| Background | Bradykinin is a nine-amino-acid vasoactive peptide which elicits numerous physiologic responses such as vasodilation, smooth muscle spasm and pain. Bradykinin is one of a family of such peptides called the kinins. These kinins are generated by proteolysis of high-molecular-weight precursors called kininogens upon activation by pathophysiologic conditions such as inflammation or allergy. The physiological actions of these kinins are brought about through their interaction with transmembrane receptors. There are two distinct bradykinin receptor subtypes; B1 and B2, both of which appear to be coupled to G-proteins. The B2 receptor subtype is found in healthy smooth muscle cells and neurons, whereas the B1 receptors are only detected following tissue injury. The B2 receptor is similar in structure to other seven-helix G-protein-coupled receptors. Bradykinin has relatively low affinity for B1 receptors, interacting primarily with the B2 receptor through which several second messenger systems are stimulated, including inositol phospholipid hydrolysis, arachidonic acid metabolism, tyrosine phosphorylation, and membrane depolarization and hyperpolarization. |