Endogenous norepinephrine can stimulate α1, α2, β1 and β2 receptors. Most drugs are created to be either agonists or antagonists that are more selective for one or two receptors. However, no substance so far has shown 100% selectivity. β-agonist agents are more selective for the β-receptors. β1-receptors are abundant in the circulation and skeletal muscle, β2 in smooth muscle (particularly in the respiratory tree).
Commonly used drugs of this group are ephedrine, salbutamol

style="color: #000040; font-family: Verdana, Arial; font-size: medium; background-color: #e4eaff;">, salmeterol, formoterol, and terbutaline.


The β-receptors in airways are β2 type.


Many tissues contain β-receptors.