Discrimination between neuropeptide Y and peptide YY in the rat tail artery by the neuropeptide Y1-selective antagonist, BIBP 3226.

1. The ability of the novel, nonpeptide, neuropeptide Y (NPY) Y1-selective antagonist, BIBP 3226 ¿(R)-N2-(diphenylacetyl)-N-[(4-hydroxyphenyl)methyl]-D-arginine amide¿, to antagonize the increase in perfusion pressure induced by NPY and peptide Y (PYY) was tested in the perfused rat tail artery, a postjunctional Y1-receptor bioassay, precontracted by 1 microM phenylephrine. 2. NPY and PYY produced a concentration-dependent enhancement of the vasoconstrictor response evoked by 1 microM phenylephrine. Although NPY and PYY are roughly equipotent, the maximal contractile response elicited by PYY was about twice that elicited by NPY. 3. Increasing concentrations of BIBP 3226 caused a parallel and rightward shift in the NPY concentration-response curve without depressing the maximal response. The contractile effect of NPY was potently inhibited in a competitive manner. The pA2 value for BIBP 3226 was 7.01 +/- 0.08, a value equivalent to that observed in the rabbit saphenous vein. Although increasing concentrations of BIBP 3226 shifted the concentration-response curve of PYY to the right without any significant decrease in the maximal vasoconstrictor response, the antagonism appeared non-competitive as the slope of the Schild plot was significantly different from unity (0.58 +/- 0.04). 4. In conclusion, these data confirm that BIBP 3226 is a potent and selective nonpeptide Y1 receptor antagonist. Moreover, they show that complex interactions occur between BIBP 3226 and postjunctional receptors activated by PYY. We postulate that BIBP 3226 might discriminate between the effects of NPY and PYY at the postjunctional level in the rat tail artery. It may be that distinct receptors for NPY and PYY exist; these may or may not allosterically interact with each other. Another working hypothesis would be that there is a single receptor complex with allosterically interacting binding sites for the two peptides.