Comparative Study of Chronotropic Effects of Catecholamines and Tyramine on the Sa Node of the Dog Heart in Situ

The relative potency of chronotropic effects of dopamine (DA), l-norepinephrine (NE), l-epinephrine (EP), l-isoproterenol (ISO) and tyramine (TY) was determined by direct application into the sinus node artery of the in situ canine heart (n = 10), which was treated with atropine. The order of potencies of chronotropic effect of these amines was ISO>NE=EP>DA≧TY. The ratio of doses required to produce 50% increase in sinus rate was ISO: NE: EP: DA: TY = 1/30: 1: 1: 10: 10. At any dose level for inducing the same grade of sinus acceleration, the duration of action of NE and EP was almost equal and short, while that of ISO was usually longer. The duration of action of DA was short at a lower dose level but became equal to ISO with a medium dose. At a larger dose to produce a maximum response, DA acted longer than ISO, which indicates direct effect of DA with induction of indirect effect of catecholamine release. This suggest a tyramine-like action of DA.