Effects of TZI-41127, a novel selective 5-lipoxygenase inhibitor, on A23187-induced pleurisy in rats

Calcium ionophore (A-23187) induced peritoneal eicosanoid biosynthesis: a rapid method to evaluate inhibitors of arachidonic acid metabolism in vivo

The present investigation characterizes calcium ionophore (A-23187) induced peritoneal eicosanoid biosynthesis in the rat. Intraperitoneal injection of A-23187 (20 μg/rat) stimulated marked biosynthesis of 6-keto-PGF_1α (6-KPA), TxB₂, LTC₄ and LTB₄, with no detectable changes on levels of PGE₂. Levels of all eicosanoids decreased rapidly after a peak which was seen as early as 5 min. Enzyme markers of cellular contents of neutrophils and mononuclear cells, MPO and NAG respectively, decreased rapidly after ionophore injection; this was followed by increases after 60 min. Indomethacin, a selective cyclooxygenase inhibitor, and zileuton and ICI D-2138, two selective 5-lipoxygenase inhibitors attenuated prostaglandin and leukotriene pathways respectively. Oral administration of zileuton (20 mg/kg, p.o.) inhibited LTB₄ biosynthesis for up to 6 h suggesting a long duration of pharmacological activity in the rats consistent with its longer half-life. The rapid onset and the magnitude of increases in levels of eicosanoids render the ionophore induced peritoneal eicosanoid biosynthesis a useful model to evaluate pharmacological profiles of inhibitors of eicosanoid pathways in vivo.

Involvement of leukotrienes in allergic pleurisy in actively sensitized rats: inhibition by the lipoxygenase inhibitor T-0757 of the increase in vascular permeability and leukotriene E4 production

The relative contributions of inflammatory mediators to the increase in vascular permeability in antigen-induced pleurisy were examined in rats actively sensitized with ovalbumin. The effects of various inhibitors were assessed on the exudate volume and plasma exudation rate in the pleural cavity. Two peaks were observed in plasma exudation rate at 0.5 and 3 h after antigen challenge. At 0.5 h, there was a marked decrease in the histamine content of the pleural cells and also a sharp increase in the LTE4 level in the exudate, which was inhibited dose-dependently by the lipoxygenase inhibitor T-0757. Indomethacin and cyproheptadine both depressed exudate volume and exudation rate, whereas T-0757 only reduced the exudation rate. At 3 h, a substantial LTE4 concentration was still detected in the exudate, and the exudation rate was depressed by T-0757 and indomethacin, but not by cyproheptadine. These results suggest that histamine is involved mainly in the early phase, and leukotrienes predominantly contribute to the later phase of exudation. Prostaglandins appear to be involved in both phases. Allergic pleurisy of rats, therefore, may be a suitable model to examine the roles of these inflammatory mediators.

Role of intracellular signalling pathways in hydrogen peroxide-induced injury to rat glomerular mesangial cells

1. Brief exposure of cultured rat glomerular mesangial cells (GMC) to H2O2 in nominally bicarbonate-free solution induced a rapid dose dependent, dantrolene-inhibitable increase in intracellular free Ca2+ from 65 +/- 6 to 203 +/- 14 nmol/L and a prolonged release of [14C]-arachidonic acid [14C]-AA which preceded the onset of cell membrane damage assessed by trypan-blue uptake. 2. Ca2+ responses were potentiated in HCO3-/CO2 containing buffers and reached values of 1145 +/- 100 nmol/L at 1 mmol/L H2O2. In HCO3-/CO2 solutions, but not HEPES buffer, H2O2-induced Ca2+ increases were markedly attenuated by verapamil (100 mumol/L) or removal of extracellular calcium. 3. Enhanced release of [14C]-AA was partially attenuated by inhibitors of key intracellular signalling mechanisms including the phospholipase-A2 (PLA2) inhibitor mepacrine (100 mumol/L), the NADPH oxidase inhibitor diphenyliodonium (10 mumol/L), the mitochondrial calcium-cycling inhibitor ruthenium red (10 mumol/L) and the iron chelator dipyridyl (100 mumol/L). Release was unaffected by protein kinase C inhibition with H7 (100 mumol/L), inositol triphosphate antagonism with neomycin (1 mmol/L) or overnight treatment with the G-protein antagonist pertussis toxin (5 micrograms/mL). 4. Several structurally diverse lipoxygenase inhibitors, including esculetin, baicalein and phenidone, over the dose range 1-100 mumol/L, also prevented [14C]-AA release and markedly protected against cell membrane damage. No drug directly scavenged H2O2 assessed by UV absorption. 5. These results indicate that H2O2 activates in GMC a complex series of interrelated pathological mechanisms which in turn contribute to a prolongation of oxidative damage beyond the time of the initial exposure. These include an increase in intracellular calcium which, depending upon conditions, appears to be mediated by release from intracellular stores as well as Ca2+ entry from the extracellular space. In turn there is a sustained release of arachidonic acid, which may partly depend on prolonged activation of PLA2 but not phospholipase C. 6. Release of [14C]-AA could be attenuated by inhibitors of NADPH oxidase, mitochondrial calcium-cycling, iron chelators and a structurally diverse range of lipoxygenase inhibitors in association with protection from H2O2-mediated cell membrane damage.

Anti-inflammatory effect of magnolol, isolated from Magnolia officinalis, on A23187-induced pleurisy in mice

In the present study, A23187-induced pleurisy in mice was used to investigate the anti-inflammatory effect of magnolol, a phenolic compound isolated from Chinese medicine Hou p'u (cortex of Magnolia officinalis). A23187-induced protein leakage was reduced by magnolol (10 mg kg-1, i.p.), indomethacin (10 mg kg-1, i.p.) and BW755C (30 mg kg-1, i.p.). A23187-induced polymorphonuclear (PMN) leucocyte infiltration in the pleural cavity was suppressed by magnolol and BW755C, while enhanced by indomethacin. Like BW755C, magnolol reduced both prostaglandin E2 (PGE2) and leukotriene B4 (LTB4) levels in the pleural fluid of A23187-induced pleurisy, while indomethacin reduced PGE2 but increased LTB4 formation. In the rat isolated peripheral neutrophil suspension, magnolol (3.7 microM) and BW755C (10 microM) also suppressed the A23187-induced thromboxane B2 (TXB2) and LTB4 formation. These results suggest that magnolol, like BW755C, might be a dual cyclo-oxygenase and lipoxygenase inhibitor. The inhibitory effect of magnolol on the A23187-induced pleurisy is proposed to be, at least partly, dependent on the reduction of the formation of eicosanoids mediators in the inflammatory site.

Evaluation of 5-lipoxygenase inhibitors, zileuton, A-78773 and ICI-D-2138 in an ionophore (A-23187)-induced pleural inflammation model in the rat

Intrapleural injection of A-23187 (10 micrograms), a calcium ionophore, elicited rapid increase in biosynthesis of prostaglandins and leukotrienes in a time-dependent manner. 6-Keto-prostaglandin-F1 alpha (6-KPA) was the principal cyclooxygenase product with modest increases in levels of thromboxane B2 and prostaglandin-E2. Orally administered indomethacin, a selective cyclooxygenase inhibitor, and three selective 5-lipoxygenase inhibitors, zileuton, A-78773 and ICI-D-2138 markedly attenuated respective arachidonate pathways with projected ED50 values of < 1-2 mg/kg. Furthermore, a single oral administration of either ICI-D-2138 or A-78773 (each 20 mg/kg, po) resulted in persistent inhibition of 5-lipoxygenase pathway for up to 24 hr. These results indicate zileuton, A-78773 and ICI-D-2138 to be potent and selective inhibitors of 5-LO and document the utility of A-23187-induced pleural inflammation in evaluating efficacy of inhibitors of arachidonic acid metabolism in vivo.