Direct Acting Catecholamines
Direct acting catecholamines include epinephrine, norepinephrine, dobutamine, and phenylephrine. 1
Classification and Mechanism
Direct acting catecholamines work by directly stimulating adrenergic receptors (α, α2, β1, β2, and β3) without requiring intermediate steps or endogenous catecholamine release. 1 This distinguishes them from indirect-acting agents (which release stored norepinephrine) and mixed-acting agents (which combine both mechanisms). 1
Non-Selective Direct Acting Agents
These agents stimulate multiple receptor types:
Epinephrine: Acts on α, β1, and β2 receptors, producing vasoconstriction, increased inotropy, increased heart rate, and bronchodilation. 1 Epinephrine is a sympathomimetic catecholamine with the chemical designation 4-[1-hydroxy-2(methylamino)ethyl]-1,2 benzenediol. 2
Norepinephrine: Primarily stimulates α and β1 receptors, causing vasoconstriction and increased cardiac contractility. 1 It is the first-line vasopressor in most shock states. 1
Dopamine: Stimulates dopaminergic, β, and α receptors in a dose-dependent manner, affecting renal/mesenteric vasodilation at low doses, cardiac contractility at intermediate doses, and peripheral vasoconstriction at high doses. 1, 3, 4
Selective Direct Acting Agents
These agents target specific receptor subtypes:
Dobutamine: Selective β1 agonist that increases myocardial contractility and is the first-line inotrope when myocardial function is depressed. 1
Phenylephrine: Selective α agonist that produces pure vasoconstriction without direct cardiac effects. 1
Clinical Significance
Only direct acting catecholamines have a role in shock management. 1 Indirect and mixed-acting agents are not recommended for critical care applications because they rely on adequate endogenous catecholamine stores, which are often depleted in shock states. 1
Important Distinction
Catecholamines are released endogenously from sympathetic nerve terminals (norepinephrine) and the adrenal medulla (epinephrine and dopamine) in response to stress, injury, and circulatory failure. 4 However, when used therapeutically in shock, only the synthetic direct acting formulations are appropriate because they bypass depleted endogenous stores and directly stimulate receptors to restore cardiovascular homeostasis. 1, 5