IN-VITRO AND IN-VIVO CHARACTERIZATION OF BRADYKININ B-2 RECEPTORS IN THE RABBIT AND THE GUINEA-PIG

A comparative study has been performed in isolated organs and in anest hetized animals, rabbits and guinea pigs, to evaluated the myotropic r esponses (in the organs) and the blood pressure changes (in the animal s) induced by bradykinin (BK) and related peptides. Antagonist affinit ies have also been estimated in vitro in terms of pA(2) and in vivo in terms of ID50, to characterize the kinin B-2 receptors in the two spe cies. Differences have been found both in the order of potency of agon ists and in the affinity of antagonists: in fact, in the rabbit, [Hyp( 3)]BK > [Aib(7)]BK, is the opposite order of what is found in the guin ea pig, namely, [Aib(7)]BK > [Hyp(3)]BK, both in vitro and in vivo. Re sults obtained with antagonists also show important differences betwee n the two species, since DArg[Hyp(3),DPhe(7),Leu(8)]BK is more active in the rabbit than in the guinea pig, while WIN-64338 is fairly active in the guinea pig and almost inactive in the rabbit. HOE-140, the lon g-acting antagonist of the B-2 receptor, shows similar affinities in v itro in the two species. In another series of experiments, peptide deg radation by angiotensin converting enzyme (ACE) has been investigated to see whether the differences of potency observed between certain pep tides interacting with the B-2 receptor were due to metabolic degradat ion. When incubated in the presence of pure ACE from rabbit lung, BK, [Hyp(3)]BK, and desArg(9)BK are readily degraded, while [Aib(7)]BK, HO E-140, and DArg[Hyp(3),DPhe(7),Leu(8)]BK are not. When applied intrave nously (i.v.), to obtain degradation by the lung, and intraarterially (i.a.), to avoid such degradation, the effect of BK (i.v.) is markedly reduced (compared with the effects i.a.), while no difference is obse rved for [Aib(7)]BK. Thus, despite its resistance to degradation by AC E, [Aib(7)]BK shows very little activity in the rabbit, suggesting tha t the major cause in the variation of affinities observed between kini n analogs is related to their pharmacodynamic properties. Taken togeth er, the results speak strongly in favor of the existence of B-2 recept or subtypes in the peripheral circulation of the rabbit and the guinea pig. Results obtained in vivo, both in pharmacological and biochemica l experiments, are in accord with the findings obtained in isolated or gans and with purified ACE enzyme.