Beta2-adrenergic agonist

Beta₂-adrenergic agonists, also known as adrenergic β₂ receptor agonists, are a class of drugs that act on the β₂ adrenergic receptor. Like other β adrenergic agonists, they cause smooth muscle relaxation. β₂ adrenergic agonists' effects on smooth muscle cause dilation of bronchial passages, vasodilation in muscle and liver, relaxation of uterine muscle, and release of insulin. They are primarily used to treat asthma and other pulmonary disorders. Bronchodilators are considered an important treatment regime for chronic obstructive pulmonary disease (COPD) and are usually used in combination with short acting medications and long acting medications in a combined inhaler.

Activation of β adrenergic receptors leads to relaxation of smooth muscle in the lung, and dilation and opening of the airways.

β adrenergic receptors are coupled to a stimulatory G protein of adenylyl cyclase. This enzyme produces the second messenger cyclic adenosine monophosphate (cAMP). In the lung, cAMP decreases calcium concentrations within cells and activates protein kinase A. Both of these changes inactivate myosin light-chain kinase and activate myosin light-chain phosphatase. In addition, β₂ agonists open large conductance calcium-activated potassium channels and thereby tend to hyperpolarize airway smooth muscle cells. The combination of decreased intracellular calcium, increased membrane potassium conductance, and decreased myosin light chain kinase activity leads to smooth muscle relaxation and bronchodilation.

Findings indicate that β₂ stimulants, especially in parenteral administration such as inhalation or injection, can induce adverse effects:

Overuse of β₂ agonists and asthma treatment without proper inhaled corticosteroid use has been associated with an increased risk of asthma exacerbations and asthma-related hospitalizations. The excipients, in particular sulfite, could contribute to the adverse effects.

All β₂ agonists are available in inhaler form, as either metered-dose inhalers which dispense an aerosolized drug and contains propellants, dry powder inhalers which dispense a powder to be inhaled, or soft mist inhalers which dispense a mist without use of propellants.

Salbutamol (INN) or albuterol (USAN) and some other β₂ agonists, such as formoterol, also are sold in a solution form for nebulization, which is more commonly used than inhalers in emergency rooms. Nebulizers continuously deliver aerosolized drug and salbutamol delivered through nebulizer was found to be more effective than IV administration.

Salbutamol and terbutaline are also both available in oral forms. In addition, several of these medications are available in intravenous forms, including both salbutamol and terbutaline. It can be used in this form in severe cases of asthma, but it is more commonly used to suppress premature labor because it also relaxes uterine muscle, thereby inhibiting contractions.