Beta-blockers

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Beta-blockers

Beta-blockers are medications used primarily for treating high bloodpressure. The usefulness of these medications rests on their ability to block the effects of a nervous system transmitter chemical known as norepinephrine and the related “fight-or-flight” hormone epinephrine. Beta-blockers are also used to treat heart-related chest pain (angina pectoris, or simply angina), abnormalities of heart rhythm, and certain other conditions.

Adrenergic receptors

Like all nervous system transmitter chemicals and many hormones, norepinephrine and epinephrine exert their effects by interacting with proteins on the target cell’s outer surface. Scientists refer to the ones on which epinephrine and norepinephrine act as adrenergic receptors, and group them into two major classes. These classes are formally known as a- and b-adrenergic receptors. However, many medical articles use the short forms “alpha receptors” and “beta receptors,” respectively.

The most fundamental distinction between alpha and beta receptors is their response or lack of response to specific synthetic chemicals. They also respond differently to their natural stimuli: alpha receptors are more responsive to norepinephrine than to epinephrine, while beta receptors respond equally to both.

Some cell types carry both alpha and beta receptors, while others carry only one, or neither. The two classes of receptors often have opposite effects. This allows the body to “fine-tune” its response by varying the relative amounts of circulating epinephrine and locally released norepinephrine in different tissues. In the circulatory system, however, both alpha and beta receptors raise blood pressure. Nevertheless, they do so in different ways: alpha receptors by constricting the blood vessels, beta receptors by increasing the force and rate of the heartbeat.

Frequently used beta-blockers
Generic name	Trade names
Frequently Used beta-blockers (Thomson Gale.)
Pindolol	Visken
Propranolol	Inderal
Timolol	Blocarden
Enter brain poorly
Atenolol	Tenormin
Nadolol	Corgard
Selective for beta1 receptors
Metoprolol	Lopressor
Acebutolol	Sectral
Block both alpha and beta receptors
Labetalol	Normodyne, Trandate

Mechanism of action

Beta-blockers are not general blood-pressure-lowering drugs, that is, they do not cause already normal blood pressure to go still lower. Nor do they usually affect the heartbeat of a person at rest, although they do limit the ability of exercise or emotion to make the heart beat more quickly and strongly.

Exactly how beta-blockers combat elevated blood pressure remains unclear. One important aspect is their ability to relax small arteries, thus allowing blood to flow more easily and with less pressure behind it. No one knows how beta-blockers do this, however, it would not be expected from their known actions. Furthermore, since relaxation occurs only after several days of beta-blocker use, it is very likely to be an indirect effect.

Scientists also know that beta-blockers reduce the kidney’s release of renin, an enzyme essential for production of the hormone angiotensin II. Since angiotensin II raises blood pressure in several ways, there can be little doubt that renin plays a significant role in regulating blood pressure. Unfortunately, researchers have found little relationship between blood pressure levels and the amount of renin circulating in the blood. This leaves them uncertain whether beta-blockers’ ability to lower blood pressure is tied to their effect on renin release.

By contrast, reasons for beta-blockers’ ability to relieve angina are obvious. This condition results from fatty deposits narrowing the arteries that carry blood to the heart muscle. As a result, the heart muscle does not get enough blood to meet its needs—especially when those needs increase because it is beating harder and faster than usual. Since beta-blockers limit the effects of exercise and emotion on the heartbeat, the gap between the amount of blood the heart receives and what it needs will be smaller. As a result, the patient will experience less pain.

Musicians, who rely on steady nerves for an outstanding performance, can also use beta-blockers in an attempt to ease their pre-performance anxiety and calm their nerves. While some benefit can be gained from their use, beta-blockers can have side effects that are more serious than the pursuit of musical excellence.

Side effects

Different parts of the body contain different beta receptor subtypes, designated beta₁ and beta₂. Receptors in the circulatory system belong to the beta₁ subclass, while those on cells lining the small airways of the lung are of the beta₂ subclass. Beta ₂ receptors help relax these small airways and therefore make breathing easier-indeed, patients with asthma and other obstructive lung diseases often inhale beta₂ —stimulating medications to help them breath more easily. Thus, patients with such diseases should not take medications that block beta₂ receptors. Fortunately, several blood pressure medications that selectively block only beta₁ receptors are now available.

Beta-blockers are also probably not the best choice of treatment for people who have diabetes along with their high blood pressure or angina. In a hypoglycemic crisis (where blood sugar drops too low), the body pours out large amounts of epinephrine to stimulate release of stored sugar into the blood stream. This epinephrine also causes a rapid, pounding heartbeat that is often the diabetic’s first indication something is wrong. Beta-blockers blunt both responses, leading to a crisis that is worse and longer-lasting than it would be otherwise.

These medications may likewise not be the best choice for people with poor circulation in their hands or feet, since beta-blockers sometimes make circulation in the extremities even worse.

About 10% of patients treated with beta-blockers may become dizzy or light-headed. More seriously, about 5% may become clinically depressed, with feelings of helplessness and hopelessness that sometimes lead to suicide. As might be expected, all such reactions are less common with beta-blockers that do not enter the brain readily.

Other moderately common side effects of beta-blockers include diarrhea, rash, slow heartbeat, and impotence or loss of sexual drive.

KEY TERMS

Alpha receptors (alpha-adrenergic receptors)— Proteins on the surface of target cells through which epinephrine and norepinephrine exert their effects.

Angina pectoris (angina)— Chest pain that occurs when blood flow to the heart is reduced, causing a shortage of oxygen. The pain is marked by a suffocating feeling.

Beta receptors (beta-adrenergic receptors)— Proteins on the surface of target cells through which epinephrine and norepinephrine exert their effects; beta receptors respond to the two substances to approximately the same extent.

Epinephrine (adrenaline)— The “flight-or-fight” hormone synthesized by the adrenal gland.

Norepinephrine (noradrenaline)— A substance that certain nerve cells release in order to produce their effects.

An additional concern with beta-blockers is their effect on blood cholesterol, they lower the amount of “good” (HDL) cholesterol, while increasing the amount of “bad” (LDL) cholesterol. They also raise the amounts of fatty materials known as triglycerides in the bloodstream; some scientists believe triglycerides may increase the risk of a heart attack to almost the same extent as cholesterol. Nevertheless, there is no concrete evidence that people treated with beta-blockers are more likely to have heart attacks than those treated with other blood-pressure medications.

While clinical evidence shows that beta-blockers are in fact very effective medications, other blood pressure drugs are prescribed more often. One reason they are under prescribed may be that physicians are wary of potential side effects when patients are given recommended doses. However, beta-blocker medications may still provide positive clinical outcomes at lower doses, making the risk of side effects lower. Some beta-blocker medications are, however, commonly used. Inderal (Propranolol), Lopressor (Metoprolol), and Ternormin (Atenolol) are widely used beta-adrenergic blocking agents. In general, the generic names for this class of drugs end with the suffix -olol, as in Propranolol.

See also Heart diseases; Hypertension.

Resources

BOOKS

Icon Health Publications. Beta Blockers: A Medical Dictionary and Annotated Research Guide to Internet References. San Diego: Icon Health Publications, 2003.