Peripherally Acting Alpha Agonists

Phenylephrine is a strong α1 agonist, which results in vasoconstriction and increased peripheral vascular resistance.  Phenylephrine is used systemically to treat septic, distributive, and neurogenic shock.  Phenylephrine is also used in local applications such as the treatment of priapism and as a nasal decongestant.

Oxymetazoline is a mixed α1 and α2 agonist. Oxymetazoline is used topically to treat nasal congestion and to reduce eye redness by causing constriction of local blood vessels.  

Class Drugs

• Phenylephrine

• Oxymetazoline

Class Mechanism of Action

• Alpha agonists (See Individual Drug)

Class Applications

• See Individual Drug

Adverse Effects

• See Individual Drug

Drug Table

Drug Name	Application	Receptor	Adverse Effects;	Image
Phenylephrine	Shock; Nasal congestion; Priapism	α1	Reflex Bradycardia	"Fennel" and "F"
Oxymetazoline	Nasal congestion; Eye redness	α1; α2	Headache; Nervousness; Rebound congestion with long-term use (>3 days)	"Ox" "Mets" "Line"

Physiology Review

Direct-Acting Adrenergic Agonists directly stimulate adrenergic receptors (α and β).  α and β receptors are found in the sympathetic nervous system.  The following table is a reminder of the effects of stimulation of α receptors.

Alpha Adrenoreceptors

Receptor Type	Physiologic Response	Signal Transduction
α1	GI Relaxation; Mydriasis (pupillary dilatation); Vasoconstriction;      - ↑ PVR;      - ↑ BP; ↑  Bladder sphincter tone	Gq, activated. Results in IP3 (Calcium influx) and DAG (PKC activation) release
α2	↓  Norepinephrine release; ↓  Insulin release; ↓  Lipolysis; ↓  Aqueous humor production; ↓  ACh release	Gi, g-protein linked receptor, which inhibits adenylyl cyclase, which decreases cAMP levels