Cerebral microvessels contain a beta 2-adrenergic receptor

The β2-adrenergic receptor antagonist ICI-118,551 blocks the constitutively activated HIF signalling in hemangioblastomas from von Hippel-Lindau disease

One of the major consequences of the lack of a functional VHL protein in von Hippel-Lindau disease, a rare cancer, is the constitutive activation of the HIF pathway. This activation ends up in the generation of Central Nervous System (CNS) Hemangioblastomas among other tumours along the lifespan of the patient. Nowadays, only surgery has been proven efficient as therapy since the systemic attempts have failed. Propranolol, a non-specific β1-and β2-adrenergic receptor antagonist, was recently designated as the first therapeutic (orphan) drug for VHL disease. Nevertheless, its β1 affinity provokes the decrease in blood pressure, being not recommended for low or regular blood pressure VHL patients. In order to overcome the β1-drawback, the properties of a high specific β2-adrenergic receptor blocker named ICI-118,551 have been studied. ICI-118,551 was able to decrease Hemangioblastomas cell viability in a specific manner, by triggering apoptosis. Moreover, ICI-118,551 also impaired the nuclear internalization of HIF-1α in Hemangioblastomas and hypoxic primary endothelial cells, reducing significantly the activation of HIF-target genes and halting the tumour-related angiogenic processes. In this work, we demonstrate the therapeutical properties of ICI-118,551 in VHL-derived CNS-Hemangioblastoma primary cultures, becoming a promising drug for VHL disease and other HIF-related diseases.

Characterization and ontogeny of PGE2 and PGF2 alpha receptors on the retinal vasculature of the pig

The vasoconstrictor effects of PGE2 and PGF2 alpha are less pronounced on retinal vessels of the newborn than of the adult pig. We tested the hypothesis that the decreased vasomotor response to these prostaglandins might be due to relatively fewer receptors and/or different receptor subtypes (in the case of PGE2) on retinal vessels of the newborn animal. Binding studies using [3H]PGE2 and [3H]PGF2 alpha revealed that PGE2 (EP) and PGF2 alpha (FP) receptor densities in retinal microvessel membrane preparations from newborn animals were approximately 25% of those found in vessels from the adult. The Kd for PGF2 alpha did not differ; however, the Kd for PGE2 was less in newborn than in adult vessels. Competition binding studies using AH 6809 (EP1 antagonist), butaprost (EP2 agonist), M/+B 28,767 (EP3 agonist), and AH 23848B (EP4 antagonist) suggested that the retinal vessels of the newborn contained approximately equal number of EP1 and EP2 receptor subtypes whereas the main receptor subtype in the adult vessels was EP1. In addition, PGE2 and butaprost produced comparable increases in adenosine 3',5'-cyclic monophosphate synthesis in newborn and adult vessels. PGE2, 17-phenyl trinor PGE2 (EP1 agonist) and PGF2 alpha caused a 2.5 to 3-fold greater increase in inositol 1,4,5-triphosphate (IP3) formation in adult than in newborn preparations. It is concluded that fewer PGF2 alpha receptors and an associated decrease in receptor-coupled IP3 formation in the retinal vessels of the newborn could lead to weaker vasoconstrictor effects of PGF2 alpha on retinal vessels of the newborn than of adult pigs; fewer EP1 receptors (associated with vasoconstriction) and a relatively greater proportion of EP2 receptors (associated with vasodilation) might be responsible for the reduced retinal vasoconstrictor effects of PGE2 in the newborn.

Ontogenic increase in PGE2 and PGF2 alpha receptor density in brain microvessels of pigs

1. The hypothesis that the relative vasoconstrictor ineffectiveness of prostaglandin E2 (PGE2) and PGF2 alpha on cerebral vessels of newborn pigs might be due to fewer receptors for these prostanoids was tested by comparing receptors for PGE2 (EP) and PGF2 alpha (FP) in cerebral microvessels from newborn and adult pigs. 2. Specific binding of [3H]-PGE2 and [3H]-PGF2 alpha to membranes prepared from brain microvessels showed that EP and FP receptor density (Bmax) in tissues from newborn animals was less than 50% of that determined in tissues from adults. By contrast, estimates of affinity (KD) were unchanged. 3. Specifically bound [3H]-PGE2 to brain microvessels from both the newborn and adult was displaced by AH 6809 (EP1-selective antagonist) by 80-90%, and only by approximately 30-35% by both 11-deoxy PGE1 (EP2/EP3 agonist) and M&B 28,767 (EP3 agonist); butaprost (EP2 agonist) was completely ineffective. 4. PGE2, 17-phenyl trinor PGE2 (EP1 agonist), PGF2 alpha and fenprostalene (PGF2 alpha analogue) caused significantly less increase in inositol 1,4,5-triphosphate (IP3) in brain microvessels from the newborn than in those from adult pigs. The stimulation of IP3 by PGE2 and 17-phenyl trinor PGE2 was almost completely inhibited by the EP1 antagonist, AH 6809. 5. PGE2, 11-deoxy PGE1 and M&B 28,767 produced small reduction of adenosine 3':5'-cyclic monophosphate (cyclic AMP) production in adult vessels but no effect in newborn tissues. 6. The lower density of EP and FP receptors in microvessels of newborn pigs compared to adults may explain the reduced ability of PGE2 and PGF2 alpha to stimulate production of IP3 in tissues from newborn animals. This in turn, may provide an explanation for previous observations demonstrating that these prostanoids elicit contraction of adult cerebral microvessels, but exert minimal effects on these vessels in newborn animals.

Adrenergic receptors coupled to adenylate cyclase in human cerebromicrovascular endothelium

Cultured endothelium derived from three microvascular fractions of human brain was used to characterize adrenergic receptors coupled to adenylate cyclase activity. Catecholamines (norepinephrine, epinephrine) and their analogs (isoproterenol, phenylephrine, 6-fluoronorepinephrine) dose-dependently stimulated endothelial production of cAMP. Antagonists for beta 1 and beta 2 receptors (propranolol, atenolol, and butoxamine) and for alpha 1-receptors (prazosin) dose-dependently blocked cAMP formation induced by the tested adrenergic agonists. Clonidine, an alpha 2 > alpha 1-agonist, also inhibited isoproterenol-stimulated production of cAMP while yohimbine (alpha 2 > alpha 1 antagonist) augmented the norepinephrine or epinephrine-induced accumulation of cAMP. Cholera toxin-induced ADP ribosylation of the stimulatory guanine nucleotide binding protein (Gs) abolished the stimulatory effect of norepinephrine, epinephrine, phenylephrine or 6-fluoronorepinephrine on cAMP formation. ADP ribosylation of the inhibitory guanine nucleotide binding protein (Gi) by pertussis toxin had no effect on either phenylephrine- or 6-fluoronorepinephrine-induced production of cAMP while it increased the norepinephrine and epinephrine-induced accumulation of cAMP. These findings represent the first documentation of beta 1-, beta 2-, alpha 1 and alpha 2-adrenergic receptors linked to adenylate cyclase in endothelium derived from human brain microvasculature. These data also indicate that activation of endothelial alpha 1 -adrenergic receptors is mediated by a signal transduction mechanism associated with Gs protein. The results strongly support the presence of various receptor-controlled adrenergic regulatory mechanisms on human cerebromicrovascular endothelium.

Epinephrine in cardiopulmonary resuscitation

This review assesses the role of epinephrine in cardiopulmonary resuscitation from the perspective of mechanisms of action, cardiac and cerebral effects, and use in human beings. We reviewed the literature from 1966 onward, using a Medline Search of the National Library of Medicine with the key words: "heart arrest," "resuscitation," and "epinephrine." Pertinent articles that represented original research were critically appraised by at least two authors. We concluded that the Advanced Cardiac Life Support recommended dose of epinephrine (1 mg or 0.007 to 0.014 mg/kg) has little scientific basis. Evidence from animal studies demonstrates that doses of 0.1 to 0.2 mg/kg are required to significantly improve myocardial and cerebral blood flow and resuscitation rates. Limited human data confirm the dose-dependent vasopressor response to epinephrine and the potential for improved immediate survival with higher doses. We suggest that randomized controlled human trials are needed to document the usefulness of higher doses of epinephrine in cardiopulmonary resuscitation.

Coexpression of beta 1- and beta 2-adrenergic receptors on bovine brain capillary endothelial cells in culture

The pharmacological study of the blood-brain barrier has often been hampered by the unavailability of a large number of pure and fully differentiated brain capillary endothelial cells. Here we describe a homogeneous culture of brain capillary endothelial cells isolated from bovine brain (BBECs), which retain at least some phenotypic characteristics of the functional blood-brain barrier: intracellular tight junctions and monoamine oxidase activity. These cells were subcultured in vitro, in the absence of any neuronal or glial influences, for greater than 100 doublings without any sign of senescence. The present study is focused on the expression of beta-adrenergic receptors on BBECs. By Northern blot hybridization, subtype-specific ligand binding, and cyclic AMP accumulation experiments, we demonstrate that beta 1- and beta 2-adrenergic receptors are coexpressed (in the respective proportions of 42 and 58%) on BBEC membranes and are functionally coupled to adenylate cyclase. This is the first report documenting a significant number of beta 1-adrenergic receptors on brain capillary endothelial cells. The results are discussed in light of the known noradrenergic innervation of brain capillaries.

Kinetics of protein phosphorylation in microvessels isolated from rat brain: modulation by second messengers

The role of second messengers in the regulation of protein phosphorylation was studied in microvessels isolated from rat cerebral cortex. The phosphoproteins were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and the kinetics of 32P incorporation into specific protein substrates were evaluated by computer-aided x-ray film densitometry. With the use of this method, Ca2+-calmodulin (CAM)-, Ca2+/phospholipid (PK C)-, cyclic GMP (cGMP)-, and cyclic AMP (cAMP)-dependent protein kinases were detected. CAM-dependent protein kinase proved to be the major phosphorylating enzyme in the microvascular fraction of the rat cerebral cortex; the activity of cGMP-dependent protein kinase was much higher than that of the cAMP-dependent one. Autophosphorylation of both the alpha- and beta-subunits of CAM-dependent protein kinase and the proteolytic fragment of the PK C enzyme was also detected. The kinetics of phosphorylation of the individual polypeptides indicate the presence in the cerebral endothelium of phosphoprotein phosphatases. The phosphorylation of proteins in the cerebral capillaries was more or less reversible; the addition of second messengers initiated a very rapid increase in 32P incorporation, followed by a slow decrease. Because the intracellular signal transducers like Ca2+ and cyclic nucleotides are frequently regulated by different vasoactive substances in the endothelial cells, the modified phosphorylation evoked by these second messengers may be related in vivo to certain changes in the transport processes of the blood-brain barrier.

The comparative effects of epinephrine versus phenylephrine on regional cerebral blood flow during cardiopulmonary resuscitation

Epinephrine in larger doses than currently recommended during cardiopulmonary resuscitation (CPR) has been shown to improve cerebral blood flow (CBF) following a 10-min arrest in a swine model. The purpose of this pilot study was to measure CBF during CPR, comparing high-dose epinephrine to a pure alpha-1 agonist, phenylephrine. Ten swine each weighing greater than 15 kg, were instrumented for regional CBF measurements using tracer microspheres. CBF was measured during normal sinus rhythm (NSR). Following 10 min of ventricular fibrillation, CPR was begun and regional CBF was again measured. Following 3 min of CPR, the swine were randomized to receive either epinephrine (0.2 mg/kg), or phenylephrine (0.1 mg/kg), through a peripheral intravenous line. Regional CBF was again measured 1 min after drug administration. Regional CBF following drug administration was compared using an analysis of covariance. Adjusted CBFs are expressed in ml/min per 100 g for epinephrine and phenylephrine, respectively: left cerebral cortex (12.5 vs. 2.3, P = 0.002); right cerebral cortex (13.0 vs. 2.8, P = 0.003); cerebellum (32.9 vs. 4.1, P = 0.004); midbrain (35.7 vs. 2.6, P = 0.0004), pons (30.3 vs. 2.9, P = O.006); medulla (49.5 vs. 13.6, P = 0.02) and cervical spinal cord (49.6 vs. 14.1, P = 0.003).

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.

Characterization of beta adrenergic receptors in human cerebral arteries and alteration of the receptors after subarachnoid hemorrhage

The nature of beta adrenergic receptors in human cerebral arteries was characterized and alteration of these receptors after subarachnoid hemorrhage was examined using a radioligand binding assay. The specific 3H-dihydroalprenolol, a beta adrenergic antagonist, binding to human cerebral arteries was saturable and of high affinity (KD = 12.3 nM) with a Bmax of 790 fmol/mg protein. Ki values and Hill coefficients of adrenergic agents for 3H-dihydroalprenolol were as follows; propranolol, 4.1 X 10(-8)M, 1.01; isoproterenol, 1.7 X 10(-6)M, 0.80; epinephrine, 8.3 X 10(-6)M, 0.48; norepinephrine, 2.3 X 10(-5)M, 0.45; metoprolol, 6.8 X 10(-8)M and 7.9 X 10(-6)M, 0.62; butoxamine, 2.2 X 10(-8)M and 2.1 X 10(-6)M, 0.43. The analysis of inhibition of specific 3H-dihydroalprenolol binding by these adrenergic agents suggests that human cerebral arteries contain a high density of beta adrenergic receptors and that the receptors are classified into two types, namely beta 1 and beta 2 adrenergic receptors. The calculated beta 1/beta 2 ratio from Hofstee plots was approximately 4/6. KD and Bmax of 3H-dihydroalprenolol binding to the cerebral arteries after subarachnoid hemorrhage were compared with those of control group. KD and Bmax of 3H-dihydroalprenolol binding of subarachnoid hemorrhage group were 13.9 nM and 1140 fmol/mg protein, respectively. The calculated beta 1/beta 2 ratio was approximately 6/4. These data suggest that the density of total beta adrenergic receptors increased without any significant change in the affinity after subarachnoid hemorrhage and that the increase of beta 1 adrenergic receptors was dominant.

Cholinergic-adrenergic receptor interactions in cerebral microvessels

Enriched capillary preparations isolated from rat cerebral cortex were used to evaluate cholinergic-adrenergic receptor interactions in cerebral endothelium. Possible receptor interactions were determined by measuring an intracellular mediator, cyclic AMP and alterations in GTP-sensitive agonist binding. Unstimulated microvessel homogenates generate 66 +/- 16 pmol/mg/10 min of cyclic AMP. Adrenergic agonists norepinephrine and isoproterenol increase cyclic AMP to 147 +/- 31 and 149 +/- 23 pmol/mg/10 min, respectively. Addition of the muscarinic agonist carbachol has no effect on basal cyclic AMP but it completely blocks the stimulation elicited by adrenergic agonists. The displacement of quinuclidinyl benzilate (QNB) by carbachol yields an IC50 of 1.5 +/- 0.45 X 10(-4) M and a Hill coefficient of 0.54 +/- 0.07, indicating a heterogeneous population of binding sites. Guanine nucleotides shift the displacement curve to the right (IC50, 4.7 +/- 0.16 X 10(-4) M) and convert the binding site population to greater homogeneity (0.76 +/- 0.18). Isoproterenol prevents both the affinity shift and binding site conversion evoked by guanine nucleotides. These data suggest that cholinergic-adrenergic interactions occur at both the level of receptor binding and the generation of an intracellular messenger. Since cyclic AMP has been purported to play a role in regulation of blood-brain barrier permeability, the existence of adrenergic-cholinergic, i.e., excitatory-inhibitory modulators of adenylate cyclase in cerebral endothelium, suggests that these receptors may mediate physiological and/or pathological alterations of cerebrovascular permeability.

Neuronal control of brain microvessel function

Cerebral capillary endothelium forms a barrier limiting and controlling the movement of ions and solutes between blood and brain. Recent anatomical, physiological and biochemical studies have suggested the possibility that capillary function may be directly controlled by neuronal structures. Alterations in neuronal systems involved in the regulation of microcirculation may account for microvascular dysfunctions which occur in different pathologic conditions.

Distribution of alpha 2-adrenergic receptors in the human brainstem: an autoradiographic study using [3H]p-aminoclonidine

The distribution of alpha 2-adrenergic receptors in the human brainstem and some forebrain areas was examined by means of an in vitro autoradiographic technique using [3H]p-aminoclonidine as a ligand. High densities of alpha 2-adrenergic receptors were observed in the dorsal motor nucleus of the Xth nerve, the nucleus of the solitary tract, the locus coeruleus and the substantia grisea centralis. Other brainstem areas presenting significant densities of alpha 2-adrenergic receptors were the substantia gelantinosa of the Vth nerve, the nucleus cuneiformis, the superficial gray layer of the superior colliculus, the cortical mantle of the inferior colliculus and the inferior olivary nuclei. Of the forebrain areas studied, the highest density was seen in the primary visual cortex. The distribution observed presents some significant differences with that previously described for the rat. The localization of alpha 2-adrenergic receptors in the human brain correlates well with the distribution of noradrenergic neurons in the human brainstem and suggests sites of action of alpha 2-adrenergic drugs in the human brain.

Treatment of acute focal cerebral ischemia with propranolol

Propranolol's potential as a protective agent against tissue injury has been noted in experimental myocardial, renal and early acute focal cerebral ischemia. The purpose of the present investigation was to study further the effects of racemic (d,l) propranolol on blood-brain barrier permeability, morphological changes, cortical electrical activity, and regional cerebral blood flow (rCBF) in experimental focal cerebral ischemia. Thirty adult cats, anesthetized with nitrous oxide, underwent 6 hours of right middle cerebral artery (MCA) occlusion. Fifteen cats were untreated. Fifteen cats were given a continuous infusion of racemic propranolol (1 mg/kg/hr) for 7 hours beginning 1 hour before MCA occlusion and a 4 mg/kg bolus immediately before occlusion, both directly into the right carotid artery. Right Sylvian rCBF did not significantly differ in the treated and untreated groups. Carbon filling defects and vital dye (i.e., Evans blue and fluorescein) extravasation were less severe in the propranolol treated animals. Light microscopic findings demonstrated no difference in infarct size between the two groups. The findings suggest that at doses given, racemic propranolol does not exert a protective effect upon cerebral tissue subjected to 6 hours of incomplete ischemia.

Endothelial cells contain beta adrenoceptors

The direct identification of beta adrenoceptors in endothelial cell cultures has not been possible until the advent of a new beta-adrenergic radioligand, [125I]iodocyanopindolol ([125I]ICYP). Using [125I]ICYP, we report the successful identification of a beta adrenoceptor in cultured bovine aortic endothelial cells. At 37 degrees C, specific binding is saturable, stable and reversible. There is a single class of binding sites (21,500 +/- 2,900 sites/cell) with an equilibrium dissociation constant (Kd) of 109 +/- 26 pM. The rate constant of association, k1, is 1.22 X 10(9) M-1 min-1 and of dissociation, k-1, is 0.01 min-1. Binding studies on monolayers of endothelial cells grown in microtiter plates yield similar data (Kd = 53 +/- 9 pM, Bmax = 20,000 +/- 1,900 sites/cell). Stereoselectivity of binding for the (-)-isomer is demonstrable for both agonists and antagonists. A series of adrenergic agonists competes with [125I]ICYP for binding with an order of potency suggesting beta 2 subselectivity ; isoproterenol (0.73 microM) greater than epinephrine (15 microM) greater than norepinephrine (71 microM). Furthermore, the beta 2 inhibitor butoxamine is more potent than the beta 1 inhibitor practolol (7.7 microM vs 22 microM respectively). The GTP analogue, Gpp(NH)p, reduces isoproterenol affinity to 1.9 microM and increases the Hill coefficient from 0.62-0.90.

Cerebrovascular smooth muscle culture. II. Characterization of adrenergic receptors linked to adenylate cyclase

Cultured and propagated smooth muscle cells contain adenylate cyclase (AC) responsive to catecholamines and their analogues. Isoproterenol and zinterol were the most effective stimulants of AC activity with EC50 = 8.5 X 10(-8)M. They were followed by epinephrine, phenylephrine and norepinephrine (EC50 = 7.5 X 10(-7)M, 6.5 X 10(-6)M and 4 X 10(-6)M, respectively). When the selective antagonists for beta 1 and beta 2 receptors (beta 1-type practolol and atenolol, beta 1/beta 2-type propranolol and beta 2-type butoxamine) were tested against isoproterenol, epinephrine and norepinephrine stimulation of AC activity, the beta 1 in contrast to beta 2 antagonists were found ineffective. The alpha-blockers (phentolamine alpha 1/alpha 2-type antagonists) and yohimbine (alpha 2-type antagonist) alone or in the presence of propranolol did not significantly inhibit the catecholamine-induced enhancement of cAMP formation. On the other hand, prazosine (alpha 1-type antagonist) blocked the stimulatory effect of epinephrine and norepinephrine on AC system. Similarly, the alpha 2-agonist, clonidine, did not affect the catecholamines' stimulated AC activity while alpha 1 agonist, phenylephrine, induced an additive enhancement of norepinephrine production of cAMP. The findings of beta-2- and alpha-1-type adrenergic receptors in the cultured cerebrovascular smooth muscle provide additional support for the implicated involvement of adrenergic innervation in the regulation of cerebral blood flow and/or systemic blood pressure.

Heart microvessels: presence of adenylate cyclase stimulated by catecholamines, prostaglandins, and adenosine

Microvessels (capillaries) were isolated in pure form from rabbit ventricle by a process which involved fine mincing of the tissue, repeated incubation with collagenase to effect cell dispersal, passage of the suspended cells over a column of glass beads, and finally concentration of the capillary fraction on a step-wise sucrose gradient. Adenylate cyclase in the capillary preparation was stimulated by beta adrenergic agonists in a potency order which suggested coupling to a beta 2 subtype adrenergic receptor. Catecholamine-stimulated activity was antagonized by methoxamine, but this did not seem to be mediated through alpha adrenergic receptor activation, since it was not reversed by the alpha adrenergic antagonist phentolamine. Adenylate cyclase was stimulated by adenosine and several adenosine analogs in a potency order which suggested enzyme coupling to a stimulatory A2 receptor. Prostaglandins were also effective stimulators of enzyme activity, those of the E and A series being more potent than members of the F series. It is possible that these agents may exert their physiological actions on the microvasculature via cyclic AMP formed in response to activation of adenylate cyclase.

Effects of vasoactive intestinal polypeptide, monoamines, prostaglandins, and 2-chloroadenosine on adenylate cyclase in rat cerebral microvessels

Adenylate cyclase in microvessels isolated from rat cerebral cortex was stimulated by guanine nucleotides, catecholamines, prostaglandin E1, prostaglandin E2, and 2-chloroadenosine. Catecholamine stimulation was mediated by interaction with beta-adrenergic receptors. The order of relative potency was: isoproterenol greater than epinephrine greater than norepinephrine. Activation of microvessel adenylate cyclase by prostaglandins E1 and E2 as well as by 2-chloroadenosine was dose related. Twenty-two peptides were tested for possible effects on the microvessel adenylate cyclase. Only vasoactive intestinal polypeptide (VIP) was stimulatory. No inhibitory action was observed. Activation by VIP required guanosine triphosphate and was dose dependent from 10 nM to 1 microM (ED50 = 0.1 microM). At 30 degrees C, stimulation of adenylate cyclase by the peptide increased linearly with time for up to 15 min. The effect of VIP was not inhibited by phentolamine or propranolol, suggesting that its action was not elicited by interaction with alpha- or beta-adrenergic receptors. Activation achieved by VIP and isoproterenol, prostaglandin E1, or 2-chloroadenosine was the sum of the individual stimulations, suggesting that receptors for VIP were distinct from those for isoproterenol, prostaglandin E1, and 2-chloroadenosine.

Identification of muscarinic receptors in rat cerebral cortical microvessels

Microvessels isolated from rat cerebral cortex consist mainly of capillaries (greater than 85%). Fresh, intact microvessel preparations have been analyzed by radioligand binding techniques for muscarinic receptors. Scatchard analysis of specific quinuclidinyl benzilate (QNB) binding indicates that microvessels possess a large number of muscarinic sites (914 fmol/mg protein) of high affinity (KD = 0.034 nM). The association and dissociation rate constants (0.37 min-1 nM-1 and 0.0067 min-1, respectively) yield an equilibrium KD of 0.018 nM. Displacement of [3H]QNB by muscarinic ligands and control substances is typical of muscarinic receptors. The results indicate that cerebral microvessels possess a large population of muscarinic receptors.

Adenylate cyclase sensitivity to catecholamines and forskolin in rat pia-arachnoid and cerebral microvessels

Adenylate cyclase in homogenates of perfused pia-arachnoid from the rat brain displayed a sensitivity for activation by dl'isoproterenol, l'epinephrine, l'norepinephrine (NE) and fenoterol that was greater than shown by partial beta2 adrenergic agonists (metaproterenol and salbutamol), or partial beta1 adrenergic agonists (tazolol and dobutamine) and an alpha adrenergic agonist (phenylephrine). The addition of the quanosine triphosphate (GTP) analog, Gpp(NH)p(5'-guanylyl imidodiphosphate) to the enzyme preparations resulted in an increased ability of all agents (except tazolol) to activate adenylate cyclase. The agent, forskolin, exerted a very potent activation of the catalytic site of adenylate cyclase in both pia-arachnoid and cerebral microvessels (capillary fraction). The order of potency for adrenergic antagonists to inhibit NE-induced stimulation of the pial enzyme was: propranolol (mixed beta) greater than butoxamine (beta2) greater than phentolamine (alpha1) greater than practolol (beta1). Subchronic injections of reserpine to rats resulted in a pial enzyme that was hyper-responsive to NE and isoproterenol. Similarly, the capillary enzyme displayed an enhanced activity to NE and dopamine (DA). Both high and low Km forms of cyclic AMP (cAMP) phosphodiesterase were detected in developing and adult pia-arachnoid and capillaries. The addition of a calmodulin fraction plus calcium ions did not elicit activation of the high Km enzyme. Moreover, adenylate cyclase during development was sensitive to activation by NE and forskolin. Thus, the pia-arachnoid possesses an adenylate cyclase receptor-coupled system with a mixed response to catecholamines, but which is primarily beta2 in nature. In addition, this system, as well as the capillaries, possesses a capacity to respond to manipulation of monoamine levels.

Cerebral endothelial cell culture. I. The presence of beta 2 and alpha 2-adrenergic receptors linked to adenylate cyclase activity

Cultured endothelial cells derived from cerebral microvessels separated from 2-day-old rat brain contain a specific beta 2 and alpha 2-adrenergic sensitive adenylate cyclase (AC). Among the various tested hormones, PGE1 and PGE2 were found to be the most potent activators, while adenosine, angiotensin I and II, gamma-aminobutyric acid and vasoactive intestinal peptide inhibited the enzyme activity. However, acetylcholine, histamine, serotonin, glycine, glutamine, bradykinin, neurotensin and vasopressin (Lysine and Arginine) had no effect on the adenylate cyclase activity in this model. The susceptibility of the cerebrovascular endothelial AC system to the vasoactive substances as well as presence of beta 2 and alpha 2-type adrenergic receptors in the cultured endothelium provides additional support for the proposed endothelial involvement in the regulation of cerebrovascular permeability and blood flow.

Subtypes of beta-adrenergic receptors in rat cerebral microvessels

The 125I-labeled iodohydroxybenzylpindolol (IHYP) binding to beta-receptors on brain micro-vessels is inhibited by isoproterenol, epinephrine and norepinephrine, with Ki values of 2 X 10(-7) M, 2.5 X 10(-6) M and 1.2 X 10(-5) M, respectively. A modified Scatchard analysis of the inhibitory effects of practolol, metroprolol and zinterol on IHYP binding has shown that the proportion of beta 2-receptors in our preparation is about 80% of the total beta-adrenergic receptor population. Our data indicate that the beta-adrenergic receptors located on cerebral microvessels are of both beta 1 and beta 2 types, with a predominance of the beta 2 type.

Adrenergic and cholinergic receptors of cerebral microvessels

The presence of alpha- and beta-adrenergic and muscarinic cholinergic receptors in cerebral microvessels of the rat and pig was assessed by ligand binding techniques. The results demonstrate the presence of specific binding to alpha 2- and beta-adrenergic receptors but no appreciable specific binding to alpha 1-adrenergic or muscarinic cholinergic receptors. beta-Adrenergic receptors of pig cerebral microvessels are similar to those of the brain and other organs in their binding characteristics to the tritiated ligand and in their stereospecificity of binding to the biologically active isomers of beta-adrenergic agonists. Further evidence derived from the differential potency of binding displacement by the various beta-adrenergic agonists and selective beta 1- and beta 2-adrenergic antagonists indicates that beta-adrenergic receptors of pig cerebral microvessels are mostly of the beta 2-subtype.