To study flow-mediated responses in a conduit vein, we investigated the physiological characteristics of endothelium-dependent acetylcholine (ACh)- and flow-induced relaxations using a conventional bioassay cascade. Cylindrical segments isolated from canine common carotid arteries and external jugular veins were perfused at a constant mean flow rate ranging from 1 to 8 ml/min. Endothelium-derived nitric oxide (NO) activity in perfusion effluent through the arterial and venous segments was measured by relaxation of endothelium-denuded arterial rings and arterial and/or venous rings precontracted by prostaglandin F2 alpha, respectively. Stimulation by a flow rate of 8 ml/min on the arterial and venous endothelial cells produced approximately 60 and 20% of the maximum relaxation in the arterial and venous rings, respectively. ACh (10(-6) and 10(-5) M) perfused through the arterial and venous segments with endothelium caused dose-related relaxations of both bioassay rings. The ACh- and flow-induced relaxations were completely reduced by mechanical removal of the endothelial cells. Pretreatment with 5 x 10(-5) M NG-nitro-L-arginine methyl ester (L-NAME) produced a significant reduction of the ACh- and flow-induced vasodilation. Additional treatment with 10(-4) M L-arginine significantly reversed the L-NAME-induced inhibition of ACh-induced relaxation but had no effect on flow-induced relaxation. When the flow rate was increased from 2 to 4 ml/min, the same concentrations of ACh produced larger dose-related relaxations than those obtained at a flow rate of 2 ml/min. Pretreatment with 25 U/ml superoxide dismutase caused no significant effect on the flow-mediated potentiation of ACh-induced relaxation. These findings suggest that venous endothelial cells of canine large vein are able to produce and release NO by stimulation of increased flow or ACh to a significantly lesser extent compared with the artery and that ACh-induced vasodilation is potentiated by an increase in shear stress up to approximately 4 dyn/cm2 loaded on the endothelial cells.