ObjectiveEndotoxin and cytokines have been reported to have both stimulatory and inhibitory effects on endothelial cell-derived nitric oxide release. This discrepancy may be explained by differential regulation of the endothelial and inducible types of nitric oxide synthase gene expression. This study aimed to investigate the differential effect of lipopolysaccharide treatment in vivo on the three isoforms (endothelial, brain-type, and inducible) of nitric oxide synthase gene expression in the rat.DesignProspective, controlled, animal trial.SettingExperimental laboratory of a postgraduate medical research institution.SubjectsNormal, anesthetized rats.InterventionsAnimals were treated with lipopolysaccharide (15 mg/kg ip), saline (1 mL/kg ip), or lipopolysaccharide plus dexamethasone (3 mg/kg ip, 50 mins before lipopolysaccharide administration) in vivo 4 hrs before experimentation.Measurements and Main ResultsThe expression of endothelial, brain-type, and inducible nitric oxide synthase mRNAs was quantified by Northern blot analysis using bovine, rat, and mouse cDNA probes, respectively. An endothelial nitric oxide synthase mRNA was detected at 4.3 kilobase in the heart, lung, and aorta, and a 10-kilobase brain-type nitric oxide synthase mRNA was detected in the brain. The endothelial and brain-type signals were strong in tissues from animals treated with saline, but were reduced by three- to four-fold in tissues from lipopolysaccharide-treated rats as estimated by optical density ratio. The 4.4-kilobase inducible nitric oxide synthase mRNA detected using the murine cDNA probe was absent or negligible in the heart, lung, and brain from saline-treated rats, but was markedly increased in the same tissues from lipopolysaccharide-treated animals. Dexamethasone significantly inhibited lipopolysaccharide-induced inducible nitric oxide synthase mRNA expression, but had no effect on the down-regulation of endothelial and brain nitric oxide synthase mRNAs.ConclusionsRats treated with lipopolysaccharide in vivo display down-regulation of endothelial nitric oxide mRNA in the heart, lung, and aorta, and brain-type nitric oxide synthase mRNA in the brain. There was a parallel up-regulation of inducible nitric oxide synthase mRNA in all tissues except in the aorta. Dexamethasone prevents the induction of inducible nitric oxide synthase mRNA, but has no effect on the down-regulation of endothelial and brain-type nitric oxide synthase mRNAs induced by lipopolysaccharide. Thus, endotoxin regulates constitutive and inducible nitric oxide synthase mRNA differentially.(Crit Care Med 1996; 24:1219-1225)