1948: Classification of Alpha and Beta Adrenergic Receptors (1948)

By the 1940s, adrenaline (epinephrine) and noradrenaline were known to produce markedly different
effects in different tissues—contraction in most blood vessels, relaxation in bronchial smooth
muscle, increased heart rate—but the reason for this tissue-dependent variation was obscure.

In 1948, Raymond Ahlquist of the Medical College of Georgia published a landmark paper in the
American Journal of Physiology proposing that the divergent actions of sympathomimetic amines
could be explained by the existence of two distinct receptor types, which he termed alpha (α)
and beta (β) adrenoreceptors. Alpha receptors mediated vasoconstriction and uterine contraction;
beta receptors mediated vasodilation, bronchodilation, and cardiac stimulation. Ahlquist's
classification was based on the rank order of potency of a series of sympathomimetic amines
across different tissues—a clever pharmacological methodology that did not require isolating
the receptors themselves.

The practical importance of Ahlquist's framework was demonstrated by James Black. Reasoning that
selective blockade of cardiac beta receptors would reduce myocardial oxygen demand in angina,
Black developed propranolol—the first clinically useful beta-blocker—at ICI Pharmaceuticals in
1964. The success of propranolol validated Ahlquist's receptor classification and established
receptor subtype selectivity as a rational drug design strategy.

Subsequent work identified beta-1 (cardiac) and beta-2 (bronchial) subtypes, alpha-1 (post-synaptic)
and alpha-2 (pre-synaptic) subtypes, and eventually characterised the molecular structure of these
receptors as members of the G-protein coupled receptor (GPCR) superfamily through the cloning work
of Robert Lefkowitz and Brian Kobilka in the 1980s, earning them the 2012 Nobel Prize in Chemistry.