Generation of nitrate during dialysis as a measure of nitric oxide synthesis

Nitric oxide production and blood pressure reduction during haemodialysis

AIM

A decrease of systolic blood pressure in excess of 20 mmHg during haemodialysis treatment (IDD) is common for haemodialysis patients. Intradialytic hypotension (IDH) is symptomatic IDD by definition. Overproduction of nitric oxide (NO) is a possible cause of IDD. Dialysate nitrate and nitrite amount can be used as an indicator of intradialysis NO production. Our aim was to find the predictor of NO production in IDD patients.

METHODS

Partial dialysate samples were collected during the whole haemodialysis session and total dialysate nitrate and nitrite amount was measured to assess the association of intradialysis NO production with blood pressure change.

RESULTS

There were 31 IDD patients and 71 patients who did not develop IDD (NIDD) included in the study. Among the IDD patients, 13 were IDH patients with a mean systolic blood pressure lower than that of the other 18 symptomless IDD patients (96.6 ± 3.4 mmHg vs 125.0 ± 3.8 mmHg, P<0.001). The median value of NO production was higher in the IDD than in the NIDD patients (447.7 μg vs 238.8 μg, P<0.001). The NO production correlated linearly with blood pressure reduction (R=0.487, P<0.001). The multivariate analysis showed that NO production was positively associated with predialysis systolic blood pressure.

CONCLUSION

Nitric oxide production during haemodialysis was higher in IDD than in NIDD patients. IDH often occurred when systolic blood pressure was reduced to below 100 mmHg. The amount of NO produced during haemodialysis, which may be associated with predialysis systolic blood pressure, can be used to predict intradialysis blood pressure decrease.

Measurement of the NO metabolites, nitrite and nitrate, in human biological fluids by GC-MS

In this article we critically review the development and application of gas chromatography-mass spectrometry (GC-MS) techniques to the measurement of the nitric oxide (NO) metabolites, nitrite and nitrate, in human biological fluids. Our focus is on the issue of the fitness of any analytical strategy to its intended purpose and the validity of the analytical results generated. The accuracy, precision, recovery, selectivity and sensitivity of the various methods are evaluated and the potential pitfalls, both specific to the methods, and general to the area, are considered. Several examples of the applications of these techniques to clinical investigations of NO physiology are also critically evaluated.

[Thrombocytopathy and blood complications in uremia]

Bleeding diathesis and thrombotic tendencies are characteristic findings in patients with end-stage renal disease. The pathogenesis of uremic bleeding tendency is related to multiple dysfunctions of the platelets. The platelet numbers may be reduced slightly, while platelet turnover is increased. The reduced adhesion of platelets to the vascular subendothelial wall is due to reduction of GPIb and altered conformational changes of GPIIb/IIIa receptors. Alterations of platelet adhesion and aggregation are caused by uremic toxins, increased platelet production of NO, PGI(2), calcium and cAMP as well as renal anemia. Correction of uremic bleeding is caused by treatment of renal anemia with recombinant human erythropoietin or darbepoetin alpha, adequate dialysis, desmopressin, cryoprecipitate, tranexamic acid, or conjugated estrogens. Thrombotic complications in uremia are caused by increased platelet aggregation and hypercoagulability. Erythrocyte-platelet-aggregates, leukocyte-platelet-aggregates and platelet microparticles are found in higher percentage in uremic patients as compared to healthy individuals. The increased expression of platelet phosphatidylserine initiates phagocytosis and coagulation. Therapy with antiplatelet drugs does not reduce vascular access thrombosis but increases bleeding complications in endstage renal disease patients. Heparin-induced thrombocytopenia (HIT type II) may develop in 0-12 % of hemodialysis patients. HIT antibody positive uremic patients mostly develop only mild thrombocytopenia and only very few thrombotic complications. Substitution of heparin by hirudin, danaparoid or regional citrate anticoagulation should be decided based on each single case.

Methylene blue, a nitric oxide inhibitor, prevents haemodialysis hypotension

BACKGROUND

Plasma nitric oxide (NO) levels have been found to be high in haemodialysis (HD) patients, especially in those prone to hypotension in dialysis. The aim of the study was to prevent dialysis hypotension episodes by i.v. administration of methylene blue (MB), an inhibitor of NO activity and/or production.

METHODS

MB was given i.v. in 18 stable HD patients with hypotensive episodes during almost every dialysis, in 18 HD patients without hypotension during dialyses, and in five healthy controls. MB was given as a bolus of 1 mg/kg bodyweight followed by a constant infusion of 0.1 mg/kg bodyweight lasting 210 min until the end of the dialysis session and only as a bolus on a non-dialysis day. Systolic and diastolic blood pressures (BP) were measured at 10-min intervals during HD sessions with or without MB and on a non-dialysis day with MB.

RESULTS

In hypotension-prone patients, MB completely prevented the hypotension during dialysis and increased both systolic and diastolic BP on non-dialysis days. In normotensive patients, MB increased BP during the first hour of dialysis and for 90 min on the non-dialysis day. The BP in the healthy controls remained unchanged. Plasma and platelet NO(2)+NO(3) (stable metabolites of NO) levels were determined. The NO(2)+NO(3) generation rate in the first post-dialysis day was calculated. The plasma and platelet NO(2)+NO(3) were higher in the hypotensive group than in the normotensive dialysis group. The generation rate of nitrates was higher (P<0.01) in the hypotensive group (1.21+/-0.13 micromol/min and 0.74+/-0.16 after MB) than in the normotensive patients (0.61+/-0.11 micromol/ min and 0.27+/-0.14 after MB). No side-effects were recorded.

CONCLUSIONS

MB is an efficient therapy in the prevention of dialysis hypotension.

Effect of hemodialysis on plasma nitric oxide levels

Nitric oxide (NO) is produced in excess in various pathological states, including sepsis and hepatic cirrhosis, and appears to be related to inflammatory status. In uremia, one would expect the levels of NO to increase. We aimed to determine whether hemodialysis (HD) would remove NO from the systemic circulation of uremic patients. Blood was collected before, after, and 1 day after HD from 12 uremic patients. Plasma nitrite and nitrate (NOx-) levels were measured by colorimetric Greiss reaction and cGMP was measured by an enzyme immunoassay kit. Our study demonstrated that uremic patients have high plasma NO levels, and HD led to a significant drop in plasma NOx- level (63 +/- 15% reduction). The level rose back to the pre-HD level on the following day. Plasma cGMP in the patients also decreased significantly after HD (27 +/- 14% reduction). In conclusion, we hypothesized that HD might be a possible approach for the removal of excess NO in pathological conditions such as sepsis and hepatic cirrhosis.

Nitrite and nitrate analyses: a clinical biochemistry perspective

OBJECTIVE

To review the assays available for measurement of nitrite and nitrate ions in body fluids and their clinical applications.

DESIGN AND METHODS

Literature searches were done of Medline and Current Contents to November 1997.

RESULTS

The influence of dietary nitrite and nitrate on the concentrations of these ions in various body fluids is reviewed. An overview is presented of the metabolism of nitric oxide (which is converted to nitrite and nitrate). Methods for measurement of the ions are reviewed. Reference values are summarized and the changes reported in various clinical conditions. These include: infection, gastroenterological conditions, hypertension, renal and cardiac disease, inflammatory diseases, transplant rejection, diseases of the central nervous system, and others. Possible effects of environmental nitrite and nitrate on disease incidence are reviewed.

CONCLUSIONS

Most studies of changes in human disease have been descriptive. Diagnostic utility is limited because the concentrations in a significant proportion of affected individuals overlap with those in controls. Changes in concentration may also be caused by diet, outside the clinical investigational setting. The role of nitrite and nitrate assays (alongside direct measurements of nitric oxide in breath) may be restricted to the monitoring of disease progression, or response to therapy in individual patients or subgroups. Associations between disease incidence and drinking water nitrate content are controversial (except for methemoglobinemia in infants).