The endothelium plays a key role in the control of vascular patency and tone. Thus, the main objective of the study was to determine the role of endothelium and its derived relaxation factors in mediating relaxation of rat thoracic aorta, in response to sulfur dioxide (SO 2 ) derivatives "1:3 M/M sodium bisulfite (NaHSO 3 ) and sodium sulfite (Na 2 SO 3 )" using Pow-erLab tissue bath system. Endothelial denudation enhanced relaxation responses of SO 2 derivatives with an IC 50 of 6.11 mM as

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... ared to control rings with an IC 50 of 6.21 mM, as well as the maximum relaxation (E max ) was increased from 62.026% ± 6.527% to 83.13% ± 14.755%. Furthermore, the relaxation responses to SO 2 derivatives in aortic rings were significantly enhanced by indomethacin, clotrimazole and methylene blue with IC 50 's of 4.8 mM, 5.33 mM and 4.01 mM, and E max were raised to 101.1% ± 6.537%, 66.92 ± 7.538 and 104.68 ± 3.575, respectively. Meanwhile, L-NAME did not alter dose-dependent relaxation of SO 2 derivatives in comparison to control aortic rings. The results of this study had shown that endothelium denudation and blocking of endothelium derived-relaxation factors enhanced vasodilator effect of SO 2 ; this may clarify the role of endothelium in the vasodilatory mechanism of SO 2 .