Endogenous nitric oxide synthase inhibitors in sickle cell disease: abnormal levels and correlations with pulmonary hypertension, desaturation, haemolysis, organ dysfunction and death

Abstract

Pulmonary hypertension (PH) in patients with sickle cell disease (SCD) is linked to intravascular haemolysis, impaired nitric oxide bioavailability, renal dysfunction, and early mortality. Asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide synthases (NOS), is associated with vascular disease in other populations. We determined the plasma concentrations for several key arginine metabolites and their relationships to clinical variables in 177 patients with SCD and 29 control subjects: ADMA, symmetric dimethylarginine (SDMA), NG-monomethyl L-arginine (L-NMMA), N-omega-hydroxy-L-arginine (NOHA), arginine and citrulline. The median ADMA was significantly higher in SCD than controls (0.94 micromol/l vs. 0.31 micromol/l, P < 0.001). Patients with homozygous SCD had a remarkably lower ratio of arginine to ADMA (50 micromol/l vs. 237, P < 0.001). ADMA correlated with markers of haemolysis, low oxygen saturation and soluble adhesion molecules. PH was associated with high levels of ADMA and related metabolites. Higher ADMA level was associated with early mortality, remaining significant in a multivariate analysis. Subjects with homozygous SCD have high systemic levels of ADMA, associated with PH and early death, implicating ADMA as a functional NOS inhibitor in these patients. These defects and others converge on the nitric oxide pathway in homozygous SCD with vasculopathy.

Trial registration: ClinicalTrials.gov NCT00011648.

Figure 1. Relationship of the NOS inhibitor ADMA to arginine metabolism in sickle cell disease

Nitric oxide synthase (NOS) converts L-arginine to the intermediate NOHA, which is then converted to citrulline and nitric oxide. In patients with sickle cell disease with relatively normal renal function (serum creatinine < 220 μmol/L, n = 161), plasma arginine levels are lower than controls (n = 29), consistent with previously published effects of plasma arginase activity. Plasma levels of the NOS inhibitor ADMA are much higher than controls, and the downstream products of NOS activity are lower than controls, including both NOHA and citrulline. Horizontal bars indicate median values, and p value is calculated by the Mann-Whitney test.

Figure 2. Higher ADMA levels associated with early mortality

Kaplan-Meier plot of survival fraction in 124 patients with homozygous sickle cell disease with serum creatinine < 220 μmol/L, divided into those with ADMA plasma levels above or below the median for this group (1.02 μmol/L). Patients with higher ADMA levels have earlier mortality (p = 0.02, Cox regression).