In vitro effect of 6-hydroxydopamine on isolated rat atria

In vitro treatment with 6-hydroxydopamine (6-OHDA), but not with oxidized 6-hydroxydopamine, caused transient, positive inotropic and chronotropic responses in the isolated rat atria. In the presence of rat blood plasma, the excitatory effect of 6-hydroxydopamine persisted unless the drug was washed out by fresh medium. 6-Hydroxydopamine failed to elicit the excitative response in the atria obtained from reserpinized rats or rats treated with 6-hydroxydopamine in vivo. In vitro treatment with 6-hydroxydopamine did not cause the development of supersensitivity to noradrenaline. Cocaine and desipramine, but not bretylium, inhibited the excitative action of 6-hydroxydopamine. No tachyphylaxis developed after repeated exposure to 6-hydroxydopamine in vitro. Propranolol, but not phentolamine, blocked the excitatory effect of 6-hydroxydopamine. It is thus concluded that in vitro treatment with 6-hydroxydopamine may cause atrial stimulation by an indirect action involving the release of catecholamines as a result of its displacement at the nerve ending.