EFFECT OF BIBP3226 ON INOSITOL PHOSPHATE ACCUMULATION AND CYTOSOLIC CALCIUM LEVEL IN CONTROL AND NPY Y-1 RECEPTOR-EXPRESSING CHO-K1 CELLS

BIBP3226 was developed as a potent, selective and competitive antagoni st for NPY Y-1 receptors by mimicking the C-terminal part of NPY. In a greement with previous studies, NPY mediated a pertussis toxin sensiti ve elevation of intracellular calcium concentration in CHO-K1 cells th at express recombinant human NPY Y-1 receptors which can be inhibited by BIBP3226. Surprisingly micromolar concentrations of BIBP3226 were f ound to induce by itself a fast increase of intracellular calcium conc entration followed by a sustained elevated level of this ion. These re sponses of BIBP3226 are not mediated by NPY receptor activation since (1) they are still present after NPY receptor activation and desensiti zation, (2) they are also evoked by the receptor inactive enantiomer B IBP3435, (3) they are not affected by pretreatment of the cells with p ertussis toxin, (4) they also occur in non-transfected CHO-KI cells. P reincubation of the cells with EGTA abolished only the sustained incre ase calcium concentration elicited by BIBP3226 suggesting that the fas t increase of intracellular calcium concentration reflects the mobiliz ation of intracellular calcium pools. The ability of thapsigargin to c ompletely inhibit BIBP3226 mediated responses, in the presence or abse nce of extracellular calcium indeed indicated that BIBP3226 mobilizes intracellular Ins(1,2,3)P-3 sensitive calcium stores. In agreement, BI BP3226 was found to activate phospholipase C since the responses were completely inhibited by U73122. Furthermore, when measured in the pres ence of 10 mM LiCl, BIBP3226 caused an increased accumulation of inosi tol phosphates. This effect of BIBP3226 is likely to be mediated by ac tivation of an until now unknown receptor or cellular target that is e ndogeneously expressed in CHO-K1 cells. (C) 1998 Elsevier Science BN. All rights reserved.