FUNCTIONAL-CHARACTERIZATION OF TACHYKININ RECEPTORS MEDIATING ION-TRANSPORT IN PORCINE JEJUNUM

In the present study, tachykinin receptors (designated NK1, NK2 and NK 3) involved in regulation of ion transport in porcine jejunum were cha racterised. Stripped tissue preparations were mounted in Ussing chambe rs and short-circuited. Substance P produced a concentration dependent increase in short-circuit current, the relationship showing a double sigmoidal form. The non-peptide NK, receptor antagonist, CP 99,994 ,3S )-3-(2-methoxybenzyl)amino-2-phenylpiperidine), caused a dextral shift of the first sigmoidal response, indicating the involvement of an NK1 receptor. This was further supported by a concentration-dependent res ponse of the NK, receptor agonist [Sar(9)Met(O-2)(11)]substance P with an EC50 value of 235.0 +/- 53.9 nM. Increasing concentrations of CP 9 9,994 (0.1, 0.3 and 1 mu M) produced a parallel dextral shift of the [ Sar(9)Met(O-2)(11)]substance P curve with a slope of the Schild regres sion significantly different from unity (1.59). The neurokinin A conce ntration-response curve, with an EC,, value of 68.87 +/- 16.23 nM, was not significantly changed by the non-peptide NK2 receptor antagonist SR 48,968 piperidino)-2-(3,4-dichlorophenyl)butyl)bezamide). In additi onal studies, the peptide NK, receptor antagonists, GR 94,800 (PhCO-Al a-Ala-DTrp-Phe-DPro-Pro-NleNH(2)) and PD 147,714 ((2,3-diOMeZ)-(S)Trp( S)alpha MePheGlyNH(2)), did not change the response to neurokinin A. H owever, CP 99,994 totally inhibited neurokinin A responses at 0.5 mu M and above. The NK2 receptor agonist, [beta-Ala(8)]neurokinin A-(4-10) , caused only an increase in short-circuit current in mu M concentrati ons, whereas the NK3 receptor agonist, senktide, did not elicit a resp onse. These results indicate, that substance P and neurokinin A mediat e ion transport in porcine jejunum through NK1 receptors. However, tac hykinins seem to activate another receptor. Two active conformers of t he NK1 receptor might be present. (C) 1998 Elsevier Science B.V. All r ights reserved.