracellular adenosine during chemical or electrical brain stimulation.

The purpose of this study was to determine the changes in adenosine and adenosine metabolites during graded electrical stimulation or kainic acid-induced activation and to assess the role of adenosine in the cerebral blood flow (CBF) response to increased brain activity. A modified brain microdialysis technique was used to sample cerebral interstitial fluid (ISF), deliver drugs locally to the brain, electrically stimulate the brain, and measure local CBF (H2 clearance). Microdialysis probes were implanted bilaterally in the caudate nuclei of ketamine-anesthetized rats. Graded electrical stimulation at 5, 15, and 30 Hz increased dialysate adenosine 1.5-fold, 2.3-fold, and 4.7-fold, respectively. Local infusion of kainic acid, an agonist of the excitatory amino acid neurotransmitter glutamate, produced a transient increase (2-fold) in dialysate adenosine and sustained increases in dialysate inosine (2-fold), hypoxanthine (4-fold) and CBF (2.4-fold). When the adenosine receptor antagonist 8(p-sulphophenyl)-theophylline (SPT, 10(-3) M) was co-administered with kainic acid, CBF increased only 1.6-fold, while the increase in dialysate adenosine was augmented by 40%. These data demonstrate that ISF adenosine increases during brain activation and suggest that adenosine contributes to active hyperemia in the brain.