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
The β-receptors in airways are β2 type.
Extra info: This is an important physiological distinction for pharmacology. If you give the β-agonist by the inhaled route, then you will only affect the local β-2 receptors. If you give the drug systemically, you can effect β1 and β2 receptors throughout the body (e.g. β1 receptors in cardiac muscle).
Many tissues contain β-receptors.
Extra info: β-receptors are located in many parts of the body including the heart, eye, bladder, lungs, smooth muscle, etc.