Acid Release during Activation of Barnea candida (Mollusca, Pelecypoda) Oocytes

Barnea caridida oocytes release acid (1.35 pmole H⁺/oocyte) upon fertilization. After artificial activation by an excess of KCl, germinal vesicle breakdown (GVBD) occurs normally and a quite similar, but not identical, acid release is recorded (1.10 pmole H⁺/oocyte). KCl activation of Barnea oocytes is completely inhibited in 100 mM sodium-acetate sea water at pH 6.5 and fertilization does not result in activation when the oocytes are transferred after one minute into 100 mM sodium-acetate sea water at pH 6.3. When D–600, a calcium transmembrane fluxes inhibitor, is added 20 seconds after fertilization, GVBD is inhibited but a normal acid release is recorded. The presence of at least 10 mM sodium ions in the external medium is required for 100% activation of these oocytes by an excess of KCl. These results suggest that while an intracellular pH increase may be a requisite for GVBD, this can not be a sufficient condition to trigger it unless a calcium influx is allowed to occur. Moreover, the acid release does not result from a Ca⁺⁺-H⁺ exchange transport but appears more likely to be due to a Na⁺-H^* exchange as it has been demontrated in sea urchin eggs.