The role of divalent cations in activation of the sea urchin egg. I. Effect of fertilization on divalent cation content.

The Ca and Mg content of unfertilized sea urchin eggs (3 and 21 mumole/ml eggs) remains remarkably constant over periods of hours, even when the eggs are suspended in Ca- or Mg-free sea water. After fertilization the Ca content of eggs in regular sea water increases sharply by about 20%, followed by a decrease to the unfertilized level by 40 minutes. However, if the fertilized eggs are washed three minutes in Ca-free sea water a sharp decrease in the Ca content occurs amounting to 30% of the total in the first 40 minutes, with little change thereafter. Suspension of the eggs in Ca-free sea water results in an even greater loss of Ca amounting to about 43% of the total in the same time interval, followed by a continuing slow loss. It is concluded that fertilization initiates the intracellular release of Ca, which is then extruded. For unwashed eggs this change is masked by the simultaneous generation of new extra-cellular coats with high affinity for Ca. Changes in the Mg content of fertilized eggs follow the same general pattern except that absorption of this divalent cation to the extracellular coats is minimal.