Lack of specific binding of bradykinin and D-Arg [Hyp3, Thi5, D-Tic, Oic8] bradykinin in membranes from rat heart.

Recent data demonstrated effects of bradykinin (BK) in the isolated perfused rat heart which could be blocked by BK antagonists. However, so far BK receptors in heart tissue have not been characterized. To search for BK receptors in rat hearts iodinated D-Arg[Hyp3,Thi5,D-Tic,Oic8]BK (Hoe 140) was used as a ligand for binding due to its high affinity and to its resistance to degradation. The labeled antagonist bound well to membranes prepared from rat ileum and could be displaced by the unlabeled antagonist as well as by BK and T-kinin in a concentration dependent manner. However, there was no specific binding detectable when membranes from rat left cardiac ventricle were used. To assure integrity of at least one other receptor in these membranes, binding of 3H-methylscopolamine and its displacement by the respective cold ligand was demonstrated. To rule out an occupation of the BK receptors by endogenous formed BK, the tissue was treated with an acidic buffer with high ionic strength to remove surface bound BK. However, this treatment did not unmask any specific binding for the BK antagonist. In view of the possibility that the structure of the labeled antagonist prevented its binding, experiments were carried out using tritiated BK itself. These experiments also failed to demonstrate specific binding sites which indicate that the structural aspects of Hoe 140 are probably not interfering in binding of the antagonist, especially since it binds to ileum. It is concluded, that there are too few binding sites for BK in the rat heart homogenate for ordinary binding studies.(ABSTRACT TRUNCATED AT 250 WORDS)