Pharmacology · Adrenergic Pharmacology

Synthetic Adrenergic Agonists

Phenylephrine · Alpha-2 Agonists · Beta-2 Bronchodilators · Tocolytics

Phenylephrine is a pure alpha-1 agonist with no beta activity — it raises blood pressure through vasoconstriction and triggers a baroreceptor-mediated reflex bradycardia; it is the preferred vasopressor for spinal anesthesia hypotension and for hypertrophic obstructive cardiomyopathy (where tachycardia is dangerous); oral formulations have extensive first-pass metabolism and are ineffective decongestants.
Clonidine and dexmedetomidine are central alpha-2 agonists that reduce sympathetic outflow from the locus coeruleus — this single mechanism explains both their antihypertensive and sedative effects; dexmedetomidine's advantage in the ICU is cooperative, arousable sedation without respiratory depression; abrupt withdrawal of either agent causes rebound hypertensive crisis.
Midodrine (prodrug → desglymidodrine, an alpha-1 agonist) combined with fludrocortisone (volume expansion) treats orthostatic hypotension by two complementary mechanisms; midodrine must not be taken within 4 hours of bedtime to avoid supine hypertension.
Terbutaline relaxes the uterus via beta-2 → cAMP → protein kinase A → myosin light-chain kinase inhibition; it is used for acute short-term tocolysis (24–34 weeks) to allow time for corticosteroid-induced fetal lung maturation; the FDA black box prohibits prolonged injectable use beyond 48–72 hours due to maternal cardiac toxicity.
Short-acting beta-2 agonists (albuterol) are rescue agents only — use more than twice weekly signals inadequate control and mandates adding an inhaled corticosteroid; long-acting beta-2 agonists (salmeterol, formoterol) must never be used without an inhaled corticosteroid in asthma (SMART trial showed increased asthma-related mortality with salmeterol monotherapy).

Drug	Class	Onset	Duration / Role	Key Rule
Albuterol	Short-acting	5–15 min	4–6 hrs — rescue only	Use >2×/week = inadequate control; add controller therapy. Monitor potassium with high-dose nebulized use.
Salmeterol	Long-acting	15–20 min	12 hrs — maintenance only, NOT rescue	Never without inhaled corticosteroid in asthma (SMART trial: increased asthma-related death).
Formoterol	Long-acting	1–3 min	12 hrs — maintenance + rescue protocols	Never without inhaled corticosteroid in asthma. Rapid onset allows dual rescue/maintenance use.

Suggested References

Author / Organization	Title	Source
Katzung BG (ed)	Basic and Clinical Pharmacology, 15th ed. Chapter 9: Adrenoceptor-Activating and Other Sympathomimetic Drugs	McGraw-Hill, 2021
Brunton LL, Knollmann BC (eds)	Goodman and Gilman's The Pharmacological Basis of Therapeutics, 14th ed. Chapter 9: Adrenergic Agonists and Antagonists	McGraw-Hill, 2023
Westfall TC, Westfall DP	Adrenergic agonists and antagonists. In: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 13th ed.	McGraw-Hill, 2018:191–224
Heesen M, Kolhr S, Rossaint R, Straube S	Prophylactic phenylephrine for caesarean section under spinal anaesthesia: systematic review and meta-analysis	Anaesthesia. 2014;69(2):143–165
Kollar C, Schneider H, Waksman J, Krusinska E	Meta-analysis of the efficacy of a single oral dose of phenylephrine 10 mg compared with placebo in adults with acute nasal congestion due to the common cold	Clin Ther. 2007;29(6):1057–1070
Kamibayashi T, Maze M	Clinical uses of alpha2-adrenergic agonists	Anesthesiology. 2000;93(5):1345–1349
Giovannitti JA Jr, Thoms SM, Crawford JJ	Alpha-2 adrenergic receptor agonists: a review of current clinical applications	Anesth Prog. 2015;62(1):31–39
Pandharipande PP, Patel MB, Barr J	Management of pain, agitation, and delirium in critically ill patients	Pol Arch Med Wewn. 2014;124(3):114–123
Low PA, Gilden JL, Freeman R, et al	Efficacy of midodrine vs placebo in neurogenic orthostatic hypotension: a randomized, double-blind multicenter study	JAMA. 1997;277(13):1046–1051
Lahrmann H, Cortelli P, Hilz M, et al	EFNS guidelines on the diagnosis and management of orthostatic hypotension	Eur J Neurol. 2006;13(9):930–936
Johnson M	Molecular mechanisms of beta2-adrenergic receptor function, response, and regulation	J Allergy Clin Immunol. 2006;117(1):18–24
Global Initiative for Asthma (GINA)	Global Strategy for Asthma Management and Prevention	Updated 2024. ginasthma.org
Nelson HS, Weiss ST, Bleecker ER, et al; SMART Study Group	The Salmeterol Multicenter Asthma Research Trial: a comparison of usual pharmacotherapy for asthma or usual pharmacotherapy plus salmeterol	Chest. 2006;129(1):15–26
Gyetvai K, Hannah ME, Hodnett ED, Ohlsson A	Tocolytics for preterm labor: a systematic review	Obstet Gynecol. 1999;94(5 Pt 2):869–884
American College of Obstetricians and Gynecologists	Practice Bulletin No. 171: Management of preterm labor	Obstet Gynecol. 2016;128(4):e155–e164
Salpeter SR, Ormiston TM, Salpeter EE	Cardioselective beta-blockers for chronic obstructive pulmonary disease	Cochrane Database Syst Rev. 2005;(4):CD003566
Zimetbaum P	Antiarrhythmic drug therapy for atrial fibrillation	Circulation. 2012;125(2):381–389