Cysteinyl-leukotrienes induce vascular endothelial growth factor production in human monocytes and bronchial smooth muscle cells.

BACKGROUND: Cysteinyl leukotrienes (cysLTs) are suggested to be implicated in the process of airway remodelling in asthma. OBJECTIVE: We investigated the potential for cysLTs to modulate vascular endothelial growth factor (VEGF) expression, a growth factor involved in the angiogenesis of airway remodelling. METHODS: VEGF mRNA and protein were quantified by real-time PCR and ELISA, respectively. VEGF promoter activation was assessed using luciferase gene-tagged promoter constructs. RESULTS: We found that LTD(4) induction of VEGF in human monocytes and bronchial smooth muscle cells is cysLT1 dependent. Stimulation of HEK293 cells stably expressing cysLT1 or cysLT2 with cysLTs showed a concentration-dependent activation of the VEGF promoter and a time-dependent increase in VEGF mRNA and protein. For the cysLT1-mediated response, mutations of hypoxia-induced factor-1 (HIF-1) sites failed to reduce cysLT-induced VEGF promoter activation and 5' deletions showed that the proximal region containing one AP-1 and four specificity protein 1 (Sp1) sites was necessary. Pretreatment with inhibitors of c-Jun N-terminal kinase (JNK) and extracellular signal-regulated kinase (ERK), but not p38, and an overexpression of dominant negative forms of c-Jun, c-Fos or Ras suggested the implication of mitogen-activated protein kinases and AP-1. Mutation of the AP-1-binding element failed to prevent VEGF transactivation suggesting that AP-1 might not act directly on the promoter. Moreover, inhibition of Sp1-dependent transcription by mithramycin completely inhibited VEGF promoter transactivation and VEGF mRNA expression by LTD(4) . Finally, mutations of Sp1 binding elements prevented VEGF promoter transactivation. CONCLUSION AND CLINICAL RELEVANCE: Our data indicate for the first time that cysLTs can transcriptionally activate VEGF production via cysLT1 receptors, with the involvement of JNK, ERK, the AP-1 complex and Sp1. These findings suggest that cysLTs may be important in the angiogenic process of airway remodelling and potentially provide a previously unknown benefit of using cysLT1 receptor antagonists in the prevention or treatment of airway remodelling in asthma.

Transforming growth factor-beta1 in asthmatic airway smooth muscle enlargement: is fibroblast growth factor-2 required?

Enlargement of airway smooth muscle (ASM) tissue around the bronchi/bronchioles is a histopathological signature of asthmatic airway remodelling and has been suggested to play a critical role in the increased lung resistance and airway hyperresponsiveness seen in asthmatic patients. The pleiotropic cytokine, TGF-beta1, is believed to contribute to several aspects of asthmatic airway remodelling and is known to influence the growth of many cell types. Increased TGF-beta1 expression/signalling and ASM growth have been shown to occur concurrently in animal models of asthma. Abundant studies further substantiate this association by showing that therapeutic strategies that reduce or prevent TGF-beta1 overexpression/signalling lead to a parallel decrease or prevention of ASM enlargement. Finally, recent findings have supported a direct link of causality between TGF-beta1 overexpression/signalling and the overgrowth of ASM tissue. To follow-up on these in vivo studies, many investigators have pursued detailed investigation of ASM in cell culture conditions, assessing the direct role of TGF-beta1 on cellular proliferation and/or hypertrophy. Inconsistencies among the in vitro studies suggest that the effect of TGF-beta1 on ASM cell proliferation/hypertrophy is contextual. A hypothesis focusing on fibroblast growth factor-2 is presented at the end of this review, which could potentially reconcile the apparent discrepancy between the conflicting in vitro findings with the consistent in vivo finding that TGF-beta1 is required for ASM enlargement in asthma.

Leukotriene D4-induced, epithelial cell-derived transforming growth factor beta1 in human bronchial smooth muscle cell proliferation.

BACKGROUND: Cysteinyl-leukotrienes (cys-LTs) orchestrate many pathognomonic features of asthma in animal models of allergic airway inflammation, including bronchial smooth muscle cell (BSMC) hyperplasia. However, because cys-LTs alone do not induce mitogenesis in monocultures of human BSMC, the effect observed in vivo seemingly involves indirect mechanisms, which are still undefined. OBJECTIVE: This study aims to investigate the regulatory role of leukotriene (LT)D(4) on TGF-beta1 expression in airway epithelial cells and the consequence of this interplay on BSMC proliferation. METHODS: HEK293 cells stably transfected with cys-LT receptor 1 (CysLT1) (293LT1) were stimulated with LTD(4) and TGF-beta1 mRNA and protein expression was measured using Northern blot and ELISA, respectively. Conditioned medium (CM) harvested from LTD(4)-treated cells was then assayed for its proliferative effect on primary human BSMC. TGF-beta1 mRNA expression was also determined in tumoural type II pneumocytes A549 and in normal human bronchial epithelial cells (NHBE) following LTD(4) stimulation. RESULTS: The results demonstrated that LTD(4)-induced TGF-beta1 mRNA production in a time- and concentration-dependent manner in 293LT1. TGF-beta1 secretion was also up-regulated and CM from LTD(4)-treated 293LT1 was shown to increase BSMC proliferation in a TGF-beta1-dependent manner. The increased expression of TGF-beta1 mRNA by LTD(4) also occured in A549 and NHBE cells via a CysLT1-dependent mechanism. CONCLUSION: In conclusion, elevated expression of cys-LTs in asthmatic airways might contribute to BSMC hyperplasia and concomitant clinical features of asthma such as airway hyperresponsiveness via a paracrine loop involving TGF-beta1 production by airway epithelial cells.

Membrane markers, target cell specificity, and sensitivity to biological response modifiers distinguish human natural cytotoxic from human natural killer cells.

In the present report, we provide evidence for the distinct existence of a human natural cytotoxic (HNC) cell. This HNC cell can be identified by the monoclonal antibody HNC-1A3 and by the absence of the T10 antigen, other antigenic markers being shared, at least in part, with natural killer (NK) cells, T cells, or monocytes. In addition, the HNC cell preferentially kills the MA-160 target, the herpes simplex virus-1-infected MA-160 cell line, and the two human tumor cell lines HEp-2 and HF-2. It has weak lytic activity against the NK-sensitive K562 cell line or its relatively NK-resistant clone I subline. The cytotoxic activity of the HNC cell is not augmented by interferon but is markedly enhanced by interleukin 2 and by a measles-virus-induced factor (MVF). Furthermore, it is not inhibited by cyclosporin A (CsA), in contrast to NK cell cytotoxicity against the K562 target cell line which is augmented by interferon, inhibited by CsA, and not affected by MVF. These data suggest that spontaneously cytotoxic cells may belong to more than one subset of human lymphocytes, and that HNC cells may be defined in man using membrane markers, target cell specificity, and sensitivity to biological response modifiers.

Platelet activating factor produced in vitro by Kaposi's sarcoma cells induces and sustains in vivo angiogenesis.

Imbalance in the network of soluble mediators may play a pivotal role in the pathogenesis of Kaposi's sarcoma (KS). In this study, we demonstrated that KS cells grown in vitro produced and in part released platelet activating factor (PAF), a powerful lipid mediator of inflammation and cell-to-cell communication. IL-1, TNF, and thrombin enhanced the synthesis of PAF. PAF receptor mRNA and specific, high affinity binding site for PAF were present in KS cells. Nanomolar concentration of PAF stimulated the chemotaxis and chemokinesis of KS cells, endothelial cells, and vascular smooth muscle cells. The migration response to PAF was inhibited by WEB 2170, a hetrazepinoic PAF receptor antagonist. Because neoangiogenesis is essential for the growth and progression of KS and since PAF can activate vascular endothelial cells, we examined the potential role of PAF as an instrumental mediator of angiogenesis associated with KS. Conditioned medium (CM) from KS cells (KS-CM) or KS cells themselves induced angiogenesis and macrophage recruitment in a murine model in which Matrigel was injected subcutaneously. These effects were inhibited by treating mice with WEB 2170. Synthetic PAF or natural PAF extracted from plasma of patients with classical KS also induced angiogenesis, which in turn was inhibited by WEB 2170. The action of PAF was amplified by expression of other angiogenic factors and chemokines: these included basic and acidic fibroblast growth factor, placental growth factor, vascular endothelial growth factor and its specific receptor flk-1, hepatocyte growth factor, KC, and macrophage inflammatory protein-2. Treatment with WEB 2170 abolished the expression of the transcripts of these molecules within Matrigel containing KS-CM. These results indicate that PAF may cooperate with other angiogenic molecules and chemokines in inducing vascular development in KS.

Expression of platelet-activating factor receptor in human carotid atherosclerotic plaques: relevance to progression of atherosclerosis.

BACKGROUND: Human monocyte-derived macrophages synthesize numerous proinflammatory and prothrombotic substances, including lipid mediators, such as platelet-activating factor (PAF), which may play a major role in the onset and perpetuation of atherosclerotic lesions. In addition, both monocytes and macrophages express PAF receptors (PAF-R). The expression of PAF-R is transcriptionally downregulated by oxidized LDL in in vitro primary cultures of monocyte/macrophages. In this study, we evaluated the expression of PAF-R in human carotid plaque tissue, in foam cells isolated from human carotid plaques, and in primary cultures of umbilical smooth muscle cells (SMCs). METHODS AND RESULTS: We show that PAF-R was expressed at low levels in foam cells compared with monocyte/macrophages in plaques, as assessed by immunohistochemical staining and in situ hybridization. In addition, low levels of mRNA were also detected by RT-PCR in isolated human carotid foam cells. A prominent finding of our study was the demonstration that contractile SMCs were positive for PAF-R, and its mRNA was extracted from primary cultures of umbilical SMCs. CONCLUSIONS: As macrophages loose their inflammatory phenotype on transformation into foam cells, they may equally loose their capacity of defense against aggression. We postulate that the diminished expression of PAF-R may be deleterious in the context of plaque formation and progression. The observation that arterial SMCs of contractile phenotype express PAF-R opens new avenues concerning the migration of these cells from media to intima and atherosclerotic plaque formation.

Immune regulation by platelet-activating factor: II. Mediation of suppression by cytokine-stimulated endothelial cells in vitro.

Human umbilical vein endothelial cells (EC) can respond to endotoxin or to the inflammatory cytokines tumor necrosis factor (TNF) and interleukin 1 (IL-1) by producing platelet-activating factor (PAF). When EC were preexposed to TNF-alpha (25 U/ml) for 1 h, and then washed, their subsequent coculture with peripheral blood mononuclear cells (PBMC) resulted in suppressed proliferative response of the latter to the mitogen Con A (P less than 0.05). This effect was completely reversed by the concomitant use of the PAF receptor antagonist BN 52021 (0.1 mM). Preexposure of EC to IL-1 beta (0.5 U/ml) induced similar effects, but IL-1 and TNF were not additive. Removal of monocytes from the PBMC population abolished the effects. On the other hand, coculture of monocytes with cytokine-preexposed EC resulted in significant induction of suppressor activity on lymphocyte proliferation. Our data indicate that EC, preexposed to inflammatory cytokines, can modulate lymphocyte functions via the production of PAF and its action on monocytes.

Differentiation-dependent modulation of TNF production by PAF in human HL-60 myeloid leukemia cells.

Platelet-activating factor (PAF) can augment tumor necrosis factor (TNF) production by human monocytes in a bimodal manner, with two peaks of activation at picomolar and micromolar concentrations. These peaks are partially associated with monocyte subsets presenting different characteristics in terms of size, density, phenotypic markers, and [Ca2+]i mobilization responses. In the present study, we used the human promyelocytic leukemia cell line HL-60, at various times during differentiation with 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] toward the monocyte lineage, in order to study the relation of cell differentiation to responsiveness to PAF in terms of cytokine production. TNF production was induced by pretreatment with interferon gamma for 24 h and treatment with muramyl dipeptide. Although detectable TNF was produced by 4 day-differentiated cells, no effect was seen with PAF (10(-16)-10(-6) M) at this or earlier stages. In contrast, 5 day-differentiated cells had a comparable baseline production of TNF but responded with a 2.5-fold increase to PAF with a single peak, maximal at 10(-8) M. Moreover, 6 day- or 7 day-differentiated HL-60 cells showed a further increase in TNF production in response to PAF, and the response was bimodal, similar to that of the less dense subset of monocytes, with peaks at 10(-14) and 10(-7) M PAF. In parallel, undifferentiated HL-60 failed to respond to PAF in terms of [Ca2+]i mobilization. The earliest responsiveness to PAF (10(-7) M) was observed by 4 days of treatment with 1,25(OH)2D3, and by day 7 the response to PAF became bimodal (10(-14) and 10(-7) M). These results indicate that myeloid cells acquire, during maturation toward the monocyte lineage, a progressive responsiveness to PAF in terms of [Ca2+]i mobilization and enhanced cytokine production, and they suggest that the heterogeneity in responses to PAF observed in normal monocytes may be related to their stage of differentiation or maturation.

Inhibition of alveolar macrophage cytotoxicity by asbestos: possible role of prostaglandins.

Asbestosis and silicosis are chronic, fibrosing lung diseases due to prolonged inhalation of asbestos fibers or silica particles. However, little is known about the implication of these toxic dusts on cell-mediated cytotoxicity. Among the first types of cells that are in contact with the dusts are the alveolar macrophages (AM). We studied the effect of different concentrations of UICC chrysotile asbestos and silica on 18-h cytotoxicity of AM against tumor necrosis factor (TNF)-resistant P815 target cells or TNF-sensitive L929 target cells. Rat AM, obtained by bronchoalveolar lavage, were incubated for 2 h with 20, 50, or 100 micrograms/ml chrysotile or silica before the addition of target cells. AM cytotoxicity was significantly inhibited at greater than 20 micrograms/ml of chrysotile. In contrast, silica did not inhibit AM-mediated cytotoxicity at any concentration used. Asbestos, but not silica, caused significant production of PGE2 by macrophages and target cells. Addition of the cyclooxygenase inhibitor indomethacin to our system abolished all inhibition by asbestos. These results suggest that the inhibition of AM-mediated cytotoxicity by chrysotile was caused by prostaglandins, and that fibrogenic particles differ in their capacity to modulate AM function.

Augmented expression of platelet-activating factor receptor gene by TNF-alpha through transcriptional activation in human monocytes.

Tumor necrosis factor alpha (TNF-alpha) is a cytokine produced by activated monocytes and often associated with platelet-activating factor (PAF) during the pathogenesis of many inflammatory and infectious diseases. PAFR is a G-protein-coupled receptor constitutively expressed on monocytes. TNF-alpha (100-400 U/mL) significantly increased PAFR mRNA expression in human monocytes. This increase was seen after 1 h of stimulation and persisted up to 24 h. Actinomycin D pretreatment studies revealed a transcriptional increase in PAFR gene expression without effect on mRNA half-life. [3H]WEB 2086 binding studies showed a significant (43%) increase in specific binding sites in 24-h-treated cells without change in receptor affinity. Increased interleukin-6 production in response to PAF was also found in 24-h TNF-alpha-pretreated monocytes. These observations provide new evidence for TNF-alpha and PAF interactions in human monocytes during inflammatory processes through up-regulation of PAFR expression by TNF-alpha.

Abnormal suppressor cell function in atopic dermatitis.

A status of suppressor cells in patients with atopic dermatitis was studied. As a group, they showed absent concanavalin A-inducible suppressor cell function as measured by proliferative responses to pokeweed mitogen and decreased function as measured by responses to phytohemagglutinin or concanavalin A. Similarly, preincubation in medium enhanced proliferative responses in normal donors but not in atopic dermatitis patients, suggesting an absence of a short-lived suppressor cell population in the latter group. Suppressor cell function correlated negatively with log10 of serum IgE concentrations. Theophylline-sensitive suppressor cell numbers were significantly decreased in atopic dermatitis patients (p less than 0.01). In vitro preincubation of normal lymphocytes with aminophylline or isoproterenol (10 microgram/ml) enhanced subsequent proliferative responses to pokeweed mitogen. In contrast, actual depression was seen with cells from atopic dermatitis patients, suggesting abnormal immunomodulatory effects of these drugs in the disease.

Identification of functional platelet-activating factor receptors on human keratinocytes.

Platelet-activating factor (PAF) is a potent inflammatory mediator that has been shown to be produced by human keratinocytes and is thought to play a role in cutaneous inflammation. Immunofluorescence and radioligand binding studies were used to characterize PAF receptors (PAF-R) on human keratinocytes and the human epidermoid cell lines A-431 and HaCaT. Indirect immunofluorescence studies demonstrated anti-PAF-R staining of primary cultures of human keratinocytes, A-431 cells, and HaCaT cells. Primary cultures of human fibroblasts and the melanoma cell line SK-30 failed to show immunostaining above that seen with control antiserum. With indirect immunofluorescence studies of sections of normal human skin, a granular anti-PAF-R staining pattern was noted on the keratinocyte cell membranes. A-431 cells readily metabolized PAF by deacetylation-reacylation at 37 degrees C, but not at 4 degrees C. Binding studies on crude membrane preparations of A-431 cells conducted at 4 degrees C demonstrated specific binding that reached saturation by 120 min. Scatchard analysis of PAF binding data revealed a single class of high-affinity (KD = 6.3 +/- 0.3 nM) PAF binding sites. The immunofluorescence and radioligand binding sites were shown to be functional PAF-Rs, as 10 pM to 1 microM PAF increased intracellular calcium in primary cultures of human keratinocytes, A-431 cells, and HaCaT cells, whereas PAF treatment of primary cultures of human fibroblasts or the melanoma cell line SK-30 did not result in changes in the intracellular calcium concentration. The structurally dissimilar PAF-R antagonists CV-6209, Ro19-3704, and alprazolam all inhibited the PAF-induced calcium changes in A-431 cells. The CV-6209 inhibition was seen at doses that competed with the PAF binding to these cells. These studies provide the first evidence for the presence of a functional PAF-R expressed on human keratinocytes, suggesting that this lipid mediator may play an important role in normal keratinocytes or in inflammatory dermatology.

Role of the Cys90, Cys95 and Cys173 residues in the structure and function of the human platelet-activating factor receptor.

Platelet-activating factor (PAF) is a potent phospholipid mediator which binds to a specific, high affinity receptor of the G protein-coupled receptor family. In the present report, we show that ligand binding to the PAF receptor is sensitive to the reducing agent dithiothreitol (DTT), suggesting the involvement of disulfide linkages in the proper PAF receptor conformation. Substitutions of Cys90, Cys95 and Cys173 to Ala or Ser demonstrated that these cysteine residues are critical for normal cell surface expression of the PAF receptor protein and ligand binding to the receptor. The Cys90 and Cys173 mutant receptors did not display any specific ligand binding, were not expressed on the cell surface but were found in the intracellular compartment. The Cys95 mutants showed specific binding and were able to stimulate low levels of inositol phosphate (IP) production. These mutants were expressed at low density on the cell surface and showed high expression intracellularly. Our results suggest that the structure and function of the PAF receptor require the conserved Cys90 and Cys173 to form a disulfide bond. Moreover, Cys95 also appears to be necessary, possibly by establishing a disulfide linkage with an as yet unidentified Cys residue. All three residues appear essential for the proper folding and surface expression of the PAF receptor protein.

Nutrition factors in relation to cellular and regulatory immune variables in a free-living elderly population.

Aging is associated with a greater susceptibility to nutrition deficiencies and to progressive senescence of the immune system. To test whether nutrition status contributes to the immunologic changes observed in elderly individuals, we examined the relationship between nutrition status and in vitro indices of immune responses in 82 healthy, free-living elderly individuals. Nutrition status was assessed by anthropometric measurements, 7 d food records, and blood concentrations of selected nutrients. Using regression analyses, we found that none of the nutrition factors was associated with cytotoxic activity of natural killer (NK) cells against the leukemic cell line K562. However, our results suggest that the dietary intakes of vitamins E and D negatively influenced the activity of interleukin 2 (IL-2) measured by a bioassay in which the CTLL cell line was used. An association may exist between particular aspects of nutrition status and regulation of immune response by IL-2. The need for further studies is emphasized.

Changes in apoptosis of human polymorphonuclear granulocytes with aging.

Many alterations with aging occur at the cellular and organic levels in the immune system ultimately leading to a decrease in the immune response. Our aim in the present work was to study apoptosis of polymorphonuclear granulocytes (PMN) with aging under various stimulations since apoptosis might play an important role in several pathologies encountered with aging. The PMN of healthy young (20-25 years) and elderly (65-85 years) subjects were examined after 24 h of sterile culture with and without stimulation. The stimulating agents included: phorbol myristate acetate (PMA), hydrogen peroxide (H2O2), N-formyl-methionyl-leucyl phenylalanine (FMLP), granulocyte-macrophage colony stimulating factor (GM-CSF), reduced glutathione (GSH), lipopolysaccharide (LPS) and interleukin 2 (IL-2). Apoptosis was assessed by traditional staining of the plates, by flow cytometric staining and DNA gel electrophoresis. It was found that without stimulation the susceptibility of PMN to apoptosis was slightly increased with aging. Under various stimulations, such as PMA. H2O2, apoptosis was almost 100%, while the treatment by FMLP, oxLDL and GSH did not change its extent in PMN obtained either from young or elderly subjects. Marked age-related changes were observed in the extent of apoptosis under stimulation with GM-CSF, IL-2 and LPS. These agents were able to significantly prevent apoptosis in PMN of young subjects, while only the GM-CSF was able to slightly modulate it in neutrophils of elderly subjects. From these results, we suggest that changes in apoptosis of PMN with aging could play a role in the increased incidence of certain immune system related pathologies of aging, such as cancer, infections and autoimmune disorders.

Purification of human monocytes by continuous gradient sedimentation in ficoll.

A new method of obtaining purified human monocytes has been developed. The peripheral blood mononuclear leukocytes are isolated by centrifugation over Ficoll--Hypaque and then further purified by sedimentation over a linear 5--10% Ficoll density gradient. In ten experiments, the average purity obtained was 77.1% macrophages and the mean yield was 22.4% of the monocytes contained in the peripheral blood leukocytes. Viability of monocytes isolated by this technique exceeded 95%. The cells were phagocytic and responded to human migration inhibitory factor.

Unusual eye abnormalities associated with congenital cytomegalovirus infection.

Seven children with congenital cytomegalovirus infection demonstrated a higher than expected incidence of "rare" ophthalmological abnormalities, including anophthalmia and Peters' anomaly. These data suggest that appropriate investigation for evidence of cytomegalovirus infection should be instituted in any child with congenital ocular defects.

Gallium-67 uptake in the lung of asbestos exposed sheep: early association with enhanced macrophage-derived fibronectin accumulation.

To evaluate the time course and mechanisms of enhanced 67Ga lung uptake in asbestosis, we exposed two groups of sheep every 2 wk to either 100 ml saline (controls) or 100 mg UICC chrysotile fibers in 100 ml saline. The sheep were evaluated periodically by pulmonary function tests (PFT), thoracic radiograph (TR), 67Ga lung scan bronchoalveolar lavage (BAL), and transbronchial lung biopsy (TLB). By month 24 of the study, 9/15 exposed sheep had developed the initial alveolitis and had significant changes in PFT, TR, and TLB. The other six exposed sheep differed from controls only by a 75% increase in BAL fibronectin until month 30, where significant changes in albumin occurred and 67Ga scan score increased. The nine sheep with alveolitis had significant sustained increases in 67Ga scan and BAL levels from month 6, associated with a 150% increase in BAL fibronectin and other parameters of disease activity changed from month 18 to 30. We concluded that in the sheep model of asbestosis, significant changes in 67Ga scan, 67Ga BAL counts, and excessive elevation of BAL fibronectin preceded other parameters of disease activity. The data suggest that excessively activated macrophages are primarily responsible for the early 67Ga lung uptake.

Role of R-type calcium channels in the response of the perfused arterial and venous mesenteric vasculature of the rat to platelet-activating factor.

1. The vasoactive properties of platelet-activating factor (PAF) were studied in the arterial and venous vasculature of the rat double-perfused mesenteric bed. Although PAF (0.01-0.3 pmol) induced a dose-dependent vasodilatation of the arterial mesenteric vasculature, it triggered only vasoconstrictions on the venous side, with an intact endothelium as bradykinin induced a significant venodilatation. 2. NG-nitro-L-arginine methyl ester (L-NAME, 100 microM), a nitric oxide synthase inhibitor, markedly reduced the vasodilatation induced by PAF in the arterial mesenteric vasculature and potentiated the contractile responses of the venous side to the same agent. 3. The PAF antagonist, WEB-2170, markedly reduced the response to PAF on both sides of the mesenteric vasculature. However, the IC50 of WEB-2170 against PAF was reached at a much higher concentration (1 x 10(-8) M) on the arterial side than on the venous side (5.3 x 10(-11) M). Furthermore, a second antagonist of PAF receptors, SRI-63441, although being less potent on the venous vasculature than WEB-2170, was equipotent in antagonizing the venoconstriction and the arterial dilatation induced by PAF (IC50 of SRI-63441, arterial side: 2.9 x 10(-9) M; venous side: 3.1 x 10(-9) M). 4. The dual L- and R-calcium channel blocker, isradipine (PN 200-110), but not the L-type calcium channel blocker, nifedipine, markedly reduced the PAF-induced vasoactive properties on both sides of the mesenteric vasculature. 5. Our results illustrate the differential vasoactive properties of PAF in the mesenteric vasculature of the rat. These vasoactive responses occur following activation of specific receptors for PAF or,alternatively, through activation of R-type calcium channels.

PAF activation of a voltage-gated R-type Ca2+ channel in human and canine aortic endothelial cells.

By the use of fura-2 and digital imaging techniques, [K]o depolarization or PAF (10(-9) M) were shown to induce a sustained increase of [Ca]i in human or canine single aortic vascular endothelial cells (VEC) that was insensitive to nifedipine but sensitive to (-)-PN200-110 or to lowering of [Ca]o. The PAF-induced effect on [Ca]i was blocked by the PAF receptor antagonist, WEB2170. Our results suggest that [K]o depolarization and PAF increase [Ca]i via the activation of R-type Ca2+ channels.