Angiographic contrast media relax isolated rabbit aorta through an endothelium-independent mechanism that may not depend on the presence of the iodine atom.

Systemically administered iodinated angiographic contrast media evoke vasodilatation through mechanisms that are at present poorly understood. In the current investigation we have evaluated the role of the vascular endothelium in responses to an iso-osmolar formation of the non-ionic dimer iodixanol and a hyperosmolar formulation of the non-ionic monomer iopromide. Isolated rabbit aortic ring preparations with endothelium intact or removed by gentle abrasion were mounted in organ baths containing oxygenated Holman's solution, and cumulative concentration-response curves for relaxation to the contrast media were constructed after pre-constriction by phenylephrine (300 nM) in the presence of indomethacin to inhibit prostaglandin synthesis. Endothelial denudation did not influence the ability of either iodixanol or iopromide to relax the aortic ring preparations. Iopromide was significantly more potent than iodixanol when expressed in terms of iodine concentration (mg I ml-1), but both agents were equipotent when expressed in terms of molarity (mM). We conclude that relaxation of isolated rabbit aortic rings to iodixanol and iopromide under conditions where there is no fluid flow is endothelium-independent, and therefore not mediated by release of the potent endogeneous nitrovasodilator endothelium-derived relaxing factor (EDRF). Furthermore, their relaxant activity under the in vitro experimental conditions employed is attributable to a direct action on vascular smooth muscle by factors in addition to osmolality, and may depend on features that are not specifically associated with the presence of the iodine atom.