Increased asymmetrical dimethyl-arginine suppresses nitric oxide production in Pseudomonas aeruginosa sepsis

Introduction

More than 750,000 patients in the United States develop sepsis annually. Previously, we have shown that plasma nitric oxide (NO) levels were approximately sevenfold higher and that arginase activity is significantly lower in methicillin-resistant Staphylococcus aureus (MRSA) sepsis than in Pseudomonas aeruginosa sepsis. In the present study, we hypothesize that increased asymmetrical dimethyl-arginine (ADMA), an endogenous inhibitor of NO synthase, is responsible for the suppressed NO production in Ps. aeruginosa sepsis.

Methods

Ewes were operatively prepared and randomized after a 7-day recovery period into control, MRSA, and Ps. aeruginosa groups (n = 6). Injury consisted of instillation of (2 to 5) × 10¹¹ CFU live MRSA or Ps. aeruginosa into the airway, and the sheep were sacrificed after 24 hours. In addition, groups of C57Bl/6J wild-type mice and iNOS knockout mice (n = 6) were nasally inoculated with (2 to 5) × 10⁸ CFU live MRSA or Ps. aeruginosa and were sacrificed after 8 hours.

Results

Ps. aeruginosa-treated sheep had a significantly higher ADMA (1.79 ± 0.14 vs. 1.16 ± 0.24 μM, P < 0.05), lower plasma NOx (6.83 ± 0.17 vs. 9.63 ± 0.64 μM, P < 0.05), and higher arginase activity (1.55 ± 0.16 vs. 1.07 ± 0.11 μM urea/μg protein, P < 0.05) compared with MRSA-treated sheep. These changes were associated with more pronounced lung injury in Ps. aeruginosa sepsis compared with MRSA sepsis (PaO₂/FiO₂: 205 ± 72 vs. 319 ± 82, P < 0.05). Ps. aeruginosa-treated mice had a significantly higher arginase activity (0.60 ± 0.12 vs. 0.16 ± 0.0038 μM urea/μg protein, P < 0.05) and higher protein oxidation with carbonyl groups (3,223 ± 440.7 vs. 1,124 ± 140.1, P < 0.05) compared with MRSA-treated mice. iNOS knockout mice treated with MRSA had significantly lower arginase activity than wild-type mice (0.21 ± 0.01 vs. 0.36 ± 0.04 μM urea/μg protein, P < 0.05).

Conclusion

Suppressed NO production in Ps. aeruginosa sepsis is caused by the increased expression of ADMA. Increased arginase activity is most probably caused by augmented oxidative stress, which results in a more severe lung injury.