ABSTRACT
During fertilization or ionophore induced activation, ascidian eggs rapidly release cell surface N-acetylglucosaminidase activity used in the block against polyspermy and undergo cortical contractions before they re-initiate meiosis. To better understand the activation process, we probed the relationship between these two processes in Ascidia ceratodes eggs by activating with different agents that increase intracellular Ca levels and under different ionic conditions. Glycosidase activity release was followed by the use of a fluorogenic substrate, and cortical contractions were followed by examining changes in cell shape with light microscopy. Ionomycin (2.7 µM) and thimerosal (1 mM) initiate glycosidase release and cortical contractions when administered in complete sea water (SW) but only the contractions in low Ca SW. Ryanodine (0.67 mM), known to raise free intracellular Ca in a number of cell types by release from the endoplasmic reticulum, causes glycosidase release but fails to initiate cortical contractions in complete SW. Thapsigargin (10 µM), which inhibits Ca dependent ATPase in the ER, causes glycosidase release but induces the contractions only about 50% of the time. These experiments show that, although glycosidase release normally precedes the ooplasmic shape changes that accompany the resumption of meiosis in ascidian eggs, they are not obligately coupled. That both processes can be induced by treatments known to raise intracellular Ca in other systems but under different conditions indicates that there may be a multiplicity of Ca requiring but functionally independent events during egg activation.
