CL/RAMP2 and CL/RAMP3 produce pharmacologically distinct adrenomedullin receptors:: a comparison of effects of adrenomedullin22-52, CGRP8-37 and BIBN4096BS

1 Adrenomedullin (AM) has two known receptors formed by the calcitonin receptor-like receptor (CL) and receptor activity-modifying protein (RAMP) 2 or 3: We report the effects of the antagonist fragments of human AM and CGRP (AM(22-52) and CGRP(8-37)) in inhibiting AM at human (h), rat (r) and mixed species CL/RAMP2 and CL/RAMP3 receptors transiently expressed in Cos 7 cells or endogenously expressed as rCL/rRAMP2 complexes by Rat 2 and L6 cells. 2 AM(22-52) (10 mum) antagonised AM at all CL/RAMP2 complexes (apparent pA(2) values: 7.34+/-0.14 (hCL/hRAMP2), 7.28+/-0.06 (Rat 2), 7.00+/-0.05 (L6), 6.25+/-0.17 (rCL/hRAMP2)). CGRP(8-37) (10 mum) resembled AM(22-52) except on the rCL/hRAMP2 complex, where it did not antagonise AM (apparent pA(2) values: 7.04+/-0.13 (hCL/hRAMP2), 6.72+/-0.06 (Rat2), 7.03+/-0.12 (L6)). 3 On CL/RAMP3 receptors, 10 mum CGRP(8-37) was an effective antagonist at all combinations (apparent pA(2) values: 6.96+/-0.08 (hCL/hRAMP3), 6.18+/-0.18 (rCL/rRAMP3), 6.48+/-0.20 (rCL/hRAMP3)). However, 10 mum AM(22-52) only antagonised AM at the hCL/hRAMP3 receptor (apparent pA(2) 6.73+/-0.14). 4 BIBN4096BS (10 mum) did not antagonise AM at any of the receptors. 5 Where investigated (all-rat and rat/human combinations), the agonist potency order on the CL/RAMP3 receptor was AMsimilar tobetaCGRP>alphaCGRP. 6 rRAMP3 showed three apparent polymorphisms, none of which altered its coding sequence. 7 This study shows that on CL/RAMP complexes, AM(22-52) has significant selectivity for the CL/RAMP2 combination over the CL/RAMP3 combination. On the mixed species receptor, CGRP(8-37) showed the opposite selectivity. Thus, depending on the species, it is possible to discriminate pharmacologically between CL/RAMP2 and CL/RAMP3 AM receptors.