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6-Nitrodopamine (6-ND) is the predominant catecholamine released from isolated vascular tissues in both mammals and reptiles, with its release being significantly reduced by the NO synthesis inhibitor, Nω-nitro-L-arginine methyl ester (L-NAME). The vasorelaxation induced by 6-ND is unaffected by either L-NAME or the soluble guanylate cyclase (sGC) inhibitor, ODQ, indicating an alternative mechanism of action. The vasorelaxant effect appears to be mediated through selective antagonism of dopamine D2 receptors rather than traditional nitric oxide (NO)-mediated pathways. This study examined the basal release of 6-ND, dopamine, noradrenaline, and adrenaline from the rabbit thoracic aorta by liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS). Additionally, the effects of 6-ND and the dopamine receptor antagonist L741,626 on relaxation responses and electric-field stimulation (EFS)-induced contractions in aortic rings were assessed. Nitric oxide pathway inhibitors, including L-NAME, ODQ, and methylene blue, were utilized to assess the involvement of this pathway in 6-ND-induced vasorelaxation. Concentration–response curves for norepinephrine, epinephrine, and dopamine were generated in the presence and absence of 6-ND and L-741,626. The rabbit isolated aorta presented the basal release of endothelium-derived dopamine and 6-ND. Furthermore, 6-nitrodopamine and L-741,626 induced concentration-dependent relaxations in endothelin-1 pre-contracted aortic rings. The relaxations were reduced by the mechanical removal of the endothelium but unaffected by pre-treatment with L-NAME, ODQ, or methylene blue. Pre-incubation with 6-ND significantly reduced dopamine-induced contractions, while noradrenaline- and adrenaline-induced contractions remained unchanged. The findings demonstrated that endothelium-derived 6-ND is the most potent endogenous relaxant of the rabbit isolated aorta, and the mechanism is independent of the NO pathway and involved the blockade of dopamine D2 receptors. [ABSTRACT FROM AUTHOR]

Background: 6‐Nitrodopamine (6‐ND) is a novel endogenous catecholamine that has a potent relaxant action on vascular smooth muscle in vitro. Objectives: To evaluate the basal release of 6‐ND and noradrenaline from rabbit‐isolated corpus cavernosum (RbCC) and its relaxing action on this tissue. Methods: Rabbit corpus cavernosa were dissected and suspended in a 5‐mL organ bath containing oxygenated Krebs‐Henseleit's solution. 6‐ND and noradrenaline release was quantified by liquid chromatography coupled to tandem mass spectrometry. The relaxant activity of 6‐ND was assessed in RbCC strips pre‐contracted with endothelin‐1 (10 nM). Results: Rabbit corpus cavernosum presented basal release of both 6‐ND (2.9 ± 0.8 ng/mL, n = 12) and noradrenaline (1.7 ± 1.3 ng/mL, n = 12). The 6‐ND release was reduced by pre‐treatment with Nω‐nitro‐l‐arginine methyl ester (l‐NAME) (100 µM), whereas that of noradrenaline was unaffected. Tetrodotoxin (TTX, 1 µM) abolished the noradrenaline release but had no effect on 6‐ND release, indicating a non‐neurogenic origin for 6‐ND. 6‐ND and the selective dopamine D2‐agonist L‐741,626 caused concentration‐dependent RbCC relaxations (pEC50 of 11 ± 0.15 and 11.15 ± 0.28, respectively). Pre‐treatment with either l‐NAME or the soluble guanylate cyclase inhibitor 1H‐[1,2,4] oxadiazolo[4,3‐a]quinoxalin‐1‐on (ODQ) (100 µM) caused a rightward shift of the concentration–response curve to 6‐ND, without affecting the L‐741,626 responses. In TTX (100 nM)‐pre‐treated preparations, neither l‐NAME nor ODQ shifted the 6‐ND concentration–response curve. Dopamine, noradrenaline, and adrenaline caused concentration‐dependent RbCC contractions. Pre‐incubation with 6‐ND concentration‐dependently inhibited the dopamine‐induced contractions, without affecting those induced by either noradrenaline or adrenaline. Discussion and conclusion: 6‐Nitrodopamine is the most potent endogenous relaxant agent in RbCC ever described and represents a novel mechanism by which NO causes corpus cavernosum smooth muscle relaxation. The finding that 6‐ND acts as a truly selective dopamine D2‐receptor antagonist indicates that the balance of dopamine and 6‐ND release/synthesis may be the main mechanism that modulates corpus cavernosum smooth muscle tonus in vivo. [ABSTRACT FROM AUTHOR]

6-Nitrodopamine (6-ND) is released from human vas deferens and plays a modulatory role in the male ejaculation. Therapeutical use of α1-adrenoceptor antagonists is associated with ejaculatory abnormalities. To evaluate the effect of α1-adrenoceptor antagonists on the contractions induced by 6-ND, dopamine, noradrenaline, and adrenaline in the human epididymal vas deferens (HEVD). HEVD strips were suspended in glass chambers containing heated and oxygenated Krebs–Henseleit's solution. Cumulative concentration–response curves to catecholamines (10 nM-300 μM) were constructed in HEVD strips pre-incubated (30 min) with doxazosin (0.1-1 nM), tamsulosin (1–10 nM), prazosin (10–100 nM) and/or silodosin (0.1–10 nM). The effects of these α1-adrenoceptor antagonists were also evaluated in the electric-field stimulation (EFS, 2–32 Hz)-induced contractions. Doxazosin (0.1 nM) caused significant reductions in 6-ND-induced HEVD contractions without affecting the contractions induced by dopamine, noradrenaline, and adrenaline. Similar results were observed with tamsulosin (1 nM) and prazosin (10 nM). At these concentrations, these α1-adrenoceptor antagonists largely reduced the EFS-induced contractions. Silodosin (1 nM) caused concentration-dependent rightward shifts of the concentration–response curves to 6-ND but had no effect on the contractions induced by dopamine and adrenaline. Silodosin (0.1 nM) only inhibited the contractions induced by noradrenaline. Silodosin at 1 nM, but not at 0.1 nM, caused significant reductions in the EFS-induced contractions. The results reinforce the concept that 6-ND plays a major role in the human vas deferens contractility and indicate that the ejaculation disorders caused by doxazosin, tamsulosin, prazosin and silodosin cause in man, may be due to inhibition of the contractions induced by 6-ND rather than by the classical catecholamines dopamine, noradrenaline, and adrenaline. [ABSTRACT FROM AUTHOR]

Introduction: The human umbilical artery (HUA), rat-isolated right atrium, and rat-isolated vas deferens present a basal release of 6-nitrodopamine (6-ND). The basal release of 6-ND from these tissues was significantly decreased (but not abolished) when the tissues were pre-incubated with Nω-nitro-L-arginine methyl ester (L-NAME). Methods: In this study, the effect of the pharmacological modulation of the redox environment on the basal release of 6-ND was investigated. The basal release of 6-ND was measured using Liquid chromatography with tandem mass spectrometry (LC-MS/MS). Results and Discussion: Pre-incubation (30min) of the tissues with GKT137831 (1 µM) caused a significant increase in the basal release of 6-ND from all tissues. In the HUA, pre-incubation with diphenyleneiodonium (DPI) (100 µM) also caused significant increases in the basal release of 6-ND. Preincubation of the HUA with hydrogen peroxide (H2O2) (100 µM) increased 6-ND basal release, whereas preincubation with catalase (1,000 U/mL) significantly decreased it. Pre-incubation of the HUA with superoxide dismutase (SOD) (250 U/mL; 30min) also significantly increased the basal release of 6-ND. Preincubation of the HUA with either allopurinol (100 µM) or uric acid (1mM) had no effect on the basal release of 6-ND. Pre-treatment of the HUAwith L-NAME (100 µM) prevented the increase in the basal release of 6-ND induced by GKT137831, diphenyleneiodonium, and H2O2. The results obtained indicate amajor role of endogenous H2O2 and peroxidases as modulators of 6-ND biosynthesis/release and a lack of peroxynitrite contribution. [ABSTRACT FROM AUTHOR]