Dynamic cerebral and systemic circulatory effects of adenosine, theophylline and dipyridamole

Methylxanthines, seizures, and excitotoxicity

Clinical evidence, in particular the wide use of theophylline as a bronchodilator, suggests that methylxanthines can cause seizures in patients without known underlying epilepsy. Theophylline is also known to be an added risk factor for seizure exacerbation in patients with epilepsy. The proconvulsant activity of methylxanthines can best be explained by their antagonizing the brain's own anticonvulsant adenosine. Recent evidence suggests that adenosine dysfunction is a pathological hallmark of epilepsy contributing to seizure generation and seizure spread. Conversely, adenosine augmentation therapies are effective in seizure suppression and prevention, whereas adenosine receptor antagonists such as methylxanthines generally exacerbate seizures. The impact of the methylxanthines caffeine and theophylline on seizures and excitotoxicity depends on timing, dose, and acute versus chronic use. New findings suggest a role of free radicals in theophylline-induced seizures, and adenosine-independent mechanisms for seizure generation have been proposed.

First-line therapy for theophylline-associated seizures

Theophylline-associated seizures (TAS) are considered a neurologic emergency, as they can sometimes be intractable and difficult to stop with standard treatments such as intravenous administration of diazepam. As a consequence, a proportion of patients who experience status epilepticus while receiving theophylline will require endotracheal intubation. The optimal first-line therapy for TAS has not yet been fully investigated. We compared 54 cases of TAS with 779 cases of non-TAS, that had presented at a single institution between 1991 and 2002. Among the 54 cases of TAS, 36 experienced generalized tonic-clonic seizures, with the remainder experiencing partial seizures. TAS occurred mainly in children under 3 years of age, and serum theophylline levels were within the therapeutic range in 78% of the cases. The duration of TAS tended to be longer than for non-TAS, and intravenous administration of diazepam was less effective in controlling TAS (45%), compared with non-TAS (68%). Many cases required repeated injections of diazepam, and 15 cases (27%) eventually required endotracheal intubation. Reports concerning the therapy for TAS were also reviewed. Theophylline is known to antagonize the effects of benzodiazepines, and this may explain why drugs such as diazepam are relatively ineffective in treating TAS. In TAS, the prompt use of barbiturates is recommended when diazepam is not effective, to avoid potential brain injury secondary to status epilepticus.

Retinal discoloration test

There are considerable variations in the size and the origin of internal carotid arteries (ICAs) of New Zealand white rabbits. A simple test that could correctly identify the ICA could facilitate intracarotid delivery of drugs and embolic material for research in these animals. Five conventional methods for identifying the ICA proved inferior to a novel retinal discoloration test (RDT). A positive RDT implies a change in color of the retinal reflex after injection of 0.2 mL of indigo-carmine blue (ICB). We conclude that RDT is a useful test for correctly identifying and isolating the ICA in this animal species.

Caffeine impairs short-term neurological outcome after concussive head injury in rats

OBJECTIVE

Adenosine is an endogenous neuroprotective agent that is released during ischemia, hypoxia, epilepsy, and ischemic brain injury. Caffeine is a receptor antagonist for adenosine that might interfere with the neuroprotective effect of adenosine in ischemic-hypoxic conditions. An investigation was undertaken to study the effect of caffeine on neurological function, edema formation, and blood-brain barrier permeability after experimental head injury in rats.

METHODS

Adult female Wistar rats classified into different groups received caffeine intraperitoneally at doses of 0, 50, 100, and 150 mg/kg body weight. Thirty minutes after the caffeine treatment, the animals were subjected to concussive head injury (CHI) administered by a controlled cortical impact device. Neurological severity score was recorded in each rat at 2 hours after CHI. Specific gravity, water content (as an indicator of edema), and blood-brain barrier impairment were analyzed in the cortical tissue surrounding the injury site. The levels of myeloperoxidase and malondialdehyde in the cortical region were measured as indicators of neutrophil infiltration and lipid peroxidation, respectively.

RESULTS

A significant increase in righting latency and neurological deficiency after CHI was observed in caffeine-treated rats as compared with untreated animals. Although no deaths occurred in the rats exposed to CHI after pretreatment with saline, pretreatment with caffeine caused significant mortality of animals after trauma in a dose-dependent manner. Caffeine also exacerbated neutrophil infiltration, edema, and disruption of blood-brain barrier in the traumatic cortex. Light microscopy of brain revealed more severe hemorrhage and neuronal degeneration in the injured hemisphere of caffeine-treated rats as compared with rats in the injury-alone group. A significant increase in malondialdehyde in the brain of injured rats treated with caffeine before CHI clearly indicated the role of oxidative stress.

CONCLUSION

Caffeine adversely affects outcome after CHI, possibly as a result of blockade of adenosine receptors. The findings also point toward the involvement of free radical-mediated neuronal damage in caffeine-induced exacerbation of neurotrauma.

Effects of theophylline on human cerebral blood flow velocity during halothane and isoflurane anaesthesia

The acute influence of therapeutic doses of theophylline on cerebral blood flow velocities during general anaesthesia induced either by halothane or isoflurane has not been elucidated previously. The aim of this study, was therefore to investigate these effects during steady state anaesthetic conditions in anaesthetized patients using transcranial Doppler sonography. Thirty-four Patients were investigated. Anaesthesia was maintained either with 1 MAC (minimum alveolar concentration) halothane (n = 16) or 1 MAC isoflurane (n = 18); FiO2: 40%; end-expiratory pCO2: normocapnia. Theophylline administration was performed before surgery by infusing 6 mg kg-1 over a period of 7.5 min. Measurements were recorded prior to theophylline administration, and immediately after 2 mg kg-1 (2.5 min), 4 mg kg-1 (5 min) and 6 mg kg-1 (7.5 min), as well as 5, 10, 15, 20, 30 and 45 min after theophylline administration. Recorded variables included blood pressure, heart rate, arterial oxygen saturation, end-expiratory pCO2, body temperatures, middle cerebral artery blood flow velocity and pulsatility index. Theophylline administration was well tolerated by all study subjects. Heart rate, blood pressure, body temperatures, arterial oxygen saturation and end-expiratory pCO2 remained constant during the whole examination period. Following theophylline, mean blood flow velocity decreased by 25% in the halothane and by 30% in the isoflurane group (P < 0.05). By the end of the examination period, mean blood flow velocity had not yet recovered to the initial values whereby in both groups the mean blood flow velocity was 26% below the initial values. In the isoflurane group, the pulsatility index increased by about 20% (P < 0.05) and remained high until the end of the examination. In the halothane group, however, there were no changes in the pulsatility index. Our results show a remarkable, significant and simultaneous decrease in blood flow velocity in the middle cerebral artery associated with theophylline administration during halothane and isoflurane anaesthesia in man.

Hypoxia stimulates cerebral blood flow in the estuarine crocodile (Crocodylus porosus)

The effect of N2 respiration on cerebral blood flow (CBF) velocity on the dorsal surface of cerebellum was examined in the estuarine crocodile, Crocodylus porosus, using epi-illumination microscopy. Twelve minutes of N2 respiration resulted in a 126% increase in CBF velocity. N2 respiration had no effect on blood pressure, indicating an underlying cerebral vasodilation. In addition, heart rate increased significantly. Systemic injections of aminophylline and the NO synthase (NOS) inhibitor nitro-L-arginine (L-NA) did not affect the hypoxia induced increase in CBF. We conclude that C. porosus responds to hypoxia with adenosine and nitric oxide (NO) independent cerebral vasodilation, and that this is likely to be a mechanism protecting the brain from energy deficiency during prolonged dives.

Adenosine receptor antagonism affects regional resting vascular resistance during rat peritoneal sepsis

BACKGROUND

To identify vascular beds where endogenous adenosine plays a significant role as a mediator of resting perfusion alterations associated with sepsis, we tested the hypothesis that adenosine receptor blockade would cause differential regional increases in vascular resistance during intraperitoneal (ip) sepsis in the rat.

MATERIALS AND METHODS

Rats (250-350 g) were catheterized and randomized to septic or nonseptic groups. Sepsis was induced with an ip injection of cecal slurry (150 mg/kg in D5W; 5 ml/kg), and baseline hemodynamics, cardiac output (CO), and blood flows (microspheres) were measured 24 h later. Animals then received the adenosine receptor antagonist 8-phenyltheophylline (8-PTH; 10 mM, 1.5 ml/kg), its vehicle (1.5 ml/kg), or normal saline (1.5 ml/kg), iv, and measurements were repeated.

RESULTS

Septic animals treated with 8-PTH had a significant increase in skeletal muscle, hepatic portal, and cerebral vascular resistance with concomitant decreases in CO when compared with vehicle at 1 min. No significant resistance changes were observed in the renal, adipose, or coronary vasculatures. Adenosine receptor blockade caused a significant increase in +dP/dt and -dP/dt during sepsis, indicating that the reduced CO was not secondary to myocardial depression.

CONCLUSIONS

These data suggest that adenosine receptor-mediated actions during sepsis affect vascular beds selectively and indicate a significant role for adenosine in resting perfusion redistribution in sepsis.

Effect of i.v. dipyridamole on cerebral blood flow, blood pressure, plasma adenosine and cAMP levels in rabbits

In response to intravenous administration of dipyridamole, the quantitative and temporal changes in plasma adenosine and cyclic AMP (cAMP) levels in relation to the changes in cerebral blood flow (CBF) and mean arterial blood pressure (MABP) have not been studied. Therefore, we investigated simultaneously the changes in CBF (hydrogen and thermal clearance methods), MABP, plasma adenosine (HPLC) and cAMP (radioimmunoassay) levels for 1 h after intravenous injection of 0.7 and 1.4 mg/kg dipyridamole in rabbits. In separate experiments, only plasma adenosine concentrations were measured to determine how and for how long intravenous administration of 0.7 mg/kg dipyridamole is able to inhibit the removal of plasma adenosine. Dipyridamole decreased MABP, increased plasma adenosine and cAMP levels in a dose-dependent manner. The dose-dependency of increases in CBF could not be demonstrated owing to the marked hypotension. The increase in plasma adenosine concentrations was biphasic. The first peak could be detected at the end of the dipyridamole injection. The second peak occurred 20 min after drug administration, simultaneously with the maximal increases in plasma cAMP level and CBF, whereas the maximal fall in MABP developed earlier. Intravenous administration of 0.7 mg/kg dipyridamole inhibited adenosine uptake only by 25%, which lasted less than 10 min. We concluded that intravenously given dipyridamole is responsible only for the initial short-lasting elevation of plasma adenosine concentration, and is able to induce vasodilation without either dipyridamole itself or adenosine necessarily gaining access to the muscular layer.

The effect of 2-chloroadenosine on lipid peroxide level during experimental cerebral ischemia-reperfusion in gerbils

Oxygen free radicals may be implicated in the pathogenesis of ischemia-reperfusion damage. It is known that 2-chloroadenosine (2-CADO) has neuromodulatory effects and prevents the neuronal damage seen in the period of postischemia reperfusion. However, direct effects of 2-CADO on lipid peroxidation have not been investigated previously. The attack on the cell membrane by free radicals leads to lipid peroxidation, which can be assayed by the malondialdehyde (MDA) level. The aim of this study was to determine the effect of 2-CADO therapy on lipid peroxidation in experimental forebrain ischemia and postischemia reperfusion in Mongolian gerbils. Cerebral ischemia was induced by a bilateral 30-mm occlusion of the common carotid arteries. 2-Chloroadenosine (0.6 mg/kg, IV) was administered 5 min subsequent to ischemia. Ischemia was followed by reperfusion for 30 min. The MDA level was measured by the thiobarbituric acid (TBA) test. Bilateral carotid artery occlusion for 30 min in gerbils resulted in no significant change in MDA level in the brain. The MDA level was higher in postischemia reperfusion than in the ischemic group. 2-Chloroadenosine treatment did not change the MDA level in the ischemic period. However, the MDA level recovered significantly upon 2-CADO therapy during reperfusion following ischemia. These results suggest that 2-CADO may offer some degree of protection against oxidative stress in cerebral ischemia-reperfusion damage.

Theophylline and hypoxic ventilatory response in the rat isolated brainstem-spinal cord

We have used the isolated brainstem-spinal cord preparation of the neonatal rat to study the effects of theophylline on the ventilatory response to hypoxia. The brainstem-spinal cord was isolated from neonatal rats (0-4 days) and superfused with mock cerebrospinal fluid (CSF), equilibrated with a gas mixture (FO2, 0.90; FCO2, 0.02; FN2, 0.08; control CSF) at 27 degrees C. We recorded phrenic nerve discharge from C4 roots, using suction electrodes, and measured respiratory frequency (fR) and the amplitude of the integrated phrenic neurogram (integral of phr). We examined how theophylline and the specific adenosine antagonist, 8-p-sulfophenyltheophylline (SPT), modify the ventilatory response to hypoxia. The response during superfusion with hypoxic CSF (FO2, 0.06) consisted of a marked decrease in fR (to 60% of control) and a slight decline in integral of phr (to 85% of control). By contrast, in the presence of theophylline (30 mg/L = 165 microM) and SPT (5 mg/L = 15 microM) in the superfusate hypoxia reduced fR only moderately (to 87% of control) and exerted virtually no effect on integral of phr (105% of control). Theophylline and SPT attenuated the rate of decrease in fR and completely blocked the decrease in integral of phr. There was no difference between the effects of theophylline and those of SPT. The results suggest that theophylline attenuates hypoxic respiratory depression, and that this effect is mediated by the blockade of adenosine.

Effects of dipyridamole in spontaneously hypertensive rabbits with diffuse chronic cerebral ischemia

The effect of intravenous dipyridamole (0.7 mg/kg) on cerebral blood flow (CBF), mean arterial blood pressure (MABP), heart rate, respiration rate, cerebral electrical activity, arterial blood gases, pH, and glucose was investigated in 14 normotensive and 14 stroke-prone spontaneously hypertensive anesthetized rabbits. CBF was measured by hydrogen and heat clearance. In both groups, MABP decreased (normotensive: -24 mm Hg, hypertensive: -47 mm Hg; ANOVA: P < 0.0001) and CBF increased (normotensive: +59 ml/100 g/min, hypertensive: +72 ml/100 g/min; ANOVA: P < 0.0002). CBF returned to the initial level 21 min later in hypertensive than in normotensive rabbits. Changes in other parameters were insignificant. In additional experiments, 30 mg/kg theophylline entirely prevented the cerebral vasodilator and systemic hypotensive effects of dipyridamole in both normotensive and hypertensive rabbits. We conclude that, in stroke-prone spontaneously hypertensive rabbits, the longer-lasting and larger CBF increase in response to dipyridamole may be attributed to reversible functional changes in the cerebral vasculature resulting from hypertension.

Effects of the acute administration of a new trimethylxanthine derivative, S 9977-2, on local cerebral blood flow and glucose utilization in the rat

S 9977-2 is a new trimethylxanthine derivative with promnesic properties. Its effects on cerebral glucose utilization and blood flow were studied by means of quantitative autoradiography. S 9977-2 was injected intravenously into adult rats at doses of 0.1, 1.0 and 10 mg/kg. At 0.1 mg/kg, S 9977-2 induced a significant increase in cerebral glucose utilization over control values in two white matter areas and in the vestibular nucleus. At 1.0 mg/kg, glucose utilization was affected in 14 areas out of the 63 studied, mainly limbic regions such as the hippocampus, raphe nuclei and locus coeruleus, as well as some posterior areas. Conversely, after the injection of 10 mg/kg S 9977-2, cerebral glucose utilization was similar to that of control rats. At the three doses tested, S9977-2 did not induce any significant variation in local rates of cerebral blood flow compared to those of controls. Likewise, S 9977-2 did not change the level of coupling between cerebral blood flow and metabolism, except at 10 mg/kg, where a relative hypoperfusion at a constant metabolic level was recorded. These data show that, at 1.0 mg/kg, S 9977-2 increased glucose utilization in hippocampal areas, an effect which may be related to the promnesic properties of this compound at the same dose. Moreover, at low doses, the lack of change in the level of coupling between cerebral blood flow and metabolism is indicative of the rather selective action of this compound, compared to that of caffeine. Thus S9977-2 should have therapeutic effects, mainly via its promnesic properties, without having many side effects.

Caffeine and the central nervous system: mechanisms of action, biochemical, metabolic and psychostimulant effects

Caffeine is the most widely consumed central-nervous-system stimulant. Three main mechanisms of action of caffeine on the central nervous system have been described. Mobilization of intracellular calcium and inhibition of specific phosphodiesterases only occur at high non-physiological concentrations of caffeine. The only likely mechanism of action of the methylxanthine is the antagonism at the level of adenosine receptors. Caffeine increases energy metabolism throughout the brain but decreases at the same time cerebral blood flow, inducing a relative brain hypoperfusion. Caffeine activates noradrenaline neurons and seems to affect the local release of dopamine. Many of the alerting effects of caffeine may be related to the action of the methylxanthine on serotonin neurons. The methylxanthine induces dose-response increases in locomotor activity in animals. Its psychostimulant action on man is, however, often subtle and not very easy to detect. The effects of caffeine on learning, memory, performance and coordination are rather related to the methylxanthine action on arousal, vigilance and fatigue. Caffeine exerts obvious effects on anxiety and sleep which vary according to individual sensitivity to the methylxanthine. However, children in general do not appear more sensitive to methylxanthine effects than adults. The central nervous system does not seem to develop a great tolerance to the effects of caffeine although dependence and withdrawal symptoms are reported.

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.

Acute dyskinesias in young asthmatics treated with theophylline

Two young children, 5 and 30 months of age, developed acute transient dyskinesias. The novel features of these patients were classic orobuccal-lingual dyskinesias, their young ages, and the association with asthma and theophylline. The movements resembled tardive dyskinesias of older patients on neuroleptics, but neither patient had any exposure to neuroleptic drugs. Choreiform movements were moderately severe and remitted several hours after discontinuation of theophylline or over days in the patient who remained on the drug. No symptomatic treatment of dyskinesia was required. Both infants otherwise had normal neurologic examinations with no clinical evidence of meningoencephalitis, seizures, or stroke. Both infants required hospitalization for respiratory distress, but not intensive care. The highest theophylline levels measured in these patients were 22 and 25 micrograms/ml and levels determined closest to the appearance of dyskinesias were lower. Urine toxicology screen for other drugs and routine blood work were normal. The infants, examined subsequently for 1 1/2-2 years for asthma, have demonstrated no reappearance of dyskinesias or other neurologic abnormalities. We propose an interaction of theophylline, hypoxemia, or other factors related to asthma in the pathophysiology of reversible dyskinesia in our patients.

Adenosine transport systems on dissociated brain cells from mouse, guinea-pig, and rat

The kinetics and sodium dependence of adenosine transport were determined using an inhibitor-stop method on dissociated cell body preparations obtained from mouse, guinea-pig and rat brain. Transport affinity (KT) values for the high affinity adenosine transport systems (KT(H] were significantly different between these three species; mean +/- SEM values were 0.34 +/- 0.1 in mouse, 0.9 +/- 0.2 in rat, and 1.5 +/- 0.5 microM in guinea-pig. The KT values for the low affinity transport system (KT(L) were not different between the three species. Brain cells from rat displayed a significantly greater maximal capacity to accumulate [3H]adenosine (Vmax) than did mouse or guinea-pig for the high affinity system, or than did mouse for the low affinity system. When sodium chloride was replaced in the transport medium with choline chloride, the KT(H) values for guinea-pig and rat were both increased by approximately 100%; only in rat did the change reach statistical significance. The sodium-dependence of adenosine transport in mouse brain was clearly absent. The differences between KT(H) values in mouse and those in guinea-pig or rat were accentuated in the absence of sodium. The differences in kinetic values, ionic requirements, and pharmacological characteristics between adenosine transporters in CNS tissues of mouse, guinea-pig and rat may help account for some of the variability noted among species in terms of their physiological responses to adenosine.

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.

Effects of caffeine, L-phenylisopropyladenosine and their combination on local cerebral blood flow in the rat

The quantitative [14C]iodoantipyrine autoradiographic method was applied to study the effects of acute administration of caffeine and L-phenylisopropyladenosine (LPIA) separately or in combination on local cerebral blood flow in the rat. After the injection of caffeine, cerebral blood flow rates were decreased in 13 out of the 61 structures studied, mainly in motor and auditory areas. The administration of LPIA induced a general decrease in local cerebral blood flow, which was significant in only 4 regions. The combined administration of caffeine and LPIA induced decreases in blood flow rates in 17 brain areas, motor, limbic and hypothalamic structures and increases in 3 limbic regions. The results confirm previous data on the effect of caffeine on cerebral circulation. The consequences of LPIA administration on blood flow may originate partly from peripheral effects and may also be the reflection of the reduction in the energy demand of the brain. Finally, LPIA also seems to be able to modulate caffeine effects on local cerebral blood flow when injected simultaneously with caffeine.

Theophylline reduces cerebral hyperaemia and enhances brain damage induced by seizures

Hippocampal and neocortical blood flows and tissue pO2 were investigated by mass spectrometry in unanaesthetized spontaneously breathing rats during kainic acid-induced seizures to determine whether adenosine is involved in the coupling of cerebral blood flow to metabolism during enhanced metabolic demand. The possible involvement of adenosine in the neuronal damage induced by seizures was also analyzed. The intrinsic effects of theophylline and the duration of the adenosine receptor blockade by this xanthine were first tested in 8 rats. Two groups of rats were then compared: one (n = 6) received kainic acid, and the other (n = 10) theophylline 15 min prior to kainic acid administration. An additional group of 10 rats was taken for classical histology 48 h after kainic acid treatment. Theophylline significantly reduced the hyperaemia observed during seizures, prevented any tissue hyperoxia and enhanced brain damage. This strongly suggests that adenosine is partly responsible for the increase in cerebral blood flow during kainic acid-induced seizures and has neuroprotective properties.

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)

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.