Local acidosis (pH 6.4) depresses reactivity of vascular smooth muscle and especially the response of human isolated saphenous veins to exogenous norepinephrine. Experiments were performed to study, under acidosis conditions, the interaction between Daflon 500 mg, a micronized fraction of 90% diosmin and 10% hesperidin, and norepinephrine on human rings of veins. Varicose veins were obtained by conservative varicose veins surgery and normal veins from patients undergoing coronary artery bypass graft surgery. Isometric tension was recorded from venous rings in organ chambers filled with Krebs-Henseleit solution (pH 7.4; 37 degrees C). Metabolic acidosis (from pH 7.4 to 6.4) was obtained by lowering the HCO3- concentration of the Krebs-Henseleit solution. Cumulative concentration-response curves for norepinephrine (10(-7) to 10(-5)M) were obtained at pH 6.4 in the presence or in the absence of Daflon 500 mg (10(-5)M) added 20 min previously to the organ bath. Under acidotic conditions, Daflon 500 mg induced a shift to the left of the concentration-response curves for norepinephrine. This potentiation was significant in both normal and varicose veins and was increased in proportion with the pathological status of the venous rings. These results support the therapeutic benefits of Daflon 500 mg in chronic venous insufficiency.