Beta-adrenoceptors are divided into three subtypes: beta1-adrenoceptors predominantly expressed on cardiac smooth muscle, beta2
adrenoceptors predominantly expressed on airway smooth muscle, and beta3-adrenoceptors predominantly expressed on adipose
tissue. Beta2-agonists cause bronchodilation. Although the beta2-adrenoceptor is the predominant adrenergic receptor in the airway
smooth muscle, it is also present on the surface of a variety of other cells, including lung epithelial and endothelial cells and in the
heart. The precise function of beta2-receptors in the heart is not known, but their presence raises the possibility that even highly
selective beta2-agonists may have cardiac effects.