Radioligand binding and functional characterisation of tachykinin receptors in chicken small intestine

Tachykinin receptors in chicken intestine were studied using radioligand bi nding and functional techniques. Mechanisms of tachykinin-induced contracti on were also investigated. Binding of [I-125]Bolton-Hunter substance p ([I- 125]BH-SP) to chicken ileal membranes was rapid, saturable, of high affinit y and to a single population of binding sites with K-d 0.72 nM and B-max 0. 48 fmol/wet weight tissue. The rank order of agonists competing for [I-125] BH-SP binding sites was [Sar(9)]SP > [Arg(3)]Sp (natural tachykinin in chic kens) > SP > [Pro(9)]SP greater than or equal to NKA > eledoisin > [Sar(9), Met(O-2)(11)]SP > > [Lys(5),MeLeu(9),Nle(10)]-NKA(4-10) > > senktide, sugge sting similarities to the mammalian NK1 receptor. The NK1 receptor antagoni st CP 99994, and NK2 receptor antagonist SR 48968 were weak competitors whi le spantide, RP 67580, GR 82334, GR 94800 and MEN 11420 were ineffective. T he radioligand [I-125]NKA showed no specific binding to ileal membranes. The potency order of most tachykinins in contacting isolated ileal longitud inal segments was in good agreement with that obtained from competition bin ding studies. Contractions to [Arg(3)]SP, NKA and senktide were greatly red uced by tetrodotoxin, suggesting that neurally-mediated responses were prim arily involved. [Arg(3)]SP and NKA acted mainly by increasing release of ac etylcholine, prostaglandins and probably tachykinins. Responses to [Arg(3)] Sp were virtually abolished by nifedipine but were unaffected by NK1 recept or antagonists. Senktide-induced contraction was inhibited by the NK3 recep tor antagonist, SR 142801, but was unaffected by atropine or L-NAME. The st udy provides evidence for a tachykinin receptor with similarities to the NK 1 receptor in the chicken small intestine. In addition, senktide may act on a receptor similar to the mammalian NK3 receptor.