Adrenergic agonists compared — receptors, hemodynamics and uses
Verified · Oct 2026Which receptors an agonist reaches decides what happens to heart rate, resistance and cardiac output: α1 alone raises pressure and slows the heart reflexly, β1 drives rate and contractility, β2 dilates vessels, and dopamine moves from dopamine receptors to β1 to α1 as its rate rises. Clonidine and the β3 agonists use the same receptor family for quite different jobs.
Receptor reach predicts the hemodynamics
α1 receptors are peripheral vasoconstrictors that raise systemic vascular resistance. β1 receptors raise contractility and heart rate. β2 receptors relax smooth muscle, dilating vessels and bronchi. Dopamine’s D1 and D2 receptors dilate the renal, mesenteric and coronary vessels. These drugs bind more receptor types at higher doses, so they can lose selectivity.
Phenylephrine is a pure α1 agonist, and midodrine works through desglymidodrine, an α1 agonist that does not stimulate cardiac β receptors. Norepinephrine has slightly more α1 than β activity; epinephrine acts on α1 and β receptors about equally; dobutamine favours β1 over β2 over α; isoproterenol is a nonselective β agonist with very low α affinity. Ephedrine acts directly on α and β receptors and also releases norepinephrine from sympathetic neurons. Outside the pressor group, clonidine stimulates central α2 receptors and reduces sympathetic outflow, and mirabegron and vibegron stimulate β3 receptors to relax the detrusor.
Comparison
Receptors and hemodynamics
| Drug | Receptors | Heart rate | Vascular resistance and blood pressure | Cardiac output |
|---|---|---|---|---|
| Norepinephrine | α1 slightly more than β; β1 more than β2 | Reflex vagal slowing | Resistance and pressure rise — more blood-pressure effect than heart-rate effect | Generally unaffected; can fall |
| Epinephrine | α1 and β about equally | Rises | Low doses: β2 vasodilation lowers resistance and diastolic pressure; higher doses: α1 vasoconstriction raises diastolic pressure | Rises |
| Phenylephrine | Pure α1 | Reflex bradycardia | Systolic, diastolic and mean pressure and peripheral resistance rise | Mixed; can fall, because afterload rises more than preload |
| Midodrine | α1 (through desglymidodrine); no cardiac β stimulation | No clinically significant change in autonomic failure | Standing, sitting and supine pressure rise | — |
| Dopamine | D1/D2, then β1, then α1 as the rate rises | Intermediate rates: β1 raises rate and contractility | Low rates: renal, mesenteric and coronary vasodilation; high rates: α1 vasoconstriction and higher pressure | — |
| Dobutamine | β1 > β2 > α | Usually no marked rise (tachycardia occasionally) | Resistance usually falls; little or no change in blood pressure | Rises, mainly through stroke volume |
| Isoproterenol | Nonselective β1 + β2; very low α affinity | Rises | Resistance and diastolic pressure fall; mean pressure typically falls | Rises |
| Ephedrine | Direct α and β agonist; also releases norepinephrine from sympathetic neurons | Rises | Resistance rises variably; blood pressure usually rises | Rises |
| Clonidine | Central α2 (and imidazoline) agonist | Falls | Less sympathetic outflow: peripheral resistance and blood pressure fall | — |
| Mirabegron, vibegron | β3 (detrusor); mirabegron has very low intrinsic activity at β1 and β2 | Mirabegron: dose-dependent rise | Mirabegron can raise blood pressure; vibegron showed no clinically significant change in an ambulatory study | — |
Two non-adrenergic comparators sit on the vasopressors page: vasopressin constricts vessels through V1 receptors, and milrinone inhibits PDE3 to raise cAMP — an inotrope and vasodilator that is not a β agonist.
— = no statement for that cell on the class pages
Choosing by situation
| Situation | Choice | Why |
|---|---|---|
| Septic shock | Norepinephrine; add vasopressin if refractory | Norepinephrine is the first choice of vasopressor per StatPearls; vasopressin raises resistance and mean pressure and lowers the norepinephrine requirement |
| Low perfusion despite norepinephrine; cardiogenic shock | Add dobutamine | It raises contractility and cardiac output with minimal effect on blood pressure; it is indicated for cardiogenic shock and heart failure |
| Anaphylaxis; cardiac arrest | Epinephrine | Its α action lessens the vasodilation and vascular leak of anaphylaxis, and its β action relaxes bronchial smooth muscle |
| Hypotension under anesthesia | Phenylephrine or ephedrine | Both are labelled for clinically important hypotension in the setting of anesthesia; phenylephrine slows the heart reflexly, while ephedrine raises heart rate and cardiac output |
| Bradydysrhythmia; electrophysiology study | Isoproterenol | Nonselective β agonism raises the rate; its use has declined with pacemakers |
| Symptomatic orthostatic hypotension | Midodrine (oral) | Its metabolite is an α1 agonist; because supine blood pressure can rise markedly, it is reserved for patients whose lives are considerably impaired despite standard care |
| Hypertension | Clonidine | Central α2 agonism lowers sympathetic outflow, heart rate and blood pressure; the transdermal system is indicated in the treatment of hypertension |
| Overactive bladder | Mirabegron or vibegron | β3 agonism relaxes the detrusor during filling and increases bladder capacity; both are labelled for OAB with urge incontinence, urgency and frequency |
Signature adverse effects
| Drug | Effect | Mechanism or management |
|---|---|---|
| Dobutamine, isoproterenol, epinephrine | Tachycardia, palpitations and tachyarrhythmias; tremor and anxiety | β1 stimulation drives rate and arrhythmias; β2 agonists cause tremor; isoproterenol also raises myocardial oxygen demand |
| Epinephrine | Hyperglycemia and raised lactate | More glycogenolysis, less glucose uptake by tissues and inhibited insulin release; isoproterenol causes less hyperglycemia |
| Phenylephrine, norepinephrine | Reflex bradycardia | The rise in pressure increases vagal activity |
| Dopamine | Gangrenous extremities and arrhythmias in excess; necrosis after extravasation | α1 vasoconstriction; stop the infusion (short half-life); phentolamine reverses extravasation vasoconstriction |
| Midodrine | Supine hypertension (boxed); scalp tingling, goosebumps, chills; urinary urgency and retention | α1 receptors on vessels, hair follicles and the bladder neck; last dose 3 to 4 hours before bedtime, head of bed raised |
| Ephedrine | Tachyphylaxis with repeated doses; serious postpartum hypertension with oxytocics | Repeated doses can lose effect; monitor blood pressure when it is given with an oxytocic |
| Clonidine | Rebound on abrupt withdrawal: nervousness, agitation, headache, tremor and a rapid rise in blood pressure | More likely after higher doses or with a continued β-blocker, which leaves α-mediated vasoconstriction unopposed |
| Mirabegron | Blood-pressure rise; urinary retention with outlet obstruction or a muscarinic antagonist | Periodic blood-pressure checks; not recommended in severe uncontrolled hypertension |
High-yield
Practice questions
Q1A 66-year-old man in the ICU is on a dopamine infusion. As the rate is increased over several hours, his blood pressure climbs and his fingers and toes become cold, pale and mottled.
Activation of which receptor best explains these changes at the higher rates?
- A.D1 receptors in the renal and mesenteric vessels
- B.β2 receptors on vascular smooth muscle
- C.β1 receptors in the heart
- D.α1 receptors on vascular smooth muscle
- E.V1 receptors on vascular smooth muscle
Show answer
D. α1 receptors on vascular smooth muscle — Dopamine’s receptor effects depend on its dose: at low rates it activates D1 and D2 receptors in the renal, mesenteric and coronary vessels, causing vasodilation; at intermediate rates β1 receptors raise heart rate and contractility; at high rates α1 receptors cause vasoconstriction and a rise in blood pressure. Excess dopamine can cause peripheral vasoconstriction severe enough to produce gangrenous extremities. Dilating vessels is what β2 receptors do, and V1 receptors are vasopressin’s, not dopamine’s.
Q2A 59-year-old woman in septic shock has received adequate fluids and is on norepinephrine, and her mean arterial pressure is now at goal. Echocardiography shows poor ventricular function, her cardiac output is low, and her skin stays cool and mottled as her lactate rises.
Which is the best addition?
- A.Phenylephrine
- B.Dobutamine
- C.Vasopressin
- D.A higher norepinephrine rate
- E.Midodrine
Show answer
B. Dobutamine — Norepinephrine is commonly used to reach the arterial-pressure goal in shock; if tissue and organ perfusion stays low, dobutamine can be added to increase cardiac output, raising contractility with minimal effect on blood pressure. More norepinephrine adds pressure rather than output, since its effect on cardiac output is generally neutral and can be a fall, and phenylephrine raises afterload more than preload and can reduce output. Vasopressin, added when hypotension is refractory, tends to decrease heart rate and cardiac output, and midodrine is an oral drug for symptomatic orthostatic hypotension.
Q3A 70-year-old man becomes hypotensive after spinal anesthesia for hip surgery. He is given three IV boluses of ephedrine, and each raises his blood pressure less than the one before.
What is the best next step?
- A.Give a larger bolus of ephedrine
- B.Add IV phentolamine
- C.Start oral midodrine
- D.Start an isoproterenol infusion
- E.Switch to a direct-acting α1 agonist such as phenylephrine
Show answer
E. Switch to a direct-acting α1 agonist such as phenylephrine — Repeated ephedrine doses can cause tachyphylaxis, and its label advises having an alternative pressor ready. Phenylephrine, a pure α1 agonist, is labelled for clinically important hypotension resulting from vasodilation in the setting of anesthesia. Adrenergic antagonists work against ephedrine: β-blockers and α-blockers such as phentolamine antagonize its pressor effect. Isoproterenol typically lowers mean arterial pressure, and midodrine is an oral drug for symptomatic orthostatic hypotension.
Q4A 2-year-old girl is brought in an hour after chewing her grandfather’s clonidine patch. At first her blood pressure is high; over the next hours she becomes drowsy, with miosis, bradycardia and a falling blood pressure.
What best explains the early rise in blood pressure?
- A.Peripheral α2 stimulation causing vasoconstriction
- B.Central α2 agonism reducing sympathetic outflow
- C.Release of stored norepinephrine from sympathetic nerve endings
- D.β1 stimulation raising cardiac output
- E.V1 receptor stimulation on vascular smooth muscle
Show answer
A. Peripheral α2 stimulation causing vasoconstriction — In clonidine overdose, cardiovascular effects often begin with early hypertension, likely due to initial peripheral α2 stimulation causing vasoconstriction, and progress to hypotension and bradycardia, with drowsiness, miosis and respiratory depression. Central α2 agonism in the brain stem is what reduces sympathetic outflow, heart rate and blood pressure — the later phase. Releasing norepinephrine from sympathetic nerves is ephedrine’s indirect action, clonidine slows the heart rather than stimulating it, and V1 receptors are vasopressin’s.
Sources
- Adrenergic Drugs — receptors, selectivity, adverse effects — StatPearls (NCBI)
- Inotropes and Vasopressors — StatPearls (NCBI)
- Isoproterenol — StatPearls (NCBI)
- Dobutamine — StatPearls (NCBI)
- Dopamine — StatPearls (NCBI)
- Phenylephrine — StatPearls (NCBI)
- Shock — vasopressor choice — StatPearls (NCBI)
- LEVOPHED (norepinephrine) — FDA label — FDA / DailyMed
- ADRENALIN (epinephrine) — FDA label — FDA / DailyMed
- Dopamine HCl Injection (Hospira) — FDA label — FDA / DailyMed
- Dobutamine Injection (Hospira) — FDA label — FDA / DailyMed
- Phenylephrine HCl Injection (Dr. Reddy’s) — FDA label — FDA / DailyMed
- VASOSTRICT (vasopressin) — FDA label — FDA / DailyMed
- Milrinone Lactate Injection (Hospira) — FDA label — FDA / DailyMed
- Isoproterenol HCl Injection (Amphastar) — FDA label — FDA / DailyMed
- Midodrine HCl Tablets (Apotex) — FDA label — FDA / DailyMed
- AKOVAZ (ephedrine sulfate injection) — FDA label — FDA / DailyMed
- Clonidine — StatPearls (NCBI)
- CATAPRES-TTS (clonidine transdermal) — FDA label — FDA / DailyMed
- MYRBETRIQ (mirabegron) — FDA label — FDA / DailyMed
- GEMTESA (vibegron) — FDA label — FDA / DailyMed
- Emerphed (ephedrine sulfate) — FDA label — FDA / DailyMed
- Current pharmacotherapy of overactive bladder (Int Braz J Urol 2021) — selectivity, tertiary vs quaternary, cognition, β3 agonists — PubMed Central (CC BY 4.0)
- Physiology, Vasopressin — StatPearls (NCBI)
- Milrinone — PDE3, lusitropy, vasodilation, adverse effects — StatPearls (NCBI)
- Detrol (tolterodine) — 5-hydroxymethyl metabolite, CYP2D6 poor metabolizers, QT (FDA label) — FDA / DailyMed
- Solifenacin Succinate Tablets — FDA label — FDA / DailyMed
- Darifenacin Extended-Release Tablets (Torrent) — M3 selectivity, CYP2D6 (FDA label) — FDA / DailyMed
- Trospium Chloride Tablets (Glenmark) — quaternary ammonium, absorption, tubular secretion (FDA label) — FDA / DailyMed
- Toviaz (fesoterodine) — esterase activation to 5-HMT, CYP2D6 (FDA label) — FDA / DailyMed
Each class page compared lists its full sources; this page reuses facts verified there.
By LearnDrugs
Educational summary for medical students. Always verify against current prescribing information and guidelines. Not medical advice.