Control of Bronchial Calibre

Adrenergic Agonists P2 Agonists

• Examples—salbutamol, terbutaline, fenoterol, pirbuterol, rimiterol

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Figure RP.1 Causes of changes in bronchial calibre

Selective p2 adrenergic receptor agonists are used in the treatment of bronchospasm and for prophylaxis. This selectivity is not absolute and high doses of these drugs will cause p1 effects (tachycardia, tremor, hyperglycaemia, increased insulin secretion and hypokalaemia).

p2 agonists reverse bronchospasm caused by histamine release, platelet activating factor, and members of the leukotriene family, particularly C4, D4 and E4. A leukotriene inhibitor, montelukast has recently been introduced as an add-on treatment for mild asthma. The drug is given orally, once a day and acts as a leukotriene inhibitor at receptor level as well as reducing leukotriene production.

The p 2 agonist salbutamol is the most widely used agent in the treatment of asthma. It is conjugated in the liver and excreted in both conjugated and unchanged forms in urine and faeces. Terbutaline is a similar agent that may have advantages in some patients due to fewer sympathomimetic side effects. Terbutaline may be used antenatally to stimulate foetal lung surfactant production. Rimiterol is a short-acting p agonist of only 2 H duration.

p2 agonists may be used as uterine relaxants for the management of premature labour, excessive contractions, or during Caesarean section to facilitate delivery. Administration for this purpose may be inhalational.

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