Dexamethasone enhances P-selectin mRNA expression in hyperoxic rat lungs

To test the hypothesis that glucocorticoid administration would diminish the lung expression of P-selectin mRNA in hyperoxia-exposed rats.

Adult male Sprague-Dawley rats were divided into 6 separate groups containing 10 to 13 animals per group.

Rats were dosed with 1 mg/kg of dexamethasone or vehicle only, ip. Immediately after dosing, animals were placed in > 95 % oxygen. Some animals were maintained in room air and are presented as 0 h of exposure to hyperoxia. Another group of animals was dosed with 10 mg/kg lipopolysaccharide (LPS) ip immediately after dosing with either dexamethasone or vehicle as above.

At 24 or 48 h, lung samples were obtained, and lung weight to body weight ratios calculated. In the LPS studies, samples were obtained 4 h after LPS dosing. In a subset of animals, lung sections were hybridized for P-selectin mRNA. All data except for hybridizations were analyzed with three-way ANOVA, with subsequent post-hoc testing. P-selectin hybridizations were quantified by counting the number of positive vessels per high-powered field, and subsequently analyzed by unpaired Student's t-test. Immunohistochemical analyses for P-selectin expression were also performed to determine whether changes in P-selectin mRNA were associated with differences in protein expression. All data are expressed as means +/- SEM.

Rats dosed with dexamethasone had higher lung/body weight ratios after 24 and 48 h of exposure to hyperoxia than did similarly exposed rats dosed only with vehicle (at 48 h, 0.87 +/- 0.04 versus 0.65 +/- 0.06, respectively, P < 0.05). The higher ratios in hyperoxic animals dosed with dexamethasone were associated with much higher levels of lung expression for P-selectin mRNA than was observed in similarly exposed rats dosed with vehicle alone (at 48 h, 3.93 +/- 1.02, versus 0.20 +/- 0.06, respectively, P < 0.01). In contrast dexamethasone dosing lead to lower lung P-selectin mRNA expression in animals exposed to LPS (1.23 +/- 1.08 in dexamethasone dosed animals versus 6.80 +/- 0.92 in vehicle only dosed animals). Consistent with the mRNA data, P-selectin immunoreactivity increased as a function of hyperoxia-exposure time in animals dosed with dexamethasone, while immunoreactivity decreased as a function of hyperoxia-exposure time in animals dosed with vehicle only.

Increased P-selectin mRNA combined with increased P-selectin protein expression in animals exposed to hyperoxia and dosed with dexamethasone suggests that enhanced expression of P-selectin may contribute to the greater lung injury and inflammation caused by hyperoxia in rats treated with dexamethasone.