Uptake of adenosine into cultured cerebral endothelium

An in vivo and in vitro comparison of the effects of vasoactive mediators on pulpal blood vessels in rat incisors

The effects of endogenous vasoactive substances were evaluated in anaesthetized rats using a laser Doppler flowmeter to monitor changes in pulpal blood flow, as well as directly in isolated pulpal arteriole preparations utilising a microperfusion and monitoring system to observe changes in vessel diameter. In anaesthetized rats, while systemic arterial blood pressure remained relatively stable, intra-arterial delivery of adrenaline (epinephrine) (A), noradrenaline (norepinephrine) (NA), phenylephrine (PHE), dopamine (DOPA), 5-hydroxytryptamine (5-HT), or endothelin-1 (ET-1) produced a dose-dependent reduction in pulpal blood flow (order of potency: ET-1>>A=NA>PHE=DOPA=5-HT); acetylcholine induced a dose-dependent increase in pulpal blood flow; histamine, isoproterenol and adenosine produced no significant changes. In isolated arteriole preparations, intraluminal delivery of A, NA, PHE, DOPA or 5-HT produced dose-dependent vasoconstriction (A=NA>PHE=DOPA=5-HT). Acetylcholine relaxed NA-precontracted vessels dose-dependently. Histamine and isoproterenol produced a small vasodilatation. Intraluminal ET-1 produced a small vasoconstriction at 10(-8)M, whereas extraluminal ET-1 produced a dose-dependent vasoconstriction from 10(-10)M and above. Intraluminal adenosine failed to dilate vessels precontracted with ET-1, whereas extraluminal adenosine caused a complete relaxation. These combined in vivo and in vitro data suggest that, in the rat incisor, the pulpal microcirculation is capable of functional regulation and that pulpal blood flow may be modulated by endothelium-related factors, metabolic (tissue-related) factors, as well as humoral (blood-borne) factors.

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.

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.

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.

The cerebral microvessels in culture, an update

Recent advances in our knowledge of the blood-brain barrier (BBB) have in part been made by studying the properties and function of cerebral endothelial cells in vitro. After an era of working with a fraction, enriched in cerebral microvessels by centrifugation, the next generation of in vitro BBB model systems was introduced, when the conditions for routinely culturing the endothelial cells were established. This review summarizes the results obtained from this rapidly growing field. It can be stated with certainty that, in addition to providing a better insight into the chemical composition of cerebral endothelial cells, much has been learned from these studies about the characteristics of transport processes and cell-to-cell interactions during the last 12 years. With the application of new technologies, the approach offers a new means of investigation, applicable not only to biochemistry and physiology but also to the drug research, and may improve the transport of substances through the BBB. The in vitro approach has been and should remain an excellent model of the BBB to help unravel the complex molecular interactions underlying and regulating the permeability of the cerebral endothelium.

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.

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.

P1-purinoceptors in the cerebrovascular bed of the goat in vivo

The possible existence and function of specific P1-purinoceptors in the cerebrovascular bed of the unanesthetized goat have been investigated. Blood flow to one cerebral hemisphere (cerebral blood flow) was measured by means of an electromagnetic flow probe previously implanted around the ipsilateral internal maxillary artery. The injection of adenosine, AMP, ADP and ATP (3-30 micrograms) directly into the internal maxillary artery increased cerebral blood flow and decreased cerebrovascular resistance in a dose-dependent manner. Continuous infusion of 8-phenyltheophylline (8-PT), 100 micrograms/min, into the internal maxillary artery did not alter the resting cerebral blood flow or the cerebrovascular resistance, but significantly inhibited the cerebral vasodilation induced by adenosine, AMP, ADP and, to a lesser degree, ATP. The acetylcholine- and histamine-induced cerebral vasodilation was unaffected by 8-PT. These results indicate that adenosine, AMP, ADP and, at least in part, ATP increase cerebral blood flow by acting on specific P1-purinoceptors located in the cerebrovascular wall. These P1-purinoceptors do not appear to be tonically activated under physiological conditions.

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.

Neuronal versus endothelial origin of vasoactive acetylcholine in pial vessels

Functional pial vessels denuded in situ of the endothelial cell layer exhibit a markedly decreased choline uptake capacity (-53%) but integrally preserved choline acetyltransferase (ChAT) activity and acetylcholine (ACh) release mechanisms. These studies demonstrate that endothelial cells possess a specific choline uptake system. However, the unimpaired ChAT activity in denuded pial vessels implies that the endothelial pool of choline is not significantly metabolized into ACh. In spite of possible differences in the mechanisms that govern release processes in endothelial and neuronal elements, taken together the findings of the present study suggest that the ACh released following depolarization of pial blood vessels originates predominantly from cholinergic perivascular nerve terminals.

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.

Neurogenic control of cerebral circulation

The cerebral vascular neuromuscular apparatus consists of a varicose perivascular nerve plexus at the adventitial-medial border and smooth muscle cells in the medial coat that are functionally connected. In addition to noradrenaline and acetylcholine, a number of putative non-adrenergic, non-cholinergic neurotransmitters have been identified in cerebral perivascular nerves, including serotonin, substance P, vasoactive intestinal polypeptide, gastrin-releasing peptide, cholecystokinin, somatostatin, neurotensin, calcitonin gene-related peptide and neuropeptide Y. The role of adenosine-5'-triphosphate as a cotransmitter with noradrenaline in some perivascular sympathetic nerves, and of endothelial cells in mediating the vasodilatation produced by some neurohumoral agents is discussed. Speculations are made about the relation between vascular neuroeffector mechanisms and migraine, including the possibility of local vasospasm by serotoninergic nerves, reactive hyperaemia involving purine nucleotides and nucleosides, release of substance P from sensory nerve collaterals during antidromic ('axon reflex') impulses and secondary release of local agents such as prostanoids, histamine and bradykinin.

Intravertebral artery adenosine fails to alter cerebral blood flow in the dog

The effect of intra-arterial adenosine on cerebral blood flow was studied in 11 anesthetized dogs. In a first group of 6 dogs, adenosine was infused into a vertebral artery for 40 minutes at a dose of 0.3 to 0.5 mg/kg/min. Blood flow was determined before, during and after the adenosine infusion using the radioactive microsphere technique. In a second group of 5 dogs, adenosine (3 +/- 1 mcg/kg/min) was infused in a similar manner after potentiating its effect with intravenous dipyridamole, and measurements before and after the intravenous dipyridamole and during and after the adenosine infusion were performed. Systemic arterial pressure and blood gases were unchanged throughout the experiment in both groups of dogs. Blood flow to the cerebral hemispheres, cerebellum, brain stem, paraspinous and temporalis muscles remained unchanged during the adenosine infusion in both groups of dogs. Adenosine has been implicated as an active agent in the vasodilatory component of cerebral autoregulation. A controversy exists as to whether intervascular or only interstitial adenosine is of physiologic importance. These findings suggest that intra-arterial adenosine does not play a significant role in the regulation of cerebral blood flow.

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.