Reversible sequestration of nitric oxide by hemoglobin during hemodialysis in end-stage renal disease.

BACKGROUND: During hemodialysis, patients whose plasma concentrations of nitric oxide (NO) products increase reportedly experience hypotension. Therefore, whether NO bound to hemoglobin (Hb) could contribute to various clinical and laboratory changes during hemodialysis was explored in patients with end-stage renal disease (ESRD). METHODS: Ten patients were studied during 3 hemodialysis treatments with samples of blood analyzed for RBC nitrosyl Hb (HbNO), L-arginine, asymmetric dimethylarginine (ADMA), plasma nitrite+nitrate (NOx), and buffy coat NO synthase (NOS) activities. RESULTS: HbNO before and during hemodialysis varied considerably. Those with higher predialysis levels had lower HbNO values during dialysis, whereas HbNO levels in those with lower levels before dialysis increased. Plasma NOx did not correlate with HbNO, but change in HbNO in the first hour and change in NOx in the first 2 hours correlated with drop in diastolic and systolic blood pressures (BP), respectively. HbNO concentrations increased in patients with >35% drop in systolic BP, whereas in those with <35% drop, HbNO concentrations decreased. HbNO levels adjusted by the hematocrit showed a drop in HbNO for the <35% group and a >3-fold increase in the >35% group. HbNO levels were higher in men than in women, and levels and changes correlated with the hematocrit, skin temperatures, plasma ADMA, arginine, and buffy coat NOS. CONCLUSIONS: In patients with >35% drop in systolic BP, NO was scavenged by Hb in the circulating RBCs, undoubtedly attenuating the degree of hypotension. These data indicate that the amount of NO that is scavenged or released by Hb in the circulating RBCS during dialysis is highly variable and reversible. Various predialysis factors relate to the concentration of HbNO before and during dialysis, which in turn influence clinical findings that occur during the interdialytic period.

An enzyme hydrolyzing methylated inhibitors of nitric oxide synthase is present in circulating human red blood cells.

N(G),N(G)-dimethyl-L-arginine (asymmetric dimethylarginine or ADMA) and N(G)-monomethyl-L-arginine (L-NMMA) are post-translationally synthesized amino acids of nuclear proteins. Upon release during protein turnover, they are not used in protein synthesis, but are excreted or metabolized by dimethylarginine dimethylaminohydrolase (DDAH) found in many tissues. DDAH is present in monocytic and polynuclear cells of blood, but no report has appeared of its presence in red blood cells (RBCs). Because methylated arginines can inhibit nitric oxide synthase (NOS) and elevations are reported in several diseases, we explored whether RBCs express this enzyme. DDAH is present in RBCs as supported by hydrolysis of both ADMA and L-NMMA, but not symmetric dimethylarginine, and by immunoprecipitation/Westem blot using a specific monoclonal antibody to human DDAH. In a pilot study of end-stage renal disease (ESRD) patients, RBC DDAH activity with ADMA as substrate correlated inversely with age (p = 0.005) and enzyme activities were higher in patients with greater diastolic blood pressure drops during hemodialysis (p = 0.02). Similar correlations were found with white cell DDAH activity. Thus, human RBCs can hydrolyze methylated arginines. These findings indicate the RBC could be used to assess the status of DDAH in various disease states.