6-nitrodopamine is a novel catecholamine that is released from vascular tissues, such as human umbilical cord vessels, aortic rings from Chelonoidis carbonarius, Pantherophis guttatus and from Callithrix spp. 6-nitrodopamine (6-ND) has been previously identified as an endogenous substance with dopamine antagonistic properties. This study aims to identify its role in platelet aggregation.

Liquid chromatography couple to mass spectrometry was used to quantify catecholamines from washed platelet samples. Optical aggregometry was used to identify the role of 6-ND, 6-cyanodopamine, dopamine, known receptor subtype selective dopaminergic agonists and antagonists have in ADP, collagen and thrombin stimulated aggregation to identify any inhibitory or potentiator role of 6-ND. Aggregations were carried out in human plasma rich platelets (PRP) and washed platelets. All subjects gave written informed consent, and the Universidade Estadual de of Campinas (Unicamp) ethics committee approved the clinical protocol (Approval Number 00812918.0.0000.5404).

Platelets released 6-nitrodopamine and 6-cyanodopamine upon stimulation by thrombin (0.1 U; n = 15), collagen (1 μg/m; n = 15), U-46619 (3 μM; n = 13), ADP (30 μM; n = 13), dopamine (10 μM; n = 12), adrenaline (10 μM; n = 12). 6-nitroadrenaline, 6-bromodopamine and 6-nitrodopa were not detected (n = 12 for each group). 6-ND does not cause or inhibit platelet aggregation and does not affect ADP, collagen or epinephrine induced PRP aggregation or thrombin-induced washed platelet aggregation. Curiously, 6-ND potentiates the aggregation induced by U-46619. 10 μM dopamine is sufficient to potentiate ADP and collagen induced aggregation at subthreshold concentrations. 6-ND reduces dopamine potentiation of ADP and collagen induced aggregation. Haloperidol and the D3 receptor antagonists PG01037 and SB277011A reduce the potentiation induced by dopamine but not basal ADP and collagen potentiation. Sumanirole, a selective D2 agonist, potentiated aggregation which was blocked by 6-ND.

The data demonstrates that platelets released 6-nitrodopamine and a new catecholamine, 6-cyanodopamine. 6-ND acts as a D2/D3 receptor antagonist. Given that both dopamine and 6-ND are released by the vascular endothelium, 6-ND modulates platelet reactivity and may play a protective role against thrombosis involving vascular dysfunction.

Felipe Caliani Mathias-Netto (88887.849884/2023-00) thanks CAPES. Rafael Campos (2022/08232-7) and Gilberto De Nucci (2019/16805-4) thanks FAPESP.