Uptake of adenosine by isolated rat brain capillaries

Blood-brain barrier endogenous transporters as therapeutic targets: a new model for small molecule CNS drug discovery

INTRODUCTION

The blood-brain barrier (BBB) limits the uptake of most drugs by brain, and the traditional approach to the BBB problem is the use of medicinal chemistry to increase drug lipid solubility, and increase lipid-mediated transport across the BBB. This review advocates a new model to CNS drug discovery of BBB-penetrating small molecules, whereby drug candidates are screened for carrier-mediated transport (CMT) across the BBB.

AREAS COVERED

CMT systems are expressed by genes within the Solute Carrier (SLC) Transporter Gene Family, which now totals > 400 transporter genes. Emphasis is placed on reconciliation of the substrate transporter profile (STP) of BBB transport in vivo with the STP of the cloned SLC transporter in vitro. This reconciliation is crucial to the identification, from sometimes a large number of candidates, of the respective SLC transporter that is responsible for BBB transport in vivo for a given class of nutrients.

EXPERT OPINION

Dual track screening of a small molecule library for drugs that have the dual properties of affinity for a neural cell drug receptor target, and affinity for a BBB CMT transporter target, can lead to a revolution in how small molecule drugs are identified in CNS drug discovery programs.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

Some aspects of purinergic signaling in the ventricular system of porcine brain

BACKGROUND

Numerous signaling pathways function in the brain ventricular system, including the most important - GABAergic, glutaminergic and dopaminergic signaling. Purinergic signalization system - comprising nucleotide receptors, nucleotidases, ATP and adenosine and their degradation products - are also present in the brain. However, the precise role of nucleotide signalling pathway in the ventricular system has been not elucidated so far. The aim of our research was the identification of all three elements of purinergic signaling pathway in the porcine brain ventricular system.

RESULTS

Besides nucleotide receptors on the ependymocytes surface, we studied purines and pyrimidines in the CSF, including mechanisms of nucleotide signaling in the swine model (Sus scrofa domestica). The results indicate presence of G proteins coupled P2Y receptors on ependymocytes and also P2X receptors engaged in fast signal transmission. Additionally we found in CSF nucleotides and adenosine in the concentration sufficient to P receptors activation. These extracellular nucleotides are metabolised by adenylate kinase and nucleotidases from at least two families: NTPDases and NPPases. A low activity of these nucleotide metabolising enzymes maintains nucleotides concentration in ventricular system in micromolar range. ATP is degraded into adenosine and inosine.

CONCLUSIONS

Our results confirm the thesis about cross-talking between brain and ventricular system functioning in physiological as well as pathological conditions. The close interaction of brain and ventricular system may elicit changes in qualitative and quantitative composition of purines and pyrimidines in CSF. These changes can be dependent on the physiological state of brain, including pathological processes in CNS.

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.

Functional characterization of adenosine transport across the BBB in mice

We investigated transport characteristics of adenosine across the blood-brain barrier (BBB) in mice. Uptake clearance across the BBB was measured by using an in situ mouse brain perfusion technique and cultured mouse brain capillary endothelial cell line (MBEC4 cells). Nucleoside transporter was cloned by RT-PCR and expressed on Xenopus laevis oocyte. Both in situ and in vitro studies revealed that the adenosine uptake is concentration-dependent, Na(+)-independent and S-(p-nitrobenzyl)-6-thioinosine (NBMPR)-sensitive. The K(t) values of in situ and in vitro studies were 31.7 +/- 13.8 microM and 11.9 +/- 2.84 microM, respectively. A good correlation was found for the inhibitory effects of nucleoside analogs to adenosine uptake between in situ and in vitro studies. RT-PCR revealed the expression of RNA of mouse equilibrative nucleoside transporter (mENT1) in mouse brain capillary and MBEC4 cells. In mENT1 expressed on X. laevis oocyte, K(t) value of adenosine transport was 6.9 +/- 2.7 microM (and comparable to those in situ and in vitro studies). In conclusion, we characterized the adenosine transport across the BBB in mice by using in situ brain perfusion technique and MBEC4 cells and found that these transports share common characteristics with mENT1-mediated transport. Transport of adenosine across the BBB in mice may be attributable to mENT1.

Transporter-mediated permeation of drugs across the blood-brain barrier

Drug distribution into the brain is strictly regulated by the presence of the blood-brain barrier (BBB) that is formed by brain capillary endothelial cells. Since the endothelial cells are connected to each other by tight junctions and lack pores and/or fenestrations, compounds must cross the membranes of the cells to enter the brain from the bloodstream. Therefore, hydrophilic compounds cannot cross the barrier in the absence of specific mechanisms such as membrane transporters or endocytosis. So, for efficient supply of hydrophilic nutrients, the BBB is equipped with membrane transport systems and some of those transporter proteins have been shown to accept drug molecules and transport them into brain. In the present review, we describe mainly the transporters that are involved in drug transfer across the BBB and have been molecularly identified. The transport systems described include transporters for amino acids, monocarboxylic acids, organic cations, hexoses, nucleosides, and peptides. Most of these transporters function in the direction of influx from blood to brain; the presence of efflux transporters from brain to blood has also been demonstrated, including P-glycoprotein, MRPs, and other unknown transporters. These efflux transporters seem to be functional for detoxication and/or prevention of nonessential compounds from entering the brain. Various drugs are transported out of the brain via such efflux transporters, resulting in the decrease of CNS side effects for drugs that have pharmacological targets in peripheral tissues or in the reduction of efficacy in CNS because of the lower delivery by efflux transport. To identify the transporters functional at the BBB and to examine the possible involvement of them in drug transports by molecular and physiological approaches will provide a rational basis for controlling drug distribution to the brain.

Effect of intravenous dipyridamole on cerebral blood flow in humans. A PET study

BACKGROUND AND PURPOSE

Dipyridamole increases the concentration of circulating adenosine, which is a potent vasodilator, by inhibition of uptake of adenosine into the erythrocytes, and hence produces coronary vasodilation. However, the effects of dipyridamole on cerebral circulation is not pronounced. This study investigates the effects of intravenous dipyridamole on cerebral blood flow (CBF) in humans with use of positron emission tomography (PET).

METHODS

In each of 13 healthy subjects, CBF was measured using (15)O-labeled water and PET at rest and during hypercapnia, hypocapnia, and dipyridamole stress; corresponding CBF values were then compared.

RESULTS

CBF values during dipyridamole stress were significantly lower than those measured at rest. The dipyridamole stress PaCO(2) was also significantly lower than the resting PaCO(2). The change in CBF during dipyridamole stress relative to PaCO(2) closely followed the relationship between CBF and PaCO(2) during hypocapnia.

CONCLUSIONS

These results indicate that the observed decrease in CBF during dipyridamole stress was caused by a decrease in PaCO(2) rather than by any direct action of dipyridamole on CBF. The decrease in PaCO(2) during dipyridamole stress was most likely due to hyperventilation, which was a side effect of adenosine. These results support the hypothesis that circulating adenosine is largely prevented from binding to adenosine receptors of cerebral vessels by the blood-brain barrier.

Saturation kinetics, specificity and NBMPR sensitivity of thymidine entry into the central nervous system

It was not until the development of a technique that could measure the brain uptake of slowly moving substrates, that the saturable transport system at the blood-brain barrier (BBB) for the pyrimidine deoxyribonucleoside, thymidine, was demonstrated. The aim of this present study was to further characterize this saturable uptake system at the blood-brain and blood-CSF barriers in terms of specificity, 6-(4-nitrobenzyl)thio-9-beta-D-ribofuranosylpurine (NBMPR) sensitivity and saturation kinetics by means of the in situ brain perfusion technique in anaesthetized guinea pigs. The results indicated that the transport system identified for [3H]thymidine can also transport other pyrimidine deoxyribonucleosides (deoxycytidine) and pyrimidine ribonucleosides (uridine) and is partially NBMPR-sensitive. In addition, guanosine, monocarboxylic acids, hexoses or amino acids were not substrates for the transport system. Further studies revealed that the transport system for [3H]thymidine at the BBB has a low affinity (Km 0.20 +/- 0.06 mM), but a relatively high capacity (Vmax 1.06 +/- 0.08 nmol min(-1) g(-1)). Overall, this study is indicative of a NBMPR-sensitive (es) facilitative transport system for [3H]thymidine and the likely presence of a NBMPR-insensitive and/or sodium-dependent transport system of the N2 (cit) type at the blood-brain and blood-CSF barriers of the guinea pig.

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.

Adenosine kinase inhibition protects brain against transient focal ischemia in rats

Endogenous adenosine released locally during cerebral ischemia is neuroprotective, and agents which decrease adenosine inactivation may potentiate its protective effects. The effects of 5'-deoxy-5-iodotubercidin (5'd-5IT), an inhibitor of the adenosine-catabolizing enzyme, adenosine kinase, were studied in male Wistar rats subjected to 2 h of transient middle cerebral artery occlusion. 5'd-5IT or the vehicle (10% DMSO in saline) was administered i.p. 30 min before, and 2 h and 6 h after the induction of middle cerebral artery occlusion. The infarct volume was determine using 2,3,5-triphenyltetrazolium chloride staining 48 h after middle cerebral artery occlusion. The infarct volume was significantly reduced in rats treated with 1.85 mg/kg x 3 (57% reduction, P < 0.001) or 1.0 mg/kg x 3 (34% reduction, P < 0.05), but not 0.3 mg/kg x 3 5'd-5IT compared to vehicle-treated rats. The reduction of infarct volume was accompanied by a significant improvement in behavioral measures of neurological deficit. These data further support a role of adenosine in neuroprotection and suggest that adenosine kinase inhibition may be a useful approach to the treatment of focal cerebral ischemia.

Identification of a saturable uptake system for deoxyribonucleosides at the blood-brain and blood-cerebrospinal fluid barriers

Substances can enter the brain either directly across the blood-brain barrier or indirectly across the choroid plexuses and arachnoid membrane (blood-CSF barrier) into the CSF and then by diffusion into the brain. Earlier studies have demonstrated a saturable thymidine uptake across the blood-CSF barrier, but not across the blood-brain barrier. In this study transport of [3H]thymidine across both barriers was measured in vivo by means of a bilateral vascular brain perfusion technique in the anaesthetised guinea-pig. This method allows simultaneous and quantitative measurement of slowly penetrating solutes into both brain and CSF, under controlled conditions of arterial inflow. The results of the present study carried out over perfusion periods of up to 30 min indicated a progressive uptake of [3H]thymidine into brain and CSF, which was found to be significantly greater than the transport of D-[14C]mannitol (a plasma space marker). Furthermore, the addition of 1 mM unlabelled thymidine in the perfusate caused saturation of [3H]thymidine uptake into both brain and CSF. In conclusion, these findings suggest that thymidine can cross both the blood-brain and blood-CSF barriers in the guinea-pig by carrier-mediated transport systems.

Effects of adenosine and its analogues on isolated intracerebral arterioles. Extraluminal and intraluminal application

We evaluated the responses of brain parenchymal arterioles to intraluminal and extraluminal application of adenosine and its analogues. Intracerebral arterioles (28.4- to 60.3-microns diameter) were isolated from Sprague-Dawley rats, cannulated with micropipettes, and perfused in vitro. Both extraluminal and intraluminal adenosine, 5'-(N-ethylcarboxamido)adenosine (NECA), R-N6-(phenylisopropyl)adenosine (R-PIA), and S-N6-(phenylisopropyl)adenosine (S-PIA) elicited concentration-dependent dilation of these arterioles, but intraluminal application was less potent and efficacious than extraluminal application. Inosine was not vasoactive. A common order of agonist potency (NECA > adenosine > R-PIA > or = S-PIA) was determined for both extraluminal and intraluminal application. Theophylline (10 microM) caused a rightward shift of the adenosine concentration-response curve and a 50-fold reduction in potency. Intraluminal theophylline was one sixth as effective as extraluminal theophylline in antagonizing the extraluminal adenosine response, whereas intraluminal 8-sulfophenyltheophylline, a polar theophylline derivative, was ineffective. Polyadenylic acid (PolyA, 1 microM), an adenosine polymer that does not penetrate the endothelium, induced a dilation of 44.2 +/- 5.3% when applied extraluminally but had no effect when infused intraluminally. The dilator effect of PolyA was antagonized by theophylline. We conclude that: (1) intraluminal adenosine and its analogues are effective dilators of intracerebral arterioles, (2) the dilator effects of both intraluminally and extraluminally applied adenosine are predominantly mediated by A2-type receptors, and (3) adenosine receptors mediating vasodilation are not present on the luminal surface of the endothelium.

Changes in pial arteriolar diameter and CSF adenosine concentrations during hypoxia

We measured the changes in pial arteriolar diameter and CSF concentrations of adenosine, inosine, and hypoxanthine during hypoxia in the absence and presence of topically applied dipyridamole (10(-6) M) and erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA; 10(-5) M). Closed cranial windows were implanted in halothane-anesthetized adult male Sprague-Dawley rats for the observation of the pial circulation and collection of CSF. The mean resting arteriolar diameter in mock CSF was 31.2 +/- 5.9 microns. Topically applied dipyridamole and EHNA, in combination, caused a slight but significant (p < 0.05) increase in resting arteriolar diameter (33.8 +/- 4.3 microns). With mock CSF, moderate hypoxia caused a 22.1 +/- 9.7% increase in pial vessel diameter. Topically applied dipyridamole and EHNA significantly (p < 0.01) potentiated pial arteriolar vasodilation in response to hypoxia. Moreover, the potentiating effects of dipyridamole and EHNA during hypoxia were completely abolished by theophylline (0.20 mumol/g, i.p.; p < 0.05), an adenosine receptor antagonist. Resting concentrations of adenosine, inosine, and hypoxanthine in the subwindow CSF were 0.18 +/- 0.09, 0.35 +/- 0.21, and 0.62 +/- 0.12 microM, respectively. In the absence of dipyridamole and EHNA, these levels were not affected by sustained moderate hypoxia (PaO2 = 36 +/- 6 mm Hg). However, in the presence of dipyridamole and EHNA, the concentration of adenosine in the CSF during hypoxia was significantly (p < 0.05) increased. Our data indicate that dipyridamole and EHNA potentiate hypoxic vasodilation of pial arterioles while simultaneously increasing extracellular adenosine levels, thus supporting the hypothesis that adenosine is involved in the regulation of cerebral blood flow.

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.

Vascular factors in the neurotoxic damage caused by 1,3-dinitrobenzene in the rat

Using a 3 x 10 mg/kg dose schedule of 1,3-dinitrobenzene (DNB) over two days in Fischer rats, we have found the following changes in vascular function and structure during the early phase of the symmetrical brain stem lesions. 1. Marked increase in cerebral blood flow generally but especially in the inferior colliculi, from 6 h after the final dose of DNB. 2. Increasing incidence of petechial haemorrhages in inferior colliculi, cerebellar roof, vestibular and superior olivary nuclei from 12 h. 3. Focal leakage of horseradish peroxidase and many sleeve-like arteriolar haemorrhages seen in vibratome sections and by scanning electron microscopy (SEM) in these regions from 12 h. 4. Periarteriolar oedema and protein leakage present in step-serial sections in these regions from 12 h, with astrocyte swelling and occasional small infarcts. These changes suggest that the vascular bed may play an important role in the pathogenesis of these lesions, perhaps in parallel with early astroglial damage. They are discussed in relation to (i) the known presence of xanthine oxidase in the vascular bed of the brain and the likelihood of "useless redox cycling' with free radical generation from this enzyme's interaction with nitroheterocyclic compounds, and (ii) the possible role of free radical damage to endothelial cells in this intoxication and in the analogous lesions of natural and experimental Wernicke's encephalopathy.

Blood-brain barrier: transport studies in isolated brain capillaries and in cultured brain endothelial cells

The development of in vitro BBB models consisting of isolated brain capillaries and cultured brain microvessel endothelial cells has made possible the study of BBB transport phenomena at the cellular level. Basic characteristics of BBB transport of endogenous and exogenous solutes and their biochemical, pharmacological, ontogenic, and pathological regulation mechanisms have been investigated. This information has led not only to a better understanding of BBB transport but also to the construction of strategies for improving drug delivery to the CNS for diagnosis and therapeutics. To elucidate the complexity of BBB transport, in vivo studies are always necessary at some point; however, in vitro systems can be useful complements to the in vivo systems. The tissue culture systems seem to be especially important in the clarification of cellular, biochemical and molecular features of BBB transport. Appropriate systems should be selected or combined, depending on the purpose of the investigation.

Adenosine transport by rat and guinea pig synaptosomes: basis for differential sensitivity to transport inhibitors

Adenosine transport by rat and guinea pig synaptosomes was studied to establish the basis for the marked differences in the potency of some transport inhibitors in these species. An analysis of transport kinetics in the presence and absence of nitrobenzylthioinosine (NBTI) using synaptosomes derived from several areas of rat and guinea pig brain indicated that at least three systems contributed to adenosine uptake, the Km values of which were approximately 0.4, 3, and 15 microM in both species. In both species, the system with the Km of 3 microM was potently (IC50 of approximately 0.3 nM) and selectively inhibited by NBTI. This NBTI-sensitive system accounted for a greater proportion of the total uptake in the guinea pig than in the rat and was inhibited by dipyridamole, mioflazine, and related compounds more potently in the guinea pig. Preliminary experiments with other species indicate that adenosine transport in the mouse is similar to that in the rat, whereas in the dog and rabbit, it is more like that in the guinea pig. In the rat, none of the systems appeared to require Na+, but the two systems possessing the higher affinities for adenosine were inhibited by veratridine- and K(+)-induced depolarization. The transport systems were active over a broad pH range, with maximal activity between pH 6.5 and 7.0. Our results are consistent with the possibility that adenosine transport systems may be differentiated into uptake and release systems.

Changes in adenosine release and blood flow in the contracting dog gracilis muscle

Ischaemic contraction of skeletal muscle increases the venous concentration of adenosine. The present investigation was undertaken to determine changes in blood flow and the release of adenosine into venous blood resulting from 5 min of free flow contractions of the isolated gracilis muscle in dogs anaesthetised with pentobarbitone sodium (42 mg.kg-1) and artificially ventilated. Arterial and venous concentrations of adenosine were measured by high performance liquid chromatography. Five-minute-contractions (induced electrically, 6 V, 1.8 ms, 4 Hz) caused significant increases in blood flow (to 304 +/- 33% of control; mean +/- SEM, n = 9) and venous plasma adenosine concentration (from 126 +/- 18 nM to 293 +/- 76 nM, equivalent to an average increase in release of 7.28 +/- 1.89 nmol.min-1 100 g-1 wet weight of muscle). The venous oxygen tension decreased from 8.33 +/- 0.48 to 3.39 +/- 0.31 kPa (62.5 +/- 3.6 to 25.4 +/- 2.3 mm Hg). This small but significant increase in venous adenosine concentration within the vasoactive range, in the face of a concomitant increase in blood flow, suggests that an increase in the interstitial adenosine concentration during free-flow exercise may contribute to the total dilatation of the resistance vessels to increase blood flow and keep its own concentration low. A significant correlation between venous adenosine concentration and vascular conductance is therefore absent. The results suggest that adenosine may contribute to sustained active hyperaemia in skeletal muscle.

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

The effects of intravenous adenosine, dipyridamole and theophylline on local cerebral blood flow were studied in conscious rabbits. Long-term quantitative blood flow measurements were performed in 5 cerebral structures together with tissue pO2 and pCO2 measurements by a mass spectrometry technique. In an additional study, the time course of the cerebrovascular changes was determined by thermal clearance. It was found that: firstly, adenosine failed to modify local blood flow except in the caudate nucleus; secondly, dipyridamole increased cerebral blood flow in all 5 structures under study, and lastly, theophylline decreased cerebral blood flow in the same 5 structures. The increase in caudate blood flow induced by adenosine was instantaneous and lasted only for the duration of the infusion, whereas the cerebrovascular changes induced by dipyridamole and theophylline were gradual and persisted after their administration. Theophylline blocked the systemic and cerebrovascular changes caused by adenosine alone and by dipyridamole alone. In anesthetized rabbits, the intracarotid infusion of adenosine showed that the caudate reaction only occurred in the ipsilateral hemisphere. Taken together, our findings suggest that the transport system for adenosine in cerebral vessels is not only species-dependent but also structure-dependent. Furthermore, perivascular adenosine helps to maintain resting cerebrovascular tone and finally, cerebral adenosine may be involved in the control of cerebral blood flow via specific adenosine receptors.

Adenosine uptake site heterogeneity in the mammalian CNS? Uptake inhibitors as probes and potential neuropharmaceuticals

Inhibitors of adenosine uptake or transport have been used clinically for some time in certain cardiovascular diseases. More recently, some of them have also been investigated for possible clinical use in combination with antimetabolites based on the observed heterogeneity of nucleoside transport in mammalian tumor cells. Such a heterogeneity of adenosine uptake and uptake sites has now also been suggested in the mammalian CNS. The aim of this article is, therefore, to review the present status of our knowledge of adenosine uptake in the mammalian CNS, compare it with our far more advanced knowledge of nucleoside transport in other mammalian cells and suggest direction of future research. The possible implications for the development of adenosine uptake inhibitors as adenosinergic neuropharmaceuticals will be discussed based on our knowledge of the physiological function of adenosine in the CNS.

Systemically administered adenosine increases caudate blood flow in rabbits

Adenosine has been proposed to be a chemical link between cerebral metabolism and blood flow. In the present study, we investigated whether the intravenous or intracarotid administration of adenosine could influence regional cerebral blood flow in anesthetized rabbits. The study was performed with the [14C]ethanol tissue sampling technique which enables quantitative, instantaneous, multiregional blood flow measurements. With either mode of adenosine administration, no change in cerebral blood flow was observed, except in the caudate nucleus in which a significant vasodilation took place. These data indicate that, in rabbits exogenous adenosine increases blood flow in highly specific brain areas, by mechanisms that are discussed.

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.

Neurochemical coupled actions of transmitters in the microvasculature of the brain

The discovery that monoamine nerves end on the central microvessels of the choroid plexus, pia-arachnoid and parenchyma has prompted an intense investigation as to their physiological and neuropathological roles. The source of the monoamine fibers to the pial vessels and choroid plexus was shown to be the superior cervical ganglion. Ganglionic stimulation causes vasoconstriction or vasodilation of pial vessels, an event depending upon the functional ratio of alpha to beta adrenergic receptors. Moreover, stimulation of the superior cervical ganglion evokes an inhibition of cerebrospinal fluid formation in choroid plexus. The locus coeruleus is the site of adrenergic nerve supply to the parenchymal capillaries and stimulation of this nucleus increases capillary permeability to small molecules and water. Neurotransmitter receptors (adrenergic, histamine, adenosine, dopamine, prostacyclin, prostaglandins and specific amino acids or neuropeptides) have been identified on microvessels and in many instances these transmitter actions are coupled to cyclic AMP synthesis. Moreover, cyclic AMP has been shown to increase the rate of capillary endothelial pinocytosis and produce brain edema. In small vessels containing smooth muscle cells cyclic AMP production improves cerebral blood flow via an initiation of vasodilatory processes. The presence of receptors for serotonin and acetylcholine have likewise been demonstrated to occur on cerebral microvessels. Limited information is available as to the receptor coupled actions of these two transmitters, but cholinergic mechanisms may act to restrict catecholamine-induced formation of cyclic AMP. Altered sensitivity of microvessels to neurotransmitters has been demonstrated following conditions of stroke, hypertension, aging, diabetes and X-irradiation.

The effects of lidoflazine and flunarizine on cerebral reactive hyperemia

Cerebral blood flow in the rat was monitored by a venous outflow technique with an extracorporeal circulation, which allows for the continuous recording of flow over periods of several hours. The bi-fluorophenyl-piperazine derivatives, lidoflazine and flunarizine, enhanced the reactive hyperemia elicited by a brief (30 s) anoxic challenge. They did not alter resting cerebral blood flow rates. Verapamil, a potent calcium slow channel blocker, decreased resting flow rates but did not alter the duration of the reactive hyperemia. As lidoflazine and flunarizine are potent inhibitors of adenosine uptake, whereas verapamil is not, the results are consistent with the hypothesis that adenosine plays a significant role in cerebral vascular autoregulation.

Adenosine deaminase inhibitors enhance cerebral anoxic hyperemia in the rat

Cerebral blood flow in the rat was monitored by a venous outflow technique with an extracorporeal circulation, which allows for the continuous recording of flow over periods of several hours. The adenosine deaminase inhibitors erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA) (1.0-100 micrograms/kg) and deoxycoformycin (0.1-1 micrograms/kg) potentiated the reactive hyperemia elicited by a brief (24-s) anoxic challenge. Basal flow rate was unaltered by EHNA administration and slightly enhanced by deoxycoformycin. The results are consistent with the hypothesis that adenosine plays a significant role in cerebral vascular regulation and suggest that low doses of these deaminase inhibitors may be useful in the treatment of cerebral vascular insufficiency.

Methylxanthine discrimination in the rat: possible benzodiazepine and adenosine mechanisms

Rats were trained to discriminate either caffeine or theophylline from saline using a two-lever discrimination paradigm. Since methylxanthines have been found to interfere with agonist binding at both adenosine and benzodiazepine (BDZ) receptors, chlordiazepoxide (CDP) and L-PIA (an adenosine analog) were tested for generalization to and blockade of both xanthine cues. Neither L-PIA nor CDP generalized to either xanthine cue, although both produced dose-related decreases in response rate. CDP, but not L-PIA, produced dose-related decreases in drug-lever responses when combined with training doses of caffeine or theophylline. Response rates indicated a complex interaction between the xanthines and both L-PIA and CDP. When combined with the caffeine training dose, pentobarbital also produced a dose-dependent decrease in response rate but not in drug lever choices. Finally, papaverine generalized to the caffeine cue in a dose-dependent fashion. In a second experiment, rats trained to discriminate CDP from saline showed no generalization in L-PIA tests. CDP-appropriate responding was not significantly affected when the CDP training dose was combined with caffeine. These data indicate that: (a) methylxanthine interactions with L-PIA and CDP on response rate likely involve blockade of adenosine mechanisms; (b) the xanthine cue does not appear to depend on interactions with adenosine receptors; and (c) the xanthine cue may involve effects on cyclic AMP activity and/or interaction with the BDZ/GABA receptor complex.

Identification of hypoxanthine transport and xanthine oxidase activity in brain capillaries

Microvessel segments were isolated from rat brain and used for studies of hypoxanthine transport and metabolism. Compared to an homogenate of cerebral cortex, the isolated microvessels were 3.7-fold enriched in xanthine oxidase. Incubation of the isolated microvessels with labeled hypoxanthine resulted in its rapid uptake followed by the slower accumulation of hypoxanthine metabolites including xanthine and uric acid. The intracellular accumulation of these metabolites was inhibited by the xanthine oxidase inhibitor allopurinol. Hypoxanthine transport into isolated capillaries was inhibited by adenine but not by representative pyrimidines or nucleosides. Similar results were obtained when blood to brain transport of hypoxanthine in vivo was measured using the intracarotid bolus injection technique. Thus, hypoxanthine is transported into brain capillaries by a transport system shared with adenine. Once inside the cell, hypoxanthine can be metabolized to xanthine and uric acid by xanthine oxidase. Since this reaction leads to the release of oxygen radicals, it is suggested that brain capillaries may be susceptible to free radical mediated damage. This would be most likely to occur in conditions where the brain hypoxanthine concentration is increased as following ischemia.

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

The purpose of these experiments was to determine the utility of a new method for monitoring CBF using a venous outflow technique with an extracorporeal circulation and to examine the effects of agents that potentiate or antagonize the actions of adenosine on the blood flow response to brief periods of anoxia. The results demonstrate the ability of the new technique to detect the increases in CBF in response to anoxia. Caffeine, an adenosine antagonist, reduced the intensity and duration of the anoxia-induced hyperemia. Dipyridamole and papaverine, inhibitors of adenosine uptake, potentiated the increase in CBF during anoxia. The results support the hypothesis that adenosine plays an important role in regulating CBF during anoxic episodes.

Demonstration of adenosine receptors on mouse cerebral smooth muscle membranes

Adenosine receptors have been identified on brain cortical membranes and microvascular preparations. However, they have not been demonstrated on specific microvascular elements in isolation. 2-3H-chloroadenosine was used as a ligand to investigate the presence of adenosine receptors on isolated mouse cerebral smooth muscle membranes. The binding studies reveal the presence of a high affinity binding site with a Kd value of 33.3 nM and a maximal binding capacity (Bmax) of 283 fmol/mg protein. These findings demonstrate that there is an adenosine receptor on cerebral smooth muscle membranes.

Capacity for energy metabolism in microvessels isolated from rat brain

Numerous methods used for the isolation of brain microvessels involve procedures which disturb the structural integrity of the cells and their organelles. In the present study, analysis of the adenylate energy charge and content as well as the incorporation of adenosine derivatives in isolated rat brain microvessels indicated a lesion of the mechanisms of energy production. The results show that experiments on isolated microvessels prepared by a mechanical homogenization exerting shear forces should be interpreted with caution when the rate of energy metabolism is a significant factor in the study.

Monoamine oxidase activity in brain microvessels determined using natural and artificial substrates: relevance to the blood-brain barrier

The possible contribution of cerebrovascular monoamine oxidase (MAO) to the blood-brain barrier to catecholamines was studied in isolated porcine and rat microvessels by determining its activity with various substrates. Michaelis-Menten kinetic constants, Km and Vmax, were determined using noradrenaline (NA) as substrate in a Tris medium. Km values were 0.25 +/- 0.05 mM in control and 0.16 +/- 0.09 mM in ultrasonically disintegrated (USD) preparations (difference not significant); Vmax in USD preparations (1.83 +/- 0.20 n.atoms O2 min-1 mg protein-1) was slightly higher (p less than 0.05) than in control preparations (1.35 +/- 0.11 n.atoms O2 min-1 mg protein-1), suggesting a certain restriction by the plasma membrane of substrate access to the enzyme. This phenomenon was confirmed in a more physiological, ionic medium; the activity was then approximately doubled for 1 mM NA, whereas that for 1 mM beta-phenylethylamine (beta-PEA), a lipid-soluble substrate, tended to decrease with USD treatment. These results show that this highly active form of MAO is unlikely to be saturated by physiological concentrations of catecholamine. It can be estimated that, for a plasma concentration of NA of 1 microM, a facilitated diffusion accelerating the entry of the catecholamine into the cells by at least 15-fold would be necessary in order to exceed the catabolic capacity of MAO. It is concluded that circulating catecholamines are not likely to cross the endothelial barrier of cerebral microvessels intact, and that the small quantities of radioactivity detected in the parenchyma in measurements of the brain uptake index essentially represent metabolites due to MAO activity.

Uptake of adenosine by isolated bovine cortex microvessels

The uptake of adenosine is studied in microvessels isolated from a bovine cortex. The KM value for adenosine uptake is 1.92 microM and the Vmax is 1.93 picomole/mg protein/10 min. This high affinity uptake system is very sensitive to inhibition by dipyridamole and papaverine. The uptake of adenosine by microvessels is also inhibited by CuCl2 and by high concentration (2 mM) of adenine nucleotides. Using a series of four xanthines is observed that the adenosine uptake system is most inhibited by 3-methyl-l-(5'-oxohexyl)-7-propylxanthine and the least by caffeine. Theophylline causes a stimulation of adenosine uptake by microvessels. The results obtained agree with the existence of the nucleoside transport system associated with the blood-brain barrier, as previously observed by in vivo studies and experiments with rat brain capillaries.

Uptake of adenosine by cultured cerebral vascular smooth muscle cells

Adenosine uptake by cerebral smooth muscle cells is a carrier-mediated process. The Km value for adenosine uptake is 10.0 microM and the Vmax is 0.95 nmol/min-mg cell protein. This uptake system is inhibited by the adenosine analog 2-chloroadenosine at low adenosine concentrations. These results prove the existence of a nucleoside transport system associated with cerebral smooth muscle.

Uptake of adenosine into cultured cerebral endothelium

Adenosine uptake by cultured cerebral endothelium is a carrier-mediated process. The Km value for adenosine uptake is 5.0 microM and the vmax is 1.15 nmol/min/mg cell protein. The uptake system is inhibited by the adenosine analog 2-chloroadenosine at low adenosine concentrations. The results prove the existence of a nucleoside transport system associated with cerebral capillary endothelium.