Platelet mobilization induced by PAF and its role in the thrombocytosis triggered by adrenaline in rats.

The injection of PAF (6 micrograms/kg, i.v.) induced, in rats, haemoconcentration accompanied by an increase in the platelet number, as attested by the counts of platelets in blood samples diluted in formalin-free EDTA solution. This increase was significant at 15 min, peaked from 1 to 4 h and returned to basal levels 24 h after the lipid administration. The release of platelets induced by PAF was inhibited dose-dependently by specific PAF receptor antagonist such as WEB 2086 (0.5-2 mg/kg), BN 52021 and 48740 RP (5-25 mg/kg). Furthermore, platelet mobilization was clearly impaired in splenectomized animals stimulated by PAF, whereas thrombocytopenia and haemoconcentration by the same stimulus were intact. It was also noted that a second injection of PAF, 24 h after the initial stimulation with the lipid, failed to induce an increase in platelet counts, indicating autodesensitization. Desensitization to PAF or pretreatment with PAF antagonists clearly prevented the increase in the platelet counts after stimulation by adrenaline (15 micrograms/kg). These findings suggest that, in rats, PAF can induce release of platelets by a spleen-dependent mechanism and that this lipid may be relevant to the thrombocytosis triggered by adrenaline.

Pharmacological modulation of Paf-induced rat pleurisy and its role in inflammation by zymosan.

1. The intrapleural injection of Paf-acether into rats caused, at 30 min, a marked exudation accompanied by a reduction in the pleural leucocyte count. At 6 h, the exudate volume had decreased and a significant increase in the total leucocyte count, particularly eosinophils was noted. 2. Two Paf-acether antagonists, WEB 2086 and 48740 RP abrogated the pleural leucopenia observed 30 min after Paf-acether administration, whereas the exudation was inhibited only by the former. Pleurisy was also reduced by about 60% with dexamethasone, by about 45% with BW 755C or LY 171883, a mixed cyclo-oxygenase/lipoxygenase inhibitor and a peptido-leukotriene antagonist respectively, and by about 30% with indomethacin, flurbiprofen or piroxicam. 3. Repeated daily intrapleural injections of Paf-acether led to a state of progressive desensitization to Paf-acether itself, whereas responsiveness to 5-hydroxytryptamine was maintained. In addition, the Paf-induced auto-desensitization was largely inhibited by WEB 2086. 4. Pleurisy induced by zymosan, but not by carrageenin, was significantly reduced in Paf-acether-desensitized animals. These results were consistent with those obtained with WEB 2086 which suppressed zymosan-induced but not carrageenin-induced pleurisy. 5. This study suggests that Paf-acether-induced pleurisy in the rat may be mediated by lipoxygenase arachidonic acid metabolites and that pleurisy induced by zymosan, but not by carrageenin, is largely dependent upon Paf-acether.

Intravenous injections of PAF-acether induce platelet aggregation in rats.

The mechanism of rat thrombocytopenia induced by i.v. injections of platelet-activating factor (PAF-acether) was investigated. Platelet counts performed after diluting the blood samples in 1% formalin in saline showed that PAF-acether (6 micrograms/kg i.v.) induced a significant thrombocytopenia in rats, which peaked within 1 h, followed by a drastic increase of platelet counts at 4 h and a return to basal levels at 24 h. At this time, it was not possible to induce thrombocytopenia with a second challenge with PAF-acether, indicating a clear state of desensitization which disappeared within five days after the first injection of PAF-acether. The pretreatment with the specific PAF-acether receptor antagonist, BN 52021 (2.5-15 mg/kg), 48740 RP (6-25 mg/kg) and WEB 2086 (0.25-1 mg/kg) blocked the thrombocytopenia dose dependently. The lipoxygenase inhibitor nordihydroguaiaretic acid, at 25-100 mg/kg, was also effective against the thrombocytopenia induced by PAF-acether, reinforcing the potential involvement of arachidonic acid derivatives in this process. Adrenal hormones may modulate this process, since adrenalectomized animals responded to PAF-acether with exacerbated thrombocytopenia. No reduction in the platelet counts was noted when the blood was diluted in formalin-free saline, indicating that unstable aggregates were formed in vivo, which tended to resolve in vitro. Our results suggest that the thrombocytopenia induced in rats by PAF results from a reversible process of intravascular platelet aggregation, probably following the secretion of platelet-activating substances released by a first-hit blood cell.

A polymorphism in exon b2 of the major breakpoint cluster region (M-bcr) identified in chronic myeloid leukaemia patients.

The BCR/ABL junctional region and the b3 exon from chronic myeloid leukaemia (CML) patients were sequenced. In all 21 samples analysed the junctional region, as well as the b3 exon of 8 b3a2 mRNA molecules, presented no differences to the already described sequences. However, we identified a polymorphic base in the b2 exon in two out of seven b3a2 samples, four out of 10 b2a2 samples and all four b3a2/b2a2 samples analysed. In the eighth position before the junctional region of BCR/ABL cDNA, a cytosine replaces thymine in these cases. The polymorphism described here could be a useful marker for the differentiation of normal and rearranged BCR alleles in heterozygotes.

Two novel genetic groups (VIIb and VIII) responsible for recent Newcastle disease outbreaks in Southern Africa, one (VIIb) of which reached Southern Europe.

34 strains of Newcastle disease virus (NDV) isolated during epizootics in the Republic of South Africa and in Mozambique between 1990 and 1995, and in Bulgaria and Turkey in 1995-1997 were identified by restriction enzyme and partial sequence analysis of the fusion (F) protein gene. The majority of isolates in southern Africa and those from Bulgaria and Turkey were placed into a novel group which has been termed VIIb. Group VIIb is part of a larger genetic cluster (VII) that also includes NDV strains from the Far East and some western European countries (VIIa). The genetic distance of 7-8, 5% between genotype VIIa and VIIb viruses excludes the existence of a direct epidemiological link between recent southern African epizootics and outbreaks in either western Europe in the 1990's or those of the Far East. Another hitherto unrecorded genotype (VIII) was also found in South Africa with descendants of putative ancestral members isolated in the 1960's. The genetic distance of recent group VIII strains from the major epizootic genotype (VIIb) is over 11%, therefore outbreaks caused by them were epidemiologically unrelated. Genotype VIII viruses must have been maintained in South Africa by endemic infections during the past decades while group VIIb appears to be introduced more recently.