Adrenergic Agonists

Writer: Mustafa A. Shahrori, MD

Keywords: Sympathomimetics, adrenergic receptors, α-receptors, β-receptors, vasopressors, bronchodilators, catecholamines, cardiovascular effects, sympathetic nervous system

Overview

Adrenergic agonists are drugs that mimic the effects of sympathetic nervous system stimulation by activating adrenergic receptors (α1, α2, β1, β2, β3) or dopamine receptors (D1, D2). These agents are classified as direct-acting (binding directly to receptors) or indirect-acting (increasing endogenous catecholamine availability). Their clinical applications span cardiovascular, pulmonary, and other systems based on their receptor selectivity profiles.

Adrenergic Receptor Classification and Neurotransmission

Adrenergic receptors (α₁, α₂, β₁, β₂, β₃) mediate effects like vasoconstriction, bronchodilation, and increased heart rate. The response to adrenergic agonists depends on receptor selectivity.

Adrenergic neurotransmission involves NE synthesis, storage (via VMAT), release, receptor binding, reuptake (via NET), and metabolism (by MAO and COMT).

For a detailed overview, refer to the article on Autonomic Receptors and Their Effects.

Direct sympathomimetics

Directly bind to and activate adrenergic receptors

Table 1: Direct-acting sympathomimetics: Receptor selectivity, Hemodynamic effects and applications

Abbreviations. HR: Heart rate, BP: Blood pressure, CO: Cardiac output, COPD: Chronic obstructive pulmonary disease, HF: Heart failure, HTN: Hypertension, D₁, D₂: Dopamine receptors, α, β: Alpha and beta adrenergic receptors

The distribution of adrenergic receptors varies significantly between tissues. This forms the basis for the therapeutic specificity of different sympathomimetic drugs. β₁-receptors are predominantly found in cardiac tissue, while β₂-receptors are predominantly in bronchial and vascular smooth muscle.

ALERT:

Epinephrine vs. Norepinephrine

●        Epinephrine: Stronger β2 effects, useful in anaphylaxis and cardiac arrest

●        Norepinephrine: Stronger α effects, better for maintaining blood pressure in septic shock

Dopamine dose-dependent effects

●        Low dose (1-5 μg/kg/min): D1 effects → renal vasodilation

●        Moderate dose (5-10 μg/kg/min): β1 effects → inotropic and chronotropic

●        High dose (>10 μg/kg/min): α effects → vasoconstriction

Albuterol vs. Salmeterol

●        Albuterol: Short-acting (4-6 hours), used for acute symptoms

●        Salmeterol: Long-acting (12+ hours), used for maintenance therapy

NOTE: Remember the reversal of epinephrine's pressor effect after α-blockade (becomes depressor due to unopposed β2-mediated vasodilation), while phenylephrine's pressor effect is only suppressed but not reversed after α-blockade (Figure 1).

Figure1. Effects of epinephrine on blood pressure in the presence of adrenergic Receptor antagonists. Two line graphs depict changes in blood pressure following epinephrine administration under different receptor blockade conditions.

Indirect Sympathomimetics

Increase synaptic norepinephrine by various mechanisms

Table 2: Indirect-acting sympathomimetics: Mechanisms and Applications

Abbreviations. ADHD: Attention-deficit hyperactivity disorder, CNS: Central nervous system, OTC: Over-the-counter, ↑: Increase, ↓: Decrease

ALERT: β-blockers should be avoided in patients with suspected cocaine intoxication as they can leave α1 effects unopposed, potentially worsening hypertension and causing coronary vasospasm.

NOTE: Cocaine causes mydriasis in eyes with intact sympathetic innervation, making it useful to confirm Horner syndrome diagnosis (lack of dilation indicates sympathetic denervation).

α2-Agonists (Sympatholytics)

Despite being agonists, these drugs reduce sympathetic output through central α2 receptor stimulation, reducing norepinephrine release and sympathetic outflow.

Table 3: Sympatholytics (α₂-Agonists): Applications and Adverse Effects

Abbreviations. ADHD: Attention-deficit hyperactivity disorder, CNS: Central nervous system, ⊕: Positive (e.g., Coombs ⊕ = Coombs test positive)

Prototype Drugs and Their Hemodynamic Profiles

1. Norepinephrine (α > β)

●        ↑↑ Mean arterial pressure (MAP)

●        ↑ Systolic and diastolic BP

●        Reflex bradycardia (baroreceptor response)

●        Minimal ↑ in cardiac output

2. Epinephrine (β > α)

●        ↑ Cardiac output

●        ↑ Heart rate

●        ↑ MAP (moderate)

●        ↑ Pulse pressure

●        Low dose: β effects predominate → possible ↓ diastolic BP

●        High dose: α effects become significant → ↑ diastolic BP

3. Isoproterenol (β1 = β2)

●        ↑↑ Heart rate

●        ↑↑ Cardiac output

●        ↓ Peripheral resistance (β2 effect)

●        ↑ Pulse pressure

●        MAP may remain unchanged or slightly ↑

Table 4: Comparing hemodynamic profiles of key sympathomimetics

Abbreviations. BP: Blood pressure, MAP: Mean arterial pressure, SVR: Systemic vascular resistance, ↔: No significant change, ↑ / ↓: Increase or decrease (variable effect depending on dose or reflex responses)

Adverse Effects

Table 5: Adverse effects of sympathomimetics by system

Abbreviations. CNS: Central Nervous System, α₁: Alpha-1 adrenergic receptor, Non-selective agents: Drugs that stimulate multiple adrenergic receptor subtypes (e.g., β₁ and β₂ or α and β), Mydriasis: Pupil dilation, Ileus: Temporary paralysis of a portion of the intestine

References

Whalen, K. (2022). Lippincott illustrated reviews: Pharmacology (8th ed.). Wolters Kluwer.