Comparison of the responses of B1 and B2 kinin receptors to agonist stimulation

The human B2 kinin receptor (B2KR), stably expressed in chinese hamster ova ry cells, responded to bradykinin stimulation with rapid (within minutes) l igand internalization and loss of cell surface receptors (sequestration). B y contrast, B1 kinin receptors (B1KR) showed almost no ligand internalizati on or receptor sequestration upon stimulation with des-Arg10-Kallidin (DAK) . The ability of the B2KR to internalize and sequester is conferred by info rmation in the cytoplasmic tail of the receptor. It is normally impossible to determine receptor affinity at 37 degrees C because of internalization a nd sequestration processes. We created a mutant B2KR, truncated at K315 of the cytoplasmic tail, that was no longer able to internalize or sequester, and compared the affinity of this mutant, and of the B1KR, at 0 degrees C a nd 37 degrees C. The B1KR receptor showed the same affinity (K-d = 0.4 nM) at both 0 degrees C and 37 degrees C. By contrast, the K315 mutant of the B 2KR showed a lower affinity ( K-d = 2.9 nM) at 37 degrees C than at 0 degre es C (K-d = 1.4 nM), indicating more rapid ligand dissociation at 37 degree s C. After ligand exposure, clones expressing B1KR exhibited a very slow di ssociation of DAK, even at 37 degrees C. Although both kinin receptor subty pes induce the generation of inositol phosphates, functional responses show ed clear differences. The response to stimulation of the B2KR comprises a r apid loss of functional responses, receptor sequestration, and ligand disso ciation, and, upon long term stimulation, downregulation. By contrast, liga nd stimulation of the B1KR, once this receptor is expressed de novo under p athological conditions, results in persistent signaling due to lack of liga nd dissociation, desensitization and receptor sequestration. Moreover, long term stimulation of this receptor actually leads to increased expression. (C) 1999 Elsevier Science B.V. All rights reserved.