Abstract

Bradykinin (BK) and kallidin (Lys-BK), liberated from kininogens by kallikreins, are ligands of the BK B₂ receptor. We investigated whether kallikreins, besides releasing peptide agonist, could also activate the receptor directly. We studied the effect of porcine and human recombinant tissue kallikrein and plasma kallikrein on [Ca²⁺]_i mobilization and [³H]arachidonic acid release from cultured cells stably transfected to express human BK B₂ receptor (CHO/B₂, MDCK/B₂, HEK/B₂), and endothelial cells were used as control cells. As with BK, the actions of kallikrein were blocked by the B₂ antagonist, HOE 140. Kallikrein was inactive on cells lacking B₂receptor. Kallikrein and BK desensitized the receptor homologously but there was no cross-desensitization. Furthermore, 50 nM human cathepsin G and 50 nM trypsin also activated the receptor; this also was blocked by HOE 140. Experiments excluded a putative kinin release by proteases. [³H]AA release by BK was reduced by 40% by added kininase I (carboxypeptidase M); however, receptor activation by tissue kallikrein, trypsin, or cathepsin G was not affected. Prokallikrein and inhibited kallikrein were inactive, suggesting cleavage of a peptide bond in the receptor. Kallikreins were active on mutated B₂receptor missing the 19 N-terminal amino acids, suggesting a type of activation different from that of thrombin receptor. Paradoxically, tissue kallikreins decreased the [³H]BK binding to the receptor with a low K _D (3 nM) and inhibited it 78%. Thus, kallikreins and some other proteases activate human BK B₂ receptor directly, independent of BK release. The BK B₂ receptor may belong to a new group of serine protease-activated receptors.