he prototype for this system -β-Adrenergic Receptor SystemAdrenergic receptorsare of four general types,α1, α2, β1, and β2, found in different tissues and defined by the differences in their affinities and responses to a group of agonists and antagonists.Agonistsare structural analogs that bind to a receptor and mimic the effects of its natural ligand.Antagonistsare analogs that bind the receptor without triggering the normal effect and thereby block the effects of agonists, including the biological ligand.