TACHYKININ PEPTIDES AFFECT DIFFERENTLY THE 2ND-MESSENGER PATHWAYS AFTER BINDING TO CHO-EXPRESSED HUMAN NK-1 RECEPTORS

The human NK-I receptor transfected in Chinese hamster ovary (CHO) cel ls was studied with use of different tachykinin analogs: Substance P, [Pro(9)]SP, [Sar(9), Met(O-2)(11)]SP, [Gly(9) Psi(CH2CH2) Leu(10)]SP, Ac-Arg;septide, septide, [Gly(9) Psi(CH2CH2) Gly(10)]SP, NKA [pGlu(6)] SP(6-11) and [Lys(5)]NKA(4-10). Binding experiments with [H-3][Pro(9)] SP discriminated two classes of peptides with either high affinity (K- i in the nanomolar range) for the human NK-1 receptor or with low affi nity (K-i in the micromolar range); this second group of peptides incl uded NKA and [pGlu(6)]SP(G-II). In spite of these differences, both pe ptide families evoked potent stimulation of phosphatidylinositol hydro lysis (EC(50) in the nanomolar range). In contrast, only NK-1 agonists , with high affinity, stimulated with great potency cyclic AMP formati on (EC(50) from 8 to 50 nM), whereas the second family of peptides wer e only weak agonists (EC(50) in the micromolar range). RP 67580, CP 96 345 and GR 94800, a NK-2 antagonist, were either competitive or uncomp etitive inhibitors of inositol phosphates or cyclic AMP formations ind uced by [Pro(9)]SP: septide or NKA, independently of the agonist or th e response studied. Thus, NKA, the presumed NK-2 endogenous peptide th at may be co-released with SP, and the enzymatically produced C-termin al fragment of SP, [pGlu(6)]SP(6-11), may trigger specific pharmacolog ical responses via the NK-1 receptor, at nanomolar concentrations, and thus regulate the action of SP at the NK-1 receptor.