Thimerosal potentiates Ca2+ release mediated by both the inositol 1,4,5-trisphosphate and the ryanodine receptors in sea urchin eggs. Implications for mechanistic studies on Ca2+ signaling

Abstract

Two intracellular Ca2+ release pathways are modulated by Ca2+ itself, namely Ca(2+)-induced Ca2+ release (classical CICR), and Ca(2+)-sensitized inositol 1,4,5-trisphosphate (InsP3)-induced Ca2+ release (CSIICR). Both of these pathways are thought to be important in generating cytosolic Ca2+ oscillations. Caffeine and the sulfhydryl reagent thimerosal (TMS) are frequently used as selective modulators of CICR and CSIICR, respectively. We have studied the modes of action of caffeine and TMS in sea urchin egg homogenates using the Ca2+ indicator fluo-3. Cyclic ADP-ribose (cADPR), the only known endogenous compound to cause Ca2+ release via the ryanodine receptor, was used as an agonist for the classical CICR pathway. cADPR and InsP3 each alone caused release of sequestered Ca2+. Coaddition of caffeine dose-dependently and dramatically enhanced Ca2+ release induced by cADPR. InsP3-induced Ca2+ release was not affected by caffeine, but it was amplified by TMS, indicating the existence of the CSIICR pathway in this preparation. Surprisingly, cADPR-induced Ca2+ release was also markedly potentiated by TMS. TMS alone caused a slow increase in Ca2+ release which was not inhibited by heparin and/or procaine, indicating that TMS-induced Ca2+ release was not due to the actions of endogenous agonists (InsP3 or cADPR) on their receptors. All observed actions of TMS were abolished by dithiothreitol. We conclude: 1) both the InsP3- and cADPR-controlled Ca2+ release pathways are potentiated by TMS, while caffeine is specific for the cADPR pathway, indicating that TMS cannot be used as a selective reagent for probing the CSIICR mechanism; and 2) the action of TMS on intracellular Ca2+ release is independent of endogenous Ca2+ channel agonists, suggesting a novel action of TMS possibly on intravesicular Ca2+ handling.