Calcium release and subsequent development induced by modification of sulfhydryl groups in porcine oocytes

The mechanism of Ca2+ release induced by modification of sulfhydryl groups and the subsequent activation of porcine oocytes were investigated, Thimero sal, a sulfhydryl-oxidizing compound, induced Ca2+ oscillation in matured o ocytes, In thimerosal-preincubated oocytes, the amount of Ca2+ released aft er microinjection of inositol 1,4,5-trisphosphate (InsP(3),) or ryanodine i ncreased strikingly, indicating that thimerosal potentiated both InsP(3)-,- and ryanodine-sensitive Ca2+ release pathways. Thimerosal also enhanced th e sensitivity of oocytes to microinjected Ca2+ so that in pretreated oocyte s a Ca2+ injection triggered a larger transient. Heparin at concentrations that normally block the InsP(3),-induced Ca2+ release were without effect; higher doses significantly increased the time leading up to the first spike . The thimerosal-induced Ca2+ release could not be blocked by procaine, and it did not require the formation of InsP, since pre-injection with neomyci n did not prevent the oscillation, Immunocytochemistry revealed that thimerosal treatment destroyed the meioti c spindle, preventing further development, an effect that could be reversed by dithiothreitol. The combined thimerosal/dithiothreitol treatment trigge red second polar body extrusion in 50% of the oocytes, and as a result of t his activation scheme similar to 15% of the in vitro- and similar to 60% of the in vivo-matured oocytes developed to blastocyst during a 7-day culture in vitro.