BASAL RELEASE OF 6-CYANODOPAMINE AND 6-NITRODOPAMINE FROM RAT ISOLATED AORTA AND THEIR ROLE IN VASCULAR RELAXATION

6-Cyanodopamine (6-CYD) and 6-nitrodopamine (6-ND) are newly described endogenous catecholamines. However, their presence and vascular actions in rat aorta remain unclear. Thoracic aortas from male Wistar rats were analyzed for basal catecholamine release by LC-MS/MS. Vascular reactivity was assessed in U-46619-precontracted rings with intact or removed endothelium and after pharmacological inhibition with N^ω-nitro-L-arginine methyl ester (L-NAME), 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), indomethacin, SQ-22536, SCH-23390, and haloperidol. Tyrosine hydroxylase (TH) expression was examined by immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH). Rat aorta basally released both 6-CYD and 6-ND, with 6-ND levels exceeding 6-CYD. Both catecholamines produced concentration-dependent relaxation, with 6-ND showing greater potency than 6-CYD (pEC₅₀: 8.9±0.5 and 8.5±0.6, respectively). Endothelium removal abolished 6-CYD relaxation and significantly reduced 6-ND relaxation. L-NAME, ODQ, indomethacin, and SQ-22536 significantly inhibited responses to 6-CYD, but not to 6-ND, indicating different mechanisms of action. SQ-22536 also attenuated forskolin- and iloprost-induced relaxation, supporting involvement of adenylyl cyclase/cAMP signaling in 6-CYD responses. SCH-23390 had no effect on 6-CYD- and 6-ND-induced relaxation. Haloperidol reduced the effect of sumanirole- and 6-CYD-induced relaxation but had no effect on 6-ND-induced relaxation, suggesting involvement of D2-like receptor in 6-CYD signaling. Tyrosine hydroxylase protein and mRNA were detected by immunohistochemistry and fluorescent in situ hybridization respectively, in endothelial cells and adventitial neural structures. Rat thoracic aorta basally releases 6-CYD and 6-ND, and although both act as potent vasodilators, they have distinct mechanisms of action. These findings shed new light on mechanisms by which the vascular endothelium regulates vascular tone.