Lipid peroxidation modifies the effect of phenolic anti-inflammatory drugs on prostaglandin biosynthesis.

The effects of phenolic anti-inflammatory drug, MK-447, on prostaglandin (PG) I2 and thromboxane (TX) A2 biosynthesis by rat dental pulp tissue were evaluated in the presence of 10 mM mannitol (MA) or 1 mM ascorbic acid with 0.3 mM Fe2+ (A + F). Although MK-447 alone at 1 and 10 microM had no significant effects, MK-447 at 100 microM stimulated both PGI2 and TXA2 biosynthesis, and suppressed the lipid peroxidation in the pulp tissue as estimated by thiobarbituric acid method. MA also reduced the lipid peroxidation, but had no effect on PG and TX production. However, in the presence of MA, the stimulatory effect of MK-447 was potentiated, and the significant effects were observed at concentrations higher than 1 microM. In contrast, A + F remarkably stimulated the lipid peroxidation, and inhibited both PG and TX biosynthesis. In the presence of A + F, MK-447 showed no stimulatory effect, and contrary, at 100 microM inhibited PG and TX production. These results suggest that the cellular levels of lipid peroxidation exert a significant influence on the effects of phenolic anti-inflammatory drugs like MK-447 on PG biosynthesis. The possible mechanism of action for such drugs has been discussed in view of the significance of lipid peroxidation in inflammatory condition.

5-Hydroxytryptamine stimulates the release of prostacyclin but not thromboxane A2 from isolated rat dental pulp.

The effect of 5-hydroxytryptamine (5-HT) on the release of prostacyclin and thromboxane (TX) A2 from isolated rat dental pulp was evaluated. 5-HT (1-1,000 microM) caused a dose-dependent and marked stimulation of the release of prostacyclin but not TXA2. Of the 5-HT-related indolealkylamines tested, only tryptamine had a similar stimulatory effect while tryptophan and 5-hydroxytryptophan had no effect. Neither histamine (100 microM) nor bradykinin (100 microM) had such an effect. Our results suggest the possible involvement of 5-HT receptors in 5-HT-induced stimulation of prostacyclin production in rat dental pulp.

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

Platelet-activating factor (PAF-acether), but not lyso PAF, stimulated the production of both PGI2 and TXA2 by rat dental pulp tissue in vitro. However, there were differences in the dose- and time-dependence of the stimulatory effects. PAF-acether antagonists, Bn 52021, CV 3988 and kadsurenone, dose dependently inhibited PAF-acether-induced PG production. BN 52021, CV 3988 also dose dependently inhibited TX production, but kadsurenone was almost without effect on TX production. Pretreatment of the tissues with PAF-acether or phorbol 12-myristate 13-acetate completely abolished the effect of the second challenge with PAF-acether. The stimulatory effects of PAF-acether and the calcium ionophore A23187 on PGI2 production were completely blocked by removal of extracellular calcium, whereas the effects on TXA2 production were not. TMB-8, an intracellular calcium antagonist, completely inhibited PAF-acether-induced PG production, whereas it slightly inhibited TX production. H-7, a protein kinase C inhibitor, and neomycin, a phospholipase C inhibitor, completely inhibited PAF-acether-induced PG and TX production, whereas W-7, a calmodulin inhibitor, did not. These results suggest that PAF-acether stimulates PGI2 and TXA2 production in rat dental pulp by interacting with distinct PAF-acether receptors, and that these receptors are coupled to independent signal transduction pathways which have a different dependence on extra- and intracellular calcium.

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.

The effects of colchicine on prostaglandin I2 and thromboxane A2 biosynthesis in the rat dental pulp.

Pulp tissues isolated from rats treated with either colchicine (0.5, 0.75 or 1.0 mg/kg) or beta-lumicolchicine (1.0 mg/kg) for 24 h were incubated in Tyrode buffer. Prostaglandin (PG) I2 and thromboxane (TX) A2 released into the medium were determined by radioimmunoassay as their stable metabolites, 6-keto-PGF1 alpha and TXB2, respectively. Colchicine generally enhanced PGI2 and TXA2 production concomitantly with hypocalcaemia, except that 0.5 mg/kg dosage had no significant effects. This enhancement was greater in PGI2 than in TXA2 synthesis, but was not observed in PGI2 release from aortic tissue. beta-Lumicolchicine (1.0 mg/kg) had no effect on PG and TX synthesis, nor on serum calcium. Kinetic studies revealed that after a lag period of more than 12 h, 1.0 mg/kg colchicine caused a progressive increase in the production of PGI2 and TXA2, which reached a maximum at 24 h and decreased thereafter gradually to the basal level within 168 h (7 days). In contrast, the hypocalcaemic effect of colchicine appeared rapidly, reached a maximum within 3 h and gradually recovered to the control level within 7 days. Colchicine at concentrations of 1-1000 microM had no stimulatory effect on PGI2 and TXA2 biosynthesis when incubated in vitro with isolated pulp tissues; it was inhibitory at higher concentrations. The enhancing effect of colchicine treatment on PGI2 and TXA2 production in dental pulp tissue may be caused indirectly by interaction of this agent with microtubules, but the precise mechanisms are unclear. Thus, a drug known to affect dentinogenesis modifies arachidonic-acid metabolism in dental pulp.

Stimulation of prostaglandin E2 biosynthesis in rat dental pulp explants in vitro by 5-hydroxytryptamine.

When excised rat dental pulp tissue was incubated in Krebs solution, prostaglandin E2 (PGE2) was released into the medium. The release was completely blocked by the PG synthetase inhibitor, indomethacin (0.1 mM). Exogenous arachidonic acid (0.01 mM), a substrate of PG synthetase, increased the amount of PGE2 production and the endogenous PGE2 biosynthesis was strongly stimulated by 5-hydroxytryptamine (5-HT) at concentrations higher than 0.01 mM. Bradykinin (0.1 mM) and histamine (0.1 mM) had no effect.