Inhibition of inducible nitric oxide synthase prevents the attenuated response to noradrenaline during human endotoxemia

Vasodilatory shock is the main cause of mortality during sepsis. Animal experiments suggest that the decreased vasopressor sensitivity present in vasodilatory shock is caused by increased levels of nitric oxide (NO). Human data on this subject are sparse. The administration of Escherichia coli endotoxin (LPS) to human volunteers induces an inflammatory response and, as clinically observed in sepsis, an attenuated vasoconstrictive response to noradrenaline. The present study investigated the effects of NO inhibitors L-NMMA and aminoguanidine on the attenuated vasoconstrictive response to noradrenaline during human endotoxemia. Thirteen human volunteers received 2 ng/kg E. coli LPS intravenously. The brachial artery was cannulated for infusion of noradrenaline and L-NMMA. The response to noradrenaline was determined by intra-arterial infusion (1–3–10–30 ng/min/dl) and determination of forearm blood flow (FBF) using venous occlusion plethysmography. The noradrenaline dose-response was determined before and 4 hours after LPS administration.

Group A (n = 6): to determine the local effect of NO inhibitor L-NMMA on noradrenaline dose-response, a dose of 0.2 mg/min/dl L-NMMA was infused intra-arterially at t = 5 hours after LPS administration. During the infusion of L-NMMA the noradrenaline dose-response was determined again.

Group B (n = 7): to assess the effect of systemic NO inhibition the selective inducible NO synthase (iNOS) inhibitor aminoguanidine was administrated i.v. at t = 1 hour post-LPS administration and continued until t = 5 hours (370 mg loading dose and 60 mg/hour continuously). Data are expressed as the mean ± SEM. FBF is expressed as the ratio of the flow in the infused/non-infused arm and is presented in percentages compared with the baseline. Differences were tested by repeated-measures ANOVA. P < 0.05 was considered to indicate significance.

LPS administration induced the expected flu-like symptoms, fever (38.3 ± 0.1°C), and a decrease in mean arterial pressure and increase in heart rate in both groups. The intrabrachial infusion of L-NMMA increased noradrenaline sensitivity after endotoxin administration (LPS alone 92 ± 5, 85 ± 4, 68 ± 11, 44 ± 13; LPS + L-NMMA 71 ± 8, 66 ± 7, 48 ± 11, 21 ± 3%, P = 0.009). The systemic inhibition of iNOS completely prevented the attenuated response to noradrenaline at t = 4 hours (before LPS 83 ± 5, 76 ± 7, 58 ± 9, 31 ± 2; after LPS + aminoguanidine 89 ± 3, 82 ± 4, 57 ± 5, 34 ± 5%, P = NS).

The local administration of nonselective NO-inhibitor L-NMMA restores the attenuated noradrenaline sensitivity during human endotoxemia. Moreover, systemic iNOS inhibition by aminoguanidine completely prevents an attenuated vasoconstrictor response. The present study indicates that noradrenaline insensitivity during human endotoxemia is mediated by induction of inducible NO synthase.