Hyporeactivity of mesenteric vascular bed in endotoxin-treated rats

Vascular reactivity and activation of the nitric oxide (NO) pathway were investigated in perfused mesenteric vascular bed removed from rats 5 h after i.p. injection of bacterial lipopolysaccharide (E. coli lipopolysaccharide, 30 mg kg ^-1). Lipopolysaccharide treatment induced hyporesponsiveness to noradrenaline. Maximal noradrenaline-induced vasoconstriction was significantly reduced in lipopolysaccharide-treated vs. untreated preparations. Continuous infusion of L-arginine (L-Arg) (0.2 mM) enhanced noradrenaline hyporeactivity of lipopolysaccharide-treated rats. N(ω)-Nitro-L-arginine methyl ester (L-NAME) (0.2 mM), a non-selective inhibitor of NO synthase, failed to completely restore the noradrenaline hyporeactivity of lipopolysaccharide-treated + L-Arg-infused mesenteric vascular bed. After L-NAME treatment, Methylene blue (10 μM), a guanylate cyclase inhibitor, produced no additional increase of noradrenaline vasoconstriction in lipopolysaccharide-treated + L-Arg-infused mesenteric vascular bed, suggesting that an NO-independent activation of guanylate cyclase may be excluded. In lipopolysaccharide-treated preparations, L-Arg (0.2 mM) elicited a significant increase in nitrite production, which was antagonized by L-NAME. In conclusion, lipopolysaccharide-induced noradrenaline hyporesponsiveness of rat resistance vessels can only be partially explained by NO overproduction. Other mechanisms, probably related to vasoconstriction, may be involved.