Comparative Cardiovascular Effects of Tyramine, Ephedrine, and Norepinephrine in Man.

• The observation that tissues depleted of norepinephrine no longer respond normally to certain sympathomimetic amines" led Burn and Rand to hypothesize that these amines exert their effect by releasing norepinephrine from storage sites in the postganglionic adrenergic nerve endings. On the basis of their effect on denervated or reserpine-pretreated tissues, the sympathomimetic amines have been classified into three groups: (1) those which have a direct action on receptor sites; (2) those which act indirectly by releasing norepinephrine; and (3) those with a mixed effect.