Phospholipase inhibition and prostacyclin generation by gastric muscularis and mucosa layers

Inhibition of eicosanoid biosynthesis by glucocorticoids in humans

Therapeutic doses of glucocorticoids are thought to inhibit prostaglandin and leukotriene formation in humans. Several studies in animals, however, have failed to demonstrate modulation of eicosanoid biosynthesis by steroids in vivo. We administered prednisone (60 mg/day) to eight healthy volunteers and measured eicosanoid formation by a variety of cell types in vivo and ex vivo, using sensitive and specific physicochemical assays. We found that the in vivo course of prednisone failed to inhibit the synthesis of thromboxane A2, prostaglandin I2 (prostacyclin), prostaglandin E2, and leukotriene E4 in vivo and of leukotriene B4 ex vivo. Biosynthesis of leukotriene B4, thromboxane B2, and prostaglandins F2 and E2 by macrophage-rich bronchoalveolar lavage cells was strongly suppressed. These findings indicate that therapeutic regimens of glucocorticoids suppress eicosanoid biosynthesis in human macrophages but not in a number of other cell types with steroid receptors, the capacity for eicosanoid formation, and lipocortin-like material.

Gastrointestinal damage induced by platelet-activating factor: role of leukotrienes

The role of leukotriene synthesis in the gastrointestinal damage induced by platelet-activating factor (PAF) was examined in the rat. The effects of a 20-min infusion of PAF (100 ng/kg per min) on leukotriene B4 (LTB4) and leukotriene C4 (LTC4) synthesis were examined in samples of the stomach, duodenum, jejunum, ileum and colon. Administration of PAF resulted in marked hemoconcentration and extensive hemorrhagic damage which was only observed in the corpus region of the stomach and in the small intestine. However, LTB4 synthesis was increased significantly in all regions studied, while LTC4 synthesis was increased significantly only in the duodenum. Pretreatment of the rats with dexamethasone significantly reduced the PAF-induced increase in LTB4 synthesis in all tissues studied. However, a reduction of PAF-induced damage following dexamethasone treatment was observed in the small intestine, but not the stomach. To further investigate the role of leukotrienes as mediators of PAF-induced gastrointestinal damage, the effects of a 10-min infusion of PAF (100 ng/kg per min i.v.) were compared to those of similar infusions of LTB4, LTC4 or leukotriene D4 (LTD4) (0.3-3 micrograms/kg per min). None of the doses of leukotrienes tested produced hemoconcentration or gastrointestinal damage comparable to that observed with the much lower dose of PAF, with the single exception of significant hemoconcentration observed with the highest dose of LTC4. The results of this study therefore suggest that leukotrienes are unlikely to play a major role as mediators of PAF-induced gastrointestinal damage in the rat.

The relationship between plasma cortisol and gastric mucosa prostaglandin levels in rats with stress ulcers

A correlation between the levels of plasma cortisol, gastric mucosal prostaglandins and the degree of gastric ulceration produced by stress in rats was examined. Four groups of rats were evaluated. Group A, the cold stressed group; group B, their controls; group C, those receiving intraperitoneal indomethacin; and group D, those receiving intraperitoneal saline. Group A developed stress ulcers and their gastric mucosal prostaglandin levels (prostaglandin E2 (PGE2) and prostacyclin levels, as measured by its stable metabolite 6-keto-prostaglandin F1 alpha (6-keto-PGF-1 alpha, but not thromboxane) were significantly reduced when compared with their respective non-stressed controls. The plasma cortisol levels in both group A and B increased slightly in the first hour but there was no statistical difference between the two groups and there was no change at 2, 3 or 4 h of stress. Group C (indomethacin) also developed ulcers and had low gastric mucosal prostaglandin levels when compared with group D (saline). The plasma cortisol levels did not alter in either group C or D. It has been postulated that stress ulcers may involve the depletion of gastric mucosal prostaglandin levels which, in turn, may be a consequence of a higher plasma cortisol level. A decrease in gastric prostaglandins independent of any change of plasma cortisol levels was demonstrated in this study and the mechanism of production of stress gastric ulcers remains obscure.

Glucocorticoid-induced gastric mucosal damage: inhibition of leukotriene, but not prostaglandin biosynthesis

The effects of daily administration of ulcerogenic doses of the glucocorticoids, dexamethasone and prednisolone, on gastric prostaglandin and leukotriene synthesis were examined in the rat. Significant gastric damage was not observed until the fourth day of treatment with dexamethasone and until the sixth day of treatment with prednisolone. However, the onset of gastric damage was not accompanied by any significant effect of these drugs on gastric 6-keto prostaglandin Fl alpha synthesis. Conversely, both drugs caused a reduction in gastric leukotriene C4 synthesis, with dexamethasone producing a highly significant (p less than 0.001) effect. Furthermore, there was a highly significant (p less than 0.001) correlation between the ability of these drugs to inhibit gastric leukotriene C4 synthesis, and their ability to reduce gastric tissue levels of the neutrophilic enzyme myeloperoxidase. Thus, glucocorticoid-induced gastric damage does not appear to be related to inhibitory effects of these drugs on prostaglandin synthesis. Whether or not effects of these drugs on gastric leukotriene C4 synthesis and myeloperoxidase activity are relevant to the mechanism of ulceration is unclear.