Medical Pharmacology Topics   

Preliminary Outline

Alpha and Beta Antagonist          Labetalol          Carvediol Alpha Antagonists   Alpha-1 and -2 Antagonists      Irreversible          Phenoxybenzamine          Dibenzamine      Competitive          Phentolamine          Tolazoline   Alpha-1 Antagonists          Prazosin          Terasozin          Doxazosin          Tamsulosin   Alpha-2 Antagonists          Yohimbine   Beta-1 and -2 Antagonists          Propanolol          Esmolol Beta-1 and -2 Partial Agonist          Pindolol   Beta-1  Antagonists          Metoprolol

Adrenergic Pharmacology: Antagonist Drugs

Non-Selective Antagonists

Labetalol and carvediol are both alpha and beta antagonist used in the treatment of hypertension and congestive heart failure, respectively. Carvediol is also a free radical scavenger and has shown to reduce mortality and morbidity from congestive heart failure.

Alpha Antagonists

Effects of alpha receptor antagonists include hypotension, increased heart rate (reflex + a2), nasal congestion, decreased ejaculation and sedation/depression. Nonselective alpha antagonist, known as alpha blockers are used to manage patients with pheochromocytoma. Examples of irreversible alpha blockers are phenoxybenzamine and dibenzamine.  Examples of competitive alpha blockers are phentolamine and tolazoline.

Alpha blockers side effects are related to reflex and alpha mediated cardiac stimulation. Alpha-2 receptor blockade at the heart muscle prevents norepinephrine reuptake and increases its cardiac effect (beta-1). At the same time, the decreased blood pressure (due to alpha-1 blockade at vasculature) triggers a parasympathetic increase in heart rate to compensate (reflex).

Prazosin, terazosin, doxazosin and tamsulosin are a1 selective antagonists. They decrease total peripheral resistance and sympathetic stimulation of smooth muscle, and are used for the treatment of hypertension, congestive heart failure and benign prostatic hyperplasia (BPH). The first dose of these agents can cause hypotension and/or syncope, so it is recommended to be taken before bed time.

Yohimbine is an alpha-2 blocker in the periphery and CNS. It increases sympathetic nerve activity and blockade of negative feedback at sympathetic neuroeffector junctions to increase norepinephrine release. Currently there is no clinical use for this agent.

Beta Antagonists

Beta adrenergic antagonists (beta blockers) depress membrane excitability. It is not well understood how they lower blood pressure in hypertensive patients, while having no pressor effect on normal individuals (may be related to their blocking of renin secretion).  They are indicated for the treatment of  angina, cardiac arrhythmia, hypertension and post-myocardial infraction. Additional indications include glaucoma, pheochromocytoma, migraine, performance anxiety, alcohol and opiate withdrawal and hyperthyroidism.

Propanolol is a beta blocker used for the treatment of hypertension, angina, cardiac arrhythmias, ischemic heart disease, and other cardiovascular conditions. Potential side effects include cardiac depression, bronchoconstriction, peripheral circulation problems (Raynaud's phenomenon) and CNS effects like sedation, nightmares and insomnia. Propanolol has a very low bioavailability and short plasma half-life.

Metoprolol is a more cardioselective beta blocker (b1) with a longer plasma half-life and more resistant to hepatic degradation than propanolol. Esmolol is an ultra-short acting beta blocker used to limit catecholamine-mediated cardiac stimulation, especially during surgery.

Pindolol is a partial agonist of beta receptors, reducing heart activity with less cardiac depression and bronchoconstriction, while increasing peripheral resistance.

Continue to "Cholinergic Receptors" or take a quiz: [Q1].

Need more practice? Answer the review questions below.

Questions:

1- List 2 non-selective adrenergic antagonists.

2- What are the clinical uses of labetalol?

3- What are the clinical uses of carvediol?

4- What characteristics of carvediol are different from other adrenergic antagonists?

5- List 5 effects of a-adrenergic antagonists?

6- What is the main clinical use of non-specific a-adrenergic antagonists?

7- List 2 irreversible a-adrenergic antagonists.

8- List 2 competitive a-adrenergic antagonists.

9- What are the effects of a2-adrenergic blockade in heart muscle?

10- List 4 a1-adrenergic antagonists.

11- What are the effects of a1-adrenergic antagonists?

12- What are the clinical uses of a1-adrenergic antagonists?

13- What is the main side effect of a1-adrenergic antagonists?

14- List 3 effects of beta blockers.

15- What are the clinical uses of beta blockers?

16- List 3 non-selective and 1 b1 selectve beta blockers.

17- What are the clinical uses of propanolol?

18- What are the side effects of propanolol?

19- What characteristics of metroprolol makes it different from other beta blockers?

20- What characteristics of esmolol makes it different from other beta blockers?

21- What is the main clinical use of esmolol?

22- What characteristics of pindolol makes it different from other beta blockers?

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Answers:

1- List 2 non-selective adrenergic antagonists.
labetalol
carvediol

2- What are the clinical uses of labetalol?
hypertension

3- What are the clinical uses of carvediol?
congestive heart failure

4- What characteristics of carvediol are different from other adrenergic antagonists?
Is a free radical scavenger, and has shown to reduce mortality and morbidity from congestive heart failure.

5- List 5 effects of a-adrenergic antagonists?
hypotension
increased heart rate (reflex + a2)
nasal congestion
decreased ability to eyaculate
sedation/depression

6- What is the main clinical use of non-specific a-adrenergic antagonists?
Management of pheochromocytoma.

7- List 2 irreversible a-adrenergic antagonists.
phenoxybenzamine
dibenzamine

8- List 2 competitive a-adrenergic antagonists.
phentolamine
tolazoline

9- What are the effects of a2-adrenergic blockade in heart muscle?
Alpha-2 receptor blockade at the heart muscle prevents norepinephrine reuptake and increases its cardiac effect (b1). At the same time, the decreased blood pressure (due to a1 blockade at vasculature) triggers a parasympathetic increase in heart rate to compensate (reflex).

10- List 4 a1-adrenergic antagonists.
prazosin
terazosin
doxazosin
tamsulosin

11- What are the effects of a1-adrenergic antagonists?
decrease total periferal resistance
decrease sympathetic stimulation of smooth muscle

12- What are the clinical uses of a1-adrenergic antagonists?
hypertension
congestive heart failure
bening prostatic hyperplasia

13- What is the main side effect of a1-adrenergic antagonists?
hypotension and/or syncope

14- List 3 effects of beta blockers.
depress membrane excitability
lower blood pressure in hypertensive patients but not normal individuals
block renin secretion (b1)

15- What are the clinical uses of beta blockers?
angina, cardiac failure
cardiac arrhythmia
hypertension
post-myocardial infarction
glaucoma
pheochromocytoma
migrane
performance anxiety
alcohol and opiate withdrawal
hypethyroidsm
esential tremor

16- List 3 non-selective and 1 b1 selectve beta blockers.
propanolol
esmolol
pindolol
metroprolol - b1

17- What are the clinical uses of propanolol?
hypertension
angina, ischemic heart disease
cardiac arrhythmias
esential tremor

18- What are the side effects of propanolol?
cardiac depression
bronchoconstriction
peripheral circulation problems (Raynaud's sydrome)
sedation
nightmares, insomnia

19- What characteristics of metroprolol makes it different from other beta blockers?
more cardio-selective (b1)
longer plasma half-life
more resistant to hepatic degradation

20- What characteristics of esmolol makes it different from other beta blockers?
ultra-short action

21- What is the main clinical use of esmolol?
limit catecholamine-mediated cardiac stimulation, especially during surgery

22- What characteristics of pindolol makes it different from other beta blockers?
partial agonist of beta receptors, reducing heart activity with less cardiac depression and bronchoconstriction, while increasing peripheral resistance