Figure 16-15 Location of dopamine-1 (DA₁ ) receptors, α₁ - and α₂ -adrenoceptors on postganglionic vascular effector cells, and DA₂ receptors and α₂ -adrenoceptors on the prejunctional sympathetic nerve terminal. When dopamine is administered, activation of DA₁ receptors causes vasodilation, whereas activation of DA₂ receptors causes inhibition (-) of norepinephrine (NE) release from storage granules. A larger dose of dopamine activates α₁ - and α₂ -adrenoceptors on the postjunctional effector cells to cause vasoconstriction and on α₂ -adrenoceptors on the prejunctional sympathetic terminal to inhibit release of NE. NE released from the prejunctional sympathetic terminal also acts on α₁ - and α₂ -adrenoceptors. (From Goldberg LI, Rajfer SI: Dopamine receptors: Applications in clinical cardiology. Circulation 72:245, 1985.)