TACHYKININ NK2 RECEPTORS - CHARACTERIZATION IN THE RAT SMALL-INTESTINE BY THE USE OF A PEPTIDE AGONIST AND A NONPEPTIDE ANTAGONIST AS RADIOLIGANDS

The tachykinin NK2 receptors present in membranes of rat small intesti ne were characterized by means of tachykinin receptor agonists and ant agonists, by using the natural agonist, NKA, or the nonpeptide antagon ist SR 48968, as radioligands. The affinity of the antagonists was ind ependent of the radioligand used, whereas the NK2 receptor selective a gonists showed a different binding profile according to the radioligan d. In particular, when using [I-125]NKA, NKA and NKA((4-10)) bound to a high affinity site, whilst [beta-Ala(8)]NKA((4-10)) and GR 64349 bou nd to a high and a low affinity site; the high affinity site was still detected in the presence of 100 mu M GppNHp which produced a strong i nhibition of the specific binding of [I-125]NKA. On the other hand, wh en using [H-3]SR 48968, NKA bound to a high affinity site, [beta-Ala(8 )]NKA((4-10)) bound to a low affinity site and NKA((4-10)) and GR 6434 9 bound to a high and a low affinity site; in this case, the high affi nity site was no longer detected in the presence of 1 mu M GppNHp, whi ch did not reduce the specific binding of [H-3]SR 48968 to NK2 recepto rs. We interpreted these data as an indication that in the rat small i ntestine membranes [I-125]NKA labels multiple conformations of G prote in-coupled NK2 receptor which are distinguished by the use of selectiv e receptor agonists, but not by peptide or nonpeptide antagonists.