EVIDENCE FOR TACHYKININ NK-2 RECEPTORS IN GUINEA-PIG AIRWAYS FROM BINDING AND FUNCTIONAL-STUDIES, USING [I-125] [LYS(5),TYR(I-2)(7),MELEU(9),NIE(10)]-NKA(4-10)

The potent contractile responses of guinea-pig airways to neurokinin A (NKA) and neuropeptide gamma (NPgamma) are thought to be mediated by NK-2 receptors. However, NK-2 binding sites are not detectable using t he radioligand [I-125]-iodohistidyl-NKA. Here, a novel, highly selecti ve iodinated radioligand, [I-125]-[Lys5,Tyr(I2)7, MeLeu9,Nle10]-NKA(4- 10), and a number of related peptides have been used to characterize N K-2 receptors on guinea-pig airways, using binding and functional stud ies. Specific binding of [I-125]-[Lys5,Tyr(I2)7,MeLeu9,Nle10]-NKA(4-10 ), was saturable and to a single high affinity site, with K(D) 1.29 +/ - 0.36 nM (n = 4). The rank order of potency for tachykinins and analo gues as competitors for the binding was: [Lys5,Tyr(I2)7,MeLeu9,Nle10]- NKA(4-10) greater-than-or-equal-to NPgamma greater-than-or-equal-to [L ys5,MeLeu9,Nle10]-NKA(4-10) > NKA greater-than-or-equal-to SR 48968 >> MDL 29913 greater-than-or-equal-to substance P (SP) = [I-127]-Bolton- Hunter NKA (BHNKA) greater-than-or-equal-to MEN 10207 > neurokinin B ( NKB). Septide, [DPro9,Pro10,Trp11]-SP, the NK-1 selective ligands [Sar 9,Met(O2)11]-SP, [Pro9]-SP and CP 96345, the NK-3 selective senktide, and calcitonin gene-related peptide (CGRP) were weak or ineffective. O n guinea-pig isolated bronchi, the potency order of contractile agonis ts was: [Lys5,MeLeu9,Nle10]-NKA(4-10) > NKA greater-than-or-equal-to N Pgamma greater-than-or-equal-to [Lys5,Tyr7,MeLeu9, Nle10]-NKA(4-10) gr eater-than-or-equal-to septide = BHNKA greater-than-or-equal-to [Lys5, Tyr(I2)7,MeLeu9,Nle10]-NKA(4-10) greater-than-or-equal-to [Sar9,Met(O2 )11]-SP greater-than-or-equal-to NKB = [Pro9]-SP greater-than-or-equal -to SP >> senktide. All NK-2 receptor preferring agonists, except BHNK A, had similar potencies in both systems. In conclusion, binding sites for [I-125]-[Lys5,Tyr(I2)7, MeLeu9,Nle10]-NKA(4-10) probably represen t the NK-2-like receptors mediating contractions to NK-2 preferring ta chykinins. NK-1 receptors also mediate contractile responses but NK-3 receptors appear to be negligible in this tissue.