Homocysteine induced impairment of nitric oxide-dependent vasorelaxation is reversible by the superoxide dismutase mimetic TEMPOL.

BACKGROUND: Elevated plasma homocysteine concentrations in renal patients are associated with accelerated cardiovascular disease. The mechanism(s) by which homocysteine acts remains unclear however, evidence implicates a role involving endothelial dysfunction.
METHODS: Rat femoral arteries after acute or 4-h pre-incubation with racemic D,L-homocysteine (100 microM) were mounted on a myograph, pre-constricted with phenylephrine (10 microM) and responses to acetylcholine-dependent vasorelaxation examined. The incubations were repeated in the presence of indomethacin (10 microM), omega-nitro-L-arginine methyl ester (100 microM), L-arginine (100 microM), tetrahydrobiopterin (1 microM), catalase (1200 U/ml), ebselen, a peroxynitrite chelator (20 microM) and TEMPOL, a superoxide dismutase mimetic (1 mM). Results are shown as means+/-standard error, expressed as per cent relaxation to acetylcholine added (nmol/l).
RESULTS: Increasing concentrations of homocysteine had no affect when added directly to basally relaxed or pre-constricted freshly isolated vessels. However, 4-h pre-incubation with or without homocysteine significantly shifted the acetylcholine EC(50) (EC(50) was defined as the concentration of acetylcholine that caused relaxation of the phenylephrine contracted tissue by 50%), control((4 h)) = 74.7 nmol/l+/-10.5 vs 100 microM D,L-homocysteine((4 h)) = 159.9 nmol/l+/-20.6; P<0.05) without affecting maximal relaxation. Response to endothelial independent relaxation was unaffected. Indomethacin, indomethacin and omega-nitro-L-arginine methyl ester, l-arginine and tetrahydrobiopterin, catalase and ebselen had no effect on the EC(50) in homocysteine-exposed arteries. However, TEMPOL normalized vasorelaxation in homocysteine-treated arteries (75.2 nmol/l+/-14.6) but had no effect on the 4-h control group. Moreover, washing TEMPOL from the treated vessels restored endothelial dysfunction in D,L-homocysteine-treated vessels (163.9 nmol/l+/-34.1).
CONCLUSIONS: We conclude that homocysteine causes endothelial dysfunction by up-regulating a potential superoxide generating system resulting in reduced nitric oxide bio-availability.