Markers of nitric oxide are associated with sepsis severity: an observational study

BACKGROUND Nitric oxide (NO) regulates processes involved in sepsis progression, including vascular function and pathogen defense. Direct NO measurement in patients is unfeasible because of its short half-life. Surrogate markers for NO bioavailability are substrates of NO generating synthase (NOS): L-arginine (lArg) and homoarginine (hArg) together with the inhibitory competitive substrate asymmetric dimethylarginine (ADMA). In immune cells ADMA is cleaved by dimethylarginine-dimethylaminohydrolase-2 (DDAH2). The aim of this study was to investigate whether concentrations of surrogate markers for NO bioavailability are associated with sepsis severity. METHOD This single-center, prospective study involved 25 controls and 100 patients with surgical trauma (n = 20), sepsis (n = 63), or septic shock (n = 17) according to the Sepsis-3 definition. Plasma lArg, hArg, and ADMA concentrations were measured by mass spectrometry and peripheral blood mononuclear cells (PBMCs) were analyzed for DDAH2 expression. RESULTS lArg concentrations did not differ between groups. Median (IQR) hArg concentrations were significantly lower in patient groups than controls, being 1.89 (1.30-2.29) μmol/L (P < 0.01), with the greatest difference in the septic shock group, being 0.74 (0.36-1.44) μmol/L. In contrast median ADMA concentrations were significantly higher in patient groups compared to controls, being 0.57 (0.46-0.65) μmol/L (P < 0.01), with the highest levels in the septic shock group, being 0.89 (0.56-1.39) μmol/L. The ratio of hArg:ADMA was inversely correlated with disease severity as determined by the Sequential Organ Failure Assessment (SOFA) score. Receiver-operating characteristic analysis for the presence or absence of septic shock revealed equally high sensitivity and specificity for the hArg:ADMA ratio compared to the SOFA score. DDAH2 expression was lower in patients than controls and lowest in the subgroup of patients with increasing SOFA. CONCLUSIONS In patients with sepsis, plasma hArg concentrations are decreased and ADMA concentrations are increased. Both metabolites affect NO metabolism and our findings suggest reduced NO bioavailability in sepsis. In addition, reduced expression of DDAH2 in immune cells was observed and may not only contribute to blunted NO signaling but also to subsequent impaired pathogen defense.