Regulation of the fertilization current in ascidian oocytes by intracellular second messengers.

Neomycin, injected into ascidian oocytes to a final concentration of 10-50 mM, inhibits both the fertilization current and the surface contraction, showing that phosphoinositide hydrolysis is required for these early activation events. Sperm-activated fertilization currents are not inhibited in the presence of 100 micrograms/ml intracellular heparin, suggesting that these currents are not directly gated by InsP3. The sulfhydryl reagent thimerosal at 100 microM, in contrast, significantly increases the fertilization current presumably by sensitizing the channel receptor. Since heparin inhibits the surface contraction, InsP3 receptors are shown to play a role in the propagation of the activation response in ascidian oocyte. Depleting intracellular calcium stores by microinjecting 50 mM EGTA into oocytes does not activate fertilization channels; however, subsequent fertilization of these EGTA loaded oocytes leads to a significantly larger and faster fertilization current. Thus in contrast to somatic cells studied to date, second messenger operated plasma membrane channels in ascidian oocytes are not gated by calcium released from intracellular stores.