Stimulation of phospholipase C in cultured microvascular endothelial cells from human frontal lobe by histamine, endothelin and purinoceptor agonists

Endothelin

The endothelins comprise three structurally similar 21-amino acid peptides. Endothelin-1 and -2 activate two G-protein coupled receptors, ETA and ETB, with equal affinity, whereas endothelin-3 has a lower affinity for the ETA subtype. Genes encoding the peptides are present only among vertebrates. The ligand-receptor signaling pathway is a vertebrate innovation and may reflect the evolution of endothelin-1 as the most potent vasoconstrictor in the human cardiovascular system with remarkably long lasting action. Highly selective peptide ETA and ETB antagonists and ETB agonists together with radiolabeled analogs have accurately delineated endothelin pharmacology in humans and animal models, although surprisingly no ETA agonist has been discovered. ET antagonists (bosentan, ambrisentan) have revolutionized the treatment of pulmonary arterial hypertension, with the next generation of antagonists exhibiting improved efficacy (macitentan). Clinical trials continue to explore new applications, particularly in renal failure and for reducing proteinuria in diabetic nephropathy. Translational studies suggest a potential benefit of ETB agonists in chemotherapy and neuroprotection. However, demonstrating clinical efficacy of combined inhibitors of the endothelin converting enzyme and neutral endopeptidase has proved elusive. Over 28 genetic modifications have been made to the ET system in mice through global or cell-specific knockouts, knock ins, or alterations in gene expression of endothelin ligands or their target receptors. These studies have identified key roles for the endothelin isoforms and new therapeutic targets in development, fluid-electrolyte homeostasis, and cardiovascular and neuronal function. For the future, novel pharmacological strategies are emerging via small molecule epigenetic modulators, biologicals such as ETB monoclonal antibodies and the potential of signaling pathway biased agonists and antagonists.

Evidence of promiscuous endothelial binding by Plasmodium falciparum-infected erythrocytes

The adhesion of infected red blood cells (iRBCs) to human endothelium is considered a key event in the pathogenesis of cerebral malaria and other life‐threatening complications caused by the most prevalent malaria parasite Plasmodium falciparum. In the past 30 years, 14 endothelial receptors for iRBCs have been identified. Exposing 10 additional surface proteins of endothelial cells to a mixture of P. falciparum isolates from three Ghanaian malaria patients, we identified seven new iRBC receptors, all expressed in brain vessels. This finding strongly suggests that endothelial binding of P. falciparum iRBCs is promiscuous and may use a combination of endothelial surface moieties.

Endothelin

In humans, the endothelins (ETs) comprise a family of three 21-amino-acid peptides, ET-1, ET-2 and ET-3. ET-1 is synthesised from a biologically inactive precursor, Big ET-1, by an unusual hydrolysis of the Trp21 -Val22 bond by the endothelin converting enzyme (ECE-1). In humans, there are four isoforms (ECE-1a-d) derived from a single gene by the action of alternative promoters. Structurally, they differ only in the amino acid sequence of the extreme N-terminus. A second enzyme, ECE-2, also exists as four isoforms and differs from ECE-1 in requiring an acidic pH for optimal activity. Human chymase can also cleave Big ET-1 to ET-1, which is cleaved, in turn, to the mature peptide as an alternative pathway. ET-1 is the principal isoform in the human cardiovascular system and remains one of the most potent constrictors of human vessels discovered. ET-1 is unusual in being released from a dual secretory pathway. The peptide is continuously released from vascular endothelial cells by the constitutive pathway, producing intense constriction of the underlying smooth muscle and contributing to the maintenance of endogenous vascular tone. ET-1 is also released from endothelial cell-specific storage granules (Weibel-Palade bodies) in response to external stimuli. ETs mediate their action by activating two G protein-coupled receptor sub-types, ETA and ET(B). Two therapeutic strategies have emerged to oppose the actions of ET-1, namely inhibition of the synthetic enzyme by combined ECE/neutral endopeptidase inhibitors such as SLV306, and receptor antagonists such as bosentan. The ET system is up-regulated in atherosclerosis, and ET antagonists may be of benefit in reducing blood pressure in essential hypertension. Bosentan, the first ET antagonist approved for clinical use, represents a significant new therapeutic strategy in the treatment of pulmonary arterial hypertension (PAH).

Endothelin ETB receptor signaling in the median eminence and subfornical organ of the rat brain

We investigated the effect of endothelins (ETs) on receptor-mediated phosphoinositides (PI) turnover in whole subfornical organ (SFO) and median eminence (ME). Consistent with the presence of a high density of binding sites in the SFO and the ME of the rat brain, our results show an increase in PI hydrolysis induced by ETs in each structure, in a dose-dependent manner and with similar ED50 values. In addition, IRL 1620, a selective ETB receptor agonist, increased the inositol monophosphate (InsP1) accumulation in the SFO and the ME in a similar degree as ETs. With the use of selective agonists and antagonists of both endothelin receptor subtypes, we characterized the receptor subtype involved in ET-induced phosphoinositide metabolism. The addition of two selective ETA receptor antagonists, BQ 123 or BQ 610, did not alter the ETs-induced increase in the PI metabolism. While, IRL 1620- and ET3-induced InsP1 accumulation was completely blocked by BQ 788, a selective ETB receptor antagonist, in both brain structures evaluated. Our results demonstrate that in the SFO and the ME of the rat brain, stimulation of phosphoinositide turnover constitutes one of the signaling pathways of ETs, and this action is mediated through ETB receptor activation. These results support the concept that endothelin could play a role in the regulation of brain functions.

The pharmacology of nucleotide receptors on primary rat brain endothelial cells grown on a biological extracellular matrix: effects on intracellular calcium concentration

1. Brain capillary endothelial cells express a variety of nucleotide receptors, but differences have been reported between culture models. This study reports examination of nucleotide receptors on primary cultured rat brain capillary endothelial cells (RBCEC) grown on a biological extracellular matrix (ECM) to produce a more differentiated phenotype. 2. Fura-2 fluorescence ratio imaging was used to monitor intracellular free calcium concentration [Ca(2+)](i). ATP, UTP, and 2-methylthioATP (2-MeSATP) increased [Ca(2+)](i) to similar levels, while 2-MeSADP, ADP and adenosine gave smaller responses. 3. Removal of extracellular calcium caused no significant change in the [Ca(2+)](i) response to 2-MeSATP, evidence that the response was mediated by a metabotropic (P2Y) receptor. 4. All cells tested responded to ATP, UTP, 2-MeSATP and ADP, while 63% responded to adenosine and 50% to 2-MeSADP. No cells responded to alpha, beta-methyleneATP. Cells grown on rat tail collagen instead of ECM gave smaller and less uniform [Ca(2+)](i) responses, suggesting that the differentiating effect of the ECM contributed to a more uniform receptor profile. 5. The [Ca(2+)](i) response to the P2Y(1)-selective agonist 2-MeSADP was abolished in the presence of the subtype-selective antagonist adenosine 3'-phosphate 5'-phosphosulphate (PAPS). 6. The P2Y(2) antagonist suramin completely blocked the response to ATP and inhibited the response to UTP by 66%. 7. The A(1) subtype-selective adenosine receptor agonist N(6)-Cyclopentyladenosine (CPA) gave a small but characteristic [Ca(2+)](i) response, while A(2A) and A(2B) subtype-selective agonists failed to generate [Ca(2+)](i) changes. 8. The results are consistent with the presence on RBCEC of a P2Y(2)-like receptor coupled to phospholipase C, and a P2Y(1)-like receptor mobilizing intracellular Ca(2+). The role of multiple nucleotide receptors in the function of the brain endothelium is discussed.

Inflammatory mediators and modulation of blood-brain barrier permeability

1. Unlike some interfaces between the blood and the nervous system (e.g., nerve perineurium), the brain endothelium forming the blood-brain barrier can be modulated by a range of inflammatory mediators. The mechanisms underlying this modulation are reviewed, and the implications for therapy of the brain discussed. 2. Methods for measuring blood-brain barrier permeability in situ include the use of radiolabeled tracers in parenchymal vessels and measurements of transendothelial resistance and rate of loss of fluorescent dye in single pial microvessels. In vitro studies on culture models provide details of the signal transduction mechanisms involved. 3. Routes for penetration of polar solutes across the brain endothelium include the paracellular tight junctional pathway (usually very tight) and vesicular mechanisms. Inflammatory mediators have been reported to influence both pathways, but the clearest evidence is for modulation of tight junctions. 4. In addition to the brain endothelium, cell types involved in inflammatory reactions include several closely associated cells including pericytes, astrocytes, smooth muscle, microglia, mast cells, and neurons. In situ it is often difficult to identify the site of action of a vasoactive agent. In vitro models of brain endothelium are experimentally simpler but may also lack important features generated in situ by cell:cell interaction (e.g. induction, signaling). 5. Many inflammatory agents increase both endothelial permeability and vessel diameter, together contributing to significant leak across the blood-brain barrier and cerebral edema. This review concentrates on changes in endothelial permeability by focusing on studies in which changes in vessel diameter are minimized. 6. Bradykinin (Bk) increases blood-brain barrier permeability by acting on B2 receptors. The downstream events reported include elevation of [Ca2+]i, activation of phospholipase A2, release of arachidonic acid, and production of free radicals, with evidence that IL-1 beta potentiates the actions of Bk in ischemia. 7. Serotonin (5HT) has been reported to increase blood-brain barrier permeability in some but not all studies. Where barrier opening was seen, there was evidence for activation of 5-HT2 receptors and a calcium-dependent permeability increase. 8. Histamine is one of the few central nervous system neurotransmitters found to cause consistent blood-brain barrier opening. The earlier literature was unclear, but studies of pial vessels and cultured endothelium reveal increased permeability mediated by H2 receptors and elevation of [Ca2+]i and an H1 receptor-mediated reduction in permeability coupled to an elevation of cAMP. 9. Brain endothelial cells express nucleotide receptors for ATP, UTP, and ADP, with activation causing increased blood-brain barrier permeability. The effects are mediated predominantly via a P2U (P2Y2) G-protein-coupled receptor causing an elevation of [Ca2+]i; a P2Y1 receptor acting via inhibition of adenyl cyclase has been reported in some in vitro preparations. 10. Arachidonic acid is elevated in some neural pathologies and causes gross opening of the blood-brain barrier to large molecules including proteins. There is evidence that arachidonic acid acts via generation of free radicals in the course of its metabolism by cyclooxygenase and lipoxygenase pathways. 11. The mechanisms described reveal a range of interrelated pathways by which influences from the brain side or the blood side can modulate blood-brain barrier permeability. Knowledge of the mechanisms is already being exploited for deliberate opening of the blood-brain barrier for drug delivery to the brain, and the pathways capable of reducing permeability hold promise for therapeutic treatment of inflammation and cerebral edema.

Effects of N,N-diethyl-2-[4-(phenylmethyl)phenoxy]ethanamine on the blood-brain barrier permeability in the rat

Histamine plays a role in the regulation of the blood-brain barrier function. In this study, effects of N, N-diethyl-2-[4-(phenylmethyl)phenoxy]ethanamine (DPPE), an intracellular histamine binding site antagonist on the cerebrovascular permeability were investigated in control and post-ischemic male Wistar rats. Intravenous administration of DPPE, in a dose of 1 and 5 mg/kg, was not followed by any major clinical change, but 20 mg/kg proved to be toxic. A significantly (P<0.05) increased permeability for sodium fluorescein (MW=376) was seen in hippocampus, striatum, and cerebellum, but not in parietal cortex, of rats 2 h after the injection of 5 mg/kg DPPE, whereas no increase was measured later. There was a more intense (5- to 12-fold) and prolonged elevation in Evan's blue-labeled albumin (MW=67,000) extravasation 2, 4, and 8 h after 5 mg/kg DPPE administration in each brain region. In parietal cortex, a dose-dependent increase in albumin extravasation developed 4 h after intravenous injection of 1, 5, and 20 mg/kg DPPE, but doses applied resulted in no significant change in sodium fluorescein permeability. Cerebral ischemia-reperfusion evoked by four-vessel occlusion caused a significant (P<0.05) increase in the permeability for albumin in each region, but few changes in that of sodium fluorescein. DPPE treatment failed to prevent the ischemia-reperfusion-induced changes in the blood-brain barrier permeability. In conclusion, DPPE induced an increased permeability in the rat, which supports a role for histamine, as an intracellular messenger, in the regulation of the blood-brain barrier characteristics.

P2 purinoceptors in cultured bovine middle cerebral artery endothelial cells

Extracellular adenosine triphosphate (ATP) plays an important role in the regulation of endothelial function. However, its receptors and their signal-transduction pathways in major cerebral arterial endothelial cells are largely unknown. This study was undertaken functionally to classify the P2 purinoceptors in cultured bovine middle cerebral artery endothelial cells by using [Ca2+]i microfluorimetry. The rank order of potency to increase [Ca2+]i was 2-methylthio-ATP approximately ATP approximately uridine triphosphate (UTP) > adenosine diphosphate (ADP) >> adenosine monophosphate (AMP) > alpha,beta-methylene-ATP > adenosine, suggesting that the effect was mediated by both P2y and P2u receptors. ATP, 2-methylthio-ATP, and UTP mobilized Ca2+ from intracellular stores and triggered Ca2+ entry. The effects of ATP, 2-methylthio-ATP, and UTP were reduced by phospholipase C inhibitor 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate (NCDC), but only the effects of ATP and UTP were attenuated by pertussis toxin, indicating that P2y and P2u receptors may activate the same effector mechanisms by coupling to different G proteins. The [Ca2+]i entry caused by UTP was significantly reduced by the receptor-regulated Ca2+ channel blocker SK&F 96365, by P-450 inhibitor econazole and by inorganic Ca2+ entry blocker lanthanum. P2-receptor antagonists suramin, pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), and reactive blue 2 reduced the effects of ATP and 2-methylthio-ATP, but not those of UTP, in a concentration-dependent manner. These studies suggest a coexistence of P2y and P2u receptors in cultured bovine middle cerebral artery endothelial cells.

Age-related changes in the blood-brain barrier

Aging of the cerebral microcirculation results in significant alteration in the blood-brain barrier (BBB). The barrier function appears to remain intact in older animals, although it may be more susceptible to disruption by external factors (hypertension) and drugs (haloperidol). While overall transport processes do not change with age, aging animals and humans have altered BBB function of select carrier mediated transport systems including the transport of choline, glucose, butyrate and triiodothyronine. These age-related changes are the result of either alteration in the carrier molecules or the physiochemical properties of the cerebral microvessels. At the present time, it is not known whether changes in the BBB contribute to the age-related neurodegenerative diseases or are merely epiphenomena of aging.

Endothelin stimulates phosphoinositide hydrolysis and PAF synthesis in brain microvessels

Treatment of brain microvessels with the three endothelin (ET) isoforms resulted in an increase of phosphoinositide turnover by activation of phospholipase C in a dose- and time-dependent manner. Both ET-1 and ET-2 are maximally effective, whereas the effect evoked by ET-3 was smaller. Concomitantly, there was an enhanced production of a platelet-activating factor (PAF)-like material. This was identified by standard and biological probes in platelets, such as induction of aggregation, phosphatidic acid (PA) production, increase of endogenous protein phosphorylation, and reversal of these responses by a PAF antagonist. The effects evoked by endothelins on phosphoinositide metabolism and PAF production were, to a certain extent, dependent on the presence of extracellular Ca2+. In addition, ET induced changes in Ca2+ dynamics, evoking an initial and rapid intracellular mobilization and influx of Ca2+ and, later, a maintained Ca2+ influx. These findings contribute to the understanding of the pathophysiological role of ET in the blood-brain barrier (BBB).

Characterization of signaling pathways of P2Y and P2U purinoceptors in bovine pulmonary artery endothelial cells

The actions of ATP on the endothelium are mediated by P2 purinoceptors. We have shown that P2Y and P2U purinoceptors coexist in bovine pulmonary artery endothelial cells (CPAE), where they induce phosphoinositide (PI) turnover and Ca2+ mobilization. The relative order of potency (based on the threshold concentration) of nucleotide analogues (1-100 microM) in stimulating the accumulation of inositol phosphate (IP) was 2-methylthio-ATP (2MeSATP) = 2-methylthio-ADP (2MeSADP) > or = 2ClATP > UTP = ATP = ADP. alpha, beta-methylene ATP, beta, gamma-methylene ATP, UDP, adenosine-5'-tetraphospho-5'-adenosine, and adenosine-5'-pentaphospho-5'-adenosine had no effect at concentrations as high as 100 microM. At maximal concentrations, the IP responses to 2MeSATP and UTP were additive, whereas those to ATP and either 2MeSATP or UTP were not. Moreover, the maximal response to 2MeSADP was additive to that to UTP but not to that of 2MeSATP. Pretreatment with pertussis toxin slightly inhibited 2MeSATP- and UTP-stimulated IP generation by 15%. Under Ca(2+)-free conditions, UTP-induced IP formation was inhibited more markedly than that induced by 2MeSATP. Short-term treatment of the cells with phorbol 12-myristate-13-acetate (PMA) resulted in a dose-dependent inhibition of 2MeSATP-induced IP formation greater and more sensitive than that induced by UTP; similar results were obtained for the sensitivity of inhibition by suramin and reactive blue. Stimulation of the cells with either 2MeSATP or UTP induced a rapid increase in intracellular Ca2+ level, followed by a slow decrease to basal levels, followed by Ca2+ level oscillation. In the absence of extracellular Ca2+, [Ca2+]i responses were quantitatively less and did not show the slow phase and oscillation. Together these results suggest that both P2Y and P2U purinoceptors are expressed in bovine pulmonary artery endothelial cells and are coupled to phospholipase C (PLC) activation and Ca2+ mobilization through pertussis toxininsensitive G proteins.

Evidence for a discrete UTP receptor in cardiac endothelial cells

1. We have examined the effects of various purine and pyrimidine nucleotides upon cells cultured from guinea-pig cardiac endothelium (CEC), and find the P2Y-agonist 2-methylthioadenosine triphosphate (2MeSATP) to be a potent (EC50 = 85 +/- 10.2 nM) stimulator of increase in intracellular calcium concentrations, while uridine 5'-triphosphate (UTP) and adenosine 5'-triphosphate (ATP) are less potent but equipotent with one another (EC50s = 2.1 +/- 0.3 and 1.8 +/- 0.2 microM, respectively). 2. While the P2Y receptor exhibited rapid homologous desensitization, this had no effect upon subsequent responsiveness of CEC to either ATP or UTP. Effects of maximal concentrations of ATP and UTP were not only additive, but did not cross-desensitize. Responses to UTP (but not to ATP or 2MeSATP) were blocked by treatment with pertussis toxin (PTX); all three nucleotides appeared to liberate calcium from an intracellular pool. 3. Suramin (30 microM) significantly (P < 0.05) increased the EC50 for ATP-dependent increases in intracellular calcium (5.3 +/- 2.2 microM vs. 2.0 +/- 0.9 microM in the absence of suramin), while it completely blocked the response to 2MeSATP. Suramin had no effect upon responses to UTP at concentrations of 100 microM. 4. We conclude that in addition to the P2Y and P2U subtypes of the ATP receptor, an additional receptor responsive to UTP but exhibiting no affinity for purine nucleotides is present in CEC; this "pyrimidine receptor' liberates intracellular calcium via a G-protein, and may partly mediate the contractile response to UTP in the coronary vasculature.

Histamine-induced prolonged depolarization in rat supraoptic neurons: G-protein-mediated, Ca(2+)-independent suppression of K+ leakage conductance

Ionic mechanisms responsible for histamine-induced prolonged depolarization in supraoptic nucleus neurons were investigated using whole-cell patch recordings in horizontally prepared brain slices from adult male rats. Bath application of histamine (1-10 microM) in control medium induced membrane depolarization in nine of 12 phasically firing, putative vasopressin cells, but not in continuous firing, putative oxytocin cells (none of five cells). Depolarization, usually accompanied by increased firing rate, started within 20 s after histamine reached the slices, lasting for 3-13 min, after which they repolarized, and this was repeatable upon washout. Chelation of intracellular Ca2+ with 11 mM 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetate and perfusion of slices with Ca(2+)-free medium blocked neither histamine-induced membrane depolarizations nor increased firing rates in 24 of 30 cells recorded. Depolarizations were always associated with decreases in membrane conductance. Following treatment with promethazine (H1 receptor antagonist) in six cells excited previously by histamine, subsequent application induced neither membrane depolarization nor increased firing. H1 receptor agonists mimicked histamine-induced depolarization (four of six cells) but the H2 receptor agonist, dimaprit (10 microM), had no effect (all of nine cells). In medium containing 0 mM Ca2+, 2 mM Co2+ and 1-2 microM tetrodotoxin, with internal Ca2+ chelation, bath application of histamine induced an apparent inward current in 15 of 20 supraoptic neurons tested. The peak of inward current evoked by 1-10 microM histamine at holding potentials around -50 mV varied from 10 to 50 pA (27.3 +/- 0.3 pA, mean +/- S.E.M.). Ramp voltage tests revealed that this inward current decreased as membrane potential was hyperpolarized and had a reversal potential of -90.1 +/- 3.8 mV (n = 10). Subtraction of current obtained before from that during histamine application revealed a current that was linear against membrane potential. Increasing external K+ concentration or introduction of K+ channel blockers in the medium attenuated or abolished histamine-induced inward current at membrane potentials close to -50 mV. When external Cl- concentration was reduced, histamine-induced inward current was still seen in five of seven supraoptic cells tested. Neither inward current nor change in conductance was observed following bath application of histamine in 11 of 12 neurons recorded using patch pipettes containing guanosine 5'-O-(2-thiodiphosphate), and in seven of eight neurons using pipettes containing guanosine 5'-O-(3-thiotriphosphate). These results suggest that histamine depolarizes supraoptic neurons, at least in part, by inhibiting a K+ leakage current mediated by H1 receptors linked to GTP-binding proteins and Ca(2+)-independent pathways. This study provides initial evidence for the second messengers regulating K+ leakage current.