Suppression by dexamethasone of inducible nitric oxide synthase protein expression in vivo: a possible role for lipocortin 1.

Western blot and densitometric analysis of organ homogenates from lipopolysaccharide (LPS)-treated rats (1-10 mg kg(-1), i.p.) exhibited a strong induction of inducible nitric oxide synthase (iNOS) expression seen at all the doses tested (1, 3, and 10 mg kg(-1), n = 3). In particular, 3 hr after challenge of rats with LPS, iNOS was detectable in the liver, kidney, aorta, spleen and lung. Dexamethasone (DEX) (0.1-1 mg kg(-1); -1 hr) dose-dependently reduced iNOS expression in lung homogenates after exposure to LPS (1 mg kg(-1); P < 0.05). A partial reversal of DEX-induced suppression of iNOS expression in lung homogenates 3 hr after challenge with LPS was observed in rats which received a specific anti-lipocortin 1 sheep serum (LCS3; 1 mL kg(-1) 24 hr prior to the steroid), with an inhibition of 35+/-8%, as compared to animals passively immunised with normal sheep serum where dexamethasone exhibited an inhibition of 60+/-7% (n = 4). Peritoneal macrophages collected from rats treated with LPS (1 mg kg(-1); 3 hr) and cultured for 16 hr, released significant amounts of nitrite (51+/-1 microM) into the cell supernatants; this was reduced (-70+/-6%) after pre-treatment with dexamethasone (0.3 mg kg(-1)) and this effect was neutralised if animals were passively immunised with LCS3 (P < 0.01; n = 4). Thus lipocortin 1 mediates, at least in part, the inhibitory action exerted by dexamethasone on both iNOS protein expression in lung and iNOS activity (as measured by nitrite release) in primary peritoneal cells of rats.