In treating asthma, preferential activation of b2 receptors without stimulation of b1 receptors in the heart is desirable. Drugs with preferential affinity for b2 receptors over b1 receptors have been developed, but selectivity is not absolute and is lost at high concentrations. Administration by inhalation of small doses of a b2 agonist in aerosol form leads to effective activation of b2 receptors in the bronchi but very low systemic drug concentrations. Consequently, there is less potential to activate cardiac b1 receptors or to stimulate b2 receptors in skeletal muscle (which can cause tremor). The use of b agonists for the treatment of asthma is discussed in Chapter 27.
METAPROTERENOL Metaproterenol (called orciprenaline in Europe), terbutaline, and fenoterol belong to the structural class of resorcinol bronchodilators that have hydroxyl groups at positions 3 and 5 of the phenyl ring (rather than at positions 3 and 4 as in catechols; see Table 10-1). Consequently, these agents are resistant to methylation by COMT, and a substantial fraction (40%) is absorbed in active form after oral administration. Metaproterenol is excreted primarily as glucuronic acid conjugates. Metaproterenol is b2 selective, although probably less selective than albuterol or terbutaline; thus, metaproterenol is more prone to cause cardiac stimulation.
Effects occur within minutes of inhalation and persist for several hours. After oral administration, onset of action is slower, but effects last 3 to 4 hours. Metaproterenol (alupent, others) is used for the long-term treatment of obstructive airway diseases, asthma, and for treatment of acute bron-chospasm (see Chapter 27). Side effects are similar to the short- and intermediate-acting sympa-thomimetic bronchodilators.
Was this article helpful?
If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.