Distinct stimulatory effect of platelet-activating factor on prostaglandin I2 and thromboxane A2 biosynthesis by rat dental pulp

Heterogeneous platelet-activating factor (PAF) receptors and calcium increase in platelets and macrophages

We used the increase in cytosolic Ca2+ levels, [Ca2+]i, as a way to characterize PAF (platelet-activating factor, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) receptors in human platelets and rat and human macrophages. [Ca2+] was measured by means of the fluorescent probe fura-2/acetoxymethylester. PAF recognized heterogeneous receptors in human macrophages only (curve slope <1). The PAF antagonist SCH 37370 (1-acetyl-4(8-chloro-5,6-dihydro-11H-benzo[5.6]cyclohepta[1,2-b]pyridine -11-ylidine)piperidine) abolished [Ca2+]i elevation in human platelets, while in rat and human macrophages the maximal inhibition was 76% and 85%, respectively. On the contrary, the antagonist WEB 2086 (3-[4-(2-chlorophenyl)-9-methyl-6Hthieno[3,2-f] [1,2,4]triazolo-[4,3-a] [1,4]-diazepin-2-yl]-1-(4-morpholiny)-1-propanon, apafant) totally inhibited the effect of PAF in both platelets and macrophages. The WEB 2086 concentration-response curves had a slope <1 in the three cell types, indicating interaction with heterogeneous receptors. Accordingly, 3H-WEB 2086 bound to two different classes of sites. Both phases of [Ca2+]i elevation (influx or release) were equally affected by the antagonists. These data support the notions that: 1) PAF receptors are heterogeneous; 2) the two antagonists have a different selectivity toward the receptor subtypes: WEB 2086 recognizes two different receptors both in platelets and in macrophages, while SCH 37370 does not discriminate between receptor subtypes in platelets, and only interacts with one subtype in macrophages; and 3) both SCH 37370 and WEB 2086 display different potencies in rat and human macrophages.

Increased prostaglandin I2 and thromboxane A2 production by rat dental pulp after intravenous administration of endotoxin

The effect of systemic endotoxin (lipopolysaccharide from Escherichia coli 0111:B4) on prostaglandin I2 (PGI2) and thromboxane A2 (TXA2) production by rat dental pulp was investigated. Intravenous injection of endotoxin increased ex vivo production of both PGI2 and TXA2 by the pulp tissue, when determined by radioimmunoassay. A significant effect on PGI2 and TXA2 production was observed with endotoxin doses of greater than 2 and 0.4 mg/kg, respectively. A significant increase was also observed at 30 min after injection of 10 mg/kg endotoxin, reaching a maximum after 60 min for both PG and TX production. Endotoxin (10 mg/kg for 60 min) also increased TXA2 but not PGI2 production in lung tissue, but had no effect in jejunal tissue. Indomethacin (10 microM) completely inhibited PGI2 and TXA2 production by the pulp of physiological saline- and endotoxin-treated rats. Further, arachidonic acids (10 microM) significantly increased PG and TX production by the pulp of saline- but not of endotoxin-treated rats. Endotoxin (100 micrograms/ml) had no in vitro effect on PG or TX production when incubated with isolated pulp, lung and jejunal tissues, suggesting that the endotoxin-induced increases in PG and TX production are an indirect effect. The endotoxin-induced increase in TXA2 production, but not in PGI2 production, by the pulp tissue was significantly suppressed by WEB 2170, a platelet-activating factor (PAF) antagonist. These results indicate that arachidonate metabolism in pulp tissue is susceptible to endotoxaemia in comparison with the lung and jejunum, and further suggest that the endotoxin-induced increase in, at least, TXA2 production by the pulp is mediated by PAF.

Platelet activating factor potentiates rat paw oedema induced by different phlogogen agents

Carrageenin oedema is enhanced by the simultaneous injection in the rat paw of platelet activating factor (PAF). The enhancement of carrageenin oedema is observed throughout the time course of the experiments. This enhancement is also present when the oedema-producing agent is dextran, cellulose sulphate, histamine, 5-hydroxytryptamine, bradykinin or prostaglandin E2. Both verapamil and BN 52021 abolished the enhancement induced by PAF without modifying significantly carrageenin oedema. In essential fatty acid deficient (EFAD) rats depleted of kininogen and amines, carrageenin oedema is not modified by PAF. These findings suggest that PAF interacts with other inflammatory mediators regulating the formation of oedema induced by irritants injected locally.

Platelet activating factor and the responses of rat isolated stomach strip to prostaglandin E2

The effect of platelet activating factor (PAF) on contractions evoked by acetylcholine, 5-hydroxytryptamine (5-HT) and prostaglandin E2 (PGE2) was studied in-vitro on rat stomach strip. Addition of PAF to the organ bath increased PGE- but not 5-HT- or acetylcholine-evoked responses. The effect of PAF was unaffected by atropine, methysergide or indomethacin, but prevented by a specific PAF receptor antagonist BN 52021. The data support a specific interaction between PAF and PGE2 on rat stomach strip.