Net calcium and acid release at fertilization in eggs of sea urchins and ascidians

Sea urchin eggs lose about 10-30% of their total calcium content upon fertilization. We have investigated the mechanism of this calcium-loss with an ion-selective vibrating probe system. Upon fertilization of Arbacia punctulata and Lytechinus pictus eggs we could measure a calcium efflux signal with an average duration of 204 +/- 26 s and 146 +/- 46 s, respectively. Measurements of hydrogen ion signals in normal and in low sodium media showed that the release of cortical vesicle material from these eggs lasts for about 30 and 50 s, respectively. The data indicate that most of the calcium that is lost from sea urchin eggs originates from the cytosol in which it is released during fertilization and then pumped out through the plasma membrane. Calcium loss due to cortical granule release accounts for less than 14% of the total loss measured. We also measured a substantial post-fertilization calcium efflux in eggs of Phallusia mammilata, with an average duration of 265 +/- 18 s followed by smaller periodic effluxes that corresponded to oscillations in the [Ca2+]i during contractile waves in these eggs. These data, together with the lack of cortical granules in ascidian eggs, indicate that Phallusia eggs also pump out a substantial amount of calcium through the plasma membrane after fertilization.