The actions of bradykinin (BK) in mammals are mediated through the activation of the B1 and B2 BK receptors. The only BK receptor that has been cloned from a non-mammalian species is a B2-like receptor from the chicken (termed the ornithokinin receptor). Pharmacological studies have demonstrated the presence of BK receptors in tissues of teleost fishes, such as trout and cod, but the ligand-binding properties of these receptors differ appreciably from those of the mammalian and chicken receptors. We report here the cloning of a B2-like receptor in zebrafish that shares 35% identity with human B2 and 30% identity with human B1. Phylogenetic analyses confirm a closer relationship with B2 than B1. The receptor gene was mapped to linkage group 17, which is syntenic to the human B2—B1 gene region. After functional expression of the zebrafish B2 receptor in mammalian cells, nanomolar concentrations of trout BK ([Arg0,Trp5,Leu8]-BK) and the derivative [des-Arg0,Trp5,Leu8]-BK (where ‘des’ indicates a missing amino acid) induced a significant transient rise in intracellular free Ca2+. The B1-selective analogue [Arg0,Trp5,Leu8,des-Arg9]-BK was inactive at nanomolar concentrations. Taken together, these results strongly support the gene's identity as a piscine orthologue of the mammalian B2 receptor.
Abbreviations used: BK, bradykinin; B1, BK receptor B1; B2, BK receptor B2; bdkrb2, zebrafish BK receptor B2 gene; TM, transmembrane; LB, Luria—Bertani; BAC, bacterial artificial chromosome; RT-PCR, reverse transcriptase-PCR.