Adrenergic Agonists
Key Concepts
1. Adrenergic agonists activate the sympathetic nervous system and induce symptoms of the
fight-or-flight response.
2. The pharmacology of adrenergic drugs is more complex than that of their cholinergic
counterparts, due to the existence of the two receptor subtypes alpha and beta.
3. Drugs in this class are norepinephrine, epinephrine, and dopamine.
4. The most important action of adrenergic agonists is on the cardiovascular system: They
increase the heart rate and cardiac output.
5. They also impact the respiratory system by relaxing the bronchi and causing
bronchodilation. They impact the gastrointestinal system by slowing peristalsis and
causing constipation. They also cause urinary retention.
6. The therapeutic effect of these drugs may become an adverse effect if taken to the
extreme: They may raise blood pressure too much or excessively dry out the mucosa.
7. Adrenergic agonists may act directly by binding to adrenergic receptors (e.g.,
epinephrine, norepinephrine [NE], and dopamine).
8. Some adrenergic agonists e.g., ephedrine, amphetamine, imipramine [Tofranil], cocaine,
and phenelzine [Nardil]) act indirectly by increasing the amount of NE available at
adrenergic synapses by three means: stimulating release of NE, inhibiting reuptake of
NE, or inhibiting destruction of NE. (. A few drugs, such as ephedrine, act by both direct
and indirect means.
9. Adrenergic agonists may be classified as catecholamines or noncatecholamines.
10. Catecholamines contain a catechol and an amino group (e.g., norepinephrine, dopamine,
isoproterenol, and dobutamine).
11. Noncatecholamines include phenylephrine, terbutaline, and ephedrine. They are rapidly
destroyed by the enzymes MAO and COMT. Therefore, they have a short duration of
action.
12. Catecholamines cannot be given orally but must be administered parenterally or by
inhalation. They also cannot cross the blood–brain barrier. They are rapidly destroyed by
MAO and COMT; therefore, they have a shorter duration of action.
13. Noncatecholamines can be given orally. They do cross the blood–brain barrier. They are
not as rapidly destroyed by MAO and COMT; therefore, they have a longer duration of
action.
14. The nonselective adrenergic agonists activate both alpha and beta receptors. The specific
effects of each drug are dependent on which receptor subtypes are stimulated.
15. The first subtype is the alpha1-receptor agonists. They are used for the treatment of nasal
congestion and hypotension. They may also be used to produce mydriasis during
ophthalmic examinations.
16. The second subtype is the alpha2-receptor agonists. They are used for the treatment of
hypertension.
17. The third subtype is the beta1-receptor agonists. They are used for the treatment of
cardiac arrest, heart failure, and shock.
18. The fourth subtype is the beta2-receptor agonists. They are used for the treatment of
asthma and to reduce preterm labor.
19. Some sympathomimetics are nonselective, stimulating two or more adrenergic-receptor
subtypes (e.g., epinephrine and ephedrine). The nonselective adrenergic agonists
generally cause more autonomic-related side effects than the selective agents.
20. One of the most important applications of the nonselective adrenergic agonists is for the
pharmacotherapy of shock and other life-threatening cardiac disorders.
21. Epinephrine, norepinephrine, and dopamine are nonselective agents for shock and
anaphylaxis when blood pressure and heart rate need to be quickly restored to normal
levels.
22. Epinephrine is the prototype drug for nonselective adrenergic agonists. It is used to treat
shock and anaphylaxis and as a bronchodilator.
23. The most common adverse effects of epinephrine are nervousness, tremors, palpitations,
tachycardia, dizziness, headache, and stinging at the site of application. Serious adverse
effects include HTN, dysrhythmias, pulmonary edema, and cardiac arrest.
24. Drugs similar to epinephrine include dopamine, droxidopa, ephedrine, and NE.
25. Activation of alpha-adrenergic receptors causes a number of important physiologic
responses, most of which relate to contraction of vascular smooth muscle.
26. Alpha-adrenergic agonists are used for treatment of hypotension associated with shock
and for treatment of orthostatic hypotension.
27. Another major action of alpha-adrenergic agonists is vasoconstriction of vessels in the
nasal mucosa. These agents are used for upper respiratory tract infection or allergic
rhinitis. An intranasal agent is phenylephrine. An oral agent is phenylephedrine.
28. A third action of alpha1-adrenergic agonists is vasoconstriction of arterioles in the eye
and mydriasis. These agents are topically instilled into the eye and relieve conjunctival
congestion and redness. They are also used for ophthalmic examination but are not the
drug of choice for this purpose.
29. Activation of alpha2 receptors produces important responses in the brain rather than the
peripheral nervous system.
30. The prototype alpha-adrenergic agonist is phenylephrine (Neo-Synephrine). Side effects
are few and minor in character.
31. Drugs similar to phenylephrine are classified as intranasal decongestants or ocular
decongestants.
32. Activation of beta1 receptors results in cardiac actions typical of the fight-or-flight
response: Heart rate, force of contraction, and velocity of impulse conduction through the
myocardium all increase.
33. Drugs with significant beta1 activity may be called cardiotonic or inotropic drugs.
34. Beta2-adrenergic receptors are more widely distributed than beta1 receptors.
Pharmacologically, the most important site is in the lung.
35. Activation of beta2 receptors leads to relaxation of bronchial smooth muscle. Beta2adrenergic agonists are commonly referred to as bronchodilators and are used for
treatment of bronchial asthma and other pulmonary disorders.
36. Beta2-adrenergic agonists are also used for the treatment of preterm labor contractions.
These drugs are called tocolytics.
37. Isoproterenol (Isuprel) is the prototype nonselective beta-adrenergic agonist and is used
as a bronchodilator and as a cardiac stimulator.
38. Common adverse effects of isoproterenol include headache, nausea, vomiting, and
symptoms of CNS stimulation such as tremors, anxiety, and insomnia. Adverse effects on
the heart include serious dysrhythmias.
39. Drugs similar to isoproterenol (Isuprel) are grouped into two primary subclasses:
bronchodilators and tocolytics. Tocolytics are used for the treatment of preterm labor
contractions.
40. Adrenergic agonist therapy requires continued frequent and careful monitoring of vital
signs and urinary and cardiac output as appropriate, especially if IV administration is
used.
41. Instruct the patient to report palpitations, shortness of breath, chest pain, excessive
nervousness or tremors, headache, or urinary retention immediately.
42. Caffeine intake should be limited or eliminated to reduce nervousness, insomnia, and
tremors.
43. IV adrenergic drips may cause intense vasoconstriction if extravasated.
44. Patients who have diabetes may experience increased blood glucose.
45. Photosensitivity may occur.
46. Nasal sprays should not be used longer than 3 to 5 days without consulting the prescriber.