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General information
guide to adrenoceptor nomenclature
pharmacology glossary index

In 1913 Henry Dale observed that adrenaline constricted some blood vessels while relaxing others. It was not until 1948; however, that a scientist named Ahlquist defined 2 receptor subtypes based on rank order of potency of various catecholamines, including adrenaline, noradrenaline and isoprenaline. The 2 receptor subtypes were called a and b and were defined in order of agonist potencies as follows        

  • a -receptor Noradrenaline > Adrenaline > Isoprenaline

  • b -receptor Isoprenaline > Adrenaline > Noradrenaline
Experimentation carried using ergot alkaloids acting as selective a -receptor antagonists lead to the unmasking of the b effects of adrenaline. Selective b -antagonists were subsequently developed in 1955 and their effects not only confirmed Ahlquist initial classification but also give rise to further subdivisions of both a and b receptors.

Lands and his colleagues first demonstrated these subdivisions. Lands proposed that different b -adrenoceptor agonists differ in their relative potency in eliciting different types of b -receptor mediated responses in different tissues. Further studies with agonist and antagonist have confirmed the existence of 2 main a -receptor subtypes (a1 and a2 ) and 3 b -receptor subtypes (b1, b2 and b3).

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