Pharmacology of Beta-2 Agonists
Beta-2 agonists are medicines used in the treatment of asthma, COPD and hyperkalemia. Here, we review the primary members, how they work, and what side effects and drug interactions the medicines are linked to.
Beta-2 agonists are primarily used to treat respiratory disorders.
For example, they are used to treat:
- Asthma – where short-acting agents relieve breathlessness. Long-acting drugs can also be used, though long-acting agents are always co-administered alongside an inhaled corticosteroid (see below).
- COPD – similarly, short-acting drugs are used to relieve breathlessness, whereas long-acting agents may be considered as a second-line option.
- Hyperkalemia – as we learn from the mechanism, beta-2 agonists encourage potassium to move into the intracellular compartment. Beta-2 agonists are not used for long-term treatment but are, instead, used in emergency situations before more appropriate, long-term treatment solutions are found.
In the indications above, we referred to “short-acting” and “long-acting” drugs.
That’s because beta-2 agonists are sub-divided into the following three classes:
- Short-acting – albuterol (salbutamol), terbutaline
- Long-acting – salmeterol, formoterol
- Ultra-long acting – indacaterol, olodaterol, vilanterol
Note: albuterol is USAN; whereas salbutamol is INN.
The duration of effect determines what medicine is used for immediate, “as required”, relief, and what medicine is used for long-term maintenance.
Mechanism of action
Beta-2 agonists trigger smooth muscle relaxation; relaxing bronchial smooth muscle.
Beta-2 receptors are not just found in the bronchi, though. They are also found in the GI tract, uterus and throughout blood vessels.
The beta-2 receptor is a G-protein coupled receptor which, upon activation, triggers a signalling cascade that leads to smooth muscle relaxation; enhancing the passage of air throughout respiratory passages.
Beta-2 agonists also stimulate sodium-potassium ATPase pumps. This is what contributes to their potassium-lowering effects. Beta-2 agonism encourages potassium to move from the extracellular compartment to the intracellular compartment – reducing potassium levels in the body.
This makes beta-2 agonists an effective short-term treatment option (not least because therapeutic predictability is not reliable) in hyperkalemic emergencies.
Side effects with beta-2 agonists include:
- Tremors / shakiness
Very rarely, beta-2 agonists are associated with pulmonary edema and arrhythmias.
When we talk about the clinical pharmacology of beta-2 agonists, we need to think about the following factors:
- That long-acting drugs should only be used to treat asthma when combined with an inhaled corticosteroid. The inhaled corticosteroid is important; as, without them, long-acting agents are associated with an increased risk of asthma deaths.
- That because beta-2 agonists are associated with tachycardia, caution is warranted in patients with established cardiovascular disease, or in persons at risk of arrythmias; a caution with added urgency in hyperkalemia where high doses of beta-2 agonists may be used.
- That short-acting agents are prescribed as “as required”, whereas long-acting agents are predominantly used for maintenance therapy.
- That beta-blockers may reduce the therapeutic potential of beta-2 agonists.
- That the risk of hypokalemia increases when beta-2 agonists are taken alongside corticosteroids or theophylline.
Beta-2 agonists are important medicines in the treatment of asthma and COPD. Patients should be informed of the correct technique when using an inhaler; with the technique and inhaler checked at each consultation.
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