Beta Agonists
Mode of Action
Inhaled short-acting β-agonists produce immediate bronchodilation lasting 4-6 hours. These drugs stimulate β2 receptors present in airway smooth muscle, increasing levels of intracellular cyclic AMP, which in turn activates protein kinase A (PKA). PKA phosphorylates several proteins to reduce intracellular Ca2+ levels and promote smooth muscle relaxation. This action is enhanced by changes in the activity of plasma membrane K+ channels. Β-agonists are unlikely to mediate anti-inflammatory effects directly but, when given intermittently, can attenuate mast cell mediator release, micro vascular edema and leakage, mucus hyper secretion and cholinergic neurotransmission. Tolerance to repeated and excessive short-acting β-agonist use is described. For example, bronchoprotection against allergen challenge is lost and mast cell degranulation is enhanced after 1 week of use of a short-acting β-agonist (3).
Side Effects
Side effects occur in a dose-dependent manner and include:
1
Fine tremor,
2
Headache,
3
Pupil dilatation and
4
Nervousness,
5
Tachycardia and palpitations,
6
Hypokalaemia.
Comments
Short-acting β-agonists provide short-term symptomatic relief for asthma. Salbutamol and terbutaline are available in dry powder and aerosol forms. Intravenous preparations are reserved for emergency use. Oral preparations are primarily used for patients who cannot use inhaled medications. Bambuterol can be used for patients with nocturnal symptoms but inhaled long acting β-agonists are preferred. Subcutaneous terbutaline has been used for the long-term management of brittle asthma, but evidence of success has only been reported in small case series.
