Pharmacological analysis of dopamine action on the isolated dog atrium

The isolated right atrium of the dog was perfused with arterial blood introduced from a carotid artery of a support dog. The selective injection of dopamine, tyramine and norepinephrine into the cannulated sinus node artery induced dose-relatedly positive chronotropic and inotropic effects. However, for an equal increase in sinus rate, dopamine caused less increase in tension development than norepinephrine. Tyramine caused least increase in contractility. Effects induced by dopamine were not blocked by treatment with tetrodotoxin which blocked those induced by nicotine. Desmethylimipramine treatment significantly suppressed dopamine-induced effects and completely blocked tyramine-induced ones but rather enhanced norepinephrine-induced ones. Alprenolol inhibited effects of dopamine, tyramine and norepinephrine. From these results, it is concluded that positive chronotropic and inotropic effects of dopamine are partly due to tyramine-like effect which causes the release of norepinephrine from sympathetic storage sites.