Adrenaline acts on two kinds of receptors, alpha and beta receptors. Constriction of blood vessels by adrenaline is due to occupation of alpha receptors, and dilatation of blood vessels and increase in heart rate and force of contraction to occupation of beta receptors. Noradrenaline and isoprenaline are relatively more selective than adrenaline: noradrenaline produces predominantly alpha effects, isoprenaline largely beta effects. Alpha or beta receptors may be blocked selectively by suitable drugs. Drugs which block alpha receptors have been known for some time: they include several ergot alkaloids, phentolamine and phenoxybenzamine. The first drug developed to block beta receptors was dichloroisoprenaline, but it was not used clinically because it also has sympathomimetic activity. More recently another derivative of isoprenaline, pronethalol (Alderlin - ICI), has been introduced and tried in a number of heart disorders.
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