6.7 : Adrenergic Agonists: Mixed-Action Agents

Mixed-acting adrenergic agonists act directly by stimulating adrenoceptors and indirectly by releasing stored catecholamines from the nerve terminals.

Unlike endogenous catecholamines, mixed-acting agonists lack a catechol moiety, making them poor substrates for metabolizing enzymes, increasing their duration of action.

Both ephedrine and pseudoephedrine exhibit excellent oral bioavailability and CNS penetration but are less potent than catecholamines.

Ephedrine—isolated from the Ephedra plant—has been used as an orally active sympathomimetic for treating asthma.

As ephedrine causes cardiac stimulation and vasoconstriction, it is used for treating anesthesia-induced hypotension. However, it is a less potent bronchodilator than isoprenaline or adrenaline.

Additionally, its central stimulatory effects are responsible for its abuse as a performance enhancer.

Pseudoephedrine—an ephedrine isomer with more indirect action—is orally effective and has fewer CNS effects. It is mainly used as a decongestant.

As it is commonly used as a precursor in illegal methamphetamine production, its sale is strictly regulated.