The effects of acute and chronic radiation on endothelium- and no-dependent vascular reactivity.

Endothelium- and NO-dependent relaxation of aortic segments and basal coronary flow and response of coronary vessels to exogenous NO in the Langendorff-perfused rat hearts were examined on the 3rd, 10th, 30th and 90th days following whole body irradiation in 1 Gy dose with different dose rate. NO-mediated elevation of coronary flow and increased aortic endothelium-dependent vasodilation were found at the early stage after acute irradiation (137Cs, 9 x 10(-4) Gy/s), while vascular reactivity to exogenous NO was not changed. Chronic irradiation (137Cs, 2.3 x 10(-7) Gy/s) significantly impaired endothelium-dependent relaxation within whole experimental period and decreased NO component of basal coronary flow at the 3rd, 10th and 90th day. It was associated with attenuated reactivity to NO-donor in aorta one month after irradiation and in the coronary vessels--shortly after irradiation. In the delayed period endothelial dysfunction and the diminution of NO-dependent coronary flow were mediated by selective impairment of NO synthesis/release. Decreased survival rate of the lethally-irradiated Nwnitro-L-arginine methyl ester-treated animals revealed radiosensitizing properties of NO synthase inhibition. The data obtained indicate the involvement of NO in the alterations of vascular reactivity depending on the dose rate and on the interval after the irradiation.