Increases in intracellular Ca2+ concentration in human microglial cells in
response to platelet-activating factor (PAF) were studied using Ca2+-sensit
ive fluorescence microscopy. In normal physiological solution (PSS), PAF-in
duced transient increases in [Ca2+](i) which recovered to baseline values w
ithin 200 s. Application of PAF in zero-Ca2+ solution caused the peak respo
nse 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+ s
ignal 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 th
e latter response included significant contributions from store-operated Ca
2+ entry. The influx of divalent cations with PAF or CPA was directly measu
red 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 v
ery 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 o
nly a small part of the total Ca2+ signal. Nevertheless, Ca2+-release induc
ed by PAF (or CPA) serves as an important factor in controlling Ca2+ entry
presumably mediated by activation of store-operated-Ca2+ channels. (C) 1999
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