THE ADRENERGIC RECEPTORS

Although these neurotransmitters are chemically related, they have their own families of receptor, the dopaminergic and adrenergic receptors. In 1948 it was shown by Ahlquist that the adrenergic receptors may be divided by their responses to adrenalin and its analogues into two classes known as the α- and β-receptors. In particular only the β-receptors responded to the analogue, isoprenaline 3.34. Later, in 1967, Lands was able to show that the β-receptors could be further su β-divided into   β1- and β2-receptors. The α-receptors are divided into a1 (post-synaptic) and a2 (pre-synaptic) families.

When the catecholamine neurotransmitters bind to receptors they produce different responses that depend on the tissue concerned. In general, binding to the α-receptors brings about a constriction while binding to the  β-receptors brings about a relaxation of a response. The receptors are G-protein-coupled receptors. The structure of the binding site in the receptor contains some key amino acids. There are two serines whose primary alcohols may act as hydrogen-bond donors to the dihydroxyphenol. The side chain of an aspartic acid unit has a free carboxyl group, which may bind to the amine while there is a phenylalanine in which the aromatic ring may interact with the electron-rich aromatic catechol ring. Finally, the β-receptors may be distinguished from the α-receptors by the presence of a hydrophobic pocket into which the isopropyl group of isoprenaline can fit.  Drugs may interact with the catecholamines in various ways, which are illustrated by the following.