CHARACTERIZATION OF A NOVEL, SELECTIVE RADIOLIGAND, [I-125] [LYS(5),TYR(I(2))(7),MELEU(9),NLE(10)]NEUROKININ-A-(4-10), FOR THE TACHYKININ NK(2) RECEPTOR IN RAT FUNDUS

The tyrosyl derivative of the tachykinin NK2 selective agonist [Lys5,MeLeu9,Nle10]NKA-(4-10) was iodinated and the product [I-125][Lys5,Tyr(I2)2,MeLeu9,Nle10]NKA-(4-10) purified using reverse phase HPLC. The binding characteristics of this novel radioligand were investigated in homogenates of rat gastric fundus. Binding was saturable, reversible and to a single population of high affinity sites of K(D) 1.3 +/- 0.2 nM (n = 4). Specific binding of [I-125][Lys5,Tyr(I2)7,MeLeu9,Nle10]NKA-(4-10) was inhibited by neuropeptide gamma SR 48968 greater-than-or-equal-to neurokinin A (NKA) greater-than-or-equal-to [Lys5,MeLeu9,Nle10]NKA-(4-10) > [Lys5,Tyr7,MeLeu9,Nle10]NKA-(4-10) > neuropeptide K > [Lys5,Tyr(I2)7,MeLeu9,Nle10]NKA-(4-10) > MDL 29,913 > [I-127-Bolton-Hunter-NKA > neurokinin B > substance P (SP) >> MEN 10207 > [Sar9,Met(O2)11]SP >> senktide, indicating binding to NK2 receptors. NKA, [Lys5,MeLeu9,Nle10]NKA-(4-10) and [Lys5,Tyr(I2)7,MeLeu9,Nle10]NKA-(4-10) contracted the isolated fundus strip, with pD2 values 7.9, 7.7 and 7.4, respectively. This novel, highly selective radioligand should prove useful in characterisation studies in peripheral tissues.