Activation of Na+-dependent transport at fertilization in the sea urchin: requirements of both an early event associated with exocytosis and a later event involving increased energy metabolism.

Transport of glycine, phosphate, and thymidine has been studied in parallel in eggs and embryos of Strongylocentrotus purpuratus. Uptake of each of these substrates is shown to be sodium dependent and expressed at fertilization in this species. The data indicate that activation of unfertilized eggs by ammonia does not result in significant expression of any of these transport systems compared to fertilized eggs. Activation of unfertilized eggs by sperm or by ionophore A-23187 in seawater results in complete development of these transport systems. However, if eggs are activated by ionophore in sodium-free seawater or fertilized by sperm in complete seawater and transferred to sodium-free seawater, Na+-dependent transport does not develop. Ammonia reverses the inhibitory effect of sodium-free seawater on development of these transport systems. This reversal is sensitive to 2,4-dinitrophenol. The data suggest that transport of glycine, phosphate, and thymidine share a common mechanism of activation. Moreover, this activation requires both an early event (less than 5 min postinsemination) and a later event involving increased energy metabolism.