THIMEROSAL TRIGGERS MEIOSIS REINITIATION IN OOCYTES OF THE JAPANESE CLAM RUDITAPES-PHILIPPINARUM BY ELICITING AN INTRACELLULAR CA2+ SURGE

Ovarian oocytes of the bivalve mollusc Ruditapes philippinarum are arr ested during first meiotic prophase. Release from this blockade is tri ggered by the neurohormone serotonin (5HT or 5-hydroxytryptamine), whi ch promotes germinal vesicle breakdown and drives these oocytes to a s econd arrest in metaphase I.5HT action involves binding to a specific G protein-coupled receptor which results in a transient rise in IP3 an d in the intracellular free Ca2+ concentration. Here we analyze the cy tological effects and mode of action of the sulphydryl reagent thimero sal which could also trigger meiosis reinitiation in Ruditapes. No met aphase I spindle formed under these conditions since thimerosal was fo und to be able to preclude or reverse tubulin polymerization when appl ied to prophase- or to metaphase-arrested oocytes, respectively. Our r esults strongly suggest that the common final target for 5HT and thime rosal actions consists in a transient rise in internal free Ca2+ level that we could follow using Fluo3/AM as a probe. The effect of thimero sal in promoting oocyte maturation and increasing intracellular free C a2+ concentration was improved by excess KCI. In addition, thimerosal, but not KCl, was found to facilitate 5HT-induced maturation at subthr eshold hormone concentrations which, by themselves, did not produce an intracellular Ca2+ surge. These data suggest that thimerosal may inhi bit Ca2+ pumps of the endoplasmic reticulum and unmask the plasma memb rane voltage-sensitive Ca2+ channels which also appear after 5HT-induc ed GVBD.