Effects of anoxia on cerebral blood flow in the rat brain: evidence for a role of adenosine in autoregulation

Caffeine Modulates Spontaneous Adenosine and Oxygen Changes during Ischemia and Reperfusion

Adenosine is an endogenous neuroprotectant that modulates vasodilation in the central nervous system. Oxygen changes occur when there is an increase in local cerebral blood flow and thus are a measure of vasodilation. Transient oxygen events following rapid adenosine events have been recently discovered, but the relationship between adenosine and blood flow change during ischemia/reperfusion (I/R) has not been characterized. Caffeine is a nonselective adenosine receptor antagonist that can modulate the effects of adenosine in the brain, but how it affects adenosine and oxygen levels during I/R is also unknown. In this study, extracellular changes in adenosine and oxygen were simultaneously monitored using fast-scan cyclic voltammetry during bilateral common carotid artery occlusion (BCCAO) and the effects of a specific A2A antagonist, SCH 442416, or general antagonist, caffeine, were studied. Measurements were made in the caudate-putamen for 1 h of normoxia, followed by 30 min of BCCAO and 30 min of reperfusion. The frequency and number of both adenosine and oxygen transient events significantly increased during I/R. The specific A2A antagonist, SCH 442416 (3 mg/kg, i.p.), eliminated the increase in adenosine and oxygen events caused by I/R. The general adenosine receptor antagonist, caffeine (100 mg/kg, i.p.), decreased the frequency of adenosine and oxygen transient events during I/R. These results demonstrate that, during BCCAO, there are more rapid release events of the neuromodulator adenosine and correlated local oxygen changes, and these rapid, local effects are dampened by caffeine and other A2A antagonists.

Cerebral blood flow response in adenosine 2a receptor knockout mice during transient hypoxic hypoxia

We evaluated cerebral blood flow by laser Doppler during 30 secs of hypoxia (0.10 FiO(2)) in anesthetized, ventilated adenosine 2a receptor knockout (A2aR KO) and wild-type (WT) mice to test the hypothesis that cerebral hypoxic hyperemia in KO mice would be attenuated. We also studied the effects of selective and nonselective A2aR antagonists. During 30 secs of hypoxia, P(a)O(2) decreased significantly (P<0.05) and to a similar degree in both types of mice, whereas P(a)CO(2) remained relatively stable. However, mean arterial blood pressure (MABP) decreased to a greater extent (P<0.05) during hypoxia in KO mice (58.6+/-1.5 mm Hg) than in WT animals (76.1+/-3.2 mm Hg). Consequently, in a separate group of mice, we stabilized and matched MABP during hypoxia. Hypoxic hyperemia was attenuated by 38% (P<0.05) in KO animals whose MABP was uncontrolled, and by 81% (P<0.05) in KO animals whose MABP changes were matched to the MABP in the hypoxic WT mice. In animals treated with adenosine antagonists, hypoxic hyperemia was decreased by 44% to 48% (P<0.05) in WT mice, but was without effect in KO mice. We conclude that adenosine via A2aR is responsible for a significant proportion of the hyperemia during hypoxia.

Adenosine treatment delays postischemic hippocampal CA1 loss after cardiac arrest and resuscitation in rats

Resuscitation from cardiac arrest results in reperfusion injury that leads to increased postresuscitation mortality and delayed neuronal death. One of the many consequences of resuscitation from cardiac arrest is a derangement of energy metabolism and the loss of adenylates, impairing the tissue's ability to regain proper energy balance. In this study, we investigated the effects of adenosine (ADO) on the recovery of the brain from 12 min of ischemia using a rat model of cardiac arrest and resuscitation. Compared to the untreated group, treatment with adenosine (7.2 mg/kg) initiated immediately after resuscitation increased the proportion of rats surviving to 4 days and significantly delayed hippocampal CA1 neuronal loss. Brain blood flow was increased significantly in the adenosine-treated rats 1 h after cardiac arrest and resuscitation. Adenosine-treated rats exhibited less edema in cortex, brainstem and hippocampus during the first 48 h of recovery. Adenosine treatment significantly lowered brain temperature during recovery, and a part of the neuroprotective effects of adenosine treatment could be ascribed to adenosine-induced hypothermia. With this dose, adenosine may have a delayed transient effect on the restoration of the adenylate pool (AXP = ATP + ADP + AMP) 24 h after cardiac arrest and resuscitation. Our findings suggested that improved postischemic brain blood flow and ADO-induced hypothermia, rather than adenylate supplementation, may be the two major contributors to the neuroprotective effects of adenosine following cardiac arrest and resuscitation. Although adenosine did not prevent eventual CA1 neuronal loss in the long term, it did delay neuronal loss and promoted long-term survival. Thus, adenosine or specific agonists of adenosine receptors should be evaluated as adjuncts to broaden the window of opportunity in the treatment of the reperfusion injury following cardiac arrest and resuscitation.

Adenosine and the regulation of cerebral blood flow

OBJECTIVES

This review summarizes the 30 year effort of my collaborator and mentor Dr J. W. Phillis to establish the role of adenosine in the regulation of cerebral blood flow.

METHODS

While most of the experiments described utilized the rat cerebral cortex as a model, several different and complementary methodologies were employed. Superfusate samples were collected from the cortical surface and analysed for purines using HPLC. Laser-Doppler flowmetry was utilized to measure blood flow in the pial vasculature, while pial diameters were monitored by videomicroscopy. An additional series of experiments looked at coronary blood flow in a Langendorff preparation.

RESULTS

Adenosine is released from the cortex in response to decreased nutrient supply (hypoxia/ ischemia) and during conditions that mimic alterations in the extracellular environment associated with increased metabolism. The application of pharmacological agents that alter adenosine metabolism resulted in the appropriate alterations in ECF adenosine levels and also in blood flow. Selective blockade of the adenosine A(2A) receptor reduced the pial vasodilation evoked by hypercapnoea. Results from the isolated rat heart, utilizing similar agents, support a role for adenosine in the regulation of coronary blood flow during respiratory and metabolic acidosis.

DISCUSSION

Adenosine is released when there is a mismatch between supply and demand. If the effects of adenosine are blocked with receptor antagonists, the vasodilation is also reduced. However, the effects of adenosine on the hyperemia evoked by hypercapnoea are complicated by the arousal evoked by adenosine receptor antagonists and the effects of upstream regulation.

Cerebrospinal fluid adenosine concentration and uncoupling of cerebral blood flow and oxidative metabolism after severe head injury in humans

OBJECTIVE

Uncoupling of cerebral blood flow (CBF) and oxidative metabolism is observed after severe head injury in comatose patients; however, the mechanism(s) involved remain undefined. Adenosine can produce cerebral vasodilation and reduce neuronal activity and is a possible mediator of uncoupling. We hypothesized that cerebrospinal fluid (CSF) adenosine concentrations would be increased during uncoupling of CBF and oxidative metabolism, defined as a narrow arterio-jugular venous oxygen difference [D(a-v)O2 4 vol%] after head injury.

METHODS

Adenosine concentrations were measured using fluorescent-based high-pressure liquid chromatography in 67 CSF samples obtained from 13 comatose (Glasgow Coma Scale score 7) adult patients who sustained a severe closed head injury. At the time each sample was obtained, CBF was measured by the xenon-133 method, and blood samples were obtained for determination of D(a-v)O2.

RESULTS

CSF adenosine concentration was negatively associated with D(a-v)O2 (P < 0.05, generalized multivariate linear regression model). In addition, CSF adenosine concentration was increased when D(a-v)O2 was 4 versus > 4 vol% (38.5 [3.2-306.3] versus 14.0 [2.7-795.5] nmol/L, respectively, median [range]; P < 0.025) and in patients who died versus survivors (40.1 [6.9-306.3] versus 12.9 [2.7-795.5] nmol/L, respectively, median [range]; P < 0.001).

CONCLUSION

The association between increased CSF adenosine concentration and a reduction in global cross-brain extraction of oxygen supports a regulatory role for adenosine in the complex balance between CBF and oxidative and nonoxidative metabolism severe head injury in humans.

Superselective intraarterial papaverine administration: effect on regional cerebral blood flow in patients with arteriovenous malformations

In this study the authors determined the effect of papaverine on regional cerebral blood flow (rCBF) in the angiographically normal arteriolar beds of patients with arteriovenous malformations (AVMs) who underwent transfemoral superselective angiography. Middle cerebral artery (MCA) branch vessels were catheterized during 10 procedures performed in nine patients. The mean (+/- standard deviation) largest AVM diameter was 4.4 +/- 1 cm. Regional CBF was measured by recording the washout of a bolus of xenon-133 injected through the microcatheter. In a dose-ranging study. rCBF and MCA pressure in two patients were repeatedly measured after 3-minute infusions of papaverine at 0.07, 0.7, and 7 mg/minute. In a single-dose study, an additional eight patients received only the highest dose of papaverine administered over a 3-minute period. In the dose-ranging study, CBF increased from baseline in a dose-dependent fashion. In the single-dose study, papaverine increased in rCBF 103%, from 48 +/- 11 to 95 +/- 23 ml/100 g/minute at an MCA pressure of 55 +/- 23 mm Hg. Increase in rCBF was linearly related (y = 2.2x - 17, r2 = 0.84; p = 0.001) to baseline MCA pressure (range 22-84 mm Hg). Papaverine increases rCBF in a direct proportion to baseline MCA pressure, even at low baseline pressures. Selective infusion of vasodilators should be investigated in acute cerebral hypotension to facilitate either primary or collateral recruitment of CBF by aiding spontaneous autoregulatory vasodilation. In addition, rCBF monitoring may be useful in determining the most effective intraarterial dose of papaverine while minimizing complications due to hyperemia.

The effect of 2-chloroadenosine on the ATP level Na,K ATPase activity in experimental brain ischemia of gerbil

T1. The effect of 2-chloroadenosine, an adenosine analogue, on brain ATP level and Na,K ATPase activity in ischemia and reperfusion was studied. 2. Na,K ATPase activity decreased in both ischemia and reperfusion. Although the ATP level decreased in ischemia, it increased with reperfusion (P < 0.05). 3. It is concluded that 2-chloroadenosine treatment influenced ATP production and Na,K ATPase activity in ischemia and reperfusion (P < 0.05).

Characterization of nucleoside transport activity in rabbit cortical synaptosomes

Rabbit central nervous system (CNS) preparations have been used to study the central effects of adenosine, but little is known about the specific uptake mechanisms in rabbit brain involved in the regulation of extracellular adenosine concentrations. The present study assessed the kinetic and pharmacological characteristics of the uptake of [3H]uridine (a poorly metabolized substrate for adenosine transporters) by rabbit cortical synaptosomes, to define the transporter subtypes involved and to evaluate species variability in transporter characteristics. [3H]Uridine transport into rabbit cortical synaptosomes was mediated by two saturable, facilitated diffusion systems with characteristics compatible with the es and ei transporter subtypes identified in other mammalian species. About 65% of the total transport was mediated by the es system, and Km estimates of 320 and 94 microM were determined for [3H]uridine uptake by the es and ei transporter, respectively. These results differ significantly from the subtype ratio and kinetic characteristics reported for rat and guinea pig cortical synaptosomes, where most of the transport was mediated by an ei subtype. Dipyridamole, dilazep, nitrobenzylthioinosine, R75231, soluflazine, and mioflazine were relatively more effective as inhibitors of es-mediated uptake (compared with ei), while the substrates adenosine, cytidine, and guanosine did not distinguish between the es and ei transporters in rabbit cortical synaptosomes. These results highlight the significant species-tissue variability in nucleoside transporter characteristics and subtype expression, and emphasize the need to characterize the transporters in human CNS tissue to allow the rational development of CNS-active therapeutics based on inhibition of nucleoside transport.

Effects of adenosine and 2-chloroadenosine on cerebral collateral vessels

Adenosine is a potent cerebral vasodilator. We tested the hypothesis that dilatation of collateral vessels in cerebrum, in response to topical adenosine and 2-chloroadenosine (2-CAD), would increase blood flow to collateral-dependent cerebrum. In dogs anesthetized with halothane, a branch of the middle cerebral artery (MCA) was occluded proximally and cannulated distally. The collateral-dependent area at risk for infarction was perfused from a reservoir with microsphere-free blood, and blood flow to normal cerebrum and to cerebrum dependent on collateral flow was measured with radioactive microspheres injected into the left ventricle through a femoral artery catheter. Perfusion through the cannulated MCA branch was stopped, and flow to normal and collateral-dependent cerebrum was measured after adenosine (10(-2) M) or 2-CAD (10(-4) M) was added to the superfusate over the cerebrum. In normal cerebrum, topical application of adenosine increased flow to outer but not inner layers. Topical application of adenosine had little effect on flow to collateral-dependent tissue. In normal cerebrum, 2-CAD increased flow to outer layers, whereas flow to inner layers tended to increase. During 2-CAD, flow to outer cortical layers of collateral-dependent cerebrum increased from 140 +/- 20 ml/100 g/min (mean +/- SD) to 231 +/- 68, whereas flow to the inner collateral-dependent tissue did not change. The findings indicate that, after occlusion of a cerebral artery, topical 2-CAD increases blood flow to outer layers of collateral-dependent and normal cerebrum.(ABSTRACT TRUNCATED AT 250 WORDS)

Dipyridamole attenuates the development of iminodipropionitrile-induced dyskinetic abnormalities in rats

The present investigation was undertaken to study the effect of dipyridamole on experimental dyskinesia in rats. The movement disorders were produced by intraperitoneal administration of iminodipropionitrile (IDPN) in the dose of 100 mg/kg per day for 12 days. Dipyridamole was administered orally, daily 30 min before IDPN in the doses of 0.5 g/kg, 1 g/kg, and 1.5 g/kg bodyweight in three different groups of rats. Twenty-four hours after the last dose of IDPN, animals were observed for neurobehavioral changes including vertical and horizontal head weaving, circling, backwalking, grip strength, and righting reflex. Immediately after behavioral studies brain specimens were collected for analysis of vitamin E, conjugated dienes, and lipid hydroperoxides as indices of oxygen-derived free radical (OFR) production. Our results showed that concurrent use of dipyridamole significantly protected rats against IDPN-induced neurobehavioral changes in a dose-dependent manner. Treatment of rats with dipyridamole inhibited IDPN-induced decrease of vitamin E and increase in conjugated dienes and lipid hydroperoxides in brain. These findings suggest the involvement of OFR in dipyridamole induced protection against the development of IDPN dyskinesia. Further studies are warranted to determine the role of dipyridamole as a prophylactic agent against the drug induced dyskinetic abnormalities.

Ionic dependence of adenosine uptake into cultured astrocytes

Adenosine uptake in cultured astrocytes is dependent on various ions and energy metabolism. The Na(+)-gradient plays an important role, since nigericin, ouabain, amiloride and substitution of Na+ with choline inhibited adenosine uptake. The proton-gradient was of importance, since carbonylcyanide m-chlorophenylhydrozone (CCCP) and omeprazole also inhibited adenosine uptake. Furthermore, adenosine uptake was dependent on Cl- anion. Substitution of Cl- with isethionate, as well as DIDS or furosemide inhibited adenosine uptake. Adenosine uptake was also sensitive to Ca2+ gradient, removal of extracellular Ca2+ and calcimycin inhibited adenosine uptake. Adenosine uptake was not dependent on extracellular K+ and was not affected by valinomycin. Although, K(+)-channel openers (BRL 34195 and nicorandil) as well as the K(+)-channel antagonist, glyburide, inhibited adenosine uptake, the inhibitory effect of BRL 34915 was not antagonized by glyburide. Rotenone and 2,4-dinitrophenol also inhibited adenosine uptake. Ionic dependence and metabolic energy dependence of adenosine uptake suggest that uptake is primarily an active process.

Dipyridamole increases oxygen-glucose deprivation-induced injury in cortical cell culture

BACKGROUND AND PURPOSE

Adenosine transport inhibitors attenuate ischemic central neuronal damage in vivo, but the locus of this protective action is presently unknown. To help address the question of whether adenosine transport inhibitors have a protective effect directly on brain parenchyma, we tested the effect of the adenosine transport inhibitor dipyridamole on neuronal loss induced by oxygen-glucose deprivation in vitro.

METHODS

Murine cortical cultures were exposed to combined oxygen and glucose deprivation, N-methyl-D-aspartate, or kainate. The extracellular concentrations of glutamate and adenosine were measured by high-performance liquid chromatography; neuronal cell death was assessed by morphological examination and measurement of lactate dehydrogenase release.

RESULTS

Cultures exposed to oxygen-glucose deprivation for 30 to 75 minutes exhibited an insult-dependent increase in extracellular adenosine, followed shortly by an increase in extracellular glutamate and 24 hours later by neuronal death. Addition of the A1 receptor antagonist 8-cyclopentyltheophylline during oxygen-glucose deprivation enhanced both glutamate release and neuronal damage. Addition of 10 mumol/L dipyridamole decreased extracellular adenosine and also enhanced extracellular glutamate and neuronal death. In contrast, dipyridamole increased the levels of extracellular adenosine stimulated by N-methyl-D-aspartate or kainate.

CONCLUSIONS

These results are consistent with the idea that endogenous adenosine has a neuroprotective effect directly on cortical cells exposed to oxygen-glucose deprivation. However, inhibition of adenosine transport with dipyridamole was surprisingly not an effective strategy for enhancing this protective effect. The beneficial effects of adenosine transport inhibitors observed in vivo may be mediated indirectly--for example, by effects on the vasculature.

Use of HPLC to measure circulating adenosine levels in migrainous patients

Adenosine is a powerful natural vasodilator that could be involved in migraine. It is difficult to assay this nucleoside, however, because it has a short half-life. We have used HPLC to compare the concentrations of blood adenosine sampled in crisis-free intervals and during crisis periods in ten patients with common migraine and have compared these levels to those noted in a control population. Our sampling technique uses vacuum suction and enables rapid mixing of the blocking solution and whole venous blood. This results in reproducible HPLC assays. We also show that, during a migraine crisis, mean blood adenosine levels increase by 47%. However, the origin of this adenosine release is difficult to define.

The role of nitric oxide in the regulation of cerebral blood flow

The role of nitric oxide in the cerebral circulation under basal conditions and when exposed to hypoxic, hypercapnic and hypotensive stimuli, was studied in mechanically ventilated rats using a venous outflow technique, by examining the effects of inhibition of nitric oxide synthase with N-nitro-L-arginine methyl ester (L-NAME). L-NAME (10 or 30 mg/kg injected intravenously) raised mean arterial blood pressure by 14% and 24%, and increased cerebrovascular resistance (CVR) by 20% and 24%, respectively. Cerebral blood flow (CBF) was unaltered, as were blood gases and pH. The increases in MABP and CVR were attenuated by L-arginine (300 mg/kg). Following the administration of L-NAME, the increases in CBF elicited by ventilation with 8% oxygen for 25 s were unaltered, in comparison to control responses. L-NAME attenuated the increases in CBF and reduced the time for recovery to basal flow rates evoked by ventilation with 10% carbon dioxide. These effects were reversed by L-, but not by D-, arginine. Autoregulation by CBF during hypotensive episodes, as measured by comparisons of CVR values, was unaffected by L-NAME. The results suggest that endogenous nitric oxide is involved in the responses of the cerebral vasculature to elevated levels of CO2 in the arterial blood. Nitric oxide does not appear to play a major role in autoregulation to increases or decreases in MABP, or in hypoxia-evoked vasodilation.

Cerebral autoregulation during moderate hypothermia in rats

BACKGROUND AND PURPOSE

Little is known about the effects of hypothermia on cerebral autoregulation. The present study was designed to examine cerebral blood flow responses to controlled hemorrhagic hypotension in normothermic and hypothermic rats.

METHODS

Cortical blood flow was measured with a laser-Doppler flowmeter in halothane-anesthetized rats assigned to one of three groups: normothermic group 1 (n = 8) with a pericranial temperature of approximately 36.5 degrees C or hypothermic group 2 (n = 8) or group 3 (n = 8) with a pericranial temperature of approximately 30.5 degrees C. In group 2, a PaCO2 of approximately 40 mm Hg was maintained without correction for body temperature. To evaluate the role of PaCO2, in group 3 animals PaCO2 was kept at approximately 40 mm Hg as corrected for body temperature. In all animals, the mean arterial blood pressure was reduced by hemorrhage in increments of 10 mm Hg every 2 minutes.

RESULTS

In group 1 animals, a typical autoregulatory curve was observed with cerebral blood flow first falling at or below 75% of baseline at a mean arterial pressure of 57 +/- 15 mm Hg (mean +/- SD). Absolute normotensive cerebral blood flow in group 2 fell to < or = 75% of baseline at a mean arterial pressure of 73 +/- 21 mm Hg. In group 3, no evidence of autoregulation was seen. Cerebral blood flow reached values < or = 75% of baseline at a mean arterial pressure of 82 +/- 14 mm Hg, whereas calculated cerebrovascular resistance failed to show any compensatory vasodilation as the mean arterial pressure decreased.

CONCLUSIONS

Different PaCO2 management schemes used during hypothermia may have profound effects on cerebral blood flow and on autoregulation. If PaCO2 is maintained at 40 mm Hg after correction for temperature, autoregulation is abolished. If uncorrected PaCO2 is maintained at approximately 40 mm Hg, some degree of autoregulation is preserved, albeit with a right-shifted "knee."

Adenosine in exercise adaptation

By influencing the regulation of the mechanisms of angiogenesis, erythropoietin production, blood flow, myocardial glucose uptake, glycogenolysis, systolic blood pressure, respiration, plasma norepinephrine and epinephrine levels, adenosine may exert a significant effect on the body's adaptation response to exercise. However, adenosine's possible influence over the vasodilatory response to exercise in skeletal muscle is controversial and more research is required to resolve this issue. Various popular exercise training methods, such as cyclic training, interval training, and the 'warm down' from training may increase adenosine levels and thereby might enhance the response of adenosine-influenced adaptive mechanisms. Among the several classes of drugs which may enhance extracellular adenosine levels and thereby might augment adenosine-influenced adaptive mechanisms, are the anabolic steroidal and some readily available non-steroidal anti-inflammatory drugs (NSAIDs).

Deoxycoformycin and oxypurinol: protection against focal ischemic brain injury in the rat

We have previously demonstrated that oxypurinol (40 mg/kg i.p.), a xanthine oxidase inhibitor, can reduce focal ischemic brain injury in the rat when applied pre-ischemically. By using a model of occlusion of the middle cerebral artery (MCA) in tandem with occlusion of the ipsilateral carotid artery, the present study further demonstrates that delayed (60 min) administration of oxypurinol also exhibits a protective action on ischemic damage in the stroked rat brain. Oxypurinol significantly reduced the ischemic cerebral infarct zone by preventing the development of brain damage primarily in areas distant to the central lesion core. A corresponding amelioration of brain swelling and attenuation of neurological deficits were evident. Similar protection against focal ischemic brain damage was evident when the adenosine deaminase inhibitor, deoxycoformycin (500 micrograms/kg), was administered prior to the onset of ischemia. However, with delayed (60 min) administration deoxycoformycin had no protective effect. These findings support the hypothesis that manipulation of adenosine catabolism can be an effective therapeutic approach to the prevention or treatment of brain injuries, such as those occurring during ischemic stroke or cardiac arrest.

Enhancement of endogenous excitatory amino acids by theophylline does not modify the behavioral and histological consequences of forebrain ischemia

The neuroprotective role of endogenous adenosine during forebrain ischemia elicited by 4-vessel occlusion in rats was assessed using the adenosine antagonist, theophylline (32 mg/kg). Despite an increase in the release of glutamate in the hippocampus during ischemia, theophylline did not alter the neurological and histological outcomes. These results indicate that endogenous adenosine does not act as an endogenous neuroprotector by modulating glutamate release in this model.

Adenosine deaminase reduces hypoxic and hypercapnic dilatation of rat pial arterioles: evidence for mediation by adenosine

Rat pial arteries were observed through a closed cranial window during hypercapnic and hypoxic episodes whilst the cerebral cortex was superfused at 37 degrees C first with artificial cerebrospinal fluid (CSF) and subsequently with adenosine deaminase (ADA, 0.5-2.0 U/ml) in CSF. The results indicate that ADA attenuated hypercapnic and hypoxic dilatatory arteriolar responses by 64% and 56% respectively. Recovery was obtained by superfusing with ADA-free CSF for 1 h. We conclude that adenosine is involved in hypercapnia- and hypoxia-evoked dilation of pial arteries.

Brain adenosine and transmitter amino acid release from the ischemic rat cerebral cortex: effects of the adenosine deaminase inhibitor deoxycoformycin

The effects of a potent adenosine deaminase inhibitor, deoxycoformycin, on purine and amino acid neuro-transmitter release from the ischemic rat cerebral cortex were studied with the cortical cup technique. Cerebral ischemia (20 min) was elicited by four-vessel occlusion. Purine and amino acid releases were compared from control ischemic animals and deoxycoformycin-pretreated ischemic rats. Ischemia enhanced the release of glutamate, aspartate, and gamma-aminobutyric acid into cortical perfusates. The levels of adenosine, inosine, hypoxanthine, and xanthine in the same perfusates were also elevated during and following ischemia. Deoxycoformycin (500 micrograms/kg) enhanced ischemia-evoked release of adenosine, indicating a marked rise in the adenosine content of the interstitial fluid of the cerebral cortex. Inosine, hypoxanthine, and xanthine levels were depressed by deoxycoformycin. Deoxycoformycin pretreatment failed to alter the pattern of amino acid neurotransmitter release from the cerebral cortex in comparison with that observed in control ischemic animals. The failure of deoxycoformycin to attenuate amino acid neurotransmitter release, even though it markedly enhanced adenosine levels in the extracellular space, implies that the amino acid release during ischemia occurs via an adenosine-insensitive mechanism. Inhibition of excitotoxic amino acid release is unlikely to be responsible for the cerebroprotective actions of deoxycoformycin in the ischemic brain.

A comparison of cerebral blood flow during basal, hypotensive, hypoxic and hypercapnic conditions between normal and streptozotocin diabetic rats

This study was undertaken to determine the differences (if any) in cerebral blood flow (CBF) between streptozotocin (STZ) diabetic and normal rats. CBF was studied in connection with episodes of hypoxia, hypercapnia and hypotension as compared to the basal condition. Overall basal CBF rates in streptozotocin diabetic rats were significantly higher than in normal animals. However, initial basal flow rates prior to the first challenge were insignificantly higher in the STZ diabetic group. The higher CBF rate in STZ diabetics was also seen during the peak flows of the hypoxic and hypercapnic challenges. Furthermore, although overall CBF decreased for both the normal and STZ diabetic groups during hypotension, higher CBFs were observed in the STZ diabetic group during this challenge. The percent increase in CBF above control resulting from hypoxia or hypercapnia and the changes in CBF resulting from hypotension were not significantly different in the STZ diabetic and normal groups. The results indicate that the STZ diabetic rat regulates CBF in the same manner as the normal rat in response to hypoxia, hypercapnia and hypertension. The STZ diabetic rat executes these CBF responses at a slightly higher CBF rate. In view of the finding that the regulation of CBF is unaltered in the STZ diabetic animal, it is hypothesized that the associated hyperglycemia may be the causative agent for the cerebral ischemic susceptibility associated with long-term diabetes mellitus rather than a failure of CBF regulation.

The role of adenosine in the hyperaemic response of the hepatic artery to portal vein occlusion (the 'buffer response')

1. Adenosine has been shown to be responsible for the hyperaemic response of the hepatic artery to portal vein occlusion (the hepatic arterial 'buffer response'). 2. The effect of adenosine receptor blockade and of adenosine uptake inhibition on the hepatic arterial response to portal vein occlusion was investigated in three groups of anaesthetized dogs. 3. Venous return and arterial blood pressure were maintained during periods of portal occlusion by establishing a side-to-side portacaval shunt. Hepatic artery and portal vein blood flows were measured with electromagnetic flowmeters. 4. Hepatic arterial infusions of 8-phenyltheophylline (500 micrograms kg-1 and 3-isobutyl-1-methylxanthine min-1) and 3-isobutyl-1-methylxanthine (75 micrograms kg-1 min-1), doses sufficient to block the vasodilator response of the hepatic artery to exogenously applied adenosine, reduced the magnitude of the 'buffer response' by 50% and 75%, respectively. 5. Intravenous infusion of dipyridamole (100 micrograms kg-1 min-1), a dose sufficient to potentiate the vasodilator response of the hepatic artery to exogenously applied adenosine, had little effect on the 'buffer response'. 6. It is concluded that adenosine is an important, but not the sole, agent responsible for the hepatic arterial 'buffer response'.

The role of adenosine in the hyperaemic response of the hepatic artery to portal vein occlusion (the 'buffer response')

1. Adenosine has been shown to be responsible for the hyperaemic response of the hepatic artery to portal vein occlusion (the hepatic arterial 'buffer response'). 2. The effect of adenosine receptor blockade and of adenosine uptake inhibition on the hepatic arterial response to portal vein occlusion was investigated in three groups of anaesthetized dogs. 3. Venous return and arterial blood pressure were maintained during periods of portal occlusion by establishing a side-to-side portacaval shunt. Hepatic artery and portal vein blood flows were measured with electromagnetic flowmeters. 4. Hepatic arterial infusions of 8-phenyltheophylline (500 micrograms kg-1 and 3-isobutyl-1-methylxanthine min-1) and 3-isobutyl-1-methylxanthine (75 micrograms kg-1 min-1), doses sufficient to block the vasodilator response of the hepatic artery to exogenously applied adenosine, reduced the magnitude of the 'buffer response' by 50% and 75%, respectively. 5. Intravenous infusion of dipyridamole (100 micrograms kg-1 min-1), a dose sufficient to potentiate the vasodilator response of the hepatic artery to exogenously applied adenosine, had little effect on the 'buffer response'. 6. It is concluded that adenosine is an important, but not the sole, agent responsible for the hepatic arterial 'buffer response'.

Effect of dipyridamole on cerebral extracellular adenosine level in vivo

The effect of dipyridamole, an adenosine transport inhibitor, on cerebral extracellular adenosine concentration remains to be determined. To examine this issue, bilateral brain dialysis samples were obtained from piglet frontal cortex before, during, and after 5 min of cerebral ischemia; 10(-4) M dipyridamole was administered through one dialysis probe. On the control side, dialysate adenosine concentration increased 5.7-fold during ischemia and 15-fold during the first 5 min of reperfusion; it returned to control levels after 15 min of reperfusion. Relative to the control side, dipyridamole caused a twofold increase in basal dialysate adenosine concentration and increased dialysate adenosine concentration at 10 and 15 min of reperfusion, but no increase in dialysate adenosine occurred during and immediately after ischemia. The results indicate that, in the piglet brain, cerebral ischemia markedly elevates intracerebral extracellular adenosine concentration and that dipyridamole increases extracellular adenosine levels.

Caffeine and human cerebral blood flow: a positron emission tomography study

Positron emission tomography (PET) was used to quantify the effect of caffeine on whole brain and regional cerebral blood flow (CBF) in humans. A mean dose of 250 mg of caffeine produced approximately a 30% decrease in whole brain CBF; regional differences in caffeine effect were not observed. Pre-caffeine CBF strongly influenced the magnitude of the caffeine-induced decrease. Caffeine decreased paCO2 and increased systolic blood pressure significantly; the change in paCO2 did not account for the change in CBF. Smaller increases in diastolic blood pressure, heart rate, plasma epinephrine and norepinephrine, and subjectively reported anxiety were also observed.

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)

Effect of flunarizine on the attenuation of baroreflex by transient cerebral ischemia

Baroreflex sensitivity assessed by phenylephrine-induced reflex bradycardia was markedly decreased by 5- an 10-min global cerebral ischemia in anesthetized dogs. Flunarizine, 0.1 and 1 mg/kg i.v., administered 5 min prior to 5-min ischemia, completely inhibited the decrease in baroreflex sensitivity while such a protective effect of the latter dose was incomplete against 10-min ischemia. In contrast, papaverine, 0.5 mg/kg per min i.v., infused for 5 min prior to 5-min ischemia, failed to protect the decrease in baroreflex sensitivity. Flunarizine may possess a certain direct cerebroprotective effect in addition to its cerebrocirculatory effect.

Effects of two nucleoside transport inhibitors, dipyridamole and soluflazine, on purine release from the rat cerebral cortex

The effects of two nucleoside transport inhibitors, dipyridamole and soluflazine, on adenosine, inosine and oxypurine release from the normoxic and hypoxic/ischemic rat cerebral cortex have been studied. Dipyridamole (500 micrograms/kg) enhanced adenosine release during hypoxic/ischemic challenges in comparison with saline-injected controls. It decreased the hypoxia/ischemia-elicited releases of inosine, hypoxanthine and xanthine. Both basal and hypoxia/ischemia-elicited releases of uric acid were elevated. Soluflazine, administered topically or systemically, failed to enhance adenosine release and did not consistently alter the hypoxia/ischemia-evoked releases of inosine, hypoxanthine and xanthine. Basal release of uric acid was elevated. The failure of either drug to elevate the basal or hypoxia/ischemia-evoked releases of adenosine above predrug levels illustrates one of the problems which may be inherent in the use of bidirectional nucleoside transport inhibitors for the manipulation of adenosine levels in the cerebral interstitial fluid.

Role of adenosine in cerebral hypoxic hyperemia in the unanesthetized rabbit

The present study was undertaken to determine the importance of adenosine in the cerebrovascular response to hypoxia. The mass spectrometry method was used to investigate local blood flow, tissue pO2 and pCO2 in 3 cerebral structures: caudate nucleus (n = 8), thalamus (n = 5) and hippocampus (n = 5) in unanesthetized, spontaneously breathing rabbits. After having tested the reproducibility of the hypoxic response each animal was exposed twice to moderate hypoxia. I.v. theophylline (10 mg/kg) was administered between the first and second exposures to hypoxia. The principal finding is that in each cerebral region, the vasodilatation induced by hypoxia was significantly decreased by pretreatment with theophylline despite the low theophylline dose used. It is concluded that adenosine is partly responsible for the cerebral vasodilatation observed during hypoxia. Several other mechanisms possibly involved in this cerebrovascular response are discussed.

Extracellular increase of hypoxanthine and xanthine in the cortex and basal ganglia of fetal lambs during hypoxia-ischemia

A microdialysis procedure was used to sample purine catabolites from the extracellular compartment of two areas of the fetal brain, the cerebral cortex and the striatum region. Seven exteriorized, anesthetized fetal sheep were studied during conditions of normal oxygenation and during asphyxia induced by stepwise lowering of maternal placental perfusion pressure. Fetal cerebral blood flow (CBF) was measured with the 133Xe washout method. Somatosensory evoked potentials were recorded during tactile stimulation of the fetal snout. The purine catabolites hypoxanthine (HX), xanthine (Xan), guanosine (Gua), inosine (Ino) and adenosine (Ade) were measured in arterial and venous plasma and in the dialysate by a HPLC method. During gradually increasing asphyxia HX increased 4-6-fold both in cerebral tissue and in arterial plasma, while Xan rose 1.5-3 times in cerebral tissue and arterial plasma. Gua did not change in either tissue or blood and Ino did not rise until at the most extreme degree of asphyxia it rose 2-4-fold. Ade concentrations remained unaltered during moderate asphyxia (when CBF rose 3-fold), but demonstrated a significant (2-3-fold) augmentation at the most extreme degrees of asphyxia. The results thus suggests a significant accumulation of oxypurines, especially HX, in the interstitium of the fetal brain during asphyxia. The rise in xanthine concentration demonstrates the presence of an activity of xanthine oxidase in brain tissue. A discrepancy between the elevation of CBF and of the tissue concentration of Ade does not support a direct role of Ade in the asphyctic cerebral vasodilation in the fetus.

The effects of dipyridamole and theophylline on rat pial vessels during hypocarbia

Hypocarbia results in an increase in brain adenosine concentrations, presumably because of brain hypoxia associated with hypocarbic vasoconstriction. It was hypothesized that adenosine limits the degree of hypocarbic vasoconstriction. To test this hypothesis, the effects of dipyridamole and theophylline on CO2 reactivity during hypocarbia were investigated in anesthetized rats. Dipyridamole should reduce the vasoconstriction by potentiating adenosine action, whereas theophylline should increase the vasoconstriction by blocking adenosine receptors. Cortical pial arterioles of mechanically ventilated and anesthetized rats were displayed on a video monitor system through a closed cranial window. Arterial blood pressure and oxygen tension were stable. CO2 reactivity, formulated as 100 X [delta diameter (micron)/resting diameter (micron)]/delta PaCO2 (mmHg), in the hypocarbic phase was calculated before and after topical superfusion of dipyridamole (10(-6) M; n = 7) and theophylline (5 X 10(-5) M; n = 6). CO2 reactivity was significantly decreased after superfusion of dipyridamole (0.57 +/- 0.08; mean +/- SEM) as compared with mock cerebrospinal fluid (CSF) (0.97 +/- 0.17, p less than 0.05, n = 7). On the other hand, CO2 reactivity after superfusion of theophylline was increased (1.63 +/- 0.28) as compared with mock CSF (1.00 +/- 0.20, p less than 0.05, n = 6), indicating that adenosine is involved in hypocarbic vasoconstriction.

Effects of deoxycoformycin on adenosine, inosine, hypoxanthine, xanthine, and uric acid release from the hypoxemic rat cerebral cortex

The effects of the adenosine deaminase inhibitor, deoxycoformycin, on purine release from the rat cerebral cortex were studied with the cortical cup technique. Deoxycoformycin (5 and 500 micrograms/kg i.v.) enhanced the hypoxia/ischemia-evoked release of adenosine from the cerebral cortex, indicating a marked rise in the adenosine content of interstitial fluid in the cerebral cortex. Inosine and hypoxanthine release were attenuated at the higher dose of deoxycoformycin. Uric acid release into the cortical perfusates was enhanced at the higher dose level. These results demonstrate that low doses of deoxycoformycin can be used to elevate interstitial levels of adenosine in the brain during hypoxia, and to depress the formation of some of its metabolites. The elevation of hypoxia/ischemia-evoked adenosine levels can account for the previously reported potentiation of hypoxia-evoked increases in rat cerebral blood flow after deoxycoformycin administration. The potential therapeutic utility of these findings is discussed.

Enhancement of adenosine-induced relaxant responses of the guinea-pig isolated taenia coli and aortae by a novel nootropic agent BY-1949: comparison with dipyridamole

In muscle strips of guinea-pig taenia coli and aortae, the effects of 3-methoxy-11-methyldibenz[b,f][1,4]oxazepine-8-carboxylate (BY-1949) on the relaxant response to adenosine have been compared with those of dipyridamole. BY-1949 (3 x 10(-5)-10(-4) M) as well as dipyridamole (3 x 10(-7)-10(-5) M) resulted in a significant potentiation of relaxant responses to a cumulative administration of adenosine (10(-6)-10(-3) M), but the drugs did not affect the response to noradrenaline and acetylcholine. The results indicate that BY-1949 exerts a selective potentiating effect on reactivity to exogenous adenosine.

Theophylline effect on the cerebral blood flow response to hypoxemia

Cerebral oxygen delivery (CO2D) remains nearly constant over a wide range of cerebral perfusion pressure and arterial oxygen content. In response to a decrease in arterial oxygen content secondary to hypoxemia, cerebral blood flow (CBF) increases, a response likely mediated by the release of adenosine. We studied the effect of theophylline, a potent adenosine antagonist, on CBF and cerebral oxygen delivery (CO2D) during hypoxemia in five healthy adult male volunteers. The CBF was measured using 133Xe clearance under conditions of (1) normoxemia (O2 saturation greater than 95 percent); (2) hypoxemia (O2 saturation = 80 percent); (3) normoxemia following aminophylline (the ethylene diamine salt of theophylline) 6 mg/kg intravenously; and (4) hypoxemia following aminophylline. Aminophylline decreased CBF and CO2D during both normoxemia and hypoxemia, but did not prevent the increase in CBF accompanying hypoxemia, suggesting that the increase in CBF in response to hypoxemia may not be mediated by adenosine or that customary doses of aminophylline are insufficient to inhibit adenosine-mediated cerebral vasodilation in response to hypoxemia. The significant decrease in CBF and CO2D observed following aminophylline is potentially clinically important and should be considered in the selection of bronchodilator therapy.

Effects of nifedipine and felodipine on adenosine and inosine release from the hypoxemic rat cerebral cortex

The cerebral cortical cup technique has been used to study the effects of nifedipine and felodipine on adenosine and inosine release from the rat brain. After basal and hypoxia (8% 02)-evoked control levels of purine release had been established, these 1,4-dihydropyridine calcium antagonists were administered intraperitoneally (1 mg/kg). Both agents depressed basal levels of purine efflux and suppressed the hypoxia-evoked release of adenosine and inosine. An inhibition of the transporter that mediates purine efflux from brain cells is likely to account for the suppression of release from the cerebral cortex. A reduced release of adenosine into the interstitial space also explains the ability of both agents to block the increase in CBF evoked by hypoxic challenges.

Adenosine receptors and the nucleoside transporter in human brain vasculature

Evidence suggests that adenosine modulates neuronal and cerebral vascular functions by interacting with specific receptors on brain cells and blood vessels. Adenosine and other nucleosides are also transported across the blood-brain barrier via a saturable, carrier-mediated mechanism. Using direct ligand binding methods, we studied the two adenosine receptor subtypes, A1 and A2 and the nucleoside transporter moiety in human brain microvessels, pial vessels, choroid plexus, and cerebral cortex membranes. The following specific tritiated ligands were used: cyclohexyladenosine (CHA) for A1 receptors; 5'-N-ethylcarboxamide adenosine (NECA) for A2 receptors; nitrobenzylthioinosine (NBMPR) and dipyridamole (DPY) for nucleoside transporters. We find that cerebral microvessels, pial vessels, and choroid plexus have few, if any, A1 receptors, in contradistinction to cerebral membranes, which have a 10-20-fold higher density of A1 receptor sites. Specific high-affinity NECA binding to A2 receptors in cerebral microvessels, pial vessels, and choroid plexus was saturable and was equivalent to that of cerebral cortical membranes. The Bmax and Kd of the high-affinity NECA binding to vessel preparations were approximately 1.3 pmol/mg protein and approximately 250 nM, respectively, which is similar to our previous findings in the rat and pig. NBMPR and DPY binding were also saturable and were consistent with a single class of high-affinity binding sites. The density of nucleoside transporters was approximately four-fold higher in cerebral microvessels than in cerebral cortex, pial vessels, and choroid plexus. These results suggest that human cerebral microvessels have A2, but not A1, receptors and are particularly enriched with the adenosine transporter moiety.

Determination of rat cerebrospinal fluid concentrations of adenosine, inosine, hypoxanthine, xanthine and uric acid by high performance liquid chromatography

Isocratic reverse-phase high performance liquid chromatography techniques were developed to resolve and quantitate the purine nucleosides adenosine (Ado) and inosine (Ino) and their metabolites hypoxanthine (Hyp), xanthine (Xan), and uric acid (UA) in the cerebrospinal fluid of the rat. The moving phase composition for resolving hypoxanthine, xanthine and uric acid was a 0.22 M, pH 5.8 phosphate buffer. The moving phase composition for resolving adenosine and inosine was a 0.22 M, pH 6.8 phosphate buffer, 7% methanol (v/v) and 2.5 mM tetrabutylammonium phosphate. The observed cerebrospinal fluid concentrations in the rat were: Ado = 35 +/- 9 nM (s.e.m.), Ino = 359 +/- 85 nM, Hyp = 243 +/- 77 nM, Xan = 1340 +/- 423 nM and UA = 6130 +/- 678 nM.

Ca++ antagonists and ACAT inhibitors promote cholesterol efflux from macrophages by different mechanisms. I. Characterization of cellular lipid metabolism

The effects of the slow Ca++ channel blocker, nifedipine, and ACAT inhibitor, octimibate, on the cholesterol metabolism of cholesterol-loaded macrophages were compared. We demonstrated that apolipoprotein A-I containing high density lipoproteins (HDL) bind to specific receptor sites on macrophages, are internalized, take up cholesterol, and are then released from the cells as native lipoproteins. The ACAT inhibitor enhances HDL receptor activity and promotes HDL-mediated cholesterol efflux from cultured mouse peritoneal macrophages. In contrast, the Ca++ antagonist increases acetyl LDL-mediated cholesterol influx, abolishes the increase in HDL binding induced by cholesterol accumulation, enhances apo E synthesis, and promotes cholesterol efflux by a mechanism independent of the presence of HDL in the surrounding medium. Concomitantly, a decrease in nucleoside transporter activity, an increase in intracellular ATP hydrolysis, adenosine and cyclic AMP concentration, and a stimulation of the activities of acid and neutral cholesteryl ester hydrolase and ACAT indicated that protein kinase A-catalyzed phosphorylation reactions might be involved in the increase in cholesterol efflux. The Ca++ antagonist-induced efflux occurred only with lysosomal-associated cholesterol, while the ACAT inhibitor acted on the formation of cytoplasmic lipid droplets. The secreted lipoprotein particles contained 68% unesterified cholesterol and 21% phospholipids, 8% esterified cholesterol, and 3% triglycerides. The phospholipid components were: 72% phosphatidylcholine, 22% sphingomyelin, and 6% phosphatidylserine, phosphatidylinositol, and phosphatidylethanolamine. We conclude that macrophages release cholesterol in two ways: 1) an HDL-mediated release of unesterified cholesterol increasing upon ACAT inhibition, and 2) an HDL-independent secretion of cholesterol which can be amplified by Ca++ antagonists.

Role of adenine compounds in autonomic neurotransmission

Clearly adenine compounds exert numerous effects throughout the autonomic nervous system. The responses of various peripheral tissues to purines are summarized in Table 2. The evidence supporting a possible excitatory neurotransmitter function for ATP is very good in the vas deferens and good in both the bladder detrusor and certain blood vessels. ATP may also be an excitatory neurotransmitter in the colon, hepatocytes and frog atrium. These responses appear to be mediated by P2x-purinoceptors. There is good evidence supporting a role for ATP as an inhibitory neurotransmitter in the taenia coli and duodenum, and some support in the anal sphincter and possibly the rabbit portal vein; these responses appear to be mediated by P2y-purinoceptors. There is good evidence against ATP being an inhibitory neurotransmitter in the stomach fundic muscle and ileum. ATP (or more likely its metabolite adenosine) may act as an inhibitory neurotransmitter by interacting with postsynaptic P1-purinoceptors in cultured sympathetic neurones and also in the parasympathetic vesicle ganglion of the cat. It seems likely that ATP released from heart, platelets or vascular endothelium could be an endogenous relaxant of blood vessels through its actions on the endothelium. Although the addition of exogenous adenosine affects many tissues, evidence supporting modulatory functions for endogenous extracellular adenosine has only been clearly demonstrated in the ileum, gallbladder, vas deferens, fallopian tubes, kidney, blood vessels, carotid sinus, heart and adipose tissue. Both ATP and adenosine, released during periods of hypoxia or ischemia, could exert negative inotropic, chronotropic and dromotropic actions in the heart. In many cases, the potential sources of extracellular purines have not been established. This is particularly important when attempting to establish a neurotransmitter function for ATP in a tissue. For instance, the one outstanding piece of evidence required to confirm that ATP is an excitatory neurotransmitter released from sympathetic nerves in blood vessels is the unequivocal demonstration that it is, in fact, released from the sympathetic nerves when they are stimulated. To date, only the release of radiolabeled metabolites of ATP, possibly from post- rather than presynaptic sites, has been detected. Studies of the release of ATP are complicated by its rapid degradation extracellularly by ecto-ATPase. Unfortunately, there are no specific inhibitors of ecto-ATPase available at present, but one hopes that a suitable inhibitor will be developed shortly.(ABSTRACT TRUNCATED AT 400 WORDS)

Increases in cerebral cortical perfusate adenosine and inosine concentrations during hypoxia and ischemia

The cerebral cortical cup technique was used to monitor changes in adenosine and inosine levels in the rat cerebral cortex during periods of hypoxia, anoxia, or hemorrhagic hypotension. Basal levels of adenosine and inosine in cortical perfusates stabilized within 10 min at concentrations of 30-50 and 75-130 nM, respectively. Comparable levels were observed in normal CSF collected from the cisterna magna. Reductions in the oxygen content of the inspired air (14, 12, 8, and 5% oxygen) resulted in increases in the adenosine and inosine levels in the cortical perfusates, the magnitude of the increase being progressively more pronounced with greater reductions in the oxygen content. Cerebral anoxia/ischemia, induced by 100% nitrogen inhalation, caused a rapid increase in the adenosine and inosine contents of the cortical perfusates. Hemorrhagic hypotension (46.1 +/- 1.7 mm Hg) of 5 min duration did not result in an elevated adenosine or inosine release. The results suggest that interstitial fluid adenosine levels are likely to be in the low nM range in the normoxic animal and are capable of rapid increases during hypoxic or anoxic episodes. The findings support the adenosine hypothesis of CBF regulation.

Cardiovascular effects of microinjections of adenosine analogs into the fourth ventricle of rats

Rats were implanted with chronic indwelling cannulae into the posterior region of the fourth ventricle. After recovery from surgery, acute experiments on blood pressure were conducted under urethane anesthesia. The blood pressure and heart rate responses following administration of two adenosine analogs, NECA and L-PIA were examined. Microinjections of both analogs produced dose-dependent reductions in blood pressure and heart rate. NECA was approximately 20-fold more potent than L-PIA in reducing blood pressure and depressing heart rate. The cardiovascular effects of both analogs were antagonized by parenteral injections of caffeine. These findings show that microinjections of analogs of adenosine into the fourth ventricle can influence areas of the central nervous system involved in cardiovascular control.

Effects of dipyridamole and theophylline on reactive hyperaemia in subcutaneous adipose tissue in humans

The importance of adenosine for reactive hyperaemia in subcutaneous adipose tissue was studied in healthy volunteers, using the adenosine uptake inhibitor dipyridamole (bolus 0.1 mg/kg i.v. followed by infusion of 0.7 microgram/kg/min) and the adenosine receptor antagonist theophylline (4 or 6 mg/kg i.v.). Basal blood flow, total blood flow and hyperaemia (total minus basal flow) after a 20-min arterial occlusion were measured in the distal femoral region by the 133Xe washout technique with and without drug treatment. Basal blood flow (mean +/- SEM) was 2.4 +/- 0.3 ml/min/100 g, while total post-occlusive flow and total reactive hyperaemia were 97.3 +/- 8.4 and 61.8 +/- 6.5 ml/100 g, respectively, without drug treatment. Basal blood flow was unaffected by dipyridamole but the total flow and hyperaemia were enhanced by 49 +/- 24 and 60 +/- 31%, respectively (P less than 0.05 for both). This enhancement was due to increases in both amplitude and duration of the hyperaemia. Neither basal blood flow, total post-occlusive flow nor hyperaemia were significantly altered by theophylline. The amplitude of the enhanced hyperaemia during dipyridamole was not significantly counteracted by simultaneous theophylline treatment (6 mg/kg) but the duration of hyperaemia was reduced from 13 +/- 1 to 8 +/- 1 min (P less than 0.01). The results suggest that endogenous adenosine does not regulate basal blood flow or reactive hyperaemia of limited duration in human adipose tissue. However, reactive hyperaemia may be enhanced by pharmacological elevation of endogenous adenosine levels.

The effect of aminophylline on cerebral blood flow in patients with chronic obstructive pulmonary disease

Aminophylline has been shown to produce a reduction in cerebral blood flow (CBF) in animal models and patients with neurologic symptoms or signs. The effect of aminophylline on regional CBF (rCBF) in patients with chronic obstructive pulmonary disease (COPD) has not previously been reported to our knowledge. We studied the effect of loading and maintenance infusions of aminophylline on CBF in five subjects with moderate to severe COPD. rCBF was determined in eight homologous regions of each cerebral hemisphere at three intervals: (1) baseline; (2) following the IV loading dose of aminophylline (6.0 mg/kg body weight); and (3) early and late in the maintenance infusion (0.5 mg/kg/hr) period. Aminophylline loading caused a 26 percent reduction (p = 0.005) in mean rCBF from 40.6 +/- 5.2 (SD) ml/100 g/min to 30.1 +/- 6.0 ml/100 g/min. A 23 percent reduction (31.5 +/- 6.9 ml/100 g/min) persisted throughout the maintenance phase. Thus, aminophylline, as customarily used in subjects with COPD, is associated with a significant reduction in rCBF.

An involvement of adenosine in cerebral blood flow regulation during hypercapnia

The possibility that endogenously released adenosine, a potent vasodilator, is involved in the increase in cerebral blood flow (CBF) response to hypercapnia has been investigated in an anesthetized, paralyzed rat model. The left retroglenoid vein was cannulated and cerebral venous blood flow measured with a drop counter. Animals were ventilated with a 40% oxygen, 60% nitrogen gas mixture. At 20 min intervals, at a constant rate of flow, the inspired gas mixture was altered to 10% carbon dioxide, 40% oxygen, 50% nitrogen for periods of between 30-90 sec. This brief hypercapnic challenge induced a rapid increase in CBF in the absence of any change in MABP. An involvement of adenosine in this response was demonstrated using an adenosine antagonist, caffeine, an uptake inhibitor, dipyridamole and an adenosine deaminase inhibitor, deoxycoformycin. Caffeine (10 and 20 mg/kg i.p.) 15 min prior to hypercapnic challenges significantly decreased the peak increases in CBF. Dipyridamole (0.1 mg/kg) and deoxycoformycin (0.1 microgram/kg) enhanced the peak increases in flow. These results are consistent with an important role for adenosine in coupling PCO2 to cerebral blood flow.

Nucleoside transporter of cerebral microvessels and choroid plexus

The nucleoside transporter of cerebral microvessels and choroid plexus was identified and characterized using [3H]nitrobenzylthioinosine (NBMPR) as a specific probe. [3H]NBMPR bound reversibly and with high affinity to a single specific site in particulate fractions of cerebral microvessels, choroid plexus, and cerebral cortex of the rat and the pig. The dissociation constants (KD 0.1-0.7 nM) were similar in the various tissue preparations from each species, but the maximal binding capacities (Bmax) were about fivefold higher in cerebral microvessels and choroid plexus than in the cerebral cortex. Nitrobenzylthioguanosine and dipyridamole were the most potent competitors for [3H]NBMPR binding. Several naturally occurring nucleosides displaced specific [3H]NBMPR binding to cerebral microvessels in vitro, in a rank order that correlated well with their ability to cross the blood-brain barrier in vivo. Adenosine analogues and theophylline were less effective in displacing [3H]NBMPR binding than in displacing adenosine receptor ligands. Photoactivation of cerebral microvessels and choroid plexus bound with [3H]NBMPR followed by solubilization and polyacrylamide gel electrophoresis labeled a protein(s) with a molecular weight of approximately 60,000. These results indicate that cerebral microvessels and choroid plexus have a much higher density of the nucleoside transporter moiety than the cerebral cortex and that this nucleoside transporter has pharmacological properties and a molecular weight similar to those of erythrocytes and other mammalian tissues.