Stimulation of platelet-activating factor (PAF) receptors increases inositol phosphate production and cytosolic free Ca2+ concentrations in N1E-115 neuroblastoma cells.

Platelet-activating factor (1-O-alkyl-2-acetyl-sn-glycero-3-phosphorylcholine, PAF) has recently been recognized as an important mediator in the pathophysiology of brain injury. This study demonstrates that, in suspended populations of N1E-115 cells loaded with Indo-1, biologically relevant concentrations of PAF produce a rapid and transient elevation in cytosolic free calcium concentration ([Ca2+]i). Moreover, nanomolar concentrations of PAF increase [3H]-inositol phosphate production. Using lyso-PAF and the specific PAF-receptor antagonists BN52021 and BN50739, we show that these effects were mediated by stimulation of PAF receptors. Experiments performed in Ca2+ free medium show that PAF-induced [Ca2+]i increase is the result of an influx of Ca2+ and of the release of intracellular Ca2+ stores. Studies of Mn2+ influx argue in favour of additional pathways for the PAF-induced Ca2+ influx other than the pathway for the thapsigargin-induced Ca2+ influx. Using the whole-cell voltage-clamp technique, we observe that PAF induces an increase of Ltype Ca2+ current. However, the effects of La3+, nifedipine and KCl-induced depolarization on the PAF-induced [Ca2+]i increase suggest a minor participation of these voltage-gated Ca2+ channels in the response to PAF. Altogether the results point to the existence of a PAF-induced Ca2+ influx through receptor-operated Ca2+ permeant channels.