INDEPENDENT INITIATION OF CALCIUM-DEPENDENT GLYCOSIDASE RELEASE AND CORTICAL CONTRACTIONS DURING THE ACTIVATION OF ASCIDIAN EGGS

During fertilization or ionophore induced activation, ascidian eggs ra pidly release cell surface N-acetylglucosaminidase activity used in th e block against polyspermy and undergo cortical contractions before th ey re-initiate meiosis. To better understand the activation process, w e probed the relationship between these two processes in Ascidia cerat odes eggs by activating with different agents that increase intracellu lar Ca levels and under different ionic conditions. Glycosidase activi ty release was followed by the use of a fluorogenic substrate, and cor tical contractions were followed by examining changes in cell shape wi th light microscopy. Ionomycin (2.7 mu M) and thimerosal (1 mM) initia te glycosidase release and cortical contractions when administered in complete sea water (SW) but only the contractions in low Ca SW. Ryanod ine (0.67 mM), known to raise free intracellular Ca in a number of cel l types by release from the endoplasmic reticulum, causes glycosidase release but fails to initiate cortical contractions in complete SW. Th apsigargin (10 mu M), which inhibits Ca dependent ATPase in the ER, ca uses glycosidase release but induces the contractions only about 50% o f the time. These experiments show that, although glycosidase release normally precedes the ooplasmic shape changes that accompany the resum ption of meiosis in ascidian eggs, they are not obligately coupled. Th at both processes can be induced by treatments known to raise intracel lular Ca in other systems but under different conditions indicates tha t there may be a multiplicity of Ca requiring but functionally indepen dent events during egg activation.