Flow activates an endothelial potassium channel to release an endogenous nitrovasodilator.

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 L,N-mono-methylarginine (the antagonist of nitric-oxide synthesis). Barium chloride, depolarizing solutions of potassium, verapamil, calcium-free medium, and antagonists of the KCa 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 micro-carrier 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 KCa channel) on the endothelial cell membrane that leads to the release of nitric oxide.

Arginine restores cholinergic relaxation of hypercholesterolemic rabbit thoracic aorta.

BACKGROUND: Reduced synthesis of endothelium-derived relaxing factor (EDRF) may explain impaired endothelium-dependent vasodilation in hypercholesterolemia. Accordingly, we designed studies to determine if endothelium-dependent relaxation in hypercholesterolemic rabbits may be restored by supplying L-arginine, the precursor of EDRF. METHODS AND RESULTS: Normal or hypercholesterolemic rabbits received intravenous L-arginine (10 mg/kg/min) or vehicle for 70 minutes. Subsequently, animals were killed, thoracic aortas were harvested, and vascular rings were studied in vitro. Rings were contracted by norepinephrine and relaxed by acetylcholine chloride or sodium nitroprusside. Vasorelaxation was quantified by determining the maximal response (expressed as percent relaxation of the contraction) and the ED50 (dose of drug inducing 50% relaxation; expressed as -log M). In vessels from hypercholesterolemic animals receiving vehicle, there was a fivefold rightward shift in sensitivity to acetylcholine compared with normal animals (p = 0.05, n = 5 in each group). In vessels from hypercholesterolemic animals, L-arginine augmented the maximal response to acetylcholine (83 +/- 16% versus 60 +/- 15%, p = 0.04 versus vehicle) and increased the sensitivity to acetylcholine (ED50 value: 6.7 +/- 0.2 versus 6.2 +/- 0.2, p less than 0.05 versus vehicle). Arginine did not affect maximal and EC50 responses to acetylcholine in vessels from normal animals. Arginine did not potentiate endothelium-independent responses in either group. CONCLUSIONS: 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.