Salbutamol, fenoterol, rimiterol, reproterol, pir-buterol, salmeterol, ritodrine and terbutaline are (3-adrenoceptor agonists that are relatively selective for P2-receptors, so that cardiac (chiefly pj-receptor) effects are less prominent. Tachycardia still occurs because of atrial (sinus node) p2-receptor stimulation; the P2-adrenoceptors are less numerous in the ventricle and there is probably less risk of serious ventricular arrhythmias than with the use of nonselective catecholamines. The synthetic agonists are also longer-acting than isoprenaline because they are not substrates for catechol-O-methyltransferase, which methylates catecholamines in the liver. They are used principally in asthma, and to reduce uterine contractions in premature labour.
Salbutamol (see also Asthma)
Salbutamol (Ventolin) (t/-, 4 h) is taken orally, 2-4 mg up to 4 times/day; it also acts quickly by inhalation and the effect can last as long as 4h, which makes it suitable for both prevention and treatment of asthma. Of an inhaled dose < 20% is absorbed and can cause cardiovascular effects. It can also be given by injection, e.g. in asthma, premature labour (p2-receptor) and for cardiac inotropic (Px) effect in heart failure (where the p2 vasodilator action is also useful). Clinically important hypokalaemia can also occur (the shift of potassium into cells). The other drugs above are similar.
Salmeterol (Serevent) is a variant of salbutamol that has additional binding property to a site adjacent to the p2-adrenoceptor, which results in slow onset and long duration of action (about 12 h) (see p. 560).
Ephedrine (il/2 approx. 4 h) is a plant alkaloid with indirect sympathomimetic actions that resemble adrenaline peripherally. Centrally (in adults) it produces increased alertness, anxiety, insomnia, tremor and nausea; children may be sleepy when taking it. In practice central effects limit its use as a sympathomimetic in asthma.
Ephedrine is well absorbed when given orally and, unlike most other sympathomimetics, undergoes relatively little first-pass metabolism in the liver; it is largely excreted unchanged by the kidney. It is usually given by mouth but can be injected. It differs from adrenaline principally in that its effects come on more slowly and last longer. Tachyphylaxis occurs on repeated dosing. Ephedrine can be used as a bronchodilator, in heart block, as a mydriatic and as a mucosal vasoconstrictor, but newer drugs, which are often better for these purposes, are displacing it. It is sometimes useful in myasthenia gravis (adrenergic agents enhance cholinergic neuromuscular transmission). Pseudoephedrine is similar.
Phenylpropanolamine (norephedrine) is similar but with less CNS effect. Prolonged administration of phenylpropanolamine to women as an anorectic has been associated with pulmonary valve abnormalities and led to its withdrawal in some countries.
Amfetamine (Benzedrine) and dexamphetamine (Dexedrine) act indirectly. They are seldom used for their peripheral effects, which are similar to those of
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