The effect of polymixin B immobilized on fibers and continuous veno-venous hemodiafiltration on tetrahydrobiopterine and NO metabolites in septic patients

Introduction

Hemoperfusion with a column of polymixin B immobilized on fibers (PMX) increases the blood pressure too rapidly for the effect to be attributable to endotoxin removal. On the other hand, continuous veno-venous hemodiafiltration (CVVHDF) also has effects of increasing blood pressure in septic patients. Since inducible NO synthase (iNOS) is known to be involved in the profound hypotension, we hypothesized that a decrease of tetra-hydrobiopterine (BH4), an essential cofactor of iNOS, might account for the rapid effect of PMX and CVVHDF on blood pressure, if PMX and CVVHDF can decrease BH4. In this study we therefore measured the plasma level of BH4 and NO metabolites (NOx) in septic patients, and evaluated whether PMX and CVVHDF can decrease them.

Methods

With institutional approval and informed consent, we studied 14 septic patients (aged 67.1 ± 13.2 years). Fourteen healthy volunteers (aged 38.4 ± 10.3 years) served as controls for BH4 and NOx. Plasma BH4 was quantified by HPLC with fluorimetric detection.

Analysis

All results are expressed as the mean ± SD, with statistical evaluation by a paired t test and unpaired t test and repeated-measures one-way ANOVA followed by Fisher's PLSD for multicomparisons.

Results

The plasma level of BH4 in septic patients was indeed markedly elevated compared with that in volunteers (140.0 ± 148.1 vs 24.1 ± 4.8 pmol/ml, P < 0.01). Level of NOx was 140.3 ± 70 vs 28.7 ± 11.6 nmol/ml, P < 0.01). Comparison of BH4 and NOx concentrations at inflow and outflow of the PMX column or CVVHDF dialyzer confirmed significantly low values at the outflow.

Discussion and conclusion

Several lines of experimental and clinical evidence indicate that hyperproduction of NO by iNOS contributes to the hypotension, cardiac depression and vascular hyporeactivity. However, the efficacy of NOS inhibitor in patients with septic shock failed to find any beneficial effect in terms of the incidence of noncardiovascular organ dysfunction. A possible explanation for these puzzling results may be the use of a nonspecific NOS inhibitor, which blocks both endothelial NO synthase (eNOS) and iNOS, since eNOS is necessary to inhibit adhesion of platelets and leukocytes to the endothelium and plays a protective role against inflammation. We hypothesized that removal of BH4 might decrease iNOS activity without greatly affecting the activity of eNOS, since eNOS has access to tissue stores of BH4, but NO generation from iNOS is critically dependent on BH4 availability. The marked increase in BH4 concomitantly with NOx in septic shock patients and its reduction by PMX or CVVHDF are consistent with our hypothesis, and appear to justify further research on BH4 removal as a potential therapeutic target. In conclusion, BH4 plays the greater role in inducing excessive vasodilatation in septic shock, and PMX or CVVHDF can decrease BH4 and NOx.