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Section IIICardiovascular drugsand oral preparations (syrup 0.4 mg.ml−^1 and 2, 4 or 8 mg tablets). It is used in
the treatment of reversible lower airway obstruction and occasionally in premature
labour.Effects
Respiratory – its main effects are relaxation of bronchial smooth muscle. It reverses
hypoxic pulmonary vasoconstriction, increasing shunt, and may lead to hypox-
aemia. Adequate oxygen should, therefore, be administered with nebulized salbu-
tamol.
Cardiovascular – the administration of high doses, particularly intravenously, can
cause stimulation ofβ 1 -adrenoceptors resulting in tachycardia, which may limit
the dose. Lower doses are sometimes associated withβ 2 -mediated vasodilatation,
which may reduce the blood pressure. It may also precipitate arrhythmias, espe-
cially in the presence of hypokalaemia.
Metabolic – Na+/K+ATPase is stimulated and transports K+into cells resulting in
hypokalaemia. Blood sugar rises especially in diabetic patients and is exacerbated
byconcurrently administered steroids.
Uterus – it relaxes the gravid uterus. A small amount crosses the placenta to reach
the foetus.
Miscellaneous – a direct effect on skeletal muscle may produce tremor.Kinetics
The absorption of salbutamol from the gut is incomplete and is subject to a significant
hepatic first-pass metabolism. Following inhalation or intravenous administration,
it has a rapid onset of action. It is 10% protein bound and has a half-life of 4–6 hours.
Itis metabolized in the liver to the inactive 4-O-sulphate, which is excreted along
with salbutamol in the urine.Salmeterol
Salmeterol is a long-actingβ 2 -agonist used in the treatment of nocturnal and
exercise-induced asthma. It should not be used during acute attacks due to a rela-
tively slow onset.
Ithas a long non-polar side chain, which binds to theβ 2 -adrenoceptor giving it a
long duration of action (about 12 hours). It is 15 times more potent than salbutamol
at theβ 2 -adrenoceptor, but four times less potent at theβ 1 -adrenoceptor. It prevents
the release of histamine, leukotrienes and prostaglandin D 2 from mast cells, and also
has additional anti-inflammatory effects that differ from those induced by steroids.
Itseffects are similar to those of salbutamol.