The calcium content of cortical granules and the loss of calcium from sea urchin eggs at fertilization

In many species, fertilization triggers a wave of cortical granule exocytosis in the egg that is the consequence of an increase in intracellular free calcium concentration. We have measured the total calcium content of cortical granules from two species of sea urchins by quantitative X-ray microanalysis and spectrometric measurements. Our results show that cortical granules: (1) contain a high concentration of total calcium (around 30 and 95 mM for Paracentrotus lividus and Arbacia lixula, respectively), (2) represent a major cortical storage site of calcium in the egg (5 and 11% of total egg calcium for P. lividus and A. lixula, respectively), and (3) exchange part of their accumulated calcium by an ATP dependent mechanism. In addition we have confirmed that at fertilization, sea urchin eggs lose a sizeable amount of their calcium (7% for P. lividus and 15% for A. lixula). The kinetics and magnitude of the loss suggest that some of this calcium could be provided by cortical granules during exocytosis.