Xestospongin C empties the ER calcium store but does not inhibit InsP3-induced Ca2+ release in cultured dorsal root ganglia neurones

The action of Xestospongin C (XeC) on calcium concentration in the cytosol ([Ca2+](i)) and within the lumen of endoplasmic reticulum (ER) ([Ca2+](L)) was studied using cultured dorsal root ganglia (DRG) neurones. Application of 2.5 muM of XeC triggered a slow [Ca2+](i) transient as measured by Fura-2 video-imaging. The kinetics and amplitude of XeC-induced [Ca2+](i) response was similar to that triggered by 1 muM thapsigargin (TG). The [Ca2+](L) was monitored in cells loaded with low-affinity Ca2+ indicator Mag-Fura-2. The cytosolic portion of Mag-Fura-2 was removed by permeabilisation of the plasmalemma with saponin. Application of XeC to these permeabilised neurones resulted in a slow depletion of the ER Ca2+ store. XeC, however, failed to inhibit inositol 1,4,5-trisphosphate (InsP(3))-induced [Ca2+](L) responses. We conclude that XeC is a potent inhibitor of sarco(endo) plasmic reticulum calcium ATPase, and it cannot be regarded as a specific inhibitor of InsP(3) receptors in cultured DRG neurones. (C) 2002 Elsevier Science Ltd. All rights reserved.