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.

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We hypothesized that normal vascular reactivity could be restored in vessels from hypercholesterolemic animals by exposing them to L-arginine, the precursor of endothelium-derived relaxing factor (EDRF). Basilar arteries were harvested from New Zealand white rabbits fed normal chow or that supplemented with 2% cholesterol for 10 wk. Vessels were cannulated for perfusion at physiologic pressure. Changes in vessel diameter were monitored by videomicroscopy.In comparison to normal vessels, those from hypercholesterolemic animals vasoconstricted more to KC1, endothelin (E),and 5-hydroxytryptamine (5-HT). Conversely, vasodilation to acetylcholine (ACh) (but not that to verapamil) was significantly impaired in the hypercholesterolemic animals. In vitro administration of L-arginine (3 mM) for 45 min normalized vasodilation to ACh and vasoconstriction to E, 5-HT, and KCI in the isolated vessels from hypercholesterolemic animals. This effect was stereospecific, since D-arginine had no effect. To conclude, these data confirm that hypercholesterolemia attenuates endothelium-derived relaxation, and enhances the sensitivity of these vessels to vasoconstrictors. In vitro administration of L-arginine normalized vascular reactivity of isolated vessels from hypercholesterolemic animals. Thus, hypercholesterolemia induces a reversible endothelial dysfunction that may be corrected by supplying the precursor of EDRF, L-arginine.

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