Platelet-activating factor induced Ca2+ signaling in human microglia

Increases in intracellular Ca2+ concentration in human microglial cells in response to platelet-activating factor (PAF) were studied using Ca2+-sensitive fluorescence microscopy. In normal physiological solution (PSS), PAF-induced transient increases in [Ca2+](i) which recovered to baseline values within 200 s. Application of PAF in zero-Ca2+ solution caused the peak response to be decreased to a value near 20% of that recorded in PSS suggesting a primary contribution of Ca2+ influx for the [Ca2+](i) increase in PSS. To investigate PAF-induced Ca2+ influx, the contents of intracellular stores were modulated using the SERCA blocker cyclopiazonic acid (CPA). The Ca2+ signal induced by CPA (10 mu M) in zero-Ca2+ solution showed a peak response about 20% of the amplitude in the presence of external Ca2+, suggesting the latter response included significant contributions from store-operated Ca2+ entry. The influx of divalent cations with PAF or CPA was directly measured using Mn2+ quenching of the fluorescence signal. Although both PAF and CPA induced a similar degree of Mn2+ influx over time, the PAF effect was very rapid, whereas the CPA action was delayed and only evident about 200 s after application. Overall, the results show that the primary source of the PAF-induced increase of [Ca2+](i) in human microglia was the influx of Ca2+ from the extracellular space and intracellular Ca2+-release contributed only a small part of the total Ca2+ signal. Nevertheless, Ca2+-release induced by PAF (or CPA) serves as an important factor in controlling Ca2+ entry presumably mediated by activation of store-operated-Ca2+ channels. (C) 1999 Published by Elsevier Science B.V. All rights reserved.