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Calcium Release and Subsequent Development Induced by Modification of Sulfhydryl Groups in Porcine Oocytes¹

^a Department of Animal Sciences, University of Missouri-Columbia, Columbia, Missouri 65211

The mechanism of Ca²⁺ release induced by modification of sulfhydryl groups and the subsequent activation of porcine oocytes were investigated. Thimerosal, a sulfhydryl-oxidizing compound, induced Ca²⁺ oscillation in matured oocytes. In thimerosal-preincubated oocytes, the amount of Ca²⁺ released after microinjection of inositol 1,4,5-trisphosphate (InsP₃) or ryanodine increased strikingly, indicating that thimerosal potentiated both InsP₃- and ryanodine-sensitive Ca²⁺ release pathways. Thimerosal also enhanced the sensitivity of oocytes to microinjected Ca²⁺ so that in pretreated oocytes a Ca²⁺ injection triggered a larger transient. Heparin at concentrations that normally block the InsP₃-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 InsP₃ since preinjection with neomycin did not prevent the oscillation.

Immunocytochemistry revealed that thimerosal treatment destroyed the meiotic spindle, preventing further development, an effect that could be reversed by dithiothreitol. The combined thimerosal/dithiothreitol treatment triggered second polar body extrusion in 50% of the oocytes, and as a result of this activation scheme ~15% of the in vitro- and ~60% of the in vivo-matured oocytes developed to blastocyst during a 7-day culture in vitro.

¹ This material is based upon work supported by the Food for the 21st Century and the Cooperative State Research, Education and Extension Service, U.S. Department of Agriculture, under agreement No. 95-37203-2073. The manuscript is a contribution from the Missouri Agriculture Experiment Station Journal Series No. 12,857.

² Correspondence: Randall S. Prather, 162 Animal Science Research Center, University of Missouri-Columbia, Columbia, MO 65211. FAX: 573 882 6827; randall__prather{at}muccmail.missouri.edu

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