Inhibition of some aspects of acute inflammation of guinea-pig lung by intraperitoneal dexamethasone and mifepristone: demonstration of agonist activity of mifepristone in the guinea-pig.

We have determined the inhibitory activity of dexamethasone as an inhibitor of histamine-induced plasma protein extravasation (PPE) in guinea-pig lung and skin, and of lipopolysaccharide (LPS)-induced neutrophilia and platelet activating factor (PAF)-induced eosinophilia in guinea-pig lungs. Dexamethasone inhibited PAF-induced eosinophilia in guinea-pig lung (ED50 1.4 mg/kg i.p.). Higher doses of dexamethasone were required to inhibit LPS-induced neutrophilia (ED50 10.8 mg/kg i.p.). However, at doses up to 150 mg/kg i.p. dexamethasone did not inhibit histamine-induced plasma protein extravasation (PPE) in guinea-pig lung, but did inhibit PPE in guinea-pig skin. These preparations have previously been shown to be equally sensitive to inhibition by the beta 2-adrenoceptor agonist salmeterol. Dexamethasone inhibited PAF-induced eosinophilia (5 mg/kg) or LPS-induced neutrophilia (50 mg/kg) when given 3 h or 1 h prior to challenge. Inhibitory activity was lost when dexamethasone was administered 23 h prior to LPS or 1 h after PAF. The glucocorticoid antagonist mifepristone (1-100 mg/kg i.p.) caused dose-related inhibition of PAF-induced eosinophilia but not of LPS-induced neutrophilia. The highest dose of mifepristone used (100 mg/kg) did not reverse the inhibitory actions of dexamethasone (50 mg/kg) on LPS-induced neutrophilia. We suggest that the different inhibitory activity of dexamethasone in the preparations studied indicates differences in the sensitivity of the target cells involved to inhibition by dexamethasone. We also suggest that inhibition of PAF-induced eosinophilia by mifepristone reflects the partial agonist activity of this agent, demonstrated by others in different experimental systems.