Bradykinin receptors: Agonists, antagonists, expression, signaling, and adaptation to sustained stimulation

Bradykinin-related peptides, the kinins, are blood-derived peptides that stimulate 2 G protein-coupled receptors, the B₁ and B₂ receptors (B₁R, B₂R). The pharmacologic and molecular identities of these 2 receptor subtypes will be succinctly reviewed herein, with emphasis on drug development, receptor expression, signaling, and adaptation to persistent stimulation. Peptide and non-peptide antagonists and fluorescent ligands have been produced for each receptor. The B₂R is widely and constitutively expressed in mammalian tissues, whereas the B₁R is mostly inducible under the effect of cytokines during infection and immunopathology. The B₂R is temporarily desensitized by a cycle of phosphorylation/endocytosis followed by recycling, whereas the nonphosphorylable B₁R is relatively resistant to desensitization and translocated to caveolae on activation. Both receptor subtypes, mainly coupled to protein G G_q, phospholipase C and calcium signaling, mediate the vascular aspects of inflammation (vasodilation, edema formation). On this basis, icatibant, a peptide antagonist of the B₂R, is approved in the management of hereditary angioedema attacks. This disease is the therapeutic showcase of the kallikrein-kinin system, with an orally bioavailable B₂R antagonist under development, as well as other agents that inhibit the kinin forming protease, plasma kallikrein. Other clinical applications are still elusive despite the maturity of the medicinal chemistry efforts applied to kinin receptors.