APPENDIX VIII\BRONCHODILATORS: ? 2-ADRENOCEPTOR AGONISTS
These are drugs which make the bronchial muscles relax, and are therefore useful in asthma. There are three types of bronchodilators: |32-adrenoceptor agonists, xanthines and anticholinergics.
? 2-adrenoceptor agonists
Antagonists are drugs such as antihistamines which bind to receptors and block the effect of the natural messenger (eg histamine) that normally binds to the receptor. Agonists have the opposite effect. They bind to receptors and stimulate the cell, in the same way that the natural messenger would – in other words, they mimic the effects of that natural messenger.
The (32-adrenoceptor agonists mimic the effects of adrenaline on the bronchial muscles, by binding to receptors for adrenaline. These are called (52 adrenoceptors, hence the name of the drugs. They include salbutamol (Ventolin,
Ventodisks, Volmax, Cobutolin, Salbulin, Salbuvent, Asmaven, Aerolin-Auto), terbutaline (Bricanyl, Monovent), fenoterol (Berotec), pirbuterol (Exirel), reproterol (Bronchodil), rimiterol (Pulmadil). Sometimes such drugs are combined with corticosteroids (see below), as in Ventide, which contains salbutamol.
Of all the bronchodilators, these drugs have the most specific effects on the bronchi. They are now preferred to isoprenaline (Iso-Autohaler and Medihaler-Iso) which has a less specific effect, and tends to combine with adrenaline receptors in the heart muscles as well as those in the bronchi, sometimes causing irregular heartbeat, flushing and headaches. Isoprenaline is combined with a sympathomimetic, phenylephrine in Duo-Autohaler and Medihaler-Duo. ,
Isdetharine (Numotac) is another nonspecific 6-agonist. It is combined with phenylephrine in Bronchilator.Orciprenaline (Alupent) is a drug of the same type that is partially selective for bronchial muscles, and has similar side-effects.
Side-effects can also occur with the specific p2-adrenoceptor agonists, such as salbutamol, although they are generally less of a problem. They include tremor, nervous tension, headache, flushing and dry mouth. Taking the drugs from an inhaler reduces the side-effects by targeting the drug on the bronchi – this allows a much lower dose to be used than if the drugs were taken by mouth.
The effects of these drugs lasts for up to six hours, and the timing of doses should be geared to the patient’s needs. Learning how to operate the inhaler properly is very important, as the drug can be ineffective if the inhaler is misused.
Even if they are used at quite high doses over long periods of time there seem to be no serious ill-effects with these drugs. On the other hand, they do not reduce the sensitivity of the bronchi, as sodium cromoglycate does, so once they are discontinued their beneficial effects cease. A combination of the two drugs is sometimes used.
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