Differences in the effects of dexamethasone on macrophage nitrite production: Dependence on exposure regimen ( in vivo or in vitro) and activation stimuli

Abstract

Exposure to glucocorticoids in vitro is known to suppress the production of reactive nitrogen intermediates (RNI) by macrophages, and it has been suggested that this contributes to the anti-inflammatory action of glucocorticoids in vivo. However, the effects of glucocorticoid administration in vivo on subsequent RNI production as measured in vitro are not known. In the present study, dexamethasone was administered in vivo and was also used to treat macrophages in vitro prior to, and during, stimulation of nitrite production by interferon-γ (IFN-γ) and/or bacterial lipopolysaccharide (LPS). Macrophages were isolated 24 h after daily administration of dexamethasone (0.1–30 mg/kg/day) to female B6C3F1 mice for 3, 6, or 16 days. In most cases, these cells produced an equal or greater concentration of nitrite in response to IFN-γ, LPS, or IFN-γ plus LPS, than cells from vehicle control mice. In contrast, continuous exposure of macrophages to dexamethasone during stimulation in vitro caused dose-dependent inhibition of nitrite production. However, the inhibition was much less pronounced when LPS or IFN-γ together were used to stimulate the macrophages than when either was used separately. Similar results were noted when macrophages were exposed to dexamethasone for 24 or 72 h in vitro followed by a 0–24 h recovery period after removal of dexamethasone. Thus, immunosuppressive doses of dexamethasone in vivo do not decrease the induction of nitrite production 24 h after the last dose, whereas significant decreases are noted 24 h after termination of dexamethasone exposure in vitro. The basis for this difference is not clear, but there was no indication that administration of dexamethasone in vivo selects for a “glucocorticoid resistant” population of macrophages. These observations have implications with regard to the mechanisms of glucocorticoid-mediated anti-inflammatory and immunosuppressive action in vivo.