Beta blockers and beta-2 agonists: What are they?

Beta blockers

Beta blockers make up a heterogenous family combining several molecules that have a competitive antagonist effect (blocking the bond of other molecules) on beta-adrenergic receptors. They are mainly used in the treatment of cardiovascular diseases such as high blood pressure, angina or angina pectoris, heart failure, myocardial infarction (heart attack), in case of tachycardia or cardiac rhythm disorders (class II arrhythmia).

There are two types of beta-adrenergic receptors: β-1 and β-2 receptors.

In blocking beta receptors, beta blockers will be able to have several effects. The most recognised are the following:

• A negative chronotropic effect: decrease in the frequency of cardiac contraction;
• A negative inotropic effect: decrease in the force of cardiac contraction;
• A negative dromotropic effect: decrease in the conducting speed of myocardial nerve fibres;
• A negative bathmotropic effect: decrease in the muscular response to nerve stimulation;
• A decrease in renin secretion (hormone responsable for the increase in blood pressure).

All these effects lead to an antihypertensive effect by beta blockers (mainly inotropic and chronotropic effects). In addition, they help to reduce cardiac workload and output. Myocardial oxygen demand is therefore reduced which is beneficial in coronary heart conditions (angina, etc).

They are also used in the treatment of tachycardia due to hyperthyroidism and in the treatment for glaucoma.

Finally, paradoxically, beta blockers are used in the treatment of heart failure. This is due to the hyperstimulation of beta receptors which reduces their reactivity. Blocking these receptors with beta blockers removes this hyperstimulation and thus improves the responsiveness of these receptors.

There is also a distinction to be made between cardioselective beta blockers from non-cardioselective beta blocker. The former bind only to beta-1 receptors located in the heart, while the latter bind to all beta receptors, regardless of their location. Thus, non-cardioselective beta blockers have an effect on the bronchi, inducing bronchoconstriction. As a result, they are contraindicated in asthma patients who need bronchodilation in order to to be able to go through an attack.

This pharmaceutical class includes molecules such as: propranolol, atenolol, bisoprolol, etc.

Beta2-adrenergic agonists (Beta stimulants)

Asthma is a common respiratory condition characterised by an inflammation of the bronchial tubes and bronchoconstriction.

Rapid-acting beta-adrenergic agonists are the standard treatment for asthma attacks. There are also long-acting beta stimulants used as a background treatment.

Among the rapid-acting drugs, we find salbutamol and terbutaline.

The long-acting drugs include salmeterol, formoterol and bambuterol, among others.

Their benefit-risk balance by inhalation is very high. This is because when administered in this way, the concentration of beta-adrenergic agonists in the blood will be low. Therefore, their effect will primarily be bronchial.

These drugs have a selective action on β-2 adrenergic receptors, mainly in the bronchial area but also in the uterine, vascular and cardiac areas. 

Through their binding to bronchial receptors, beta2-adrenergic agonists provoke a cascade of reactions leading to relaxation of the bronchial muscle and therefore to bronchodilation within a few minutes. This effect lasts about 4 hours for those with a short-acting duration and 12 to 24 hours for those with a long-acting duration.

However, due to their vascular and cardiac effect, they can cause tachycardia (rapid heart rate). They are therefore contraindicated for patients with severe heart disease and uncontrolled high blood pressure.

Conclusion

These two therapeutic classes are well known and understood (their therapeutic effects as well as their side effects). Non-cardioselective beta blockers are therefore contraindicated for asthmatic patients. Indeed, if the latter are treated with beta2-adrenergic agonists, their effectiveness will be greatly reduced as a consequence of blocking beta receptors. In addition, beta2-adrenergic agonists can induce tachycardia, so they should be used with caution in patients with cardiovascular disorders such as high blood pressure and arrhythmia.

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