Lipopolysaccharide alters vasodilation to atrial natriuretic peptide via nitric oxide and endothelin-1: time-dependent effects.

Nitric oxide (NO) induces vascular relaxation via cGMP in vascular smooth muscle (VSM) and is an important mediator of vascular tone during sepsis, as endothelial NO synthase (eNOS) may be upregulated during the early stages. Atrial natriuretic peptide (ANP) also stimulates cGMP via eNOS hence, this study aimed to investigate the role of NO in time-dependent altered vascular responses to ANP during the first 4h of exposure to bacterial lipopolysaccharide (LPS). We used male rat saphenous arteries [internal relaxed diameter 63-152 microm, n=48], mounted on a wire myograph and pre-constricted with phenylephrine. At 2h in the presence of LPS, there was increased relaxation to ANP in arteries exposed to LPS [16.3+/-2.4%, P<0.05]. However the response to ANP was not altered by the NOS inhibitor Nomega-nitro-l-arginine methyl ester (L-NAME, 10(-4)M) and following denudation (vessels without endothelium). At 4h there was no longer increased relaxation to ANP in the presence of LPS. Moreover the vasodilator response to ANP was significantly reduced following L-NAME or denudation [4.4+/-1.0% and 4.3+/-1.1% respectively, P<0.05]. However, the non-specific endothelin-1 (ET-1) receptor antagonist Bosentan [10(-5)M] increased dilatation in LPS exposed arteries at 1 and 2h, reaching significance at 4h [14.0+/-3.4%, P<0.05]. In summary, an endothelial and NO dependent mechanism is responsible for increased relaxation to ANP following 2h exposure to LPS. However after 4h an endothelial and NO independent process involving ET-1 is responsible for decreased relaxation to ANP. The enhanced response to ANP may exacerbate early systemic vasodilatation during early sepsis.

Reduced vascular response to phenylephrine during exposure to lipopolysaccharide in vitro involves nitric oxide and endothelin 1.

Nitric oxide (NO) and endothelin 1 (ET-1) increase significantly during the first 4 h of Escherichia coli lipopolysaccharide (LPS) exposure. The aim of this study was to investigate the role of these mediators in the reduced response to phenylephrine treatment. We used male rat saphenous arteries (internal relaxed diameter, 63-152 microm; n = 48), mounted on a wire myograph and subsequently treated with LPS. At 1 h, LPS (dose, 50 microg mL(-1)) significantly (P < 0.05) inhibited constriction to phenylephrine (concentration, 10(-1)M to 10(-6)M) (LPS concentration required for half maximal response [EC50], 10.82 +/- 1.08microM; Control EC50, 5.07 +/- 0.34microM). However, by removing the endothelium (denuded) or adding Nomega-nitro-L-arginine methyl ester (L-NAME; concentration, 10(-4) microM), the response to phenylephrine treatment was significantly improved compared with LPS only-treated arteries (LPS + denuded EC50, 7.04 +/- 1.12microM; LPS + L-NAME EC50, 2.64 +/- 0.63microM). On the other hand, denudation did not restore constriction to phenylephrine at 2 and 4 h. However, L-NAME and the nonspecific ET-1 receptor antagonist bosentan (concentration, 10(-5)M) improved constriction to phenylephrine in LPS-treated arteries (P < 0.05) at 4 h (LPS EC50, 998.50 +/- 447.10microM; LPS + L-NAME EC50, 65.23 +/- 25.61microM; LPS + bosentan EC50, 63.65 +/- 25.33microM). We conclude that endothelium-dependent mechanisms have an early role in the reduced responsiveness of vascular smooth muscle to vasoconstrictors during simulated septic conditions. Shortly after exposure to LPS (duration, 1 h), endothelium-derived NO seemed to have a role in reduced arterial constriction to phenylephrine, but later (4 h) ET-1 and endothelium-independent increase in NO seemed to contribute further to the loss of response.