Interleukin-1 is a potent growth factor for immature rat sertoli cells

Testes from rats of different maturational ages were explored for presence of paracrine sertoli cell growth factors. Pubertal and adult testes contained a 17 kDa protein, with potent stimulatory effect on immature Sertoli cell multiplication in vitro. The bioactivity of this protein was mimicked by rat interleukin-1 (IL-1) and neutralized by IL-1 receptor antagonist. A receptor-mediated action was further supported by the demonstration of IL-1 receptor type I mRNA and protein expression in the cultured sertoli cells and in intact immature rat testes. IL-1alpha showed higher efficacy in stimulating proliferation than IL-1beta and follicle-stimulating hormone (FSH), and displayed synergistic action in combination with FSH. As IL-1alpha is constitutively produced by the rat testis and IL-1beta readily inducible by proinflammatory stimuli, our results suggest that IL-1 may serve as a growth factor for Sertoli cells under physiological and pathophysiological conditions.     

Monocyte chemoattractant protein-1-induced angiogenesis is mediated by vascular endothelial growth factor-A

Monocyte chemoattractant protein-1 (MCP-1) has been recognized as an angiogenic chemokine. In the present study, we investigated the detailed mechanism by which MCP-1 induces angiogenesis. We found that MCP-1 up-regulated hypoxia-inducible factor 1 alpha (HIF-1 alpha) gene expression in human aortic endothelial cells (HAECs), which induced vascular endothelial growth factor-A(165) (VEGF-A(165)) expression in the aortic wall and HAECs through activation of p42/44 mitogen-activated protein kinase (MAPK). In vivo angiogenesis assay using chick chorioallantoic membrane (CAM) showed that MCP-1-induced angiogenesis was as potent as that induced by VEGF-A(165) and completely inhibited by a VEGF inhibitor, Flt(2-11). The inhibition of RhoA small G protein did not affect MCP-1-induced VEGF-A(165) production and secretion but completely blocked both MCP-1- and VEGF-A-induced new vessel formation, as determined by CAM assay. These results suggest that MCP-1-induced angiogenesis is composed largely of 2 sequential steps: the induction of VEGF-A gene expression by MCP-1 and the subsequent VEGF-A-induced angiogenesis.     

Hypotension produced by platelet-activating factor is reversed by thyrotropin-releasing hormone

Platelet-activating factor (PAF), a vasoactive phospholipid implicated in anaphylactic reactions, causes severe hypotension in experimental animals that is highly resistant to pharmacological therapy. In the present studies, we showed that PAF (1 nmol/600 g body weight, IV) produced profound hypotension in unanesthetized guinea pigs that was promptly and completely reversed by thyrotropin-releasing hormone (TRH) (2 mg/kg, IV) or by the synthetic TRH analog MK771 (2 mg/kg, IV). TRH also reversed this hypotension when administered intracerebroventricularly (ICV) at a dose (0.02 mg/kg) that was systemically ineffective. The opiate receptor antagonist naloxone (5 mg/kg) was less effective than TRH in reversing the cardiovascular consequences of PAF administration. These data suggest that TRH reverses PAF-induced shock through central receptor-mediated mechanisms. This therapeutic action of TRH may partially account for the beneficial cardiovascular effects of this peptide in anaphylactic shock.     

Interleukin-6 receptor shedding is enhanced by interleukin-1beta and tumor necrosis factor alpha and is partially mediated by tumor necrosis factor alpha-converting enzyme in osteoblast-like cells

OBJECTIVE: Interleukin-6 (IL-6) and soluble IL-6 receptor (sIL-6R) activation of gp130 represents an alternative pathway for osteoclast development in inflammatory conditions. The goal of the present study was to investigate changes in sIL-6R levels in response to the inflammatory cytokines IL-1beta and tumor necrosis factor alpha (TNFalpha) and to determine the role of TNFalpha-converting enzyme (TACE) in this process. METHODS: Levels of sIL-6R in the culture media of MG63 and SAOS-2 osteoblast-like cell lines after exposure to various agents were determined by immunoassay. TACE protein levels were measured by Western immunoblotting. Cells were transfected with small interfering RNA (siRNA) or with an expression plasmid for IL-6R and TACE to determine the potential involvement of TACE in IL-6R shedding. RESULTS: IL-1beta and TNFalpha increased the levels of sIL-6R in the culture media of MG63 osteoblast-like cells. This effect was not influenced by cycloheximide or 5,6-dichlorobenzimidazole riboside but was markedly inhibited by the calcium chelator EGTA and by the TACE and matrix metalloproteinase inhibitor hydroxamate (Ru36156). IL-1beta and TNFalpha had no influence on the alternatively spliced form of IL-6R RNA. Levels of sIL-6R were reduced when MG63 cells were transiently transfected with TACE siRNA. Transfection of SAOS-2 cells with expression plasmids for IL-6R and TACE produced a dose-dependent increase in sIL-6R levels. CONCLUSION: IL-1beta- and TNFalpha-mediated induction of IL-6R shedding in osteoblast-like cells is at least partly dependent on TACE activation.     

H(+)-induced vasodilation of rat aorta is mediated by alterations in intracellular calcium sequestration

Acidosis induces vasodilation both in vivo and in vitro. Although it is commonly surmised that acidosis alters contractility by affecting contractile proteins and calcium entry, the exact role of these mechanisms in acidosis-induced vasodilation has not been determined. In the present study, we demonstrated that a novel mechanism, involving increased calcium sequestration into intracellular sites sensitive to norepinephrine, mediates the vasodilation associated with relatively modest decreases in pH. The effects of changing pH from 7.4 to 7.0 on tension development, 45Ca fluxes, and the norepinephrine-releasable intracellular calcium stores were studied in isolated rat aorta. Acute acidification produced marked endothelium-independent dilations of aortic rings that had been precontracted with norepinephrine. In contrast, this maneuver had only modest effects on contractions elicited by 80 mM KCl or phorbol ester. Acidification in this range did not alter basal or norepinephrine-stimulated undirectional 45Ca influx, nor did it reduce the norepinephrine-induced net gain in 45Ca content. Furthermore, neither norepinephrine-stimulated 45Ca efflux nor the peak contractile response to norepinephrine in calcium-free buffer was affected, although in this setting, the duration of the phasic contractile response was shortened. When calcium was restored to tissues exposed to norepinephrine in calcium-free buffer, acidification slowed the rate of tension development without altering 45Ca uptake, thus changing the relation between tension development and calcium entry. These effects of acidification were shown to be associated with an increase in the amount of calcium sequestered into the norepinephrine-sensitive intracellular calcium store. These findings clearly indicate that acidification, within a range that has no effect on other aspects of smooth muscle activation, elicits vasodilation by stimulating intracellular calcium sequestration. This action may represent a predominant mechanism whereby acidosis alters vascular smooth muscle contractility.     

Endothelin stimulates platelet-activating factor synthesis by cultured rat kupffer cells

Endothelins are potent peptide mediators that elicit glycogenolytic and vasoconstrictor actions in the liver. Endothelins were found to stimulate the synthesis and release of the lipid mediator platelet-activating factor in cultured rat Kupffer cells. Endothelin-mediated synthesis of platelet-activating factor required extracellular calcium in that the calcium chelator, EGTA and nifedipine, a calcium ion channel blocker, inhibited platelet-activating factor synthesis. The phospholipase A₂ inhibitor, 4-bromophenacyl bromide, strongly inhibited endothelin-induced platelet activating factor synthesis. Endothelin-stimulated platelet activating factor synthesis was inhibited after treatment of Kupffer cells with cholera toxin, whereas pertussis toxin inhibited only this response to endothelin-1. Agents that elevate intracellular cyclic AMP levels were found to inhibit endothelin-induced platelet-activating factor synthesis in Kupffer cells. Staurosporine, a protein kinase C inhibitor minimized endothelin-induced platelet-activating factor synthesis but phorbol myristate acetate, an activator of protein kinase C, did not affect endothelin-induced platelet activating factor synthesis. Thus, the current study demonstrates that activation of an endothelin receptor in cultured rat Kupffer cells results in the synthesis and release of platelet-activating factor. The importance of endothelin-mediated platelet-activating factor synthesis relates to the mechanism of intercellular signaling occurring between endothelial cells (i.e., the site of endothelin synthesis) and Kupffer cells (i.e., the site of formation of secondary mediators such as platelet-activating factor and eicosanoids) within the rat liver exposed to various types of pathophysiological episodes.     

Interleukin-1 is a chemotactic factor for human T-lymphocytes

Macrophages and T-lymphocytes physically interact in the lung in disorders such as sarcoidosis to initiate and/or maintain cellular immune responses. In these studies, we demonstrated that natural interleukin-1 (IL-1), as well as recombinant IL-1 beta, a polypeptide released from stimulated macrophages, is a potent chemotactic factor that is relatively specific for helper T-cells. This chemotactic activity is blocked by a species-specific anti-IL-1. Compared with its capacity to augment proliferation of phytohemagglutinin-stimulated murine thymocytes, IL-1 is more active as a chemotactic factor for mature T-cells. These studies suggest that stimulated lung macrophages, as well as other macrophages, may enhance their interaction with circulating T-lymphocytes via IL-1, which acts as both a chemoattractant and an initiator of T-lymphocyte activation.     

Interleukin-1 stimulates corticotropin-releasing factor gene expression in rat hypothalamus

To examine the effect of interleukin-1 (IL-1) on CRF and POMC gene expression, recombinant human IL-1 alpha and -beta were ip injected in rats. The plasma ACTH level showed a dose-related increase at 2 h after the injection of 0.5 and 2 micrograms IL-1 alpha and -beta, and also showed a sustained increase from 1 h until 5 h after the injection of 2 micrograms of IL-1 beta. CRF contents in the medial basal hypothalamus and ACTH contents in the anterior pituitary (AP) decreased at 2 h after the injection of 2 micrograms of IL-1 alpha and -beta, and such decreased levels were maintained until 5 h after the injection of 2 micrograms of IL-1 beta. The levels of CRF mRNA in the hypothalamus and POMC mRNA in AP significantly increased 3 h after the injection of 2 micrograms IL-1 alpha and -beta, and these levels were still higher at 5 h after the injection of 2 micrograms of IL-1 beta compared with those of the control. There was no significant change in the ACTH content and POMC mRNA levels in the intermediate-posterior pituitary or the hypothalamus or in the CRF contents and CRF mRNA levels in the cerebral cortex. These results indicate that acute administration of IL-1 alpha and -beta stimulates gene expression of hypothalamic CRF and CRF release, which causes the stimulation of ACTH release and POMC gene expression in AP.     

Interferon alpha-induced apoptosis on rat preneoplastic liver is mediated by hepatocytic transforming growth factor beta(1)

In previous work we showed that interferon alfa-2b (IFN-alpha2b) increases apoptosis on rat hepatic preneoplastic foci. The aim of this study was to determine if transforming growth factor beta1 (TGF-beta1) was involved in the programmed cell death on the foci. Animals were divided into 6 groups: subjected to a 2-phase model (diethylnitrosamine plus 2-acetylaminofluorene) of preneoplasia development (group 1); treated with IFN-alpha2b during the 2 phases (group 2); treated with IFN-alpha2b during initiation with diethylnitrosamine (group 3); treated with IFN-alpha2b during 2-acetylaminofluorene administration (group 4); subjected only to an initiation stage (group 5); and treated with IFN-alpha2b during the initiation period (group 6). Serum TGF-beta1 levels were increased in IFN-alpha2b-treated rats. Immunohistochemical studies showed that IFN-alpha2b significantly increased the quantity of TGF-beta1-positive hepatocytes in groups 2 to 4. Phosphorylated-Smads-2/3 (p-Smads-2/3) proteins in liver nuclear extracts were significantly elevated. To determine the source of TGF-beta1, isolated hepatocytes, Kupffer cells, and peritoneal macrophages from animals in groups 1 and 5 were cultured with or without IFN-alpha2b. IFN-alpha2b stimulus induced several-fold increases of TGF-beta1 secretion from hepatocytes. Neither Kupffer cells nor peritoneal macrophages secreted detectable TGF-beta1 levels when they were treated with IFN-alpha2b. IFN-alpha2b-stimulated cultured hepatocytes from preneoplastic livers showed enhanced apoptosis, measured by fluorescence microscopy and caspase-3 activity. They presented higher nuclear accumulation of p-Smads-2/3, indicating increased TGF-beta1 signaling. When anti-TGF-beta1 was added to the culture media, TGF-beta1 activation and apoptosis induced by IFN-alpha2b were blocked. In conclusion, IFN-alpha2b-induced production of TGF-beta1 by hepatocytes from preneoplastic liver is involved in the apoptotic elimination of altered hepatic foci.     

Modulation of leukocyte adhesion in rat mesenteric venules by aspirin and salicylate.

Erythrocyte velocity, vessel diameter, leukocyte rolling velocity, and number of adherent and emigrated leukocytes were measured in postcapillary venules both before and during superfusion of rat mesentery with either aspirin or sodium salicylate. In some experiments, animals were treated with either a leukotriene (LT)-synthesis inhibitor (L-663,536), an LTD4 antagonist (MK-571), an LTB4 antagonist (SC-41930), misoprostol, or prostaglandin (PG) I2, then the aspirin protocol was repeated. Superfusion of aspirin but not sodium salicylate resulted in increased leukocyte adherence and a reduced leukocyte rolling velocity but did not affect leukocyte emigration. Aspirin-induced leukocyte adhesion was effectively prevented by the LT-synthesis inhibitor and LTB4 antagonist but not by the LTD4 antagonist. Misoprostol and PGI2 also prevented the aspirin-induced adhesion responses. Superfusion of the mesentery with either platelet-activating factor (PAF) or LTB4 enhanced leukocyte adherence and emigration while reducing leukocyte rolling velocity. Sodium salicylate prevented all of the adhesion responses elicited by LTB4. Although salicylate did not affect the PAF-induced leukocyte adherence and rolling responses, it completely prevented the increased leukocyte emigration. These results indicate that aspirin promotes, whereas sodium salicylate inhibits, leukocyte-endothelial cell adhesive interactions at therapeutically relevant concentrations.     

Oropouche orthobunyavirus infection is mediated by the cellular host factor Lrp1

Oropouche orthobunyavirus (OROV; Peribunyaviridae) is a mosquito-transmitted virus that causes widespread human febrile illness in South America, with occasional progression to neurologic effects. Host factors mediating the cellular entry of OROV are undefined. Here, we show that OROV uses the host protein low-density lipoprotein–related protein 1 (Lrp1) for efficient cellular infection. Cells from evolutionarily distinct species lacking Lrp1 were less permissive to OROV infection than cells with Lrp1. Treatment of cells with either the high-affinity Lrp1 ligand receptor-associated protein (RAP) or recombinant ectodomain truncations of Lrp1 significantly reduced OROV infection. In addition, chimeric vesicular stomatitis virus (VSV) expressing OROV glycoproteins (VSV-OROV) bound to the Lrp1 ectodomain in vitro. Furthermore, we demonstrate the biological relevance of the OROV-Lrp1 interaction in a proof-of-concept mouse study in which treatment of mice with RAP at the time of infection reduced tissue viral load and promoted survival from an otherwise lethal infection. These results with OROV, along with the recent finding of Lrp1 as an entry factor for Rift Valley fever virus, highlight the broader significance of Lrp1 in cellular infection by diverse bunyaviruses. Shared strategies for entry, such as the critical function of Lrp1 defined here, provide a foundation for the development of pan-bunyaviral therapeutics.

Bunyaviruses are a large group of related viruses with single-stranded, segmented, negative, or ambisense RNA genomes (1, 2). Within the order Bunyavirales, the Peribunyaviridae family contains viruses that infect humans and animals with confirmed or potential zoonotic transmission (2, 3). Oropouche virus (OROV; Orthobunyavirus genus; Simbu serogroup) is found primarily in the South American regions of Brazil, Trinidad, Peru, Panama, and Tobago (4). OROV has caused more than 30 epidemics, resulting in excess of 500,000 total cases of human febrile illness, making it the second most common arboviral disease in Brazil, behind Dengue fever (4–6). The true case number is likely higher as clinical testing for OROV is lacking and patients are often misdiagnosed as having Chikungunya or Dengue fevers (4). The arthropod vectors for OROV include Culicoides midges and Culex mosquitoes. In humans, OROV causes a febrile illness that manifests as fever, intense headache, myalgia, joint pain, retro-orbital pain, and photophobia, which can further develop into encephalitis or meningitis (5–7). Systemic infection manifests as rash, nausea, vomiting, and diarrhea. Viremia and leukopenia are common features (6), and virus can be detected in the cerebrospinal fluid (8, 9). In mice, the virus replicates in the liver and spleen after either subcutaneous or intracerebral infection (10).Due to the broad cellular tropism and ability to infect a variety of species, bunyaviruses are thought to use multiple receptors or attachment factors for entry and/or a protein that is widely expressed across different tissues and conserved across species. Recently, using a CRISPR-Cas9 screen, the conserved host protein low-density lipoprotein receptor (LDLR)-related protein-1 (Lrp1) was reported to mediate cellular infection with Rift Valley fever virus (RVFV), a phlebovirus within the Bunyavirales order (11). Lrp1 (also known as alpha-2-macroglobulin receptor or CD91) is a highly conserved multifunctional member of the LDLR family of transmembrane surface proteins. Lrp1 is important for ligand endocytosis, cell signaling, lipoprotein metabolism, blood–brain barrier maintenance, and angiogenesis (12–15). Homozygous deletion of Lrp1 is embryonically lethal in mice (16), further supporting the critical nature of Lrp1 in homeostatic functions.The M segment of Bunyavirales encodes the surface glycoproteins Gn and Gc, which form heterodimers and multimerize on the surface of the virion. Few studies have been conducted on the binding and entry mechanisms facilitated by OROV Gn/Gc. Given the conserved nature of Lrp1 across taxonomically distinct species and its expression in different tissues, we investigated whether OROV, a bunyavirus distantly related to RVFV, also requires Lrp1 for efficient infection of host cells. Despite having a similar genome organization among members of the Bunyavirales, many of the virally encoded sequences show little sequence homology. Therefore, studies to define similar host protein usage by these two distantly related viruses would have significant implications for pan-bunyavirus therapeutic and diagnostic development.Here, we used Lrp1 knockout (KO) cell lines to show that OROV infection is decreased compared to parental cells expressing Lrp1. Pretreatment of cells with varying concentrations of the high-affinity Lrp1-binding protein receptor-associated protein (RAP) significantly reduced OROV infection. Zika virus (ZIKV), an arbovirus outside the Bunyavirales order, was unaffected by the loss of Lrp1 or by treatment with Lrp1-binding RAP. Chimeric virions expressing OROV glycoproteins bound to the Lrp1 ectodomain. Finally, the role of Lrp1 in OROV infection was validated in vivo, whereby RAP treatment was able to reduce viral tissue titers and rescue mice from lethal intracerebral infection with OROV. Based on our findings, Lrp1 is a host factor for multiple bunyaviruses, presenting a potential therapeutic approach to address this important group of emerging arboviruses. This work also paves the way for future studies to understand the mechanism of OROV binding to Lrp1.     

Gene induction by coagulation factor Xa is mediated by activation of protease-activated receptor 1

Cell signaling by coagulation factor Xa (Xa) contributes to pro-inflammatory responses in vivo. This study characterizes the signaling mechanism of Xa in a HeLa cell line that expresses protease-activated receptor 1 (PAR-1) but not PAR-2, -3, or -4. Xa induced NF-kappaB in HeLa cells efficiently but with delayed kinetics compared to thrombin. This delay caused no difference in gene expression patterns, as determined by high-density microarray analysis. Both proteases prominently induced the angiogenesis-promoting gene Cyr61 and connective tissue growth factor. Inhibition of PAR-1 cleavage abolished MAP kinase phosphorylation and gene induction by Xa, demonstrating that Xa signals through PAR-1 and not through a novel member of the PAR family. Activation of cell surface prothrombin with the snake venom enzyme Ecarin also produced PAR-1-dependent signaling. However, though the response to Ecarin was completely blocked by the thrombin inhibitor hirudin, the response to Xa was not. This suggests that the Xa response is not mediated by locally generated thrombin. The concentration dependence of Xa for PAR-1 activation is consistent with previously characterized Xa-mediated PAR-2 signaling, suggesting that local concentration of Xa on the cell surface, rather than sequence-specific recognition of the PAR scissile bond, determines receptor cleavage. This study demonstrates that PAR-1 cleavage by Xa can elicit the same cellular response as thrombin, but mechanistic differences in receptor recognition may be crucial for specific roles for Xa in signaling during spatial or temporal separation from thrombin generation.     

The corticotropin-releasing factor release in rat hypophysial portal blood is mediated by brain catecholamines

In order to study the involvement of the hypothalamic corticotropin-releasing factor (CRF) in catecholamine-induced adrenocorticotropin (ACTH) secretion, we have measured CRF levels in rat hypophysial portal blood (HPB) after the pharmacological destruction of the ventral noradrenergic bundle (VNAB), using 6-hydroxydopamine (6-OHDA) stereotaxically injected into the VNAB. CRF levels in HPB were measured by radioimmunoassay, and the effects of 6-OHDA injection were controlled by the determination of catecholamine concentrations in the total hypothalamus. VNAB lesions induced a dramatic decrease in norepinephrine and epinephrine hypothalamic concentration. The CRF levels in HPB were also significantly reduced. These results suggest that central catecholamines exert a direct stimulatory control on the CRF release and play a major role in stress-induced ACTH secretion.     

Interleukin-1-induced interleukin-6 is required for the inhibition of proteoglycan synthesis by interleukin-1 in human articular cartilage

Cartilage from normal controls, patients with osteoarthritis, and patients with rheumatoid arthritis produced no interleukin-6 (IL-6) in culture. However, IL-1 induced massive production of IL-6 (up to 135 ng/ml) in cartilage from all 3 sources, in a dose-dependent manner (in some cases, a peak value was reached). The levels of induced IL-6 were similar to those found in rheumatoid arthritis synovial fluid. At IL-1 concentrations that induced almost complete inhibition of proteoglycan (PG) synthesis, IL-6 production could still be increased considerably. Exogenous IL-6 inhibited PG synthesis by up to 25%. IL-1-induced inhibition of PG synthesis was reversed by antibodies against recombinant human IL-6. These results suggest that IL-6 is required for the IL-1-induced inhibition of PG synthesis.     

Mouse islet cell lysis mediated by interleukin-1-induced Fas

Summary This study was conducted to investigate the possible involvement of Fas in β-cell death in insulitis of Type 1 (insulin-dependent) diabetes mellitus. Although primary cultured Balb/c mouse islet cells did not express Fas mRNA, 4–12 hours of treatment with 10²-10⁵ U/l of mouse interleukin-1 α (IL-1α) induced the expression of Fas mRNA. Surface Fas expression was detected by immunofluorescence flow cytometry using a non-cytolytic anti-Fas monoclonal antibody after 6 or 12 h of incubation with 10³ U/l of IL-1α. Primary islet cells were resistant to an agonistic anti-Fas monoclonal antibody. However, 12 h pretreatment with IL-1α sensitized islet cells to its cytolytic effect. Significant cell death was observed 24 h after the addition of anti-Fas, and progressively increased until 72 h, when specific ⁵¹Cr release was 72 ± 6 %. Agarose gel electrophoresis of DNA extracted from cells exposed to IL-1α and agonistic anti-Fas showed internucleosomal DNA fragmentation, a hallmark of apoptotic cell death. Since the Fas antibody showed no cross-reactive activity of tumour necrosis factor (TNF), the cytotoxic effect was not mediated by TNF receptors. A protein synthesis inhibitor cycloheximide augmented Fas-mediated islet cell death. The Fas-mediated killing of islet cells was not l-arginine-dependent, or blocked by N^G-monomethyl-l-arginine. β-TC1 cells also expressed Fas mRNA when exposed to IL-1α or IL-1α plus interferon-γ. These observations suggest that Fas-mediated apoptosis may be a mechanism of islet cell death in autoimmune insulitis. [Diabetologia (1996) 39: 1306–1312] Received: 5 April 1996 and in revised form: 7 August 1996     

Endotoxin-induced ascites formation in the rat: partial mediation by platelet-activating factor

Systemic endotoxemia has been observed in patients with acute and chronic liver failure, and bacterial endotoxin is known to increase vascular permeability. We investigated in the normal rat the effects of intraportal endotoxin administration and the possible mediation of these effects by platelet-activating factor. Injection of endotoxin lipopolysaccharide (10 and 25 mg per kg) in the rat resulted in rapid ascites formation, as well as systemic hypotension, hemoconcentration and acute erosions of the gastrointestinal mucosa. These effects were significantly attenuated by pretreatment with L652,731 and CF-3988, specific platelet-activating factor antagonists. Administration of 25 mg per kg endotoxin also resulted in significant elevations of platelet-activating factor biosynthesis in vitro by samples of duodenum, liver and lung. The effects of endotoxin were mimicked by intraportal infusion of platelet-activating factor (50 ng per kg per min), which induced ascites and gastrointestinal lesions. Platelet-activating factor reduced circulating plasma volume and increased peritoneal permeability to albumin as assessed by the ascites to plasma ratio of labeled albumin. These results, therefore, support a role for platelet-activating factor in mediating endotoxin-induced ascites and gastrointestinal erosions.