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 μM) antagonised AM at all CL/RAMP2 complexes (apparent pA 2 values: 7.34±0.14 (hCL/hRAMP2), 7.28±0.06 (Rat2), 7.00±0.05 (L6), 6.25±0.17(rCL/hRAMP2)). CGRP 8-37 (10 μM) 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 μM 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 μm AM 22-52 only antagonised AM at the hCL/hRAMP3 receptor (apparent PA 2 6.73±0.14). 4. BIBN4096BS (10 μM) 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 AM∼βCGRP>αCGRP. 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.
- calcitonin receptor-like receptor