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 intestine were characterized by means of tachykinin receptor agonists and antagonists, by using the natural agonist, NKA, or the nonpeptide antagonist SR 48968, as radioligands. The affinity of the antagonists was independent of the radioligand used, whereas the NK2 receptor selective agonists showed a different binding profile according to the radioligand. In particular, when using [125I]NKA, NKA and NKA(4-10) bound to a high affinity site, whilst [beta-Ala8]NKA(4-10) and GR 64349 bound to a high and a low affinity site; the high affinity site was still detected in the presence of 100 microM GppNHp which produced a strong inhibition of the specific binding of [125I]NKA. On the other hand, when using [3H]SR 48968, NKA bound to a high affinity site, [beta-Ala8]NKA(4-10) bound to a low affinity site and NKA(4-10) and GR 64349 bound to a high and a low affinity site; in this case, the high affinity site was no longer detected in the presence of 1 microM GppNHp, which did not reduce the specific binding of [3H]SR 48968 to NK2 receptors. We interpreted these data as an indication that in the rat small intestine membranes [125I]NKA labels multiple conformations of G protein-coupled NK2 receptor which are distinguished by the use of selective receptor agonists, but not by peptide or nonpeptide antagonists.