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Aim: The present study was designed to explore the endogenous production and localization of the sulfur dioxide (SO2)/aspartate aminotransferase pathway in vas- cular tissues of rats and to examine its vasorelaxant effect on isolated aortic rings, as well as the possible mechanisms. Methods: The content of SO2 in the samples was determined by using high performance liquid chromatography with fluores- cence detection. Aspartate aminotransferase activity and its gene expression were measured by an enzymatic method and quantitative RT-PCR, respectively. Aspar- tate aminotransferase mRNA location in aorta was detected by in situ hybridization. The vasorelaxant effect of SO2 on isolated aortic rings of the rats was investigated in vitro. L-type calcium channel blocker, nicardipine, and L-type calcium channel agonist, Bay K8644, were used to explore the mechanisms by which SO2 relaxed the aortic rings. Results: Aorta had the highest SO2 content among the vascular tissues tested (P<0.01). The aortic aspartate aminotransferase mRNA located in endothelia and vascular smooth muscle cells beneath the endothelial layer. Furthermore, a physiological dose of the SO2 derivatives (Na2SO3/NaHSO3) re- laxed isolated artery rings slightly, whereas higher doses (1-12 mmol/L) relaxed rings in a concentration-dependent manner. Pretreatment with nicardipine elimi- nated the vasorelaxant response of the norepinephrine-contracted rings to SO2 completely. Incubation with nicardipine or SO2 derivatives successfully prevented vasoconstriction induced by Bay K8644. Conclusion: Endogenous SO2 and its derivatives have a vasorelaxant function, the mechanisms of which might involve the inhibition of the L-type calcium channel.