Mechanism of action of the non-steroidal anti-inflammatory drug flufenamate on [Ca2+](i) and Ca2+-activated currents in neurons

We have shown previously that the non-steroidal anti-inflammatory drug flufenamate (FFA) causes a maintained increase in [Ca2+](i) and transient increases in a Ca2+-activated nonselective cation current (I-CAN) and a Ca2+-activated slow, outward Cl- current (I-o-slow) in molluscan neurons [Shaw T., Lee R.J., Partridge L.D. Action of diphenylamine carboxylate derivatives, a family of non-steroidal anti-inflammatory drugs, on [Ca2+](i) and Ca2+-activated channels in neurons. Neurosci Lett 1995; 190. 121-124]. Here we demonstrate that pretreatment of neurons with 10 mu M thapsigargin eliminates the FFA-induced increase in [Ca2+](i) and substantially reduces both I-CAN and I-o-slow supporting the hypothesis that the FFA-induced increase in [Ca2+](i) results primarily from Ca2+ release from a thapsigargin-sensitive intracellular store. The [Ca2+](i) response appears to be sustained, not by influx of extracellular Ca2+, but by inhibitory effects of FFA on Ca2+ removal from the cytosol. Inhibition of Ca2+ efflux may be an important component of the FFA-induced activation of both I-CAN and I-o-slow, as Ca2+ release by thapsigargin alone is not sufficient to activate either current. Our data also demonstrate that the effects of FFA on [Ca2+](i), I-CAN and I-o-slow are reversible and suggest that protein phosphorylation as well as an increase in [Ca2+](i) are involved in the FFA-induced activation of I-o-slow. Effects on neuronal Ca2+ handling as well as activation of I-CAN or I-o-slow may partially explain the analgesic effects of FFA.