6-Cyanodopamine as an endogenous modulator of heart chronotropism and inotropism

Rat isolated atria and ventricles release endothelium-derived 6-nitrodopamine, which induces potent positive chronotropic and inotropic responses. 6-Cyanodopamine is released from rabbit isolated atria and ventricles; however, it is not known whether this novel catecholamine has any action on the isolated heart. Therefore, it was investigated whether rat isolated ventricles release 6-cyanodopamine and its action on the rat isolated heart. Basal release of 6-cyanodopamine was assessed by LC-MS/MS method. Tyrosine hydroxylase expression was measured by both immunohistochemistry and fluorescence in situ hybridization (FISH). Chronotropic and inotropic effects were evaluated in isolated atria and Langendorff's preparation, respectively. Rat isolated ventricles presented basal release of 6-cyanodopamine, which was unaffected by pre-treatment with the voltage-gated sodium channel blocker tetrodotoxin. Immunohistochemistry and FISH assays identified tyrosine hydroxylase expression in both endothelium and cardiomyocytes. 6-Cyanodopamine at 10 and 100 pM significantly increased the atrial rate, which was maintained even at 30 min after washing the preparation. In the Langendorff's preparation, 1-min infusion of 6-cyanodopamine (10 and 100 pM) significantly increased heart frequency, left ventricular developed pressure (LVDP), and maximal rate of rise of the left ventricular pressure (dP/dt_max). Bolus injection of noradrenaline at 1 pmol had no effect on any of these parameters; however, in the presence of 6-cyanodopamine (0.01 pM), noradrenaline (1 pmol) significantly increased heart frequency, LVDP, and dP/dt(max). The results indicate that 6-cyanodopamine is a potent endogenous catecholamine mediating both chronotropism and inotropism in the rat isolated heart. 6-Cyanodopamine may have potential therapeutic effect in heart failure and may be useful as a biomarker of cardio-renal diseases.