DEPENDENCE OF ENDOTOXIN-INDUCED VASCULAR HYPOREACTIVITY ON EXTRACELLULAR L-ARGININE

1 The dependence on extracellular L-arginine of vascular hyporeactivity induced by bacterial lipopolysaccharide (LPS) was studied in vivo in rats infused with LPS and in vitro in endothelium-denuded rat thoracic aortic rings exposed to LPS. 2 Infusion of LPS during 50 min at a dose of 10 mg kg-1 h-1 produced a significant impairment of the pressor effect of noradrenaline, while in tissues collected 60 min after the start of LPS infusion, no significant alteration in either plasma arginine concentration or aortic arginine content was found compared to saline-infused controls (where plasma arginine was 78.5 +/- 7 muM and aortic arginine 394 +/- 124 nmol g-1 tissue). 3 Incubation of isolated, endothelium-denuded aortic rings with LPS (10 mug ml-1) in the absence of L-arginine for 4 h at 37-degrees-C produced a 6 fold (P<0.01) rightward shift in the noradrenaline concentration-effect curve compared to polymyxin B (1 mug ml-1, a LPS neutralizing agent) and reduced by 15% the maximum observed tension. 4 The presence of L-arginine (100 muM) during the incubation with LPS and throughout the following contraction experiments caused a 15 fold (P<0.01) increase in the EC50 of noradrenaline and greater depression (45%) of the maximum observed tension compared to polymyxin B-treated controls. Responses in control, non LPS-treated rings were unaffected by the presence of L-arginine. 5 The addition of L-arginine to rings incubated with LPS in the absence of L-arginine and maximally precontracted with noradrenaline (10 muM) induced a dose-dependent relaxation. The EC50 of L-arginine was 8.0 +/- 0.3 muM. 6 The reactivity of LPS-treated rings to noradrenaline both in the absence and presence of L-arginine was restored to control levels by N(G)-nitro-L-arginine methyl ester (L-NAME, 300 muM), an inhibitor of NO production and by methylene blue (3 muM), an inhibitor of guanylate cyclase. 7 Incubation of isolated aortae in the absence of L-arginine did not significantly decrease the tissue arginine content, whether LPS (10 mug ml-1) was present or not. Similarly, the presence of L-arginine (100 muM) in the incubation medium did not modify the tissue arginine content. 8 These results show that the LPS-induced impairment of vasoconstriction elicited by noradrenaline is dependent on extracellular L-arginine, although the tissue arginine content is not depleted after LPS pretreatment, and that circulating L-arginine is sufficient to activate maximally the vascular L-arginine/NO pathway in endotoxaemic rats.