beta-2 agonist
A beta-2 agonist is a bronchodilator that pushes the 'relax' button on airway muscle. It mimics the body's own adrenaline-type signal at a specific switch — the beta-2 receptor — telling the muscle wrapped around the airways to loosen its grip.
Activating the beta-2 receptor, a G-protein-coupled receptor, stimulates adenylyl cyclase to raise cyclic AMP inside airway smooth muscle, which relaxes the muscle and widens the airway. The drugs are split by duration: short-acting agents (salbutamol/albuterol) give quick relief during attacks, while long-acting agents (salmeterol, formoterol) provide steady control and are combined with an inhaled steroid for regular use.
Because beta-2 receptors also exist elsewhere, predictable side effects include tremor, palpitations, a faster heart rate and a fall in blood potassium, generally mild with inhaled doses. Importantly, long-acting beta-2 agonists should not be used alone in asthma — used without an inhaled steroid they can mask worsening inflammation and have been linked to worse outcomes, so they are always paired with anti-inflammatory therapy.
A combination inhaler delivers formoterol with a corticosteroid, so the long-acting beta-2 agonist relaxes the airways while the steroid calms inflammation.
Formoterol activates beta-2 receptors to raise cyclic AMP and relax airway muscle.
A long-acting beta-2 agonist should never be the sole asthma treatment; it must accompany an inhaled corticosteroid to control the underlying inflammation.