Salbutamol and short-acting beta2 agonists

Salbutamol and terbutaline are the standard short-acting beta2 agonists (SABAs), drugs that imitate adrenaline at beta2 receptors. Airway smooth muscle (the muscle in the airway wall that tightens during an asthma attack) carries these receptors, and adrenaline circulating in the blood normally acts on them too, so salbutamol copies adrenaline's action in the airway.

The receptor is linked to a Gs protein, which is a switch protein in the cell membrane. When it is switched on, it stimulates adenylate cyclase (the enzyme that makes cAMP), so the cell makes more cAMP. The rise in cAMP lowers the calcium inside the cells and relaxes the muscle, so the airway widens. This widening is bronchodilation, and it begins within minutes.

The short-acting antimuscarinic ipratropium also gives bronchodilation, but by blocking muscarinic M3 receptors (the receptors that acetylcholine from the vagus nerve uses to tighten the airway), not by adding a relaxing signal. Theophylline, kept for difficult-to-control asthma, blocks phosphodiesterase (the enzyme that breaks down cAMP).

Because SABAs act within minutes, they are the classic reliever: a medicine inhaled when needed for quick relief of symptoms, with an effect that lasts 4 to 6 hours. They are also given in acute exacerbations (sudden worsenings such as an asthma attack), often by nebuliser (a device that gives the medicine as a fine mist to breathe in), and before exercise. They do not treat the airway inflammation underneath, so needing them often is a marker of poor asthma control. Current UK guidance now recommends moving away from a SABA used alone: in people aged 12 and over with newly diagnosed asthma, it recommends a low-dose inhaled corticosteroid (a steroid that damps down airway inflammation) with formoterol (a long-acting beta2 agonist that acts quickly), taken as needed for symptom relief.