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 Antagonist Drugs

Effects of alpha receptor antagonists include hypotension, increased heart rate (reflex + alpha-2), 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 alpha-1 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 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. Beta blockers

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 (beta-1) 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.

Labetalol and carvediol are both an 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.

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