The effect of elevated homocysteine levels on adrenergic vasoconstriction of human resistance arteries: The role of the endothelium and reactive oxygen species

Abstract

Purpose: This study investigated the effect of elevated homocysteine levels on adrenergic contraction of human resistance arteries and tested the hypothesis that homocysteine-induced generation of reactive oxygen species contributes to vascular reactivity changes. Methods: Small (<200 μm) subcutaneous arteries were cannulated and pressurized in an arteriograph chamber that allowed the measurement of lumen diameter. Two arteries from the same patient were obtained; one was perfused and superfused (intraluminal pressure = 50 mm Hg) with physiologic saline solution (control, n = 6), and the other was perfused and superfused with physiologic saline solution plus 200 μmol/L homocysteine (HC, n = 6); the reactivity to adrenergic stimulation was assessed. Another group of arteries was incubated in 200 μmol/L homocysteine plus 1200 U/mL superoxide dismutase and 120 U/mL catalase (HC + SC, n = 6), and the reactivity to norepinephrine was determined. The vasoreactivity of homocysteine was further assessed in intact (n = 6) and denuded (n = 6) arteries that were precontracted with an intermediate concentration of norepinephrine and homocysteine (20-200 μmol/L) added to the bath while the lumen diameter was continuously recorded. Results: Sensitivity to norepinephrine was diminished in HC arteries, which increased the median effective concentration (EC50) from 0.24 ± 0.06 μmol/L in control arteries to 0.65 ± 0.10 μmol/L in HC arteries (P .05) or in sensitivity to acetylcholine (EC50 = 19 ± 7 nmol/L vs 12 ± 3 nmol/L; P > .05). Arteries in the presence of superoxide dismutase and catalase had similarly impaired reactivity to norepinephrine as did homocysteine arteries (EC50, 0.58 ± 0.15 μmol/L; P > .05 vs HC, P < .01 vs control). Conclusion: An elevated homocysteine level in vitro diminishes adrenergic contraction, with a differential endothelial versus smooth muscle influence that appears unrelated to the generation of reactive oxygen species.