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Indications

Beta-adrenergic receptors are transmembrane glycoprotein structures that elicit a response inside the cell when interacting with catecholamines. They belong to a major receptor family (R7G) containing other receptors responding to substances other than catecholamines, and these receptors are coupled with guanine nucleotide (GTP) binding proteins (G proteins). Beta receptors divide into three subtypes; beta-1 (B1), beta-2 (B2), and beta-3 (B3). Other adrenergic receptors are alpha-1 and alpha-2 receptors.

Beta 2 receptors are predominantly present in airway smooth muscles. They also exist on cardiac muscles, uterine muscles, alveolar type II cells, mast cells, mucous glands, epithelial cells, vascular endothelium, eosinophils, lymphocytes, and skeletal muscles.

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Natural catecholamines are nonspecific in their effects on the body, as they can stimulate all types and subtypes of adrenergic receptors. However, different catecholamines and synthetic drugs have affinities towards different receptor subtypes. Therefore, some synthetic drugs are considered “selective” to a specific adrenergic receptor or a subtype while other compounds are not.

Natural hormones stimulate B2 receptors in the body as well as by synthetic compounds; epinephrine (adrenaline) is the most effective natural catecholamine agonist of B2, while norepinephrine (noradrenaline) is less effective on it, and epinephrine is the hormone responsible for B2 receptor stimulation in the physiological state. Synthetic agonists were sought to selectively stimulate specific receptors, unlike natural hormones, which have a low selectivity profile to minimize adverse effects. Beta receptors can be blocked by using synthetic drugs, but B2-selective blockers are yet to be found.

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Beta-2 Receptor Agonists

These drugs are designed to mimic the natural effect of epinephrine and norepinephrine hormones on the body, thus called sympathomimetics, but to limit the stimulation to B2 receptors as much as possible to reduce adverse effects. They are mainly used to manage respiratory disorders such as chronic obstructive pulmonary disease (COPD) and asthma.[1]

B2 agonists further classify into short-acting, long-acting, and ultra-long-acting drugs.

Some FDA-approved short-acting B2 agonists (SABAs) are albuterol, levalbuterol, metaproterenol, and terbutaline, and they are prescribed for bronchospasm caused by COPD, bronchial asthma, or emphysema.

Some SABAs have off-label uses; albuterol is used in the treatment of hyperkalemia, and terbutaline is administered to postpone preterm labor and manage vascular extravasation.[2][3]

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Long-acting B2 agonists (LABAs) are indicated as maintenance treatment of bronchoconstriction in patients with COPD, chronic bronchitis, and emphysema. FDA-approved LABAs include salmeterol, formoterol, and arformoterol.

Recently, clinical trials have started on ultra-long-acting B2 agonists (ULABAs) to find their effects as a long-term once-daily management method for COPD and asthma. Olodaterol is an example of an FDA-approved ULABA.

A dangerous off-label use for some B2 agonists is by bodybuilders and athletes for their anabolic and lipolytic properties.[4][5]

Beta-2 Receptor Antagonists (Blockers)

B2 antagonists are the compounds used to block the activation of B2 receptors. There are no FDA-approved selective B2 antagonists. Butoxamine is a non-FDA-approved B2-selective blocker used exclusively for research purposes as it has no clinical use.[6]

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