6-NitroDopamine is an endogenous modulator of rat heart chronotropism

6-Nitrodopamine (6-ND) is released by rat vas deferens and exerts a potent contractile response that is antagonized by tricyclic antidepressants and α₁-, β₁- and β₁/β₂-adrenoceptor antagonists. The release of 6-ND, noradrenaline, adrenaline and dopamine from rat isolated right atria was assessed by tandem mass spectrometry. The effects of the catecholamines were evaluated in both rat isolated right atria and in anaesthetized rats. 6-ND was the major catecholamine released from the isolated atria and the release was significantly reduced in nitric oxide synthase inhibitor L-NAME pre-treated atria or in atria obtained from L-NAME chronically treated animals, but unaffected by tetrodotoxin. 6-ND (1 pM) significantly increased the atrial frequency, being 100 times more potent than noradrenaline and adrenaline. Selective β₁-blockers reduced the atrial frequency only at concentrations that prevented the increases in atrial frequency induced by 6-ND 1pM. Conversely, β₁-blockade did not affect dopamine (10 nM), noradrenaline (100 pM) or adrenaline (100 pM) effect. The reductions in atrial frequency induced by the β₁-adrenoceptor antagonists were absent in L-NAME pre-treated atria and in atria obtained from chronic L-NAME-treated animals. Tetrodotoxin did not prevent the reduction in atrial frequency induced by L-NAME or by β₁-blockers treated preparations. In anaesthetized rats, at 1 pmol/kg, only 6-ND caused a significant increase in heart rate. Inhibition of 6-ND synthesis by chronic L-NAME treatment reduced both atrial frequency and heart rate. The results indicate that 6-ND is a major modulator of rat heart chronotropism and the reduction in heart rate caused by β₁-blockers are due to selective blockade of 6-ND receptor.