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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.

Mixed-action adrenergic agonists, like ephedrine and pseudoephedrine, directly and indirectly affect adrenergic receptors. These agents stimulate adrenoceptors and indirectly release stored neurotransmitters, amplifying the adrenergic response.

Ephedrine and pseudoephedrine lack a catecholamine group, making them less susceptible to degradation by metabolic enzymes. They have increased oral bioavailability and lipophilicity, resulting in a longer duration of action. Their response is reduced by prior treatment with catecholamine-depleting agents like guanethidine or reserpine.

Ephedrine, the first orally active sympathomimetic, is used for asthma and nasal congestion. However, its potential for secondary abuse and the availability of safer alternatives have limited its use. Pseudoephedrine is primarily used as a nasal decongestant but is strictly regulated due to its potential for the illicit synthesis of methamphetamine.

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Adrenergic Agonists Mixed action Agents Ephedrine Pseudoephedrine Adrenoceptors Neurotransmitters Oral Bioavailability Lipophilicity Duration Of Action Sympathomimetic Asthma Treatment Nasal Congestion Secondary Abuse Nasal Decongestant Methamphetamine Synthesis

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