Structure-activity relationships of neuromedin U. IV. Absolute requirement of the arginine residue at position 7 of dog neuromedin U-8 for contractile activity

To examine the role of both Arg residues at positions 5 and 7 of dog neuromedin U-8 (d-NMU-8; pGlu1-Phe-Leu-Phe-Arg5-Pro-Arg7-Asn8-NH2) for smooth muscle contractile activity on isolated chicken crop, d-NMU-8 analogs were synthesized where either Arg residue was systematically replaced by various amino acids X: Ala, Thr, Glu, Gln, Lys, Orn, His, citrulline (Cit) or homoarginine (Har)]. All X5]-d-NMU-8, except for Glu5]- and Des-Arg5]-d-NMU-8, were full agonists, although their affinities to NMU receptors were decreased. No X7]-d-NMU-8 showed contractile activity even at concentrations of 10(-5) mol/l, except for Har7]-d-NMU-8, which retained weak biological activity. These analogs had no antagonistic activity against porcine neuromedin U-8 (p-NMU-8). The results revealed that Arg7 of d-NMU-8 is indispensable for receptor binding and activation to induce smooth muscle contraction, and the guanidino group of the side chain at position 7, but not at position 5, is strictly recognized by NMU receptors in the chicken crop.