Diesel exhaust particles cause an impairment of endothelium-dependent vasor
elaxation and are associated with cardiopulmonary-related diseases and mort
ality, but the mechanistic details are poorly understood. Since we reported
previously that phenanthraquinone, an environmental chemical contained in
diesel exhaust particles, suppresses neuronal nitric oxide synthase (nNOS)
activity by shunting electrons away from the normal catalytic pathway, it w
as hypothesized that phenanthraquinone inhibits endothelial NOS (eNOS) acti
vity and affects vascular tone. Therefore, the effects of phenanthraquinone
on eNOS activity, endothelium-dependent relaxation, and blood pressure wer
e examined in the present study. Phenanthraquinone inhibited NO formation e
valuated by citrulline formed by total membrane fraction of bovine aortic e
ndothelial cells with an IC50 value of 0.6 muM. A kinetic study revealed th
at phenanthraquinone is a competitive inhibitor with respect to NADPH and a
noncompetitive inhibitor with respect to L-arginine. Endothelium-dependent
relaxation of rat aortic rings by ACh was significantly inhibited by phena
nthraquinone (5 muM), whereas the endothelium-independent relaxation by nit
roglycerin was not. Furthermore, an intraperitoneal injection of phenanthra
quinone (0.36 mmol/ kg) to rats resulted in an elevation of blood pressure
(1.4-fold, P < 0.01); under this condition, plasma levels of stable NO meta
bolites, nitrite/nitrate, in phenanthraquinone-treated rats was reduced to
68% of control levels. The present findings suggest that phenanthraquinone
has a potent inhibitory action on eNOS activity via a similar mechanism rep
orted for nNOS, thereby causing the suppression of NO-mediated vasorelaxati
on and elevation of blood pressure.