Flow-mediated vasodilation is endothelium dependent. We hypothesized that flow activates a potassium channel on the endothelium, and that activation of this channel leads to the release of the endogenous nitrovasodilator, nitric oxide. To test this hypothesis, rabbit iliac arteries were perfused at varying flow rates, at a constant pressure of 60 mm Hg. Increments in flow induced proportional increases in vessel diameter, which were abolished by LN-mono-methylarginine (the antagonist of nitric-oxide synthesis). Barium chloride, depolarizing solutions of potassium, verapamil, calcium-free medium, and antagonists of the Kc. channel (charybdotoxin, iberiotoxin) also blocked flow-mediated vasodilation. Conversely, responses to other agonists of endothelium-dependent and independent vasodilation were unaffected by charybdotoxin or iberiotoxin. To confirm that flow activated a specific potassium channel to induce the release of nitric oxide, endothelial cells cultured on microcarrier beads were added to a flow chamber containing a vascular ring without endothelium. Flow-stimulated endothelial cells released a diffusible vasodilator, the degree of vasorelaxation was dependent upon the flow rate. Relaxation was abrogated by barium, tetraethylammonium ion, or charybdotoxin, but was not affected by apamin, glybenclamide, tetrodotoxin, or ouabain.The data suggest that transmission of a hyperpolarizing current from endothelium to the vascular smooth muscle is not necessary for flow-mediated vasodilation. Flow activates a potassium channel (possibly the Kc. channel) on the endothelial cell membrane that leads to the release of nitric oxide. (J. Clin.

We conclude that the endothelium-dependent relaxation is normalized in hypercholesterolemic rabbit thoracic aorta by in vivo exposure to L-arginine, the precursor for EDRF.

Recent evidence suggests that endothelium-derived relaxing factor exhibits properties of nitric oxide. Like nitric oxide, it inhibits platelet function and mediates its effects by elevating intracellular cyclic GMP. In this study we have investigated the role of reduced thiol in the mechanism of action of endothelium-derived relaxing factor on platelets. Bovine aortic endothelial cells were grown on microcarrier beads and pretreated with aspirin before use. Endothelial cells stimulated with bradykinin or exposed to stirred medium expressed a dose-dependent inhibition of platelet aggregation that was potentiated by the reduced thiol, N-acetylcysteine. Endothelial cell-mediated platelet inhibition was attenuated by methylene blue. Inhibition of platelet aggregation by endothelial cells was associated with a rise in platelet intracellular cyclic GMP, an effect that was enhanced by N-acetylcysteine. These data show that 1) the reduced thiol N-acetylcysteine potentiates platelet inhibition by endothelium-derived relaxing factor and 2) this effect is associated with increasing intracellular platelet cyclic GMP levels.

We designed a novel system to study flow-mediated endothelium-dependent vasodilation. Vascular rings of rabbit thoracic aorta were mounted for isometric tension recording in a flow chamber filled with physiological saline solution. The flow chamber contained a stir bar and was mounted on a magnetic stirrer to induce vortical flow. Norepinephrine (NE, 10(-6) M) induced contraction of the vascular rings. Bovine endothelial cells on microcarrier beads added to the chamber had little effect on contraction to NE in the absence of flow. Flow induced endothelium-dependent relaxation of the vascular rings that was dependent on the flow rate. Relaxations were annulled or reversed to a contraction with methylene blue, bovine hemoglobin, or N-monomethyl-L-arginine. Conversely, N-acetyl-L-cysteine augmented the flow-mediated relaxation. Furthermore, in the presence of N-acetyl-L-cysteine, the half-life of the endothelium-dependent relaxing factor was increased. In conclusion, the stimulus of flow induces the release by endothelial cells of a diffusible, short-lived factor with the attributes of a nitrovasodilator. The action of this endogenous vasodilator is augmented by the reduced thiol N-acetyl-L-cysteine.