ABSTRACT
The contribution of endothelium-derived relaxing factor ( E D R F / N O ) and endotheliumderived hyperpolarizing factor ( E D H F ) to the impairment of endothelium-dependent relaxation caused by acetylcholine was examined in the rabbit thoracic aorta after irradiation. The changes in membrane potential and contractile force in response to acetylcholine were recorded during contractions to phenylephrine in the presence or absence of N^-nitro-L-arginine and indomethacin. The acetylcholine-induced relaxa tions were smaller in tissues obtained from irradiated animals than in those from healthy rabbits. Aortic rings from irradiated animals showed enhanced responses to authentic N O compared with non-irradiated animals. The acetylcholine-stimulated release of N O , detected by chemiluminescence, was not different in irradiated and nonirradiated vascular tissues. The relaxation to acetylcholine in non-irradiated animals was only partly reduced in the presence of N^-nitro-L-arginine and indomethacin, indicating a sizable EDHF-mediated component. A similar experiment in aortas from irradiated animals showed that the EDHF-dependent component of the acetylcholineinduced relaxation was unchanged while the E D R F / N O component was decreased. The EDRF/NO-dependent component, but not the EDHF-dependent component of the acetylcholine-induced relaxation was restored by the treatment of irradiated animals with the antioxidant ^-tocopherol acetate. Electrophysiological studies demonstrated that application of acetylcholine hyperpolarized both healthy and irradiated rabbit aortas but that the peak amplitude of hyperpolarization was smaller after irradiation. In the presence of N G -nitro-L-arginine and indomethacin the amplitudes of hyperpolarization were similar in healthy and irradiated tissues. In the presence of N^-nitro-L-arginine and indomethacin, the acetylcholine-induced relaxations and hyperpolarizations in both tissues were unaffected by glibenclamide but reduced by apamin combined with char ybdotoxin. They were almost completely blocked by tetraethylammonium, a non-selective inhibitor of calcium-dependent potassium channels, or when the tissues were contracted with potassium chloride instead of phenylephrine. These results are consistent with the hypothesis that the depression of endothelial function following ionized irradiation are due mainly to an alteration in E D R F / N O metabolism but not to a decrease in E D H F . Since the release and/or action of E D H F are relatively resistant to radiationinduced oxidative stress, E D H F seems unlikely to be a metabolite of arachidonic acid formed by cytochrome P450 monoxygenase or generated by other oxygen-dependent enzymes.
