We tested the hypotheses that 1) inducible nitric oxide synthase (iNOS) med
iates ozone (O-3)-induced lung hyperpermeability and 2) mRNA levels of the
gene for iNOS (Nos2) are modulated by Toll-like receptor 4 (Tlr4) during O-
3 exposure. Pretreatment of O-3-susceptible C57BL/6J mice with a specific i
nhibitor of total NOS (N-G-monomethyl-L-arginine) significantly decreased t
he mean lavageable protein concentration (a marker of lung permeability) in
duced by O-3 (0.3 parts/million for 72 h) compared with vehicle control mic
e. Furthermore, lavageable protein in C57BL/B6 mice with targeted disruptio
n of Nos2 [Nos2(-/-)] was 50% less than the protein in wildtype [Nos2(+/+)]
mice after O-3. To determine whether Tlr4 modulates Nos2 mRNA levels, we s
tudied C3H/HeJ (HeJ) and C3H/HeOuJ mice that differ only at a missense muta
tion in Tlr4 that confers resistance to O-3-induced lung hyperpermeability
in the HeJ strain. Nos2 and Tlr4 mRNA levels were significantly reduced and
correlated in resistant HeJ mice after O-3 relative to those in susceptibl
e C3H/HeOuJ mice. Together, the results are consistent with an important ro
le for iNOS in O-3-induced lung hyperpermeability and suggest that Nos2 mRN
A levels are mediated through Tlr4.