Structure-activity studies of B1 receptor-related peptides. Antagonists.

We tested several peptides related to des-Arg9-bradykinin as stimulants or inhibitors of B1 (rabbit aorta, human umbilical vein) and B2 (rabbit jugular vein, guinea pig ileum, human umbilical vein) receptors. We also incubated the compounds with purified angiotensin-converting enzyme from rabbit lung to test their resistance to degradation. We evaluated apparent affinities (in terms of the affinity constant pA2) of compounds and their potential residual agonistic activities (alpha E). Bradykinin and des-Arg9-bradykinin were used as agonists for the B2 and B1 receptors, respectively. Degradation of peptides by the angiotensin-converting enzyme was prevented in the presence of a D-residue in position 7 of des-Arg9-bradykinin. Replacement of Pro7 with D-Tic combined with Leu, Ile, Ala, or D-Tic in position 8 led to weak B1 receptor antagonists, some of which had strong residual agonistic activities on the B2 receptor preparations. The use of D-beta Nal in position 7, combined with Ile in position 8 and AcLys at the N-terminal (eg, AcLys[D-beta Nal7, Ile8]des-Arg9-bradykinin) gave the most active B1 receptor antagonist (pA2 of 8.5 on rabbit aorta and human umbilical vein), which is also partially resistant to enzymatic degradation. Extension of the N-terminal end by Sar-Tyr-epsilon Ahx (used for labeling purposes) and even cold-labeling of Tyr with iodine were compatible with high, selective, and specific antagonism of the B1 receptors. We compared some compounds with some already known B1 receptor antagonists to underline the novelty of new peptidic compounds.