DIFFERENT CALCIUM-DEPENDENT PATHWAYS CONTROL FERTILIZATION-TRIGGERED GLYCOSIDE RELEASE AND THE CORTICAL CONTRACTION IN ASCIDIAN EGGS

Fertilisation of ascidian eggs induces the rapid release of a cell sur face N-acetylglycosaminidase that blocks sperm binding to vitelline co at sperm receptors resulting in a block to polyspermy. Fertilisation a lso triggers a large contraction of the egg (thus stimulating ooplasmi c segregation) that is completed within 5 min of insemination. In eggs of the ascidian Phallusia mammillata, glycosidase release and cortica l contractions are blocked by BAPTA-AM [bis-(o-aminophenoxy)-ethane-N, N,N',N'-tetraacetic acid, tetra(acetoxymethyl)-ester], a cell-permeant calcium chelator, indicating that both processes are probably depende nt on a rise in intracellular calcium levels. Both glycosidase release and the cortical contraction are induced by treatment of the egg with the protein synthesis inhibitor emetine, while only the glycosidase r elease is induced by isoproterenol, carbachol or acetylcholine. Previo us work with ryanodine demonstrated that ryanodine also caused glycosi dase release but not the cortical contraction. Inversely, activation b y ionomycin in calcium-free sea water causes cortical contractions but not glycosidase release. Thus the two processes can be activated inde pendently. Dextran-coupled (10 kDa) calcium green-1 injected eggs show an increase in intracellular calcium 30-40 s before the cortical cont raction is triggered by fertilisation or ionomycin-induced activation. This confirms previous findings that the cortical contraction is a co nsequence of the activation calcium wave triggered by the sperm. The e xtracellular calcium requirement for the glycosidase release suggests that calcium influx may be more important for this phase of egg activa tion. Thus activation of ascidian eggs appears to involve two independ ent pathways involving calcium.