The relation between ionized calcium and cortical granule exocytosis in eggs of the sea urchin Echinus esculentus

By subjecting sea urchin eggs to intense, short-duration, electric fields the permeability to low relative molecular mass substances is markedly increased. After such treatment, the extracellular space markers $^{22}$Na$^{+}$ and [$^{14}$C]mannitol penetrate into the interior of the egg and localized destruction of the oolemma is apparent. The technique permits the rapid introduction of low relative molecular mass substances into the interior of the egg. We have employed it to investigate the efficacy of various buffered calcium concentrations in bringing about exocytosis of cortical granules of the egg. Eggs rendered permeable in the presence of EGTA (free Ca < 10$^{-8}$M) retain a full complement of cortical granules and appear little different in cortical ultrastructure from unfertilized eggs, as judged by scanning and transmission electron microscopy. The proportion of cortical granules remaining in the egg cortex 30 s after application of an electric field in the presence of higher concentrations of calcium decreases progressively as the free calcium concentration introduced into the egg interior is increased from 0.5 to 6 $\mu $M. The disappearance of the cortical granules is attributed to their having undergone exocytosis, since the changes in cortical ultrastructure that result from treatment with micromolar calcium concentrations are demonstrated to be similar to the changes that result from exocytosis of the cortical granules in intact eggs after fertilization.