In Vivo Administration of Endotoxin and Tumor Necrosis Factor-α Produce Different Effects on Constitutive and Inducible Nitric Oxide Synthase Activity in Rat Neutrophils and Aorta Ex Vivo

Abstract

Tumor necrosis factor-α (TNF-α) inhibits release of nitric oxide (NO) in vitro by stimulating the degradation of constitutive NO synthase (cNOS III) mRNA. However, TNF-α is believed to be the cytokine mediator of the hypotension and upregulation of inducible NO synthase (iNOS II) produced by gram-negative bacterial endotoxin (LPS). Some in vivo effects of TNF-α are opposite to those which occur in vitro. This study tested the hypothesis that in vivo administration of exogenous TNF-α and endogenously released TNF-α induce iNOS II activity and inhibit cNOS III activity, and thereby mediate the acute phase effects of LPS on blood pressure and the NO system in the rat. We show that LPS produces acute phase hypotension in ketamine anesthetized rats. The hypotension was associated with elevation of biologically active TNF-α in plasma, increased production of RNI (NO₂ ^- and NO₃ ^- anion) in rat neutrophils (PMN) and suppression of RNI production by A23187 (1 μM) stimulated thoracic aorta (RTA) ex vivo. TNA-α (10⁶ U/ml, iv) did not produce acute phase hypotension but initially raised arterial blood pressure and heart rate (HR), did not increase RNI production by PMN, and inhibited RNI production by A23187 stimulated RTA ex vivo. Pretreatment of rats with the Immunex monomeric soluble P75 receptor binding protein for TNF-α (TNFsr, 0.5 mg/kg, iv) 15 min prior to LPS administration decreased circulating TNF-α from 92,137 ± 12,456 U/ml to undetectable levels as determined by the L929 bioassay. However, LPS-induced increases in RNI in PMN was enhanced and LPS-induced decreases in RNI production by RTA was inhibited by TNFsr. Thus, in vivo administration of TNF-α does not mimic the hemodynamic and NO-inducing effects of LPS. However, TNF-α mediates in part LPS-induced inhibition of RNI production by RTA.