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Using patch-clamp single channel recording techniques, we reported a Platelet-Activating Factor "PAF"-induced activation of large conductance Ca(2+)-activated K+ "BK(Ca)" channels in N1E-115 cells. This activation was only observed in cell-attached configuration and was blocked by the PAF antagonist BN50739 or removal of calcium from the bath. Nanomolar concentration of PAF produced a transient hyperpolarization observed in whole-cell current clamp configuration which was blocked by the bath application of BN50739 or iberiotoxin. Our results suggest that the PAF-induced hyperpolarization is mediated by an activation of BK(Ca) channels coupled to specific PAF receptors. This coupling is not direct and results from the PAF-induced elevation in cytosolic free Ca2+ concentrations that we have previously described in N1E-115 cells (Cell Calcium 1995, 17, 442-452).

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.